C15 Instruction Manual Issue 8

Wind Tunnel

Instruction Manual C15 ISSUE 8 July 2010

Table of Contents Copyright and Trademarks ...................................................................................... 1 General Overview ....................................................................................................... 2 Equipment Diagrams................................................................................................... 3 Important Safety Information....................................................................................... 4 Introduction.............................................................................................................. 4 Electrical Safety....................................................................................................... 4 Noise ....................................................................................................................... 4 Moving or Rotating Components ............................................................................. 4 Fast-Moving Air Streams ......................................................................................... 5 Heavy Equipment .................................................................................................... 5 Water Borne Hazards .............................................................................................. 6 Description .................................................................................................................. 7 Overview.................................................................................................................. 7 Important note on pressure measurement using the tunnel .................................... 7 Working Section ...................................................................................................... 8 IFD7......................................................................................................................... 8 Static Pressure Sensor............................................................................................ 9 Manometers............................................................................................................. 9 Circular Hatch........................................................................................................ 10 Small Hatch ........................................................................................................... 11 Roof Tappings ....................................................................................................... 11 Fan ........................................................................................................................ 12 Flow Visualisation.................................................................................................. 12 C15-11 Inclined Manometer Bank (optional accessory)........................................ 13 C15-12 Electronic Manometer (optional accessory).............................................. 15 C15-13 Lift and Drag Balance (optional accessory) .............................................. 16 C15-14 Pitot Static Tube (optional accessory) ...................................................... 17 C15-15 Wake Survey Rake (optional accessory).................................................. 18 C15-20 Lift and Drag Aerofoil (optional accessory)............................................... 18 ii

Table of Contents C15-21 Pressure Wing (optional accessory) ......................................................... 18 C15- 22 Drag Models (optional accessory) ........................................................... 19 C15- 23 Pressure Cylinder (optional accessory) ................................................... 19 C15-24 Bernoulli Apparatus (optional accessory) ................................................. 20 C15-25 Boundary Layer Plates (optional accessory) ............................................ 20 C15-26 Project Kit (optional accessory) ................................................................ 21 Installation ................................................................................................................. 22 Advisory................................................................................................................. 22 Installation Process ............................................................................................... 22 Electrical Wiring Diagram ...................................................................................... 28 Operation .................................................................................................................. 29 Operating the Software.......................................................................................... 29 Operating the Equipment....................................................................................... 39 Equipment Specifications.......................................................................................... 54 Overall Dimensions ............................................................................................... 54 Electrical Supply .................................................................................................... 54 Mains Water Supply .............................................................................................. 54 Connection to Drain............................................................................................... 54 Clearance .............................................................................................................. 54 USB Channel Numbers ......................................................................................... 54 Available Accessories............................................................................................ 56 C15-10 Motor Rating ............................................................................................. 56 C15-11 Manometer................................................................................................ 56 C15-12 Manometer................................................................................................ 57 C15-13 Lift and Drag Balance ............................................................................... 57 Requirements for the production of models of the student’s own design .............. 57 Environmental Conditions...................................................................................... 57 Routine Maintenance ................................................................................................ 59 Responsibility ........................................................................................................ 59

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Armfield Instruction Manual General.................................................................................................................. 59 Cleaning ................................................................................................................ 59 RCD Test............................................................................................................... 59 Fan check .............................................................................................................. 60 Replenishing the Manometer Reservoir ................................................................ 60 Lubrication ............................................................................................................. 60 Spares ................................................................................................................... 60 Calibration ............................................................................................................. 60 Laboratory Teaching Exercises................................................................................. 61 Index to Exercises ................................................................................................. 61 Introduction............................................................................................................ 61 Nomenclature ........................................................................................................ 61 Exercise A - Conversion of head measurement to pressure measurement.............. 64 Exercise B - Static pressure, dynamic pressure and total pressure.......................... 69 Exercise C - Effect of change in cross section and application of the Bernoulli equation .................................................................................................................... 73 Exercise D - Flow around a cylinder ......................................................................... 78 Exercise E - Drag forces on bluff and streamlined bodies ...................................... 83 Exercise F - Flow and pressure distribution around a symmetrical aerofoil at different angles of attack ......................................................................................................... 89 Exercise G - Lift and Drag forces on a symmetrical aerofoil at different angles of attack......................................................................................................................... 95 Exercise H - Laminar and Turbulent Boundary Layer Development..................... 103 Exercise I - Project Work......................................................................................... 109 Contact Details for Further Information ................................................................... 111

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Disclaimer This document and all the information contained within it is proprietary to Armfield Limited. This document must not be used for any purpose other than that for which it is supplied and its contents must not be reproduced, modified, adapted, published, translated or disclosed to any third party, in whole or in part, without the prior written permission of Armfield Limited. Should you have any queries or comments, please contact the Armfield Customer Support helpdesk (Monday to Friday: 0800 – 1800 GMT). Contact details are as follows: United Kingdom

International

(0) 1425 478781 (calls charged at local rate)

+44 (0) 1425 478781 (international rates apply)

Email: [email protected] Fax: +44 (0) 1425 470916

Copyright and Trademarks Copyright © 2009 Armfield Limited. All rights reserved. Any technical documentation made available by Armfield Limited is the copyright work of Armfield Limited and wholly owned by Armfield Limited. Brands and product names mentioned in this manual may be trademarks or registered trademarks of their respective companies and are hereby acknowledged.

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General Overview The C15-10 is a small wind tunnel designed for bench top operation, with a square, transparent working section and a variable-speed fan for wind speed control. The operating range is nominally 0 – 32 m/s with no model installed in the working section. The maximum velocity achievable will vary with the type of model installed and depends on the blockage created by the model (most of the models available for use in the tunnel are designed for use at lower velocity). The tunnel is designed with an inlet flow straightener and contraction ratio to give well developed air flow through the working section. Wind tunnels are a useful tool for studying air flow around bodies. The available range of accessories is designed so that all the standard demonstrations of flow around bodies can be performed, including a visual indication of flow path as well as measurement of static and total pressures, lift and drag. The tunnel incorporates an Armfield IFD7 interface, which provides connection to a suitable PC. The supplied Armfield C15-304 software provides sensor output logging and fan control as well as performing any required calculations for each demonstration. More detailed information may be found in the Description section, Operation section, Equipment Specifications section and Laboratory Teaching Exercises sections of this instruction manual.

The C15-10 Wind Tunnel with C15-11 Inclined Manometer Bank

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Equipment Diagrams

Figure 1 Front View of C15-10 Wind Tunnel (shown with C15-11 Inclined Manometer Bank)

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Important Safety Information Introduction All practical work areas and laboratories should be covered by local safety regulations which must be followed at all times. It is the responsibility of the owner to ensure that all users are made aware of relevant local regulations, and that the apparatus is operated in accordance with those regulations. If requested then Armfield can supply a typical set of standard laboratory safety rules, but these are guidelines only and should be modified as required. Supervision of users should be provided whenever appropriate. Your C15 Wind Tunnel has been designed to be safe in use when installed, operated and maintained in accordance with the instructions in this manual. As with any piece of sophisticated equipment, dangers exist if the equipment is misused, mishandled or badly maintained.

Electrical Safety The equipment described in this Instruction Manual operates from a mains voltage electrical supply. It must be connected to a supply of the same frequency and voltage as marked on the equipment or the mains lead. If in doubt, consult a qualified electrician or contact Armfield. The equipment must not be operated with any of the panels removed. To give increased operator protection, the unit incorporates a Residual Current Device (RCD), alternatively called an Earth Leakage Circuit Breaker, as an integral part of this equipment. If through misuse or accident the equipment becomes electrically dangerous, the RCD will switch off the electrical supply and reduce the severity of any electric shock received by an operator to a level which, under normal circumstances, will not cause injury to that person. At least once each month, check that the RCD is operating correctly by pressing the TEST button. The circuit breaker MUST trip when the button is pressed. Failure to trip means that the operator is not protected and the equipment must be checked and repaired by a competent electrician before it is used.

Noise This equipment generates noise when running. 

It is advisable to switch off the equipment before giving verbal instructions.

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Ensure that all local noise regulations are followed when positioning the apparatus for use.

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Depending on operator comfort, duration of operation and local noise regulations, ear defenders may be required. Noise emissions should be measured with the tunnel in its operational location.

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Noise levels should be rechecked if the equipment is repositioned, as the new surroundings will absorb and reflect sound differently.

Moving or Rotating Components This apparatus has moving or rotating components. 4

Important Safety Information 

Do not remove any protective guards while the equipment is in operation.

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When operating the apparatus ensure that long hair is tied back out of the way, and that clothing and jewelry cannot come into contact with any moving parts. Dangling items such as necklaces or neckties must be removed or secured so that they cannot become entangled in the equipment.

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Do not touch any moving components while the apparatus is in use, or insert any item into any moving or rotating section of the equipment, unless specifically instructed to do so in the Operational or Experimental sections of this manual.

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Ensure that the apparatus is switched off and that all moving parts have come to rest before handling the equipment before changing the model in use.

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All models used must be firmly secured in place, with no loose components that could become detached in use.

Fast-Moving Air Streams This apparatus generates fast moving air streams at inlet and outlet. 

Ensure that the equipment is positioned so that there are no obstructions to air entering or leaving the Wind Tunnel.

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Be aware that air will be moving quickly at the inlet and outlet of the wind tunnel. There is a risk that light objects may be sucked into the inlet or blown over up to several meters from the outlet.

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To avoid possible damage to eyesight, avoid looking directly into the outlet when the wind tunnel is in operation.

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All loose clothing such as neckties, scarves and long hair must be securely fastened.

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Ensure that the tunnel is positioned appropriately.

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All models used must be firmly secured in place, with no loose components that could become detached in use.

Heavy Equipment This apparatus is heavy. 

The apparatus should be placed in a location that is sufficiently strong to support its weight, as described in the Installation section of the manual.

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Use lifting tackle, where possible, to install the equipment. The equipment may be securely fastened to a pallet for lifting/carrying to the installation location using a fork-lift or similar. The equipment MUST be supported by the metal frame during lifting, NOT by the tunnel itself. Where manual lifting is necessary, two or more people may be required for safety, and all should be made aware of safe lifting techniques to avoid strained backs, crushed toes, and similar injuries.

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Safety shoes and/or gloves should be worn when appropriate.

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Armfield Instruction Manual

Water Borne Hazards The equipment described in this instruction manual involves the use of water, which under certain conditions can create a health hazard due to infection by harmful micro-organisms. For example, the microscopic bacterium called Legionella pneumophila will feed on any scale, rust, algae or sludge in water and will breed rapidly if the temperature of water is between 20 and 45°C. Any water containing this bacterium which is sprayed or splashed creating air-borne droplets can produce a form of pneumonia called Legionnaires Disease which is potentially fatal. Legionella is not the only harmful micro-organism which can infect water, but it serves as a useful example of the need for cleanliness. Under the COSHH regulations, the following precautions must be observed: 

Any water contained within the product must not be allowed to stagnate, ie. the water must be changed regularly.

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Any rust, sludge, scale or algae on which micro-organisms can feed must be removed regularly, i.e. the equipment must be cleaned regularly.

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Where practicable the water should be maintained at a temperature below 20°C. If this is not practicable then the water should be disinfected if it is safe and appropriate to do so. Note that other hazards may exist in the handling of biocides used to disinfect the water.

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A scheme should be prepared for preventing or controlling the risk incorporating all of the actions listed above.

Further details on preventing infection are contained in the publication “The Control of Legionellosis including Legionnaires Disease” - Health and Safety Series booklet HS (G) 70.

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Description Where necessary, refer to the drawings in the Equipment Diagrams section.

Overview The C15-10 is a small wind tunnel with a square working section (15) that is designed for bench top operation. Air is drawn in through the working section by a variable speed fan (10) located at the discharge end of the tunnel. A honeycomb type flow straightener (2) and a 9.4:1 contraction ratio (3) ensure well developed air flow through the working section. Accessories include an electronic manometer bank, which, together with the supplied IFD7 Electrical Console and C15-304 software, allows full electronic monitoring and recording of the measured pressures on a suitable PC (not supplied).

Important note on pressure measurement using the tunnel To minimise turbulence inside the working section the fan is mounted at the discharge end of the tunnel so that air is sucked through the working section. When the fan is operating the pressure inside the working section is therefore subatmospheric and any static pressure measurement will be slightly below atmospheric pressure. When using the C15-11 Inclined manometer, the bottom of each tube is connected to a common water reservoir and the top of appropriate tubes are connected to the tunnel or a model inside the tunnel. At atmospheric pressure (no air flow) each manometer tube will indicate the same level at the bottom of the tube that is coincident with the level in the water reservoir. As the air velocity increases the static pressure falls inside the tunnel and water is drawn up the relevant tubes i.e. lower pressure results in larger readings on the manometer. The left hand manometer tube on C15-11 is connected to the static pressure tapping at the rear of the working section to provide a datum for measurements inside the tunnel. This measurement can also used for calculating the air velocity at the entrance to the working section. Any manometer tube left disconnected is open to atmosphere and therefore shows the atmospheric datum. Absolute pressures in the tunnel may be determined by relating the tunnel datum to the atmospheric datum then adding the measured barometric pressure. Total pressure, as the sum of the static and dynamic pressures, will be higher than the static pressure and will therefore give a smaller differential between the (subatmospheric) reading and the outside air pressure, and thus a lower reading on the manometer than that for static pressure. For example, when using the C15-14 Pitot Static tube the static tapping will register higher on the manometer than the total pressure tapping. This is the opposite of normal convention when a Pitot Static tube is used in free air (where the total head reading would be greater than the static head reading). An illustration is provided below.

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Graph of pressure differentials in the C15 wind tunnel (Not to scale)

Ignoring frictional losses the Total pressure (stagnation pressure) in the free stream will be equal to the atmospheric pressure so Total pressure measured using the optional C15-14 will be very close to the atmospheric pressure indicated in unused tubes of the manometer. Note that when the absolute local total pressure is greater than the absolute local static pressure, the manometer reading for total pressure will be lower than the reading for static pressure. N.B. Usually local static pressure = tunnel static pressure. Exceptions occur when the cross-sectional area at the point of measurement is modified, for example when using the C15-24 Bernoulli Apparatus (Venturi). Pressures in the tunnel are sub-atmospheric due to the increased velocity and reduced cross-sectional area. The effect of changing velocity and area on fluid pressure is described by the Bernoulli Equation and is investigated in Exercise C.

Working Section The working section (15) is 150 mm (6”) square and constructed from clear acrylic to give good visibility of the models in operation. The overall length of the working section is 455mm. Appropriate model / instrumentation mounting points are included in the side wall and roof of the working section. The entire base of the working section is also removable to allow the insertion of large or complex models such as the C15-24 Bernoulli Apparatus, C15-25 Boundary Layer Plate or alternative models constructed by the user.

IFD7 The C15-10 Wind Tunnel is supplied with the Armfield IFD7 Electrical Console (11) fitted, allowing the equipment to be controlled from a suitable PC (not supplied) via a USB port.

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Description

Static Pressure Sensor An electronic pressure sensor (5) mounted in a tapping through the side wall at the rear of the working section measures the static pressure inside the working section, allowing the instantaneous air velocity to be calculated and displayed on the computer. The support plug incorporating the pressure sensor can be interchanged with the upstream blanking plug in the roof to allow measurement of the static pressure when using the optional Bernoulli Apparatus C15-24 (see Roof Tappings).

Manometers A manometer bank is required for use with some of the models. Two options are available: a 13 tube inclined water manometer (C15-11) or a sixteen channel electronic manometer (C15-12).

C15-11 Water Manometer

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C15-12 Electronic Manometer

With the C15-12 electronic manometer the readings and data logging are integrated with the wind tunnel operational software. With the C15-11 water manometer the readings can still be integrated and recorded, but need to be entered into the computer manually. Both manometers incorporate quick release connectors that allow appropriate models or instruments to be connected in seconds. For further details about the two alternative manometers refer to C15-11 Inclined Manometer Bank (optional accessory) and C15-12 Electronic Manometer (optional accessory).

Circular Hatch Many of the optional models are mounted through a circular opening (7), 120 mm diameter, in the front wall of the working section. These models are permanently mounted on a hatch cover to seal the opening (flush with the inside wall of the working section to avoid disturbing the flow). The hatch cover is secured by quick release clamps on the side wall of the working section allowing rapid change from one model to another. Where necessary the hatches incorporate an angular scale allowing the model to be manually rotated to known angles. The standard hatch cover supplied with the C15-10 Wind Tunnel incorporates a central boss with a hole, locating slot and clamping screw. This feature allows optional models such as C15-20 or C15-22 to be mounted securely in the working section when performing flow visualisation studies or when used in conjunction with the Wake survey rake (C15-15). This avoids unnecessary handling of the C15-13 Lift & Drag Balance and allows these models to be used where C15-13 is not available. A plain, clear acrylic hatch cover is supplied with the Project kit (C15-26). This can be modified as required by the user to mount alternative models.

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Description

Small Hatch A second, smaller hatch (8) behind the model mounting position allows the optional Wake Survey Rake (C15-15) to be installed downstream of the various optional models. A plain hatch cover is installed until this option is fitted.

Roof Tappings Three tappings (6) in the roof of the working section allow the flow visualisation system (supplied with C15-10) or the Pitot Static tube (option C15-14) to be inserted. These tappings are located at the start of the working section, upstream and downstream of the model mounting position. Each tapping incorporates a blanking plug, flush with the inside wall of the working section, that can be fitted when the tapping is not used to avoid disturbances in the working section.

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Fan Air flow through the working section is generated by a fan (10) located at the outlet end of the wind tunnel. The fan is fitted with a protective grill on the outside to prevent personnel from coming into contact with the rotating blades. Care must be taken when installing a model to ensure that the model is secure before starting the fan. A model that is not secure could be sucked into the rotating fan blades causing damage to the model and damage to the fan.

Flow Visualisation The working section incorporates a simple technique for flow visualisation around any of the optional models. A lightweight twine follows the flow contour around the model and shows if and where boundary layer separation (breakaway) occurs and where the flow becomes turbulent or reverses.

The twine passes through a stainless steel ‘L’ shaped tube that is mounted in a support plug that can be located in the roof of the working section at three alternative positions, i.e. the start of the working section (the usual position) and upstream and downstream of the model mounting position. The support plug incorporates an ‘O’ ring to retain the tube where it is positioned. A simple adjustment arrangement allows the length and position of the twine to be varied. The vertical position of the twine can be varied by sliding the ‘L’ shaped tube up or down in the support plug. The horizontal position of the twine can be varied by rotating the ‘L’ shaped tube in the support plug. The length of the twine can be varied by allowing more or less twine to pass through the tube then securing the twine to the tube by sliding the ‘O’ ring over the end of the tube. Adjustment of the length is best carried out when the Wind Tunnel is operating. The end of the twine should be tied to the ’O’ ring before operating the fan so that the twine cannot accidentally enter the working section and become entangled with the fan.

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Description

C15-11 Inclined Manometer Bank (optional accessory) A bank of 13 transparent tubes inclined at 30° to measure small pressure differences (0 – 160 mm H 2 O) using water as the working fluid for safe operation and convenience in use. When installed on the wind tunnel, the manometer is located inside the frame below the test section to the left hand side of the IFD7 Electrical Console.

The C15-11 manometer (12) incorporates a water reservoir with a screw operated displacer (13) to allow rapid adjustment of the datum level in the manometer. Any change in the level in one tube affects the level in all of the other tubes because they are connected to the common reservoir. After each adjustment to the model, the wind speed etc. the displacer should be screwed up or down as required to restore the tube(s) at atmospheric pressure to the original datum. All readings can then be recorded relative to a common datum.

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The manometer incorporates quick release connectors on the side for rapid connection to appropriate models and instruments. The 10 way connector is connected to tubes 1 to 10 and the two separate connectors are connected to tubes 11 and 12.

A sliding cursor is fitted to each manometer tube. These can be slid along the tubes to record the different water levels. The reading is then preserved when a change is made allowing comparison of results. Alternatively a set of readings can be preserved when the fan is switched off. The cursors also make the calculation of differential readings easier and help to reduce parallax error. All of the cursors can be slid to the bottom or top of the tubes when not required.

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Description Each bold engraved line on the backboard corresponds to 10 mm H 2 O, each fine line corresponds to 2 mm H 2 O (the reading is magnified by a factor of x2 because the tube is inclined at 30°). For conversion to alternative engineering units: 1 mm H 2 O = 9.80665 Pascals (N/m2) 1 mm H 2 O = 0.001 422 334 PSI 1 mm H 2 O = 0.039 37 Inches H 2 O As described already (see Important note on pressure measurement using the tunnel), the static pressure in the working section will be sub-atmospheric when the fan is operating. Reducing pressure will be displayed as increasing head on the inclined manometer because the tappings in the working section are connected to the top of each manometer tube and reduced pressure will suck water up the tube. Stagnation pressure in the working section will be very close to atmospheric pressure, allowing for frictional losses, i.e. a low reading on the manometer when the fan is in operation. The relative values can be converted to absolute values if an illustration of typical pressure behaviour is required.

C15-12 Electronic Manometer (optional accessory) An electronic console incorporating 16 differential pressure sensors, each with a range of 0-178 mm H 2 O. When installed on the wind tunnel, the electronic manometer is located inside the frame below the test section to the left hand side of the IFD7 Electrical Console. The electronic manometer can be secured to the frame by transferring one of the straps from the IFD7 to the C15-12 (Two straps are fitted to the IFD7 for shipping but only one is required in normal use). The electrical supply for the manometer is obtained from the outlet socket on the front of the IFD7.

A common tapping ensures that all of the differential pressure sensors are referenced to atmospheric pressure. Quick release connectors (7x single and 1x 10way) allow for rapid connection to models and instruments. The electronic manometer connects to the control PC using a second USB port on the PC, and the readings are fully integrated with the wind tunnel control software for ease of use (Use C15-12 version of the software). As described already (see Important note on pressure measurement using the tunnel ), the stagnation pressure in the working section will be very close to atmospheric pressure, allowing for frictional losses, when the fan is in operation. To match the results from the C15-11 inclined manometer, static pressure readings below atmospheric pressure are displayed as positive values so static pressure will be greater than the corresponding total pressure readings. The relative values can be converted to absolute values if an illustration of typical pressure behaviour is required.

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C15-13 Lift and Drag Balance (optional accessory) A 2-component, electronic balance used to measure the lift and drag on appropriate models (Not used with models having multiple internal tapping points). The lift and drag models connect to the balance with a fixing pin for correct alignment. Electronic sensors are used to measure the lift and drag forces, the drag being measured directly and the lift by a reduction in the model weight. The model being tested can also be rotated on the mounting and the angle of rotation measured electronically.

The readings from the lift and drag sensors and the rotation sensor are displayed on the control software screen running on the PC, and are available for data logging. All three readings should be zeroed in the software before taking measurements as follows:

The Lift reading should be zeroed with the weight of the model resting on the balance. The Drag reading should be zeroed with no rearward force on the balance. The rotation reading should be zeroed with the model at zero angle of attack (cursor on the body at mid position – two lines aligned). The balance is designed to accommodate C15-20 or C15-22 but can also accommodate alternative models manufactured by the user. A transit screw ensures that the lever arm is isolated from the load cells to prevent damage to the cells during transport or handling. It is important to clamp the lever arm at all times when the balance is not fitted in the wall of the tunnel.

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Description

Note: To avoid unnecessary handling of the C15-13 Lift & Drag Balance, optional models C15-20 and C15-22 can be mounted directly in the working section via the large circular hatch when performing flow visualisation studies or when used in conjunction with the Wake survey rake (C15-15).

C15-14 Pitot Static Tube (optional accessory)

A miniature Pitot Static tube mounted in a support plug that can be located in the roof of the working section at three alternative positions, i.e. the start of the working section and upstream and downstream of the model mounting position. The support plug incorporates an ‘O’ ring to retain the Pitot Tube where it is positioned and allows the tube to traverse over the full height of the working section to measure the velocity profile inside the working section of the tunnel. The Pitot Static tube is constructed from two concentric stainless steel tubes. The inner tube is open at the tip and measures the Total head. The outer tube incorporates a ring of small holes in the side that measure the static head. The overall diameter of the Pitot Static tube is 4 mm to give a stiff assembly without unduly disturbing the airflow downstream and the ‘L’ shaped arrangement, with the tip pointing into the flow, gives minimal disturbance at the point of measurement. The two flexible tubes from the Pitot Static tube incorporate a quick release connector that allows it to be connected to one of the optional manometers. The Pitot Static is of Prandtl design and may be used with a negligible correction up to angles of yaw of at least 5 degrees.

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C15-15 Wake Survey Rake (optional accessory) The rake consists of 10 stainless steel tubes positioned vertically in a row and pointing towards the airflow. The rake is mounted downstream of the model being used via the small access hatch in the side wall of the working section. The tubes are mounted at a fixed pitch of 5mm and are connected via flexible tubing to a multi-way quick release connector to suit the C15-11 or C15-12 manometers.

When used with models such as the C15-16 Pressure Wing, readings can be taken from the pressure tappings on the model and the Wake Survey Rake without changing any settings by simply swapping the quick release connector on the appropriate manometer.

C15-20 Lift and Drag Aerofoil (optional accessory) A plain symmetrical aerofoil to NACA 0015 profile, incorporating a mounting rod that allows it to be installed on the C15-13 Lift & Drag Balance, thus allowing the lift and drag to be measured at different angles of attack. The aerofoil has the same section as C15-21 to allow direct comparison of lift characteristics with the pressure distribution. A location peg on the support shaft ensures that the aerofoil is correctly positioned when fitted to the lever arm on C15-13. To avoid unnecessary handling of the C15-13 Lift & Drag Balance, the C15-20 can be mounted directly in the working section when performing flow visualisation studies or when used in conjunction with the Wake survey rake (C15-15).

C15-21 Pressure Wing (optional accessory)

A symmetrical aerofoil incorporating 10 tapping points distributed around the wing profile that allow the pressure distribution to be measured from the leading edge to the trailing edge. The wing is mounted in the horizontal plane through the side of the 18

Description working section, and the angle of attack is adjustable by rotating the circular hatch. Although only instrumented on one side, the effective pressure distribution on both surfaces can be obtained by inclining the aerofoil at positive and negative angles of attack. Machined to NACA 0015 profile, the aerofoil has the same section as C15-20 to allow direct comparison of pressure distribution with the lift characteristics. The tapping points are all flush with the surface of the aerofoil and connected via flexible tubing to a multi-way quick release connector to suit the C15-11 or C15-12 manometers. The NACA 0015 is one of a standard series of aerofoils. The 00 indicates that the two faces are symmetrical. The 15 indicates that the airfoil has a 15% thickness to chord (width) ratio, i.e. its thickness is 15% of its chord. This ratio is fairly typical for low-speed aerofoils, and possible applications include boat rudders as well as aircraft wings.

C15- 22 Drag Models (optional accessory) Seven different models are provided for use with the C15-13 lift and drag balance for investigations into the influence of shape on the drag forces. Five models are supplied with a common equatorial diameter of 50mm, thus all presenting the same cross section to the airflow: Sphere Hemisphere, convex to airflow Hemisphere, concave to airflow Circular disk Streamlined shape Additionally a dimpled golf ball and plain sphere of 43mm diameter are supplied to demonstrate the difference in drag force due to the dimples. This smaller sphere can also be compared with the larger sphere to show the change in drag due to the cross sectional area. A spare support rod is supplied for drag calibration purposes. To avoid unnecessary handling of the C15-13 Lift & Drag Balance, the models supplied with C15-22 can be mounted directly in the working section when performing flow visualisation studies or when used in conjunction with the Wake survey rake (C15-15).

C15- 23 Pressure Cylinder (optional accessory)

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Armfield Instruction Manual A plain cylinder, 30mm diameter, incorporating 10 equi-spaced tapping points around half of the circumference that allow the pressure distribution around the cylinder to be measured. The cylinder is mounted in the horizontal plane through the side of the working section and can be rotated through 180° to plot the pressure distribution over the whole circumference. The tapping points are all flush with the surface of the cylinder and connected via flexible tubing to a multi-way quick release connector to suit the C15-11 or C15-12 manometers.

C15-24 Bernoulli Apparatus (optional accessory) A Venturi profile that is installed in the working section of the tunnel via the removable floor. The Venturi incorporates 11 pressure tappings in the floor, connected via flexible tubing to quick release connectors to suit the C15-11 or C1512 manometers. The Venturi occupies the full height of the working section and the width varies from full width at the inlet and outlet to 100 mm at the throat. It is manufactured from clear acrylic for full visualisation. By itself the C15-24 may be used to show the variation in static pressure with change in cross section, but when used in conjunction with the Pitot Static tube (C15-14) the Bernoulli equation can be fully demonstrated. When using C15-24, the static pressure sensor should be moved from the tapping in the rear wall to the upstream tapping in the roof of the working section to avoid errors in the static pressure measurement caused by the wall of the Venturi downstream of the rear tapping.

C15-25 Boundary Layer Plates (optional accessory)

A flat plate is mounted vertically in the working section via a removable floor panel incorporating a horizontal slot. A special flattened Pitot tube, mounted on a traversing micrometer, allows the air velocity to be measured at different distances from the surface of the plate. The plate can be moved relative to the Pitot tube to allow the velocity profile to be measured at any position between the leading edge and the trailing edge of the plate. The special Pitot tube allows the average air velocity to be measured over a relatively small change in height. A solid rod downstream of the Pitot tip ensures that the operator is aware when the tip is touching the plate and avoids damage to the fragile tip by preventing excessive movement. 20

Description A smooth plate and artificially roughened plate are included to show the difference between the development of laminar and turbulent boundary layers. The flexible tubing from the Pitot tube incorporates a quick release connector to suit the C15-11 or C15-12 manometers.

C15-26 Project Kit (optional accessory) The Project Kit provides a range of mountings suitable for models of the students’ own design. These mountings are made to fit the working section, so that students may concentrate on the design of the model itself. The kit also includes a selection of suitable flexible tubing for connecting tapping points to sensors, and connectors for use with the optional manometers.

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Installation Advisory Before operating the equipment, it must be unpacked, assembled and installed as described in the steps that follow. Safe use of the equipment depends on following the correct installation procedure.

Installation Process 1. This equipment is heavy. It may be most convenient to move the equipment to its final location before unpacking it. 2. Remove all packing from the equipment. Retain all cables, thumbscrews, and other loose items (Most accessories may be unpacked after the initial installation is complete).

3. Check all items against the Advice Note included with the equipment. If any items are missing, inform Armfield or the local agent immediately. The IFD7, blank front circular hatches and clear acrylic floor are all fitted prior to shipping. Check that these are all present. 4. Check that the static pressure sensor is fitted to the tapping on the rear face of the working section (at the inlet end). A flexible tube from the side of this tapping allows the static tapping to be connected to a C15-11 or C15-12 manometer (when supplied). 5. All items are inspected before dispatch. Check all items for damage received in transit, and inform Armfield or the local agent of any damage or breakages. 6. The C15-10 should be placed on a firm, level surface that is sufficiently strong to support the weight of the tunnel and accessories.

22

Installation 7. With a spirit level placed along the metal frame at the base of the equipment, adjust the feet until the tunnel is horizontal.

8. Check the fan is free from obstructions and that the blades can rotate freely. Check that the fixings on the guard are securely fastened.

9. Check the honeycomb insert in the tunnel inlet to ensure all holes are clear of obstructions.

10. For C15-11

23

Armfield Instruction Manual If using the C15-11, this should be fixed to the frame, to the left of the IFD7, using the thumb nuts supplied. The flexible tube from the static should be connected to the top of the left hand manometer tube.

11. For C15-12 If using the C15-12, this should be located to the left of the IFD7. The C15-12 can be secured to the frame using one of the straps from the IFD7 (Two straps are fitted to IFD7 to ensure safe transit).

12. Check that the cables from the fan motor and sensors on the tunnel are securely connected to the sockets front of the IFD7.

24

Installation 13. Install the C15-304 software on a suitable PC then restart the PC. Connect the IFD7 to the PC using the USB cable supplied. If using the C15-12, connect this to one USB port on the PC before connecting the IFD7 to a second USB port. Check that the red and green indicator lights on the IFD7 (and the C15-12 if used) are illuminated. The PC should automatically detect the connected USB device(s) and install the correct driver(s).

14. Run the C15-304 software. Two versions of the software are installed, one version for use with the C15-11 Inclined Manometer Bank and the other version for use with the C15-12 Electronic Manometer.

15. For version C15-B only This version of the C15 Wind Tunnel is supplied with a loose transformer to step-up the local 115/120V electrical supply to 220/240V to suit the equipment. The transformer should be located adjacent to a 115/120V electrical outlet socket in the laboratory. Connect the mains lead from the rear of the IFD7 to the 220V outlet socket on the front of the transformer.

25

Armfield Instruction Manual Connect the transformer to the 115/120V mains outlet socket in the laboratory then switch on the electrical supply. Go to step 17. 16. For versions C15-A and C15-G Connect the IFD7 to a suitable mains electricity supply.

17. If using C15-12, this should be connected to the 240V outlet socket on the IFD7 using the mains lead supplied with C15-12.

18. Switch on the IFD7. A warning message will indicate if the IFD7 needs configuring to suit the equipment. Refer to the IFD7 Instruction Manual for further details.

The fan should NOT start at this stage. If it does so then suspect an electrical fault; shut down the equipment and contact Armfield or your local agent for assistance.

26

Installation

19. Run the appropriate C15-304 software (C15-11 if using the inclined manometer bank, C15-12 if using the electronic manometer) and select the Project Work exercise. Check that ‘IFD: OK’ is indicated in the bottom right of the software window.

20. In the software, select the ‘Fan On’ button on the mimic diagram. Check that the ‘Watchdog Enabled’ indicator on the mimic diagram is activated.

21. Use the arrow keys beside the fan speed box to gradually increase the fan speed. Check that the fan begins to operate. Check that the indicated static pressure reading increases. Check that a velocity reading is indicated.

27


22. Set the fan speed back to zero. Select ‘Fan On’ to set the IFD7 to standby. Switch off the power switch on the front of the IFD7. Exit the software.

The basic operation of the C15 Wind Tunnel has been confirmed. Refer to the Operation section for further information.

Electrical Wiring Diagram Click on the relevant link to invoke the Wiring Diagram: Wiring Diagram ACM31434 Printed Versions of this Instruction Manual Please note, all wiring diagrams are appended at the rear of this manual

28

Operation Where necessary, refer to the drawings in the Equipment Diagrams section.

Operating the Software Note: The diagrams in this section are included as typical examples and may not relate specifically to the individual product described in this instruction manual. The Armfield Software is a powerful Educational and Data Logging tool with a wide range of features. Some of the major features are highlighted below, to assist users, but full details on the software and how to use it are provided in the presentations and Help text incorporated in the Software. Help on Using the Software or Using the Equipment is available by clicking the appropriate topic in the Help drop-down menu from the upper toolbar when operating the software as shown:

Before operating the software ensure that the equipment has been connected to the IFD7 Interface and the IFD7 has been connected to a suitable PC using a USB lead. For further information on these actions refer to the Operational manual. Load the software. If multiple experiments are available then a menu will be displayed listing the options. Wait for the presentation screen to open fully as shown:

Before proceeding to operate the software ensure that IFD: OK is displayed at the bottom of the screen. If IFD:ERROR is displayed check the USB connection between the IFD7 and the PC and confirm that the red and green LED’s are both illuminated. If the problem persists then check that the driver is installed correctly (refer to the Operational manual).

29

Armfield Instruction Manual A warning message will be displayed if the IFD7 has not been configured to match the product in use. Refer to the Operational manual for further information if the IFD7 needs to be configured.

Presentation Screen - Basics and Navigation As stated above, the software starts with the Presentation Screen displayed. The user is met by a simple presentation which gives them an overview of the capabilities of the equipment and software and explains in simple terms how to navigate around the software and summarizes the major facilities complete with direct links to detailed context sensitive ‘help’ texts. To view the presentations click Next or click the required topic in the left hand pane as appropriate. Click More while displaying any of the topics to display a Help index related to that topic. To return to the Presentation screen at any time click the View Presentation icon from the main tool bar or click Presentation from the dropdown menu as shown:

For more detailed information about the presentations refer to the Help available via the upper toolbar when operating the software.

Toolbar A toolbar is displayed at the top of the screen at all times, so users can jump immediately to the facility they require, as shown:

The upper menu expands as a dropdown menu when the cursor is placed over a name. The lower row of icons (standard for all Armfield Software) allows a particular function to be selected. To aid recognition, pop-up text names appear when the cursor is placed over the icon.

Mimic Diagram The Mimic Diagram is the most commonly used screen and gives a pictorial representation of the equipment, with continuously updated display boxes for all the various sensor readings, calculated variables etc. directly in engineering units.

30

Operation

To view the Mimic Diagram click the View Diagram icon or click Diagram from the View drop-down menu as shown:

from the main tool bar

A Mimic diagram is displayed, similar to the diagram as shown:

The details in the diagram will vary depending on the experiment chosen if multiple experiments are available. In addition to measured variables such as Temperature, Pressure and Flowrate (from a direct reading flowmeter), calculated data such as Motor Torque, Motor Speed and Discharge / Volume flowrate (from pressure drop across an orifice plate) are continuously displayed in data boxes with a white background. These are automatically updated and cannot be changed by the user. Manual data input boxes with a coloured background allow constants such as Orifice Cd and Atmospheric Pressure to be changed by over-typing the default value, if required. The data boxes associated with some pressure sensors include a Zero button alongside. This button is used to compensate for any drift in the zero value, which is 31

Armfield Instruction Manual an inherent characteristic of pressure sensors. Pressing the Zero button just before starting a set of readings resets the zero measurement and allows accurate pressure measurements to be taken referenced to atmospheric pressure. This action must be carried out before the motor is switched on otherwise the pressure readings will be offset. The mimic diagram associated with some products includes the facility to select different experiments or different accessories, usually on the left hand side of the screen, as shown:

Clicking on the appropriate accessory or exercise will change the associated mimic diagram, table, graphs etc to suit the exercise being performed.

Control Facilities in the Mimic Diagram A Power On button allows the motor to be switched off or on as required. The button always defaults to off at startup. Clicking this button switches the power on (1) and off (0) alternately. A box marked Motor Setting allows the speed of the motor to be varied from 0 to 100% either stepwise, by typing in values, or using the up / down arrows as appropriate. It is usual to operate the equipment with the motor initially set to 100%, then reduce the setting as required to investigate the effect of reduced speed on performance of the equipment. When the software and hardware are functioning correctly together, the green LED marked Watchdog Enabled will alternate On and Off. If the Watchdog stops alternating then this indicates a loss of communication between the hardware and software that must be investigated. Details on the operation of any automatic PID Control loops in the software are included later in this section.

32

Operation

Data Logging Facilities in the Mimic Diagram There are two types of sampling available in the software, namely Automatic or Manual. In Automatic logging, samples are taken regularly at a preset but variable interval. In Manual logging, a single set of samples is taken only when requested by the operator (useful when conditions have to be changed and the equipment allowed to stabilize at a new condition before taking a set of readings). The type of logging will default to manual or automatic logging as appropriate to the type of product being operated. Manual logging is selected when obtaining performance data from a machine where conditions need to stabilize after changing appropriate settings. To record a set of set of data values from each of the measurement sensors click the main toolbar. One set of data will be recorded each time the

icon from the icon is clicked.

Automatic logging is selected when transients need to be recorded so that they can be plotted against time. Click the the

icon from the toolbar to start recording, click

icon from the toolbar to stop recording.

The type of logging can be configured by clicking Configure in the Sample dropdown menu from the upper toolbar as shown:

In addition to the choice of Manual or Automatic sampling, the parameters for Automatic sampling can also be set. Namely, the time interval between samples can be set to the required number of minutes or seconds. Continuous sampling can be selected, with no time limit or sampling for a fixed duration can be set to the required number of hours, minutes or seconds as shown:

33


Tabular Display To view the Table screen click the View Table icon click Table from the View dropdown menu as shown:

from the main tool bar or

The data is displayed in a tabular format, similar to the screen as shown:

As the data is sampled, it is stored in spreadsheet format, updated each time the data is sampled. The table also contains columns for the calculated values. New sheets can be added to the spreadsheet for different data runs by clicking the icon from the main toolbar. Sheets can be renamed by double clicking on the sheet name at the bottom left corner of the screen (initially Run 1, Run 2 etc) then entering the required name. For more detailed information about Data Logging and changing the settings within the software refer to the Help available via the upper toolbar when operating the software.

34

Operation

Graphical Display When several samples have been recorded, they can be viewed in graphical format. from the main To view the data in Graphical format click the View graph icon tool bar or click Graph from the View drop-down menu as shown:

The results are displayed in a graphical format as shown:

(The actual graph displayed will depend on the product selected and the exercise that is being conducted, the data that has been logged and the parameter(s) that has been selected). Powerful and flexible graph plotting tools are available in the software, allowing the user full choice over what is displayed, including dual y axes, points or lines, displaying data from different runs, etc. Formatting and scaling is done automatically by default, but can be changed manually if required.

35

Armfield Instruction Manual To change the data displayed on the Graph click Graph Data from the Format dropdown menu as shown:

The available parameters (Series of data) are displayed in the left hand pane as shown:

Two axes are available for plotting, allowing series with different scaling to be presented on the same x axis. To select a series for plotting, click the appropriate series in the left pane so that it is highlighted then click the appropriate right-facing arrow to move the series into one of the windows in the right hand pane. Multiple series with the same scaling can be plotted simultaneously by moving them all into the same window in the right pane. To remove a series from the graph, click the appropriate series in the right pane so that it is highlighted then click the appropriate left-facing arrow to move the series into the left pane. The X-Axis Content is chosen by default to suit the exercise. The content can be changed if appropriate by opening the drop down menu at the top of the window. The format of the graphs, scaling of the axes etc. can be changed if required by clicking Graph in the Format drop-down menu as shown:

36

Operation

For more detailed information about changing these settings refer to the Help available via the upper toolbar when operating the software.

PID Control Where appropriate, the software associated with some products will include a single or multiple PID control loops whereby a function on the product can be manually or automatically controlled using the PC by measuring an appropriate variable and varying a function such as a heater power or pump speed. The PID loop can be accessed by clicking the box labelled PID or Control depending on the particular software:

A PID screen is then displayed as shown:

37


The Mode of operation always defaults to Manual control and 0% output when the software is loaded to ensure safe operation of the equipment. If appropriate, the operator can retain manual operation and simply vary the value from 0 to 100% in the Manual Output box, then clicking Apply. Alternatively, the PID loop can be changed to Automatic operation by clicking the Automatic button. If any of the PID settings need to be changed from the default values then these should be adjusted individually before clicking the Apply button. The controller can be restored to manual operation at any time by clicking the Manual button. The value in the Manual Output box can be changed as required before clicking the Apply button. Settings associated with Automatic Operation such as the Setpoint, Proportional Band, Integral Time, Derivative Time and Cycle Time (if appropriate) can be changed by the operator as required before clicking the Apply button. Clicking Calculations displays the calculations associated with the PID loop to aid understanding and optimization of the loop when changing settings as shown:

38

Operation

Clicking Settings returns the screen to the PID settings. Clicking OK closes the PID screen but leaves the loop running in the background. In some instances the Process Variable, Control variable and Control Action can be varied to suit different exercises, however, in most instances these boxes are locked to suit a particular exercise. Where the variables can be changed the options available can be selected via a drop-down menu.

Advanced Features The software incorporates advanced features such as the facility to recalibrate the sensor inputs from within the software without resorting to electrical adjustments of the hardware. For more detailed information about these advanced functions within the software refer to the Help available via the upper toolbar when operating the software.

Operating the Equipment Preparation of the tunnel for use Before switching on the Wind Tunnel: 

Ensure that the required model and/or measuring instrument has been securely installed in the working section



Any adjustable or removable features must be fastened or clamped to prevent movement.



Ensure that all appropriate hatches, covers etc have been secured.



Ensure that nothing is obstructing the inlet or the outlet of the wind tunnel.



Ensure that the IFD7 Electrical Console has been connected to the USB port on the control PC.

39

Armfield Instruction Manual Note: When using the optional C15-12 Electronic Manometer ensure that the USB lead from the C15-12 is connected to the PC followed by the USB lead from the IFD7 to ensure correct operation. 

Switch on the RCD at the rear of the IFD7 Electrical Console.



Switch on the mains power switch at the front of the IFD7 Electrical Console.



Load the appropriate exercise from the C15 software. Note: Different software should be loaded to suit operation with either C15-11 or C15-12. If not using a manometer supplied by Armfield then the C15-11 software should be loaded.



Where the exercise offers a choice of multiple models, select the correct model.

Starting up Switch on power to the fan by clicking the ‘Fan On’ button in the software. The button will indicate ‘1’ when the fan is ready for use. Gradually increase the fan speed using the software control box until the required air velocity is indicated. Always start at low velocity then gradually increase the velocity, checking readings on the manometers, lift/drag balance etc. to ensure that everything is assembled and connected correctly. The setting may be adjusted using the up and down arrow buttons, or a value may be typed directly into the box using the keyboard. Note: When using the C15-11 Inclined Manometer it is important to ensure that the water level in any of the tubes does not reach the top manifold as water will be drawn into the flexible tubing. This will not present a safety hazard but water in the tubing will affect the accuracy of subsequent readings on the manometer. If water does enter the flexible tubing it will be necessary to disconnect the tubing from the quick release connector(s) on the manometer and blow through the flexible tubing to remove the blockage. This problem can easily be avoided by gradually increasing the speed of the fan while watching the readings on the manometer.

Adjusting the air velocity The air velocity is always adjusted from the software by altering the fan speed. Fan speed is set as a percentage of the maximum speed, between 0% and 100%. The corresponding static head in the tunnel (given as the differential head between the tunnel and atmosphere) and air velocity (in meters per second) are then displayed on the software mimic diagram. Check that the correct exercise is loaded and that the ‘Fan On’ button has been selected to indicate ‘1’ (power supply on). Wind speed in the test section can then be adjusted by clicking on the raise or lower arrows until the required speed is indicated, or a value may be typed directly into the fan speed box. When making adjustments it will take a few moments for the fan speed to settle due to the inertia of the fan impeller, so allow it to stabilize before checking the resulting air velocity. Continue to make small adjustments to the fan setting until the required air velocity is achieved. The maximum air velocity that can be achieved will depend on the type of model that has been installed. Note that models such as the pressure wing C15-21 will vary the air velocity as the model is rotated to different angles of attack because the blockage 40

Operation in the working section will change. Operation at large angle of attack will severely reduce the air velocity through the tunnel. If constant air velocity is required at different model settings then the fan speed can be adjusted after making changes to the model, starting at the maximum speed available when the model is causing the maximum blockage.

Measuring the air velocity using the PC The instantaneous average air velocity in the test section is indicated on the PC when viewing the diagram. The air velocity is indicated in units of m/s and is calculated from the static pressure measured at the tunnel wall. Note: Before the tunnel is used for quantitative results it will be necessary to check the accuracy of this indicator since no account is taken of the velocity profile across the working section. The calibration may also be used as a student exercise. Sensor calibration is described in the software Help Text.

Measuring the air velocity using the inclined manometer bank Tube 1 indicates static head at the start of the working section. Tubes 11 & 12 indicate atmospheric head when not connected to a model or instrument. Velocity = √(2 *ρ manometer * 9.81 * Head differential / ρ air ) m/s Where Head differential = Δh = difference between static head and atmospheric head (in meters) ρ manometer and ρ air are both temperature-dependant and thus a reading for ambient temperature is required for accuracy. The densities are then automatically calculated by the software, although they may also be determined from standard reference tables if desired. If a quick, approximate air velocity is required for some reason then values of 1000 kg/m³ for water and 1 kg/m³ for air may be sufficient, i.e. V ≈ 140 √Δh

Installing models and sensors via the large circular hatch Before removing or installing a model or instrument, ensure that the air velocity is zero and the fan has stopped rotating. If a model is already in position, the flexible tubing from the model should be disconnected from the quick-release connector(s) on the manometer before the hatch is removed. On models of the student’s own design, tappings should be disconnected at whichever position is appropriate to the design. The hatch is secured with two swinging latches. Support any model or cover in position, and loosen but do not completely remove the two thumb nuts securing the latches. Swing the latches aside, and withdraw the hatch and any attached model horizontally until hatch and model are both clear of the working section side. Insert the new model horizontally through the circular opening, taking care not to damage the model, the working section, or any model already in position within the working section. Fit the circular hatch on which the model is fitted into the hole, and support the model in place while swinging the latches into place to secure it and tightening the thumb nuts by hand.

41

Armfield Instruction Manual Connect the flexible tubing from the model or instrument to the required manometer via the quick release connector(s). If no hatch-mounted model or sensor is to be used, the basic blank hatch cover should be fitted in the same way as described for fitting a model.

Installing models via the removable floor Before removing or installing a model or instrument, ensure that the air velocity is zero and the fan has stopped rotating. If a model is already in position, the flexible tubing from the model should be disconnected from the quick-release connector(s) on the manometer before the floor is removed. On models of the student’s own design, tappings should be disconnected at whichever position is appropriate to the design. The floor is held in position using eight thumbnuts. Remove all but two opposing corner nuts and place the nuts in a safe place. Support the floor securely before removing the final two nuts then lower the floor until the model is clear of the working section sides. Lift the new model upwards into position, and support it while securing it in position with two nuts placed at opposite corners. Replace the other six thumb nuts, tightening them by hand. Connect the flexible tubing from the model or instrument to the required manometer via the quick release connector(s). If no floor-mounted model is to be used, the basic blank floor should be fitted in the same way as described for fitting a model.

Installing the flow visualisation tube or Pitot Static tube The procedure for inserting the flow visualisation system and the Pitot tube is identical. However, the flow visualisation tube is usually fitted at the far upstream end of the working section while the Pitot tube is more commonly fitted centrally or downstream within the within the working section (depending on the model used). The installation positions are fitted with blanking plugs when not in use. Loosen but do not remove the screw securing the appropriate blanking plug. Then remove the blanking plug and put in a safe place (it is not recommended that plugs are left on top of the wind tunnel, as vibration may cause them to travel). Insert the short arm of the ‘L’ section tube into the tapping hole then carefully pass the tube through the hole until the long arm of the ‘L’ is upright within the tapping. Lower the tube until the support plug is snugly fitted into the tapping. Tighten the screw to secure the support plug. After fitting the optional Pitot tube (C15-14), the two flexible tubes should be connected to the quick-release fittings on the manometer (see Using the C15-14 Pitot Static Tube (requires C15-11 or C15-12) for more detail).

Using the flow visualisation system The flow visualisation system consists of a height- and angle-adjustable tube which supports a long thread. The thread is light enough to be lifted easily by the air flow, and takes up the path of the air layer within which it is positioned by the setting of the support tube. 42

Operation

Vertical movement The vertical position of the support tube is adjusted by loosening the screw in the roof tapping, carefully raising or lowering the tube within the tapping, and securing the tube in the new position by re-tightening the screw. The screw should only be tightened sufficiently to hold the tube in position. Overtightening can damage the tube.

Horizontal movement The tube is L-shaped, and the horizontal position of the thread may therefore be adjusted by rotating the support tube within the tapping so that the short length of the ‘L’ brings the thread across to one side or the other. Loosen the tapping screw before rotating the tube, and tighten the screw afterwards.

Adjusting length of thread The length of thread can be adjusted to obtain the best visualisation (for example to demonstrate wake turbulence). Length can often be best adjusted during tunnel operation, and it is safe to make adjustments while the tunnel is in operation. The thread is secured with a rubber ‘O’-ring around the top of the tube. Slide this ring up and off the tube, then shorten the thread by pulling the external end upwards through the tube, or lengthen it by feeding more thread into the tube. When the thread length is correct gently push the ‘O’-ring back onto the end of the tube to secure the thread at the required length. Note: The end of the thread should be tied to the ‘O’-ring at all times so that the thread cannot accidentally enter the tunnel and wrap around the rotating fan.

Using the C15-11 Inclined Manometer Bank Connecting the equipment Models or instruments can be connected to the manometer using the 10-way connector or two 1-way connectors that are located on the left hand side of the manometer.

43


The 10-way connector The 10-way connector socket has tubes labelled 1 to 10. These correspond to the similarly-labelled tubes on the C15-11 or the labelled boxes on the software mimic diagram if connecting via the electronic manometer. When using the 10-way connector ensure that the red clip on top of the fixed body is set to OPEN (left) before inserting the plug.

With the notch uppermost, push the plug in straight and horizontally (without twisting) until the two parts mate.

Set the red clip to LOCK (right) to latch the two parts of the connector.

44

Operation

Before removing the plug ensure that the red clip is set to OPEN then pull the plug out horizontally without twisting.

The 1-way connectors The 1-way connectors are used with accessories such as the C15-14 Pitot static tube. The two tubes connect to the single-way connectors labelled A and B. Note that the 10-way or 1-way quick release connections can be changed while the wind tunnel is operating allowing different readings to be taken without changing the configuration of the model. For example, the pressure distribution on the surface of the aerofoil (C15-21) can be displayed on the manometer, followed by the pressure distribution from the Wake Survey Rake (C15-15) without changing the air velocity, or angle of attack.

Priming Priming is only required with the C15-11 inclined manometer bank (Other sensors do not require the use of water). The manometer should be properly primed before use, as trapped air bubbles will affect the accuracy of the results.

Adjusting levels A water reservoir with a screw operated displacer allows rapid adjustment of the datum level in the manometer tubes. Any change in the level in one tube affects the level in all of the other tubes because they are connected to a common reservoir. After each adjustment to the model, the wind speed etc. the displacer should be screwed up or down as required to restore the tube(s) at atmospheric pressure to the original datum. All readings can then be recorded relative to a common datum. Adjustment of the datum level does not affect the differentials between the various tubes i.e. the level in all tubes rises or falls by the same amount when the displacer is adjusted. Note that the water level will stop changing when the displacer becomes fully submerged.

Using cursors Each manometer tube is fitted with an adjustable cursor ring which may be positioned anywhere along the length of the tube. These are close-fitting, so that they will remain in position unless deliberately moved. The cursors should be moved either to the top of the bottom of each column when not in use, to allow maximum visibility of the columns.

45

Armfield Instruction Manual Manometers must be read from the base of the meniscus, and therefore it is important to position the cursors so that the top edge of the cursor aligns with the bottom of the meniscus curve. When used, the cursors allow the position of the water in the column to be marked before some change is made. For example, when using C15-21, the cursors could be used to mark the pressure distribution across the wing while at a low angle of attack, allowing direct comparison with the change in levels when the wing is operated at a high angle of attack.

Manual entry of results on PC when using C15-11 Two versions of the software are included as standard, one intended for use with the C15-12, and one for use with the C15-11 (which is also useable with any other compatible manometer bank). The software for the C15-11 includes manual data entry boxes where the readings from the manometer bank can be entered. Values should be entered in millimetres, with the value from each column being entered into the box with the same numerical label. When using the C15-12 Electronic Manometer Bank the C15-12 version of the software must be used. The electrical supply to the C15-12 is obtained from the IFD7 and the electrical outlet socket on the IFD7 is controlled by the C15-12 software. For this reason the electrical mains switch on the C15-12 will not illuminate (even when switched on) until an appropriate exercise has been loaded from the C15-12 software menu.

Using the C15-12 Electronic Manometer Bank

All of the differential pressure sensors inside the electronic manometer are connected to a common reference port. This ensures that all pressure measurements are relative to the current atmospheric pressure. For this reason the reference port is left open to atmosphere in normal use. Note that readings from the pressure sensors will be POSITIVE for pressures LOWER than atmospheric so that results match those obtained using the Inclined Manometer C15-11. The electronic manometer offers the same 10-way connector which is fitted to the C15-11 inclined manometer bank. For instructions on connecting to the 10-way connector, refer to section 3.10.2. The software displays each sensor reading on the mimic diagram in boxes labelled as marked around the socket. The electronic manometer also has six single-way connectors, numbered 11 to 16. Generally in the experiments presented in this manual, connectors 11 and 12 are taken as equivalent to connectors A and B on the inclined manometer bank. Note that the quick release connections can be changed while the wind tunnel is operating allowing different readings to be taken without changing the configuration of the model. For example, the pressure distribution on the surface of the aerofoil (C15-21) can be displayed on the manometer, followed by the pressure distribution 46

Operation from the Wake Survey Rake (C15-15) without changing the air velocity, or angle of attack.

Automatic logging of results on PC when using C15-12 When connected to the software via its own USB cable (in addition to the USB connection from the IFD7 to the PC), the sensor output resulting from all connections to the C15-12 are read by the computer. The relevant signals for each exercise are displayed on the software mimic diagram, and these can then be recorded to the results table in the software using the

icon.

If for some reason there is a need to record the manometer outputs at regular intervals over a period of time, this is possible in the Project Work exercise. Automatic Sampling can be set using the Sample Configuration window (selected by choosing ‘Configure…’ from the ‘Sample’ menu, by setting ‘Sampling Operation’ to ‘Automatic’. The required sample interval can also be set in this window, as can the required duration. Sampling starts when the

icon is selected. If the duration is set to

‘Continuous’, then sampling will continue until manually stopped using the

icon.

Using the C15-13 Lift and Drag Balance (requires C15-20 or C15-22) The lift and drag balance is designed to be mounted in the front wall of the working section and is mounted permanently onto a large circular hatch. The cable from the balance connects to the socket on the front of the black interface box located at the rear of the equipment. The signals from the balance are then passed via the IFD7 to the PC where the sensor outputs are displayed on the appropriate software mimic diagram. The lift and drag balance is a delicate instrument and must be handled carefully at all times. It must only be used with the accessories supplied by Armfield or models made for in it in accordance with the guidelines provided in the Specifications section (page 26). Models used with the C15-13 incorporate a rod with a locating pin which fits into a slot in the metal collar on the end of the lever arm within the balance. The rod is secured into the collar by tightening the screw. The locating pin ensures that the model is installed in the correct orientation relative to the balance and also ensures that the rod is inserted to the correct depth, positioning the model on the centreline of the tunnel.

Collar and screw to locate and secure model to balance

47


The locating pin on the model is inserted fully into the slot in the end of the lever arm

The screw is then tightened to secure the rod into the lever arm

A transit screw is fitted to the balance in order to lock the lever arm when the balance is not in active use (i.e. when it is in storage, being moved, or a model being installed or removed). The transit screw is readily accessible on the side of the balance body. Locking the lever arm prevents damage to the load cells by ensuring that there is no load placed on them resulting from movement of the lever arm. The transit screw must be tightened at all times when the balance is not being used to take readings.

The lift and drag balance measures the vertical lift component (perpendicular to the working section floor) acting on the model, and the drag component acting on the model (in the same direction as the airflow, parallel to the working section floor and walls). It also measures the angle of attack of the lift and drag aerofoil, when this is in use, or of any student-made model that can be set at a range of angles. 48

Operation Note that the balance does not measure any turning moment on the model. All three readings from the C15-13 balance should be zeroed in the software before taking measurements as follows: The Lift reading should be zeroed with the weight of the model resting on the balance. The Drag reading should be zeroed with no rearward force on the balance. The rotation reading should be zeroed with the model at zero angle of attack (cursor on the body at mid position – two lines aligned). When measuring small changes in lift or drag force it may be necessary to tap the tunnel wall lightly using a finger to reduce the effects of stiction and obtain accurate results. Note: To avoid unnecessary handling of the C15-13 Lift & Drag Balance, optional models C15-20 and C15-22 can be mounted directly in the working section via the large circular hatch when performing flow visualisation studies or when used in conjunction with the Wake survey rake (C15-15).

Using the C15-14 Pitot Static Tube (requires C15-11 or C15-12) The Pitot static tube simultaneously measures the total head and the static head of an air stream in which the Pitot tube tip is positioned. The Pitot Static tube is constructed from two concentric stainless steel tubes. The inner tube is open at the tip and measures the Total head. The outer tube incorporates a ring of small holes in the side that measure the static head. The overall diameter of the Pitot Static tube is 4 mm to give a stiff assembly without unduly disturbing the airflow downstream and the ‘L’ shaped arrangement, with the tip pointing into the flow, gives minimal disturbance at the point of measurement.

The C15-14 may be mounted in any of the three roof tappings on the working section. It will typically be mounted either in the central roof tapping (to take readings around a wall- or floor-mounted model), or in the downstream roof tapping (to take pressure readings across the wake of a model.

49

Armfield Instruction Manual To mount the Pitot tube, remove the blanking plug from the roof tapping and feed the short arm of the tube through the tapping. Pivot the tube through 90° while the elbow is positioned at the tapping, then lower the tube into the working section until the support plug fits into the tapping point. The support plug should then be secured in place using the screw. The height of the Pitot tube is adjusted by sliding it through the ‘O’ ring in the support plug. The two flexible tubes from the Pitot static tube should be attached to the quick release connectors on the manometer. The static head connection (side arm) should connect to socket 11, and the total head connection (top tube) should connect to socket 12.

Converting head readings to pressure readings When connected to a manometer, a Pitot tube measured the differential heads between the tapping points and the datum, the datum typically being atmosphere or the static pressure of the working section. In most of the experiments presented in this manual, atmospheric pressure is used as the datum and readings are given with respect to this datum. If a pressure reading is required, the head measurement must be converted to a pressure using the equation ΔP = ρgh where ΔP = the differential pressure, ρ = the density of the fluid, in this instance water, at the ambient temperature T g = acceleration due to gravity, usually 9.81m/s² h = the head difference, measured in metres (1mm = 0.001m) If an absolute pressure (relative to zero pressure) is required, then the effect of the datum pressure must be included in the calculation. Pressures inside the working section are typically lower than the datum pressure and thus the differential pressure must be subtracted from the datum pressure to obtain the absolute pressure, e.g. P absolute = P atmospheric – ΔP

Calculating flow velocity at the Pitot tube tip Total pressure may be defined as the sum of the static pressure and the dynamic pressure. Hence it is possible to determine the dynamic pressure of the flow at the point of measurement: P dynamic = P total - P static The readings from the manometer are relative to atmosphere, i.e. they are differential pressures- see Important note on pressure measurement using the tunnel. The dynamic pressure may be calculated from the differential pressure readings as follows: P total = P atmos – ΔP total P static = P atmos – ΔP static P dynamic = (P atmos – ΔP total ) – (P atmos – ΔP static ) = ΔP static - ΔP total

50

Operation (Note that as the static pressure in the tunnel is sub-atmospheric, where the absolute total pressure is greater than the absolute static pressure, the differential total pressure reading will be smaller than the differential static pressure). The dynamic pressure may be used to calculate the velocity at the point of measurement:

which becomes When using the C15-12, if the manometer is connected as described in the experiments then the total and static pressures and the velocity at the Pitot tip will be automatically calculated by the software.

Using the C15-15 Wake Survey Rake (requires C15-11 or C15-12) The wave survey rake is designed for mounting in the small hatch on the front wall of the tunnel, locating it just behind any model mounted via the large hatch. Remove the two thumb nuts securing the small hatch cover in place, and place the blank hatch cover in a safe place. Insert the rake survey rake through the small hatch with the tips of the rake pointing upstream (towards the inlet end of the working section). Slot the mounting plate over the positioning studs and secure the plate in place using the thumb nuts. The rake is designed so that when mounted as described, the centre of the rake is aligned with the centre point or zero-angle centreline of models mounted through the large hatch. It will therefore cross the wake downstream of the model, allowing the pressure changes across the wake and therefore the changes in velocity to be measured. The wake survey rake connects to the appropriate manometer using the 10-way connector (See The 10-way connector). Some models which may be used with the rake may also require the same connection to the manometer for other measurements. The connections must be changed over to obtain readings from both the model and the wake survey rake. This can be carried out with the tunnel operating so that both sets of readings can be taken without changing the airspeed or changing the position of the model itself. Refer to the exercise for the model in use for a suggested procedure. The software exercises designed for use with the rake have a selection box where either the model or the rake may be selected. Selecting the correct setting ensures that the software handles the manometer data in the correct manner.

Installing the C15-25 Boundary Layer Plates When installing the smooth or the roughened plate onto the floor panel, the locating studs should be inserted with a wiping or sliding action to minimize damage to the foam seals inside the slot. The plate should be locked in position by tightening the thumb nuts supplied onto the locating studs. Either plate must be installed with the chamfer upstream and facing the rear of the working section.

51


The floor section is secured into the working section using the eight thumb nuts. The specially shaped Pitot static tube is mounted using the large circular hatch in the front of the working section.

A blanking plug that includes two studs has been supplied with the tunnel. The plate can be locked in position by rotating this blanking plug so that the studs are resting against the plate, then tightening the small screw to secure the plug in position.

52

Operation

The plate can be moved horizontally along the slot by temporarily releasing the screw on the blanking plug at the top, then loosening the thumb nuts on the locating studs beneath the plate. Both thumb nuts and blanking plug screw must be re-tightened after positioning the plate, before using the tunnel.

53

Equipment Specifications Overall Dimensions Height

-

0.460m

Depth (Front-Back)

-

0.700m

Length (End-End)

-

2.250m

Electrical Supply PRODUCT-A

PRODUCT-B

PRODUCT-G

Green/yellow lead

Earth (Ground)

Earth (Ground)

Earth (Ground)

Brown lead

Live (Hot)

Live (Hot)

Live (Hot)

Blue lead

Neutral

Neutral

Neutral

Fuse rating

10A

20A

10A

Voltage

220-240V

110-120V

220V

Frequency

50Hz

60Hz

60Hz

Mains Water Supply The equipment does not require permanent connection to a water supply. Water is only required if using the optional C15-11 inclined manometer bank, which should be filled using clean, cold water. Distilled or de-mineralised water may be used.

Connection to Drain The equipment does not require a permanent drain connection. Drainage is only required if using the optional C15-11 inclined manometer bank, which may be drained into any suitable receptacle such as a bucket after use.

Clearance The wind tunnel requires an unobstructed inlet and outlet for correct operation. Adequate clearance room must be provided at either end to ensure that air can pass freely through the tunnel. Any blockage will cause a reduction in tunnel performance.

USB Channel Numbers The Armfield Windows™-compatible software allows data logging of the sensor outputs and operation of the fan motor. However, users may prefer to write their own software for control and data logging. For the convenience of those wishing to do so, Armfield has provided additional USB drivers allowing operation of the equipment via the USB socket on IFD7. The relevant channel numbers are as follows: Channel No

Signal Function

54

Equipment Specifications

Analog Outputs from C15-12 (0-5 V dc exported from socket) Ch 0 Signal

Pressure 0 -5 – 5V = 0 – 177.8 mm

Ch 1 Signal

Pressure 1 -5 – 5V = 0 – 177.8 mm

Ch 2 Signal

Pressure 2 -5 – 5V = 0 – 177.8 mm

Ch 3 Signal

Pressure 3 -5 – 5V = 0 – 177.8 mm

Ch 4 Signal

Pressure 4 -5 – 5V = 0 – 177.8 mm

Ch 5 Signal

Pressure 5 -5 – 5V = 0 – 177.8 mm

Ch 6 Signal

Pressure 6 -5 – 5V = 0 – 177.8 mm

Ch 7 Signal

Pressure 7 -5 – 5V = 0 – 177.8 mm

Ch 8 signal

Pressure 8 -5 – 5V = 0 – 177.8 mm

Ch 9 signal

Pressure 9 -5 – 5V = 0 – 177.8 mm

Ch 10 signal

Pressure 10 -5 – 5V = 0 – 177.8 mm

Ch 11 signal

Pressure 11 -5 – 5V = 0 – 177.8 mm

Ch 12 signal

Pressure 12 -5 – 5V = 0 – 177.8 mm

Ch 13 signal

Pressure 13 -5 – 5V = 0 – 177.8 mm

Ch 14 signal

Pressure 14 -5 – 5V = 0 – 177.8 mm

Ch 15 signal

Pressure 15 -5 – 5V = 0 – 177.8 mm

Not used Analog Outputs from IFD7 (0-5V dc output from socket): Ch 0 signal

Fan speed -4.25 – 4.75V = 0 – 100%

Ch 1 signal

Not used

Ch 2 signal

Rotation -5 – 5V = 0 – 106°

Ch 3 signal

Not used

Ch 4 signal

Lift -3.5 – 3.5V = 3.7 – -0.28N

Ch 5 signal

Drag -3.5 – 3.5V = -0.28 – 3.7N

Ch 6 signal

Tunnel Static Head 0 – 5V = -48.95 – 196mm

55

Armfield Instruction Manual Ch 7 signal

Not used

Not used Digital Outputs from IFD7 (0-5V dc): Not used Ch 6 signal

Watchdog signal

Not used

Available Accessories C15-11 Inclined Manometer Bank C15-12 Electronic Manometer C15-13 Lift and Drag Balance (requires C15-20 or C15-22) C15-14 Pitot Static Tube (requires C15-11 or C15-12) C15-15 Wake Survey Rake (requires C15-11 or C15-12) C15-20 Lift and Drag Aerofoil (requires C15-13 for Lift & Drag measurements but can be mounted directly into the tunnel for flow visualisation studies or Wake studies using C15-15) C15-21 Pressure Wing (requires C15-11 or C15-12) C15- 22 Drag Models (requires C15-13 for Lift & Drag measurements but can be mounted directly into the tunnel for flow visualisation studies or Wake studies using C15-15) C15- 23 Pressure Cylinder (requires C15-11 or C15-12) C15-24 Bernoulli Apparatus (requires C15-11 or C15-12) C15-25 Boundary Layer Plates (requires C15-11 or C15-12) C15-26 Project Kit

C15-10 Motor Rating 0.51 kW 3 phase, 220VΔ (maximum speed 50 Hz through inverter)

C15-11 Manometer Number of tubes

13 total

Tube length

320mm

Inclination

30°

Measuring range

0 – 160 mm H 2 O (0 – 6.3” H 2 O)

56

Equipment Specifications

C15-12 Manometer Number of channels

16

Measuring range

0 – 178 mm H 2 O (0 – 7” H 2 O)

C15-13 Lift and Drag Balance Range of lift measurement

:

0 – 5.4 N (1.2 lb.f)

Range of drag measurement :

0 – 5.4 N (1.2 lb.f)

Rotation

> +/- 45°

Requirements for the production of models of the student’s own design Models for use on the wind tunnel must be: 

The correct size to fit in the working section without damage to either the wind tunnel or the model.



Of the correct dimensions to mount securely via one of the mounting positions provided.



Produced using materials and construction techniques which will ensure that the model remains in place, secure and complete in use.



Light enough to avoid warping the tunnel or damaging the mounting points.



Equal to or less than 350g acting at the centreline of the model, if it is to be used with the lift and drag balance.



Mounting rod 4 mm diameter for models to fit the circular hatch on C15-10 or the C15-13 Lift & Drag Balance. Note that a locating pin is not necessary provided that the orientation of the model is adjusted before tightening the clamping screw to secure the mounting rod.



Connected to the appropriate single-way or ten-way connector using suitable flexible tubing, if the model is to be used with the inclined manometer bank or electronic manometer.

The Project Kit C15-26 provides appropriate mountings, tubing and connectors to ensure that these will be compatible with the wind tunnel. Suggestions for possible model designs and investigations are given in Exercise I.

Environmental Conditions This equipment has been designed for operation in the following environmental conditions. Operation outside of these conditions may result reduced performance, damage to the equipment or hazard to the operator. a. Indoor use; b. Altitude up to 2000 m; c. Temperature 5 °C to 40 °C;

57

Armfield Instruction Manual d. Maximum relative humidity 80 % for temperatures up to 31 °C, decreasing linearly to 50 % relative humidity at 40 °C; e. Mains supply voltage fluctuations up to ±10 % of the nominal voltage; f.

Transient over-voltages typically present on the MAINS supply; NOTE: The normal level of transient over-voltages is impulse withstand (overvoltage) category II of IEC 60364-4-443;

g. Pollution degree 2. Normally only nonconductive pollution occurs. Temporary conductivity caused by condensation is to be expected. Typical of an office or laboratory environment

58

Routine Maintenance Responsibility To preserve the life and efficient operation of the equipment it is important that the equipment is properly maintained. Regular maintenance of the equipment is the responsibility of the end user and must be performed by qualified personnel who understand the operation of the equipment.

General The equipment should be disconnected from the electrical supply when not in use. Water should be drained from the equipment before storage.

Cleaning The equipment should be cleaned as necessary using a damp, soft, non-shedding cloth or sponge. A mild detergent may be used if required. Cleaners intended for plastic baths and showers are also suitable. Avoid the use of cleaners intended specifically for glass. Hatches, supports, and models without tappings may be cleaned in warm water using a mild detergent and either dried using a soft cloth or tissue, or placed in a suitable location to air dry. Inspect for watermarking after air drying, and polish off with a soft cloth if required. Models with tappings should not be immersed. They may be cleaned with a damp cloth, taking care to avoid clogging the tappings, and should be thoroughly dried before use. Water in the tappings or flexible connecting tubes will cause inaccuracies in the pressure readings and must be removed by blowing through the tubing using a source of compressed air. The Lift and Drag balance should be wiped clean with a dry cloth or sponge. If this is not sufficient, a slightly damp cloth and a small amount of mild detergent may be used with care. The electrical components must not be wetted. Abrasives and solvents must be avoided at all times. The lift and drag balance (if present) must be removed before cleaning the tunnel, and the IFD7 Electrical Console and Electronic manometer C15-12 (if fitted) should be protected from drips or splashes of water. Avoid wetting or splashing the internal fan motor. If the inside of the inclined manometer bank becomes fouled or scaled then this must be carefully cleaned with a suitable mild biocide and/or descaler, with several changes of fresh water being circulated through afterwards. Multiple treatments may be required.

RCD Test The equipment is protected by an RCD that is installed at the rear of the IFD7 Electrical Console. Test the RCD by pressing the TEST button at least once a month. If the RCD button does not trip when the Test button is pressed then the equipment must not be used and should be checked by a competent electrician.

59


Fan check The fan must be checked at regular intervals to ensure it is securely mounted. The equipment must be switched off before checking the fan. The fan cover may then be removed and the fan tested to see that it is secure on the shaft. The fan cover must be replaced and secured before the equipment is used.

Replenishing the Manometer Reservoir Water is volatile and some will evaporate from the C15-11 inclined manometer bank during normal use. Before filling the reservoir it is suggested that the old water is fully drained and the vessel wiped clean. The manometer reservoir may be replenished by removing the top / displacer and filling the reservoir with clean water until all manometer tubes are filled to the first bold graduation. A few drops of wetting agent added to the water will reduce surface tension and reduce any meniscus inside the tubes to give clearer readings.

Lubrication No lubrication of the motor bearings is required.

Spares Applications for spares should be sent to Armfield Ltd, Bridge House, West Street, Ringwood, Hampshire, BH24 1DY, England, stating the serial number shown on the name plate at the rear of the equipment.

Calibration The calibration potentiometers for the electronic circuits on the C7 Wind Tunnel are located on the black electronic interface box (14) that is located at the rear of the frame below the exit from the working section. These potentiometers are set prior to despatch of the equipment and should not require further adjustment. If recalibration is necessary then the appropriate potentiometers should be adjusted as follows: VR1 Lift SPAN adjustment VR5 Static pressure SPAN adjustment VR2 Lift ZERO adjustment VR4 Drag ZERO adjustment VR3 Drag SPAN adjustment VR6 Static pressure ZERO adjustment Note: The Lift sensor on the C15-13 Lift and Drag Balance measures the reducing weight/force of the model as the lift increases. For this reason the sensor should be calibrated as follows: VR1 Adjusted to give a reading of 4.1 N (full scale) with the beam clamped (No load on sensor) VR2 Adjusted to give a reading of (4.1 – load from calibration weight) with the beam unclamped and calibration weight on plain model mounting rod at centreline of tunnel. E.g. if calibration weight is 336 gm (3.3 N) then adjust VR2 to give reading of 0.8 N (4.1-3.3). 60

Laboratory Teaching Exercises Index to Exercises Exercise A - Conversion of head measurement to pressure measurement Exercise B - Static pressure, dynamic pressure and total pressure Exercise C - Effect of change in cross section and application of the Bernoulli equation Exercise D - Flow around a cylinder Exercise E - Drag forces on bluff and streamlined bodies Exercise F - Flow and pressure distribution around a symmetrical aerofoil at different angles of attack Exercise G - Lift and Drag forces on a symmetrical aerofoil at different angles of attack Exercise H - Laminar and Turbulent Boundary Layer Development Exercise I - Project Work

Introduction The following teaching exercises are designed to introduce students to the basic principles of airflow and aerodynamics using the C15-10 Wind Tunnel and associated optional accessories. Many of the exercises are interrelated. For example ‘Flow patterns and pressure distribution around a cylinder’ (C15-23) is related to ‘Boundary layer demonstration using a flat plate’ (C15-25). Similarly the drag is also related to the effect of the boundary layer and the velocity distribution behind the cylinder can be measured using the Wake Survey Rake (C15-15). The exercises assume that all available optional models and instruments are available to the user. However, the structure of the exercises allows parts to be omitted when these are not available. A project work exercise is included that allows the user to test alternative models or instruments. The associated software includes manual entry of results from C15-11, and automated logging of results from C15-12 or C15-13.

Nomenclature Name

Symbol Unit

Definition

Ambient Temperature

t

°C

Ambient temperature of the room. Measured.

Atmospheric Pressure

P atmos

Pa

Ambient pressure of the laboratory. Measured.

Density of Water

w

kg/m³

The density of water at given temperature. Gained from standard reference tables using 61

Armfield Instruction Manual the ambient temperature.

Density of Air

a

Tunnel Static Head H (Differential Head)

Tunnel Static Pressure (Differential Pressure)

P

kg/m³

The density of air at given temperature, assuming sea level. Gained from standard reference tables using the ambient temperature.

The head difference between the static pressure at the tunnel wall and the ambient mmH 2 O atmospheric pressure, expressed as an equivalent height of water. Measured.

Pa

The pressure difference between the static pressure at the tunnel wall and the ambient atmospheric pressure. Calculated from the head difference: P = 9.80665H assuming 1 mm H 2 O = 9.80665 Pa

Air Velocity

v

m/s

Free stream velocity of the air in the wind tunnel. v = √(2 ρ w g H / ρ air )

Kinematic Viscosity 

m2/s

The Kinematic Viscosity of air at given temperature. Gained from standard reference tables using the ambient temperature. Reynolds Number is a dimensionless number that represents the relationship between the shear forces and inertial forces in a fluid.

where L is a representative dimension, e.g. Reynolds Number

Re

-

Tunnel: L = tunnel width Aerofoil: L = chord Bluff body: L = body diameter There are some conventions to selecting L, for example in wind tunnel tests of aircraft models to be scaled up to full size: Aircraft Model: L = span

Rotation

62

r

°

The angle of the aerofoil chord from the horizontal.

Laboratory Teaching Exercises

Lift

L

N

The force acting directly upwards on the model. Measured.

Drag

D

N

The force acting on the body in the direction of the air flow. Measured.

Head

H1, H2 etc.

mm

Difference between tapping point and atmosphere, expressed as a head of water. Measured.

Pressure

P1, P2 etc.

Pa

Difference between tapping point and atmosphere, expressed as a pressure. Calculated from the head.

Pitot Static Head

H static

mm

Head measurement from the side tapping of the Pitot static tube.

Pitot Total head

H total

mm

Head measurement from the tip of the Pitot static tube.

Pitot Dynamic Head

H dynamic

mm

H dynamic = H total - H static

Coefficient of Lift

CL

-

Coefficient of Drag C D

-

Form Drag Coefficient

C D0

-

C D = C D0 when L = 0

Induced Drag Coefficient

C DL

-

C DL = kC L ² = C D - C D0

Induced Drag Factor

k

-

k = C DL / C L ²

CL = L / ½V²S = lcos / ½V²S C D = D / ½V²S = C D0 + C DL

63

Exercise A - Conversion of head measurement to pressure measurement Objective To convert a head measurement using a manometer to an equivalent pressure reading. To demonstrate the use of a static pressure reading to determine tunnel air velocity.

Method By measuring the differential static head within the wind tunnel at a range of air velocities, and then converting this to a pressure figure using the appropriate equation. By calculating the air velocity in the tunnel using the appropriate equation and comparing the result to that generated by the computer.

Equipment Required C15-10 Wind Tunnel with IFD7 PC (not supplied) running C15-304 software Thermometer or similar temperature sensor (to measure ambient temperature) Barometer or similar (to measure ambient pressure)* C15-11 Inclined Manometer Bank or C15-12 Electronic Manometer Bank *If a barometer is not available then an approximate figure may sometimes be obtained from a local weather station, airport or docks, or on the Internet from one of the many available weather pages.

Theory Calculation of tunnel air velocity Velocity in the working section is related to the static pressure inside the working section by the relationship V = (2 ΔP / ρ air )0.5 Conversion of engineering units ∆P is a measurement of pressure. Within the Système International d'unités (S.I.) system, the unit for pressure is the Pascal, with one Pascal defined as a force of 1 Newton applied over an area of one square meter, i.e. 1Pa = 1N/m² However, other units have been used as standard both historically and in several countries who have not changed to S.I. units. In the case of the C15 wind tunnel, 64

Exercise A pressures are measured using a water manometer which gives the pressure as a height of water within a tube. Water has a mass, M, and under the effect of gravity, g (as, for example, in a typical laboratory), this water exerts a downwards force F: F = Mg The mass of the water is equal to the density of the water, ρ, multiplied by its volume, V: M = ρV F = ρVg Any given volume of water will exert this force over a horizontal plane area equivalent to the volume divided by the height of the surface above the plane. A=V/h Pressure P = Force / Area, and thus: P = ρVg / (V/h) P = ρgh It can be seen that this final form of the equation includes neither the volume of the fluid nor the area it exerts a force over, and thus the only figures required to calculate the pressure are the height of fluid, the density of the fluid, and the acceleration due to gravity. It also means that any manometer should give an identical reading regardless of the cross-sectional area of its tubes, providing the same fluid is used to fill it. Manometer readings are taken in metres or millimetres, but are often referred to as a ‘head reading’, referring to the head of liquid (i.e. the height of liquid above a datum). In some instances in generalised descriptions, direct manometer readings may be used as an equivalent term to pressure. It is important to realise that for calculations the direct manometer reading is not a pressure reading. Manometers may be filled with other liquids, such as mercury. It is possible to convert values given as a head of water into an equivalent head of mercury using a conversion factor (which in this instance assumes a constant value for the density of both water and mercury, which is not strictly accurate but is close enough to be satisfactory for many purposes). For this exercise, the conversion is taken to be 1 mmHg

=

13.3 mmH 2 O

x in mmH 2 O = 0.075x in mmH 2 O A differential pressure, ∆P, can be determined by measuring the equivalent head difference, ∆H. A differential pressure is the difference in pressure between two points at which the pressure is measured. If water (or any incompressible liquid) is enclosed in a U-tube, the top of one side of the U may be connected to the first pressure measurement point, and the top of the other side may be connected to the second measurement point. As a result of the different forces produced on the water 65

Armfield Instruction Manual surface by the two different pressures, the water in the U will be displaced by an equivalent amount. The vertical distance between the two heights can then be measured to give a differential head, ∆H. For compatibility with the inclined manometer bank, the electronic manometer is calibrated to give readings in millimetres of water.

Measurement of differential head Using the Inclined Manometer or Electronic Manometer, ∆P =  m g ∆H where  m = manometer fluid density (in kg/m3) g

= gravitational constant (= 9.81 m/s²)

∆H = vertical difference in manometer heights between static pressure and atmospheric pressure (in m) Note that the engraved scale compensates for the magnification of the inclined manometer. Each bold line corresponds to10 mm H 2 O and each light line corresponds to 2 mm H 2 O. ∆H

=

d sin 

d

=

indicated difference in levels on the tilted manometer.



=

angle of inclination to the horizontal (30°)

where

Sin 30° = 0.5 so ∆H = 0.5 d (so 10 mm H 2 O is indicated by 20 mm on scale)

Calculation of tunnel air velocity The theoretical air velocity can be calculated from V = (2 ρ man g Δh / ρ air )0.5 This will be checked in exercise B using a Pitot Static Tube to measure the local air velocity.

Equipment Set Up Note: Additional information is available in the Operation section if required. The tunnel should be set up with no models or other accessories in place. The three top tappings must be blanked with plugs, and the two plain hatch covers should be secured in position over the front hatches. Ensure that the floor is fitted. Check the surroundings to see that there is no obstruction at the inlet or outlet of the tunnel and that there are no loose objects nearby which could cause a hazard. For most experiments the reading for static pressure in the tunnel will be sent directly to the software via the IFD7 from the dedicated pressure sensor mounted on the wind tunnel frame. For this experiment, however, the reading will be taken using a manometer (if using the C15-12 electronic manometer then for this particular

66

Exercise A experiment the sensor reading will be displayed on the mimic diagram screen as a representation of a water manometer). The single tube from the pressure tapping on working section side wall (near the inlet) should be connected to the 1-way quick-release fitting on the manometer (Socket 11). Push the end of the tube straight into the socket, without bending or twisting, until it clicks into place. All other sockets should be left open to atmosphere. If using the C15-11 inclined manometer bank, check that the manometer has been filled and primed, with a convenient water level and no trapped air bubbles. Check that the IFD7 is connected to a suitable mains electrical supply and to the USB socket of a suitable PC. The PC should be switched on and the appropriate software version run (C15-11 version or C15-12 version depending on the manometer used). Select ‘Exercise A’ and ensure that ‘IFD: OK’ is displayed in the bottom right-hand corner. Switch on the IFD7 using the mains switch on the front.

Procedure Check that the fan is set to 0%, then switch it out of standby mode by selecting the ‘Fan On’ button on the mimic diagram. Check that the static pressure manometer reading (column 11) is the same as the atmospheric reading (e.g. column 12). Measure the ambient temperature in Celsius and pressure of the laboratory in Pascals and enter the results in the appropriate boxes on the mimic diagram. If using the C15-11 inclined manometer, take a reading for the water level in columns 11 and 12 and enter the result in the boxes provided on the mimic diagram. Transfer the readings for zero air velocity to the results table by selecting the

icon.

Gradually set the fan to 10% in 1% increments by using the up arrows. This allows the fan to start up gradually. Check that all fittings on the tunnel remain secure and that there is no safety hazard due to the inlet and outlet air streams. Allow time for the fan to stabilise at 10%. When the manometer readings have settled, take another pair of readings by selecting the icon. Increase the fan setting in 10% increments, typing the fan setting into the box on the mimic diagram (i.e. 20%, 30% etc) and taking a pair of readings at each setting using the icon. Allow the system to stabilise at each setting before taking measurements. Be aware of the surroundings when operating the wind tunnel, keeping safety in mind at all times. Take a final pair of readings at 100% fan setting. Gradually shut down the fan: Type in a value of 50% for the fan setting. When the fan has slowed, type in a value of 20%. Once the fan has slowed again, reduce the fan speed to 0% by using the arrow keys. Set the fan to Standby by selecting the ‘Fan On button in the software.

67

Armfield Instruction Manual Save the software results by selecting ‘Save As…’ from the File menu. Give the results a suitable name for future reference, such as the equipment code, experiment letter and date. Switch off the mains switch on the IFD7.

Results The software records the sensor data and corresponding calculations under the following headings:

Students should perform their own calculations on the recorded column readings (or an appropriate representative selection of those readings), and compare the results to those calculated by the computer.

Conclusion In your write-up, discuss the use of a manometer to measure pressure and pressure difference. How are pressure and head related? What would affect the choice of a suitable fluid for use in a manometer? Comment on the use of static pressure to determine air velocity. What are the possible sources of error? Suggest methods for reducing or eliminating those errors, and methods for checking the accuracy of the velocity measurement. Include a discussion of any errors inherent in the checking processes suggested.

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Exercise B - Static pressure, dynamic pressure and total pressure Objective To demonstrate the difference between Static pressure, Dynamic pressure and Total pressure and how Dynamic pressure can be used to determine air velocity. To show how velocity varies in the test section because of the velocity profile.

Method Using a Pitot Static tube to determine the Dynamic pressure by measuring the Total pressure and Static pressure in the test section. Traversing the Pitot Static tube vertically across the test section to determine the velocity profile.

Equipment Required C15-10 Wind Tunnel C15-14 Pitot Static tube PC (not supplied) running C15-304 software Thermometer or similar temperature sensor (to measure ambient temperature) Barometer or similar (to measure ambient pressure)* C15-11 Inclined manometer or C15-12 Electronic Manometer *If a barometer is not available then an approximate figure may sometimes be obtained from a local weather station, airport or docks, or on the Internet from one of the many available weather pages.

Optional Equipment C15-15 Wake Survey Rake if the C15-14 Pitot Static tube is not available

Theory As already presented in Exercise A, velocity in the working section is related to the static pressure inside the working section by the relationship V = (2 ΔP / ρ air )0.5 ∆P can be determined by measuring ∆H using the Inclined Manometer when ∆P =  m g ∆H where

 m = manometer fluid density (in kg/m3) g

=

gravitational constant (= 9.81 m/s²)

∆H = true difference in manometer heights (in m)

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Armfield Instruction Manual = d sin  where d =

indicated difference in levels on the tilted manometer.

 =

angle of inclination to the horizontal (30°)

Sin 30° = 0.5 so ∆H = 0.5 d (so 10 mm H 2 O is indicated by 20 mm on scale) The theoretical air velocity can be calculated from V = (2 ρ man g Δh / ρ air )0.5 Application of Bernoulli's Equation to the Pitot Static Tube provides the relationship:

This assumes that the flow is incompressible at the low velocities experienced within the wind tunnel (negligible correction if v < 100 m/s). where

∆P = difference in pressure between the total and static tappings (N/m2)  a = density of air (kg/m3) v = the point velocity (m/s) ∆P is measured using the inclined manometer as for the air velocity: ∆P =  m g ∆h

Equipment Set Up Note: Additional information is available in the Operation section if required. The tunnel should be set up with the two plain hatch covers secured in position over the front hatches. The Pitot static tube should be fitted to the middle of the three roof tappings, with the short arm pointing towards the tunnel inlet. The other two tappings should be fitted with blanking plugs. Ensure that the floor is fitted. Check the surroundings to see that there is no obstruction at the inlet or outlet of the tunnel and that there are no loose objects nearby which could cause a hazard. In this experiment the reading for static pressure in the tunnel is sent directly to the software via the IFD7, from the dedicated pressure sensor mounted on the wind tunnel frame. The single tube from the pressure tapping on working section side wall (near the inlet) should be connected to the 1-way quick-release fitting on the black box fitted to the tunnel frame. The two tubes connected to the Pitot tube should be connected to the manometer. Fit the static head connection (from the side arm on the Pitot tube) to socket 11 on the manometer, and the total head connection (from the top end of the Pitot tube) to socket 12. If using the C15-11 inclined manometer bank, check that the manometer has been filled and primed, with a convenient water level and no trapped air bubbles. If using

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Exercise B the C15-12 electronic manometer, check that the manometer is connected to a suitable PC with the USB cable. Check that the IFD7 is connected to a suitable mains electrical supply and to the USB socket of a suitable PC. The PC should be switched on and the appropriate software version run (C15-11 version or C15-12 version depending on the manometer used). Select ‘Exercise B’ and ensure that ‘IFD: OK’ is displayed in the bottom right-hand corner. Switch on the IFD7 using the mains switch on the front.

Procedure Lift the Pitot tube within the tunnel so that the sensor arm is positioned close to the top of the tunnel. Secure it gently in place with the screw. Check that the fan is set to 0%, then switch it out of standby mode by selecting the ‘Fan On’ button on the mimic diagram. Check that the static pressure manometer reading (column 11) is the same as the atmospheric reading (e.g. column 12). Select the results table within the software and rename the sheet to ‘50%’. Measure the ambient temperature in Celsius and pressure of the laboratory in Pascals and enter the results in the appropriate boxes on the mimic diagram. Gradually set the fan to 10% in 1% increments by using the up arrows. This allows the fan to start up gradually. Check that all fittings on the tunnel remain secure and that there is no safety hazard due to the inlet and outlet air streams. Gradually set the fan to 50% by typing in speed increments of 10% until 50% is reached. Be aware of the surroundings when operating the wind tunnel, keeping safety in mind at all times. Allow time for the fan to stabilise at 50%. If using the C15-11 inclined manometer, take a reading for the water level in columns 11 and 12 and enter the result in the boxes provided on the mimic diagram. Log the sensor readings by selecting the

icon.

Lower the Pitot tube approximately 20mm. Repeat the readings as before. Continue to lower the Pitot tube in 20mm increments, taking readings at each position, until it reaches its lowest position. Create a new results sheet, and rename the new sheet ‘100%’. Set the fan to 100% and allow time for it to stabilise. Repeat the readings by lifting the Pitot tube in 20mm increments until it reaches its highest position, and record the data each time. Gradually shut down the fan: Type in a value of 50% for the fan setting. When the fan has slowed, type in a value of 20%. Once the fan has slowed again, reduce the fan speed to 0% by using the arrow keys. Set the fan to Standby by selecting the ‘Fan On button in the software. 71



Students should perform their own calculations on the recorded column readings (or an appropriate representative selection of those readings), and compare the results to those calculated by the computer. Each set of sensor readings taken is tagged with a sample number by the software. For each fan speed, plot a graph of static pressure, dynamic pressure and total pressure. On the second axis, plot the tip velocity (i.e. the air velocity as calculated from the Pitot tube dynamic pressure).

Conclusion What is the relationship between static, dynamic, and total pressure? What specific factors had to be taken into account when using the C15 apparatus for this experiment? Describe the shapes of the graphs obtained. Is the shape what was expected? Discuss the reasons for the shape of graph obtained. Describe the advantages and disadvantages of the Pitot tube, and give examples of applications in which a Pitot tube or some variant of it is used.

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Exercise C - Effect of change in cross section and application of the Bernoulli equation Objective To investigate the variation in static head resulting from a change in cross-sectional area. To investigate the Bernoulli equation (if C15-14 Pitot tube is available)

Method By measuring the differential static head at a sequence of points along the length of a Venturi. By measuring the velocity head at three points along a Venturi (C15-14 only).

Equipment Required C15-10 Wind Tunnel with IFD7 PC (not supplied) running C15-304 software C15-24 Bernoulli Apparatus Thermometer or similar temperature sensor (to measure ambient temperature) Barometer or similar (to measure ambient pressure)* C15-11 Inclined Manometer Bank or C15-12 Electronic Manometer Bank *If a barometer is not available then an approximate figure may sometimes be obtained from a local weather station, airport or docks, or on the Internet from one of the many available weather pages.

Optional equipment C15-14 Pitot Static Tube

Theory Velocity in the working section is calculated as in earlier exercises: V = (2 ρ man g Δh / ρ air )0.5

Bernoulli’s Equation Bernoulli’s equation expresses the relationship between the velocity and the pressure at any given point in a fluid. It makes the assumption that air acts as a Newtonian fluid. It assumes flow is steady, and therefore cannot be applied to conditions in which flow is under acceleration or changing pressure. It also assumes mass continuity. The basic form of this equation may be stated as:

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Armfield Instruction Manual For low flow rates in air without sudden changes in pressure or velocity, it is possible to assume that the air is incompressible. In aerodynamics, air is generally assumed to act as if it were an incompressible fluid at speeds less than Mach 0.3 (i.e. 0.3 times the speed of sound in air, or very approximately 100 m/s), as treating it in this way does not usually introduce any significant errors. Air within the tunnel has a maximum velocity in the region of 32 m/s (lower with models fitted), and may thus be assumed to be incompressible. As a final assumption, the difference in pressure as a result of changing vertical elevation is assumed to be negligible across the vertical height of the working section. Bernoulli’s equation may then be expressed as:

It may be stated from examination of this equation that (if it is correct), pressure decreases as velocity increases. It may then be expected that observations and measurements will show the static pressure decreasing along the Venturi from the inlet to the throat, and increasing again along the outlet from the throat. It may also be expected that the Pitot tube will show a higher air velocity in the throat than at the inlet or outlet. This experiment will also investigate the relationship between the contraction ratio A1/A2 and the resulting velocity change, to determine the accuracy of the equation V2 = V1 * A1 / A2 To calculate the contraction ratio, the following information is required: Height of working section: 150mm Width of working section:

150mm

Area of working section:

22,500 mm²

Tapping

Width

Area

Point

(mm)

(mm²)

P1

149

22,350

1.01

P2

132.4

19,860

1.13

P3

115.8

17,370

1.30

P4

100

15,000

1.50

P5

100

15,000

1.50

P6

100

15,000

1.50

P7

109.3

16,395

1.37

P8

119.35

17,902.5

1.26

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A1/A2

Exercise C P9

129.4

19,410

1.16

P10

139.4

20,910

1.08

P11

149.4

22,410

1.004

Equipment Set Up Note: Additional information on removing and fitting the accessories is available in the Operation section. Before fitting the Venturi, ensure that any other models, sensors, or other accessories have been removed from the tunnel first. Remove the blank floor and fit the Venturi section, taking care to lift it cleanly into place without damaging either the accessory or the working section. Secure the Venturi in place. The two plain hatch covers should be secured in position over the front hatches. The Static pressure sensor should be fitted to the upstream tapping in the roof of the working section. If available the Pitot static tube should be fitted to the roof tapping at the centre of the working section, with the short arm pointing towards the tunnel inlet. Position the tip of the Pitot tube in the vertical centre of the working section. The other two roof tapping(s) should be fitted with blanking plugs. If using the C15-11 inclined manometer bank the flexible tube from the static pressure tapping should be connected to the left hand tube on the manometer marked ‘Static’. Check that the manometer has been filled and primed, with a convenient water level and no trapped air bubbles. If using the C15-12 electronic manometer, check that the manometer is connected to a suitable PC with the USB cable. The tappings along the base of the Venturi should be connected to the manometer using the 10-way connector. The two tubes connected to the Pitot tube should be connected to the manometer. Fit the static head connection (from the top end of the Pitot tube) to socket 11 on the manometer, and the total head connection (from the side arm on the Pitot tube) to socket 12. (Note that readings from tapping 11 must alternate – i.e. the Venturi and the Pitot static tube share tapping 11). Check the surroundings to see that there is no obstruction at the inlet or outlet of the tunnel and that there are no loose objects nearby which could cause a hazard. Check that the IFD7 is connected to a suitable mains electrical supply and to the USB socket of a suitable PC. The PC should be switched on and the appropriate software version run (C15-11 version or C15-12 version depending on the manometer used). Select ‘Exercise C’ and ensure that ‘IFD: OK’ is displayed in the bottom right-hand corner. Switch on the IFD7 using the mains switch on the front.

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Procedure Check that the fan is set to 0%, then switch it out of standby mode by selecting the ‘Fan On’ button on the mimic diagram. Check that the manometer readings are all the same at zero velocity. Select the results sheet in the software and rename it to ‘50%’. Measure the ambient temperature in Celsius and pressure of the laboratory in Pascals and enter the results in the appropriate boxes on the mimic diagram. Gradually set the fan to 10% in 1% increments by using the up arrows. This allows the fan to start up gradually. Check that all fittings on the tunnel remain secure and that there is no safety hazard due to the inlet and outlet air streams. Gradually set the fan to 50% by typing in speed increments of 10% until 50% is reached. Be aware of the surroundings when operating the wind tunnel, keeping safety in mind at all times. Allow time for the fan to stabilise at 50%. If using the C15-11 inclined manometer, take a reading for the water level in all the columns and enter the results on the mimic diagram. It is also possible to move the cursors along the tubes to match the readings, giving a clearer visual representation of the pressure variation along the Venturi. Log the sensor readings by selecting the

icon.

Remove the Pitot tube and blank the tapping position. Fit the Pitot to the central roof tapping, position the sensor arm in the centre of the working section, and log the sensor readings again using the icon. Move the Pitot to the downstream roof tapping, blanking the central tapping. Position it centrally in the working section as before. Log the readings. Create a new results sheet using the

icon. Name it ‘100%’.

Increase the fan setting to 100% and allow time for the fan speed to stabilise. Repeat the sensor readings at the new speed, taking one set of readings for each Pitot tube position. If time permits, further sets of readings may be taken at other fan speeds to obtain a full set of results from 10% to 100%. Create a new sheet for each speed and name it appropriately. Gradually shut down the fan: Type in a value of 50% for the fan setting. When the fan has slowed, type in a value of 20%. Once the fan has slowed again, reduce the fan speed to 0% by using the arrow keys. Set the fan to Standby by selecting the ‘Fan On button in the software. Save the software results by selecting ‘Save As…’ from the File menu. Give the results a suitable name for future reference, such as the equipment code, experiment letter and date. Switch off the mains switch on the IFD7.

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Exercise C

Results The software calculates the equivalent pressure for each head reading along the Venturi, and also calculates the air velocity indicated by each of the Pitot tube readings. Finally, it calculates the predicted air velocity at each Pitot position based on the equation V2 = V1 * A1 / A2 for comparison with the measured result using the Pitot.

Conclusion Compare the results obtained for pressure variation along the working section with the expected results. Were the expected results obtained? Using the readings for Pitot static and total head, and the corresponding calculation for velocity at that point, investigate the validity of the simplified version of the Bernoulli equation presented in the theory section. Do the Pitot static pressure readings compare well with the static pressure readings for the corresponding tapping in the base of the working section? Does the simplified equation describe the actual results obtained? Compare the air velocity calculated using the Bernoulli equation with that obtained using a simple contraction ratio calculation. How well do the two sets of results compare? Discuss any possible sources of error within the experiment, and suggest methods for reducing or eliminating them.

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Exercise D - Flow around a cylinder Objective The visualisation of flow around a bluff body at different velocities (and thus at different values of Reynolds Number). The measurement of pressure distribution around a circular cylinder at different velocities (and Reynolds Number)

Method By using a thread to indicate streamline flow around a circular cylinder positioned in an air stream. By using a tapped cylinder to measured the pressure distribution around it when positioned in a steady stream of air. By calculating the theoretical pressure distribution around the cylinder and comparing the result in graph format against the experimentally obtained values.

Equipment Required C15-10 Wind Tunnel with IFD7 and flow visualisation PC (not supplied) running C15-304 software C15-23 Pressure Cylinder C15-11 Inclined Manometer Bank or C15-12 Electronic Manometer Bank

Optional equipment C15-15 Wake Survey Rake Camera (and tripod) for recording flow visualisation

Theory Free stream velocity in the working section is calculated as in earlier exercises: V = (2ρ man g Δh/ρ air )0.5

Flow around a circular cylinder If a long circular cylinder is positioned perpendicular to a steady stream of air, the theoretical equation for the velocity at the surface, assuming no losses, is: v = 2Vsin

……(1)

where v = local velocity at the surface, and  = angle between the radius to the tapping and the free stream flow direction Using Bernoulli, the theoretical surface pressure at a point may be found: P + ½V² = p + ½v²

……(2)

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Exercise D where P = tunnel static pressure, and p = surface pressure acting on the cylinder at a point rearranging (2) and substituting in from (1): p = P + ½V²(1 - 4sin²)

……(3)

where p = p absolute and P = P absolute When using the C15, both the tunnel static pressure and the cylinder tapping pressures are measured relative to atmosphere: Absolute static pressure P abs = P atmos – P measured Absolute tapping pressure p abs = P atmos – p measured The theoretical pressure presented in the software is a differential pressure for direct comparison with the measured pressure (P = P measured and p = p measured ) (P atmos – p measured ) = (P atmos – P measured ) + ½V²(1 - 4sin²), and p measured = P measured - ½V²(1 - 4sin²) p = P - ½V²(1 - 4sin²) The tapping points are evenly distributed around half of the cylinder at 20° intervals (i.e. the tapping points are situated at  = 0°,  = 20°,  = 40°, etc. up to  = 180°).

Equipment Set Up Note: Additional information is available in the Operation section if required. The tunnel should be set up with the flow visualisation tube fitted to the upstream roof tapping. Place the arm supporting the thread a little above half height in the working section (closer to the roof than to the floor). The other two roof tappings should be fitted with blanking plugs. The pressure cylinder should be fitted through the large circular hatch, at an angle of 0° to the horizontal (i.e. the first tapping should face directly upstream and the second tapping should face directly downstream). Manually position the flow visualisation thread over the top of the cylinder. If available, the wake survey rake may be fitted in the small hatch. If the wake survey rake is not used then the small hatch should be fitted with the plain hatch cover. Ensure that the floor is fitted. Check the surroundings to see that there is no obstruction at the inlet or outlet of the tunnel and that there are no loose objects nearby which could cause a hazard. The single tube from the pressure tapping on working section side wall (near the inlet) should be connected to the 1-way quick-release fitting on the black box fitted to the tunnel frame. The 10-way connection from the pressure cylinder should be fitted to the manometer. If the wake survey rake is used then this is initially left disconnected.

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Armfield Instruction Manual If using the C15-11 inclined manometer bank, check that the manometer has been filled and primed, with a convenient water level and no trapped air bubbles. If using the C15-12 electronic manometer, check that the manometer is connected to a suitable PC with the USB cable. Check that the IFD7 is connected to a suitable mains electrical supply and to the USB socket of a suitable PC. The PC should be switched on and the appropriate software version run (C15-11 version or C15-12 version depending on the manometer used). Select ‘Exercise D’ and ensure that ‘IFD: OK’ is displayed in the bottom right-hand corner. Switch on the IFD7 using the mains switch on the front.

Camera setup If a still or video camera and tripod are available then the camera should be mounted in front of the round hatch to give a good field of view around the cylinder. A plain background behind the wind tunnel is advisable (e.g. a plain sheet of white paper may be attached to the back of the tunnel, on the outside). Select camera settings that give the fastest possible shutter speed and then the best possible depth of field at that speed. The use of flash may cause inconvenient reflections on the working section sides, so where available use sufficient lighting to avoid the need for flash For best pictures the flow visualisation thread should be in sharp focus, which can be difficult to achieve especially if using autofocus. If the camera can be pre-focussed then it is possible to temporarily insert a focus guide such as the Pitot tube arm or a similar narrow object, through the roof tapping. Once the camera is focussed on the guide, the guide should be removed and the blanking plug replaced. If possible, take a test shot and display the results at a reasonable size to check that images will be acceptable. If a tripod is not available then the camera can be hand held, but good results may be more difficult to obtain. An assistant to the camera operator is suggested who can make adjustments to the equipment as required, e.g. to position then remove any focussing guide.

Procedure Check that the fan is set to 0%, then switch it out of standby mode by selecting the ‘Fan On’ button on the mimic diagram. Check that the manometer readings are all the same at zero velocity. Select the results sheet in the software and rename it to ‘40%’. Measure the ambient temperature in Celsius and pressure of the laboratory in Pascals and enter the results in the appropriate boxes on the mimic diagram. Gradually set the fan to 10% in 1% increments by using the up arrows. This allows the fan to start up gradually. Check that all fittings on the tunnel remain secure and that there is no safety hazard due to the inlet and outlet air streams. Gradually set the fan to 40% by typing in speed increments of 10% until 40% is reached. Be aware of the surroundings when operating the wind tunnel, keeping safety in mind at all times. Allow time for the fan to stabilise at 40%.

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Exercise D Adjust the height of the flow visualisation tube and the length of the thread to give a good curve above the cylinder. Sketch or photograph the curve. Shorten the thread until the end trails immediately in the cylinder wake, investigating possible turbulence (it is difficult to visually record this unless a motion camera is available). If using the C15-11 inclined manometer, take a reading for the water level in all the columns and enter the results on the mimic diagram. It is also possible to move the cursors along the tubes to match the readings, giving a clearer visual representation of the pressure variation around the cylinder. Select ‘Cylinder’ in the ‘Model Used’ box. Log the sensor readings by selecting the

icon.

If the wake survey rake is available, create a new results sheet using the icon and rename it ‘Wake 40%’. Disconnect the pressure cylinder and connect the survey rake. Select ‘Rake’ in the ‘Model Used’ box. If using the C15-11, enter the new manometer readings on the mimic diagram. Log the sensor readings by selecting the icon. Disconnect the wake survey rake and reconnect the pressure cylinder. Create a new results sheet using the again.

icon and rename it ‘60%’. Select ‘Cylinder’

Increase the fan setting to 60%. Repeat the flow visualisation and pressure sensor logging as before. If using the wake survey rake, create a new sheet for ‘Wake 60%’, set the ‘Model Used’ to ‘Rake’ and connect the survey rake to take a set of readings. Repeat at 80%. Remember to create a new results sheet and rename it each time, and to select the correct model for each set of readings. Gradually shut down the fan: Type in a value of 50% for the fan setting. When the fan has slowed, type in a value of 20%. Once the fan has slowed again, reduce the fan speed to 0% by using the arrow keys. Set the fan to Standby by selecting the ‘Fan On button in the software. Save the software results by selecting ‘Save As…’ from the File menu. Give the results a suitable name for future reference, such as the equipment code, experiment letter and date. Switch off the mains switch on the IFD7.

Results For the Cylinder results, the software calculates the theoretical pressure at each tapping point around the cylinder, and the Reynolds number at that free stream velocity. For each fan speed setting, plot a graph of Theoretical Pressure and Surface Pressure against Tapping Position, and note the Reynolds number for each setting. For the Rake results, the software calculates the pressure and flow velocity for each prong position, and the Reynolds number at that free stream velocity. For each fan speed setting, plot a graph of pressure against position. On the second y-axis, plot the flow velocities. Note the Reynolds number for each graph.

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Conclusion Describe the visual observations made of flow around the cylinder. What shape did the streamlines form around the cylinder? How did this vary with free stream velocity? Was turbulence a significant element of the flow pattern? Did this change with Reynolds number? Include sketches or photos to illustrate your observations. How well does the theoretical prediction of surface pressure correspond to the measured pressure? Does the accuracy of the theoretical equation vary with Reynolds number? If so, how? Describe the pressure variation across the wake. Relate this to the visual observations of the streamline paths and the appearance of turbulence. How did the wake vary with free stream velocity/Reynolds number?

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Exercise E - Drag forces on bluff and streamlined bodies Objective Comparison of drag for shapes of equal equatorial diameter Visualisation of flow around different body shapes Measurement of the wake profile behind different shapes (requires C14-15)

Method By visually indicating the flow pattern around shapes of identical equatorial crosssection to the air flow direction, at a variety of free stream velocities. By measuring the drag on those shapes at a range of free stream velocities. By visualising the difference in flow around two bodies of identical shape but having different surface textures. With the C1-15, to measure the pressure and velocity variation across the wake of each model at a range of free stream velocities.

Equipment Required C15-10 Wind Tunnel with IFD7 PC (not supplied) running C15-304 software C15-22 Drag Models C15-13 Lift and Drag balance

Optional equipment C15-15 Wake Survey Rake Camera (and tripod) for recording flow visualisation results

Theory Velocity in the working section is calculated from V = (2 ρ man g Δh / ρ air )0.5

Drag forces A body moving through a fluid experiences two sorts of drag force: pressure drag, which is a result of the change in motion of the air particles and the creation of eddies and wake, and friction drag, which is the result of shear forces between the body and the layer of air moving around it. Between them, pressure drag and friction drag form the total drag on the body. The proportion of each depends on the shape of the body. If friction drag is the main component of the total drag, the body is described as streamlined. If the main component is pressure drag, the body is described as bluff (or blunt). Both types of drag vary with the Reynolds number of the flow, but the friction drag is much more sensitive to changes in Reynolds number. Friction drag therefore tends to become more significant at higher flow rates.

Flow type Flow around a body may travel in smooth layers with little or no mixing between layers, which is described as laminar flow. Alternatively flow may travel with a significant lateral component to its velocity, with eddies, mixing, and even some flow

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Armfield Instruction Manual in a reverse direction to the average. This is described as turbulent flow. In many situations, the total flow around a body will demonstrate both types of flow.

Boundary layer The development of friction drag implies that there is a force exerted by the fluid on the body. In exerting this force, the fluid itself is slowed. The slower fluid particles at the body surface then exert a drag force on the faster fluid particles further away from the body. Although in theory the boundary layer is infinite, in practice the significance of the slowing of fluid particles (the effect of viscosity) reduces with distance from the body. The region close to the body where viscous effects are significant is termed the boundary layer. It is usually assumed to be the region in which the flow velocity is less than 99% of the free stream velocity. Flow within the boundary layer may be laminar or turbulent, and may change from laminar to turbulent as it passes around the body. A change from one type to the other is known as boundary layer transition. The development of the boundary layer is affected by the type of flow within the boundary layer, the Reynolds number, the shape of the body, and the surface roughness of the body. The type of boundary layer is also affected by the Reynolds number, the surface roughness, and the presence of large disturbances in the shape or surface of the body. The boundary layer may wrap right around the body and then travel downstream in a narrow wake, or it may separate from the body at some point and travel downstream in a wide wake. The point at which separation occurs is affected by the Reynolds number, the shape of the body, and the type of flow. Some examples are illustrated below.

Low Reynolds number Smooth body

High Reynolds number Smooth body

Turbulent boundary layer (e.g. rough body)

Boundary layer manipulation Boundary layer type and separation significantly affects the drag experienced by the body. Therefore the ability to control the boundary layer characteristics if of great 84

Exercise E interest to engineers and designers in fields where drag is an important factor, such as in aircraft design. As the Reynolds number is often set by the conditions in which an object must operate, the factors that can be controlled are the shape of a body and its surface finish. It is also possible to ‘trip’ the boundary layer into turbulent conditions by placing a deliberate obstruction on an otherwise smooth surface, such as a series of bumps or a wire.

Equipment Set Up Note: Additional information is available in the Operation section if required. The tunnel should be set up with the flow visualisation tube fitted to the upstream roof tapping. Place the arm supporting the thread a little above half height in the working section (closer to the roof than to the floor). The other two roof tappings should be fitted with blanking plugs. The flat circular disk should be fitted to the lift and drag balance, and the balance should then be fitted into the large circular hatch, with the face of the model flat to the flow direction of the working section. If available, the wake survey rake may be fitted in the small hatch. If the wake survey rake is not used then the small hatch should be fitted with the plain hatch cover. Ensure that the floor is fitted. Check the surroundings to see that there is no obstruction at the inlet or outlet of the tunnel and that there are no loose objects nearby which could cause a hazard. The single tube from the pressure tapping on working section side wall (near the inlet) should be connected to the 1-way quick-release fitting on the black box fitted to the tunnel frame. Connect the cable from the lift and drag balance to the front of the IFD7. If using the wake survey rake, connect the tubes from the rake to the manometer used. If using the electronic manometer, connect the manometer to a suitable PC using the IFD cable. Check that the IFD7 is connected to a suitable mains electrical supply and to the USB socket of a suitable PC. The PC should be switched on and the appropriate software version run (C15-11 version or C15-12 version depending on the manometer used). Select ‘Exercise D’ and ensure that ‘IFD: OK’ is displayed in the bottom right-hand corner. Switch on the IFD7 using the mains switch on the front.

Camera setup If a still or video camera and tripod are available then the camera should be mounted in front of the round hatch to give a good field of view around the cylinder. A plain background behind the wind tunnel is advisable (e.g. a plain sheet of white paper may be attached to the back of the tunnel, on the outside). Select camera settings that give the fastest possible shutter speed and then the best possible depth of field at that speed. The use of flash may cause inconvenient reflections on the working section sides, so where available use sufficient lighting to avoid the need for flash For best pictures the flow visualisation thread should be in sharp focus, which can be difficult to achieve especially if using autofocus. If the camera can be pre-focussed then it is possible to temporarily insert a focus guide such as the Pitot tube arm or a similar narrow object, through the roof tapping. Once the camera is focussed on the guide, the guide should be removed and the blanking plug replaced. If possible, take

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Armfield Instruction Manual a test shot and display the results at a reasonable size to check that images will be acceptable. If a tripod is not available then the camera can be hand held, but good results may be more difficult to obtain. An assistant to the camera operator is suggested who can make adjustments to the equipment as required, e.g. to position then remove any focussing guide.

Procedure Check that the fan is set to 0%, then switch it out of standby mode by selecting the ‘Fan On’ button on the mimic diagram. Check that the manometer readings are all the same at zero velocity. Select the results sheet in the software and rename it to ‘20%’. Measure the ambient temperature in Celsius and pressure of the laboratory in Pascals and enter the results in the appropriate boxes on the mimic diagram. Select the body fitted to the drag balance in the selection box on the mimic diagram. If using the wake survey rake, set the ‘Rake’ selection to ‘Yes’. If not using the rake, check that the selection is set to ‘No’. Gradually set the fan to 20% by using the up arrows. Check that all fittings on the tunnel remain secure and that there is no safety hazard due to the inlet and outlet air streams. Allow time for the fan to stabilise. Adjust the height of the flow visualisation tube and the length of the thread to give a good curve above the model. Sketch or photograph the curve. A short written description of the observations may be inserted into the results sheet using the Notes facility, but this is probably better used to insert a reference number or code that matches with the sketch or photo made. Shorten the thread until the end trails immediately in the cylinder wake, investigating possible turbulence (it is difficult to visually record this unless a motion camera is available). Make written observations and sketches as required to describe what is observed. If using the C15-11 inclined manometer and the wake survey rake, take a reading for the water level in all the columns and enter the results on the mimic diagram. It is also possible to move the cursors along the tubes to match the readings, giving a clearer visual representation of the pressure variation around the cylinder. Log the sensor readings by selecting the

icon.

Set the fan to 30%. Allow time for the fan to stabilise, then take a new set of readings using the icon- if using the wake survey rake and the C1-11 then enter the manometer readings on the mimic diagram first. Repeat for fan speeds of 40%, 50%, and so on up to 100%. Repeat while reducing the fan speed in steps of 10% with a final reading at 20%. Set the fan to 0% and allow the fan to stop before proceeding.

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Exercise E Save the results sheet by selecting ‘Save As…’ from the ‘File’ menu, so that the data is not lost in the event of a computer failure. Give the file a suitable name so that it can be found easily at a later date, for example the equipment code, experiment letter and date. Create a new results sheet using the body type selection box.

icon, and select ‘Concave disk’ from the

Remove the lift and drag balance, and replace the flat disk with the concave disk. Repeat the flow visualisation (and wake measurements if the rake is used) for the same range of fan speeds as with the flat disk. Save the results using ‘Save’- this will add the new data to the existing file. Shut down the fan and allow it to stop. Repeat for the convex disk, sphere, and streamlined body, creating a new results sheet for each and selecting the appropriate model in the body type selection box. Always stop the fan before changing the model, and save between each set of results. Fit the small smooth sphere into the working section. Repeat the results over the previous range of fan speeds. Pay particular attention to the shape of the wake. Repeat for the small dimpled sphere (golf ball), again paying particular attention to the wake shape. Shut down the fan and it to Standby by selecting the ‘Fan On button in the software. Switch off the mains switch on the IFD7.


Plot Drag against Tunnel Velocity for each set of data. Plot Drag against Reynolds Number for each set of data. Note on each graph the model used for that set of results. If using the wake survey rake, plot the pressures 1 to 10 against Reynolds Number for each set of data. Use point values not lines on the graph. Print the graphs, and manually join the lines for the pressures at each fan speed to get a series of curves illustrating the pressure distribution across the wake at a range of Reynolds numbers.

Conclusion Describe in general terms the different boundary layer shapes observed using flow visualization. If the wake survey rake was available then describe the corresponding pressure variation across the wake. Describe any changes in behaviour as the flow 87

Armfield Instruction Manual rate increased. Note the Reynolds numbers at which any changes occurred. Was this the same regardless of whether the flow rate was increasing or decreasing? If not then discuss possible reasons for the difference. Compare and contrast the behaviour of flow around the bodies of identical diameter. Explain these differences in terms of flow type and boundary layer. Suggest reasons why the flow behaved as it did around each body, for example the presence or absence of sharp edges. Compare and contrast the behaviour of flow around the two small spheres. What was the effect of a dimpled surface compared to a smooth one? Was there a flow velocity at which the boundary layer behaviour changed on one or both spheres, and if so what was the Reynolds number at that point? If differences were observed, suggest reasons for them. Why do golf balls have dimpled surfaces? What is the likely effect on the results of the supporting rod on which the models are mounted. A spare rod is available. Describe a method by which the effect of the mounting rod could be determined and the effect on the results compensated for. Additional investigations of the effects of body shape and surface finish are possible. Refer to the Project Work exercise for some ideas.

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Exercise F - Flow and pressure distribution around a symmetrical aerofoil at different angles of attack Objective To investigate the pressure distribution around a symmetrical aerofoil at different angles. To investigate the pressure distribution across the wake behind the wing.

Method By using a symmetrical aerofoil with tappings across the chord on one surface. By changing the angle of attack of the aerofoil at a range of air velocities. By using a pressure sensor to investigate the pressures across the wake downstream of the wing.

Equipment Required C15-10 Wind Tunnel with IFD7 PC (not supplied) running C15-304 software C15-21 Pressure Wing C15-11 Inclined Manometer Bank or C15-12 Electronic Manometer Bank

Optional equipment C15-15 Wake Survey Rake (or C15-14 Pitot Static Tube if C15-15 is unavailable) Camera (and tripod) for recording flow visualisation results

Theory Velocity in the working section is calculated as V = (2 ρ man g Δh / ρ air )0.5 Pressure distribution around an aerofoil The pressure acting on the surface of an aerofoil in a steady air stream (as in steady flight) is not uniform across the chord. Taking ‘positive’ to refer to pressure greater than the static pressure of the surrounding air, there is commonly a region of positive pressure at the nose of the aerofoil, and another at the tail. The pressure around the rest of the aerofoil is typically negative, with the minimum pressure on the upper surface occurring somewhere between the point of maximum chord and the nose. The pressure distribution also varies depending on the angle of attack of the aerofoil. The point of minimum pressure tends to shift towards the nose, and the region of positive pressure at the tail increases in area and magnitude. This is illustrated in the following diagrams. Arrows pointing towards the aerofoil surface indicate pressures greater than the overall static pressure. Arrows pointing away from the aerofoil indicate pressures lower than static. The magnitude of the pressure differential is indicated by the bold line.

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0° Angle of Attack

10° Angle of Attack

20° Angle of Attack

Stall Considering the flow of air around the aerofoil as a series of layers, at low angles of attack the layers wrap smoothly around the aerofoil. As the angle of attack increases, the decrease in pressure on the upper surface will become greater, until a point is 90

Exercise F reached at which the layers of air separate from the surface. This results in turbulent air over the upper wing surface after the point of separation, and a corresponding sudden increase in pressure and a consequent loss of lift and increase in drag. The point at which separation begins to occur is known as the stall point or point of stall, and the angle at which this occurs is called the critical angle of attack. At angles of attack greater than the point at which separation begins to occur, the wing is said to be stalled or in a stall condition. For any given wind speed, the point of stall is determined by the angle of attack. However, the angle of attack at which stall occurs changes with wind speed. It is common in aviation to consider stall to be dependent on flight speed rather than angle of attack: aircraft will be described as having a particular stall speed. For a given air speed, maintaining a constant altitude will require a constant angle of attack, and maintaining steady flight at lower air speeds will require a greater angle of attack. Attempting to maintain steady flight by increasing the angle of attack while reducing the air speed will eventually lead to the critical angle being exceeded- the ‘stall speed’ being reached. Note that wing stall is completely different to engine stall.

Equipment Set Up Note: Additional information is available in the Operation section if required. The tunnel should be set up with the flow visualisation tube fitted to the upstream roof tapping. Place the arm supporting the thread a little above half height in the working section (closer to the roof than to the floor). The other two roof tappings should be fitted with blanking plugs. The pressure wing should be fitted through the large circular hatch, at an angle of 0° to the horizontal (i.e. the first tapping should face directly upstream and the second tapping should face directly downstream). Manually position the flow visualisation thread over the top of the wing. If available, the wake survey rake may be fitted in the small hatch. If the wake survey rake is not used then the small hatch should be fitted with the plain hatch cover. If using the Pitot tube, fit this through the downstream roof tapping. Ensure that the floor is fitted. Check the surroundings to see that there is no obstruction at the inlet or outlet of the tunnel and that there are no loose objects nearby which could cause a hazard. The single tube from the pressure tapping on working section side wall (near the inlet) should be connected to the 1-way quick-release fitting on the black box fitted to the tunnel frame. The 10-way connection from the pressure wing should be fitted to the manometer. If the wake survey rake is used then this is initially left disconnected. If using the C15-11 inclined manometer bank, check that the manometer has been filled and primed, with a convenient water level and no trapped air bubbles. If using the C15-12 electronic manometer, check that the manometer is connected to a suitable PC with the USB cable. Check that the IFD7 is connected to a suitable mains electrical supply and to the USB socket of a suitable PC. The PC should be switched on and the appropriate software version run (C15-11 version or C15-12 version depending on the 91

Armfield Instruction Manual manometer used). Select ‘Exercise F’ and ensure that ‘IFD: OK’ is displayed in the bottom right-hand corner. Switch on the IFD7 using the mains switch on the front.

Camera setup If a still or video camera and tripod are available then the camera should be mounted in front of the round hatch to give a good field of view around the wing. A plain background behind the wind tunnel is advisable (e.g. a plain sheet of white paper may be attached to the back of the tunnel, on the outside). Select camera settings that give the fastest possible shutter speed and then the best possible depth of field at that speed. The use of flash may cause inconvenient reflections on the working section sides, so where available use sufficient lighting to avoid the need for flash For best pictures the flow visualisation thread should be in sharp focus, which can be difficult to achieve especially if using autofocus. If the camera can be pre-focussed then it is possible to temporarily insert a focus guide such as the Pitot tube arm or a similar narrow object, through the roof tapping. Once the camera is focussed on the guide, the guide should be removed and the blanking plug replaced. If possible, take a test shot and display the results at a reasonable size to check that images will be acceptable. If a tripod is not available then the camera can be hand held, but good results may be more difficult to obtain. An assistant to the camera operator is suggested who can make adjustments to the equipment as required, e.g. to position then remove any focussing guide.

Procedure Check that the fan is set to 0%, then switch it out of standby mode by selecting the ‘Fan On’ button on the mimic diagram. Check that the manometer readings are all the same at zero velocity. Measure the ambient temperature in Celsius and pressure of the laboratory in Pascals and enter the results in the appropriate boxes on the mimic diagram. Select ‘No’ in the ‘Rake used?’ box on the mimic diagram. Gradually set the fan to 10% in 1% increments by using the up arrows. This allows the fan to start up gradually. Check that all fittings on the tunnel remain secure and that there is no safety hazard due to the inlet and outlet air streams. Gradually set the fan to 40% by typing in speed increments of 10% until 40% is reached. Be aware of the surroundings when operating the wind tunnel, keeping safety in mind at all times. Allow time for the fan to stabilise at 40%. Check the wing to see that it is in proper alignment: Adjust the angle of attack of the wing while observing the head reading for the first tapping (head reading 1, at the nose). The greatest head should be obtained when the zero reading on the scale is aligned with the central marker on the tunnel wall. If this is not in exact alignment then you will need to allow for the slight offset when setting the angle of attack. Set the wing to 0° angle of attack and enter ‘0 degrees Angle of Attack’ in the ‘Attach note’ box on the mimic diagram.

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Exercise F Adjust the height of the flow visualisation tube and the length of the thread to give a good curve above the wing. Sketch or photograph the curve. Shorten the thread until the end trails immediately in the wing wake, investigating possible turbulence (it is difficult to visually record this unless a motion camera is available). If using the C15-11 inclined manometer, take a reading for the water level in all the columns and enter the results on the mimic diagram. It is also possible to move the cursors along the tubes to match the readings, giving a clearer visual representation of the pressure variation around the cylinder. Log the sensor readings by selecting the

icon.

icon. If the wake survey rake is available, create a new results sheet using the Disconnect the pressure cylinder and connect the survey rake. Select ‘Yes’ in the ‘Rake used?’ box. If using the C15-11, enter the new manometer readings on the mimic diagram. Log the sensor readings by selecting the icon. Disconnect the wake survey rake and reconnect the pressure wing. Create a new results sheet using the

icon. Select ‘No’ again.

Increase the fan setting to 60%. Repeat the flow visualisation and pressure sensor logging as before. If using the wake survey rake, create a new sheet, set the ‘Rake used’ to ‘Yes’ and connect the survey rake to take a set of readings. Repeat at 80%. Remember to create a new results sheet and rename it each time, and to select the correct model for each set of readings. Repeat again at 100%. Set the fan back to 20%. Adjust the wing to set it at an angle of +2° from the zero point (i.e. rotated with the nose raised and the tail lowered). Enter ‘2 degrees Angle of Attack’ in the ‘Attach note’ box. Repeat the procedure as before, taking readings at 40%, 60%, 80% and 100% and using a new results sheet for each set of results. Remember to set the Rake used? to ‘Yes’ whenever taking readings using the wake survey rake, and to set it back to ‘No’ afterwards. Repeat at wing angles of +4°, +6°, and then at 7°, 8°, 9° and so on until 16°. Take further readings at 18°, 20° etc up to 30°. Note that at high angles of attack the wing will form a significant obstruction in the working section. This slightly increases the air speed for a given fan setting. The effect of this on the results obtained is small, but for accuracy it is possible to adjust the fan speed slightly to match the air velocities obtained for lower angles of attack. To obtain corresponding pressures for the underside of the wing, the wing is used at negative angles of attack. Repeat the procedure for angles of -2°, -4°, -6°, -7° etc. up to -30°, pairing with the positive angles from earlier. You need not take wake pressure readings for this part of the exercise, as they may be assumed to be an inversion of the readings for positive angles of attack. Gradually shut down the fan: Type in a value of 50% for the fan setting. When the fan has slowed, type in a value of 20%. Once the fan has slowed again, reduce the fan speed to 0% by using the arrow keys. Set the fan to Standby by selecting the ‘Fan On button in the software. 93



Plot graphs of pressure against tapping position for each fan speed and angle of attack. Pair together the graphs for positive and negative angles of attack at each fan speed, as these correspond to the equivalent upper and lower wing surfaces. If using the wake survey rake, plot the wake pressure against position (taken relative to the centreline of the wing mounting) for each fan speed at each angle of attack, and attach each graph to the corresponding wing pressure graph(s) for that fan speed and angle of attack. Match the drawings or photographs of the flow over the wing to the corresponding graphs.

Conclusion For the graphs at zero angle of attack, compare the results for increasing fan speed. What happens to the pressure distribution over and under the wing? What happens to the wake? What happens to the path of the thread? For a single fan speed, compare the results for graphs and the thread path at increasing angle of attack. How do the graphs change? Are there any sudden changes in the wing surface pressure or the wake which could correspond to the stall condition? If so, describe what happens and at which point the change occurs. How does the behaviour of the thread change during any pressure changes? How do the experimentally obtained results compare to the examples given? Mention any potential inaccuracies that may have been introduced as a result of using a wing with tappings on one surface only. Consider additional investigations that could be made using the wing. For example, discuss possible methods for measuring the air velocity at the aerofoil surface, which would then allow the dynamic pressure to be calculated.

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Exercise G - Lift and Drag forces on a symmetrical aerofoil at different angles of attack Objective To convert a head measurement using a manometer to an equivalent pressure reading. To convert head and pressure readings to alternative engineering units. To demonstrate the use of a static pressure reading to determine tunnel air velocity.

Method By measuring the differential static head within the wind tunnel at a range of air velocities, and then converting this to a pressure figure using the appropriate equation. By converting head and pressure values into alternative units using the appropriate conversion factors. By calculating the air velocity in the tunnel using the appropriate equation and compare the result to that generated by the computer.

Equipment Required C15-10 Wind Tunnel with IFD7 PC (not supplied) running C15-304 software C15-20 Lift and Drag Aerofoil C15-11 Inclined Manometer Bank or C15-12 Electronic Manometer Bank

Optional equipment Flow visualisation apparatus C15-14 Pitot Static Tube Camera and tripod

Theory Velocity in the working section is calculated from V = (2 ρ man g Δh / ρ air )0.5

Lift Lift is the component of force on an aerofoil that acts ‘upwards’. In a threedimensional situation, lift must be defined carefully. It is usually defined as acting perpendicularly to the span and chord of the aerofoil if the chord is taken as a straight line from the nose to the trailing edge, with a positive value when the force acts in the direction of the upper surface (or the surface that is most usually upwards with respect to the ground if the aerofoil can rotate through 180° or more). Lift may have components in any direction relative to a fixed ground, depending on the orientation 95

Armfield Instruction Manual of the aerofoil and the direction of the airflow. Negative values of lift may be possible depending on the angle of attack. When using the C15-20 the span of the wing is fixed parallel to the floor of the working section, and the lift is assumed to act perpendicularly to the span with no lateral component towards the tunnel side walls. The lift therefore acts directly upwards when the aerofoil is at a zero angle of attack, and acts at an angle to the vertical equal to the angle of attack of the aerofoil.

Drag Drag is the component of the force on an aerofoil that acts along the direction of the airflow, and in the same direction (For an aircraft in level flight, drag acts in the opposite direction to the direction of flight). Drag resists the movement of the aerofoil through the airstream. Drag is always a positive value or zero (in non-theoretical situations, drag will only be zero if the air velocity is also zero). Drag is a combination of the effects of friction on the surface of the aerofoil (form drag) and the component of lift acting in the drag direction (induced drag). For any given aerofoil and Reynolds number, a drag coefficient may be found which may then be used to predict the drag for that aerofoil at any other Reynolds number. C D = C D0 + C DL where C D0 is the form drag coefficient, and C DL is the induced drag coefficient C DL may be considered as a function of the coefficient of lift, C L (see Lift, later in this section): C DL = kC L ² The drag may be defined as D = ½V²SC D = ½V²SC D0 + ½V²S(kC L ²) where D = total drag,  = density of air V = velocity of air flow over aerofoil, and S = a characteristic dimension, usually the wing area = plan area of one surface of the wing = chord x span for the C15-10 When the contribution of lift to drag is zero (k = 0), the value of C D0 may be calculated directly from the measured value of D using the lift and drag balance. The combined effect of form drag and induced drag give a characteristic shape to a graph of total drag against velocity:

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Exercise G

Variation of total drag with air velocity

This curve shows that as velocity increases, drag initially falls, then reaches a minimum, then rises again. This minimum drag velocity is an important characteristic in aerodynamics as it indicates the most efficient velocity for the body (e.g. the aerofoil, wing or aircraft). This is independent of any factors due to propulsion.

Lift and drag characteristics of an aerofoil The lift and drag produced or experienced by an aerofoil varies with the air velocity and with the angle of attack. For a given angle of attack, an increase in air speed will tend to increase the magnitude of both lift and drag until the air speed is sufficiently high that compression effects become noticeable (i.e. close to supersonic speedsthe C15-10 is designed so that air speed can never reach this point). For a given air speed, the relationship between lift, and angle of attack is more complex, as illustrated below:

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Lift against  for symmetrical aerofoil

The lift generated increases until a critical angle or stall angle is reached. The lift then falls sharply until it begins to stabilise again at an even greater angle of attack; at this point the drag will be very significant. Note: It is more common in aviation to find references to ‘stall speed’ than to ‘stall angle’. The angle at which stall occurs varies depending on the velocity of the aircraft relative to the air. However, for a given aircraft at approximately constant weight and angle of attack- as occurs in level flight- stall will always occur at the same indicated air speed (as measured by pressure sensors on the aircraft, which will differ from the true air speed at altitudes above sea level as the density of the air affects the reading). This single value provides an easy figure to remember. Aircraft may have multiple stall speeds, however, depending on factors such as the undercarriage being retracted or extended, slat and flap positions, and so on.

Lift For a given aerofoil and Reynolds number, a lift coefficient may be found which may then be used to predict the lift for that aerofoil at any other Reynolds number. C L = L / ½V²S where L

= lift,

 = density of fluid V = velocity of air flow over aerofoil, and S = a characteristic dimension The characteristic dimension is taken as the chord, where the model under test is an aerofoil occupying the full width of the working section. 98

Exercise G The lift and drag balance measures the vertical lift component (the lift acting perpendicular to the tunnel floor). This may not be the total lift, as illustrated below:

Using geometry the total lift, L, may be calculated as L = lcos where l is the vertical lift component measured by the lift and drag balance, and  is the angle of attack The lift coefficient, C L , is then calculated as C L = lcos / ½V²S It may be seen that when  = 0, l = L, and thus the total lift on a symmetrical aerofoil at zero angle of attack will be the lift measured using the lift and drag balance. (N.B. this differs for an asymmetric aerofoil, which may generate a positive value of lift at zero angle of attack). The component of drag due to lift may be calculated as ltan. Now

ltan = ½V²S(kC L ²) 99

Armfield Instruction Manual It is therefore possible to find kC L ² for any given velocity and angle of attack, and thus to find the value of k.

Equipment Set Up Note: Additional information is available in the Operation section if required. Note: If Exercise F using the Pressure Wing has been performed, then the flow visualisation relevant to this exercise has already been performed. In the even that Exercise F is not available, the same flow visualisation described in that exercise may be performed here instead. Refer to the Set Up and Procedure in Exercise F that are relevant to the use of the flow visualisation equipment and to the use of a camera for recording purposes. The tunnel should be set up with the blanking plugs fitted to all three roof tappings (except as in the note above). The lift and drag aerofoil should be fitted to the lift and drag balance, and the balance should then be fitted into the large circular hatch. The small hatch should be fitted with the plain hatch cover. The cable from the lift and drag balance should be connected to the socket on the front of the IFD7. Ensure that the floor is fitted. Check the surroundings to see that there is no obstruction at the inlet or outlet of the tunnel and that there are no loose objects nearby which could cause a hazard. The single tube from the pressure tapping on working section side wall (near the inlet) should be connected to the 1-way quick-release fitting on the black box fitted to the tunnel frame. Connect the cable from the lift and drag balance to the front of the IFD7. Check that the IFD7 is connected to a suitable mains electrical supply and to the USB socket of a suitable PC. The PC should be switched on and the appropriate software version run (C15-11 version or C15-12 version depending on the manometer used). Select ‘Exercise G’ and ensure that ‘IFD: OK’ is displayed in the bottom right-hand corner. Switch on the IFD7 using the mains switch on the front.

Procedure Check that the fan is set to 0%, then switch it out of standby mode by selecting the ‘Fan On’ button on the mimic diagram. Check that the manometer readings are all the same at zero velocity. Measure the ambient temperature in Celsius and pressure of the laboratory in Pascals and enter the results in the appropriate boxes on the mimic diagram. In the software, select the ‘Zero’ button beside the ‘Lift’ data display box. This sets the datum point for zero lift (no air velocity). Gradually set the fan to 20% in 1% increments by using the up arrows. This allows the fan to start up gradually. Check that all fittings on the tunnel remain secure and that there is no safety hazard due to the inlet and outlet air streams. Be aware of the surroundings when operating the wind tunnel, keeping safety in mind at all times. Allow time for the fan to stabilise at 20%. 100

Exercise G Check the wing to see that it is in proper alignment: Adjust the angle of attack of the wing while observing the head reading for the first tapping (head reading 1, at the nose). The greatest head should be obtained when the zero reading on the scale is aligned with the central marker on the tunnel wall. If this is not in exact alignment then you will need to allow for the slight offset when setting the angle of attack. Conduct an initial investigation of the variation of lift and drag with velocity at zero angle of attack: Set the wing to 0° angle of attack and check that ‘0°’ is displayed in the ‘Angle of Attack’ box on the mimic diagram. Log the sensor readings by selecting the Increase the fan setting to 30%. Select the

icon. icon again.

Repeat at 40%, 50%, etc up to 100%, logging the data each time with the

icon.

Investigate the effect of changing angle of attack: Set the fan back to 20%. Create a new results table using the Log the sensor readings by selecting the

icon.

icon.

Adjust the wing to set it at an angle of +2° from the zero point (i.e. rotated with the nose raised and the tail lowered). Check for ‘2°’ in the ‘Angle of Attack’ box. Select the icon again. Repeat at wing angles of +4°, +6°, and then at +7°, +8°, +9° and so on until +16°, using the icon to save each set of data. Take further readings at +18°, +20° etc up to +30°. Note that at high angles of attack the wing will form a significant obstruction in the working section. This slightly increases the air speed for a given fan setting. The effect of this on the results obtained is small, but for accuracy it is possible to adjust the fan speed slightly to match the air velocities obtained for lower angles of attack. Repeat the investigation of angle of attack as before for fan speeds of 50% and 100%, using a new results sheet for each set of results. If time permits, the exercise may be repeated for negative angles of attack (-2°, -4° etc). Gradually shut down the fan: Type in a value of 50% for the fan setting. When the fan has slowed, type in a value of 20%. Once the fan has slowed again, reduce the fan speed to 0% by using the arrow keys. Set the fan to Standby by selecting the ‘Fan On button in the software. Save the software results by selecting ‘Save As…’ from the File menu. Give the results a suitable name for future reference, such as the equipment code, experiment letter and date. Switch off the mains switch on the IFD7.

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For the first set of results (at zero angle of attack with varying velocity), plot a graph of lift against velocity. On a second y-axis, plot the coefficient of lift C L . On another graph plot C D , C D0 and C DL against velocity. Plot the drag on the second Y axis. For the other sets of results (with varying angle of attack at constant velocity), on the same graph plot lift coefficient against angle of attack for every velocity. Plot a graph of k against angle of attack for two sample velocities.

Conclusion Describe the variation of coefficient of lift with velocity for the aerofoil at zero angle of attack. Describe the graph of C D . C D0 and C DL . Does it match the example given? What variation can be noted, if any? Mark the minimum drag and determine the velocity at which this occurs. Describe the general shape of the graphs of lift coefficient against angle of attack. Describe how the graphs vary with velocity. For each velocity, determine the maximum lift and the critical angle of attack.

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Exercise H - Laminar and Turbulent Boundary Layer Development Objective To measure the depth of the boundary layer on smooth and rough flat plates

Method Using a flattened Pitot tube mounted on a micrometer to measure the change in velocity associated with the boundary layer in contact with a flat plat

Equipment Required C15-10 Wind Tunnel with IFD7 PC (not supplied) running C15-304 software C15-25 Boundary Layer Plates with Pitot Tube C15-11 Inclined Manometer Bank or C15-12 Electronic Manometer Bank

Optional Equipment Wide temporary adhesive tape, e.g. masking tape Fine, stiff wire Sandpaper, glasspaper, etc. Bluetack or similar

Theory The development of friction drag implies that there is a force exerted by the fluid on the body. In exerting this force, the fluid itself is slowed. The slower fluid particles at the body surface then exert a drag force on the faster fluid particles further away from the body. Although in theory the boundary layer is infinite, in practice the significance of the slowing of fluid particles (the effect of viscosity) reduces with distance from the body. The region close to the body where viscous effects are significant is termed the boundary layer. It is usually assumed to be the region in which the flow velocity is less than 99% of the free stream velocity. Flow within the boundary layer may be laminar or turbulent, and may change from laminar to turbulent as it passes around the body. A change from one type to the other is known as boundary layer transition.

Laminar Boundary Layer In a laminar boundary layer the flow is smooth, and its behaviour may be thought of as a series of layers sliding over one another. Skin friction tends to be low and the thickness of the boundary layer tends to be small.

103


Turbulent Boundary Layer In a turbulent boundary layer the fluid moves in small eddies of varying size and strength. Skin friction is higher than in a laminar boundary layer, and the boundary layer tends to be thicker.

Rough plate A rough plate surface tends to encourage early development of a turbulent boundary layer.

Velocity Profiles By using a Pitot tube to find the velocity head of the air flow in a series of points approaching the surface of the plate, it is possible to determine the flow rates and thus to generate a velocity profile of the air passing the plate. This velocity profile varies according to whether the flow is laminar or turbulent. The velocity gradient at the surface is higher for turbulent flow, as can be seen if the gradients for a similar plate under the two conditions are plotted on the same graph.

Equipment Set Up Note: Additional information on removing and fitting the accessories is available in the Operation section. Before fitting the plate, ensure that any other models, sensors, or other accessories have been removed from the tunnel first. The smooth plate should be fitted to the plate floor section using a wiping or sliding motion as shown. The plate should be locked in position by tightening the thumb nuts supplied onto the locating studs.

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Exercise H

Remove the blank floor and fit the plate, taking care to lift it cleanly into place without damaging either the accessory or the working section. Secure the plate in place. Fit a blanking plug with two studs into the roof tapping above the plate. Slot the studs over the top of the plate, then twist the plug until the studs are touching the plate sides. Tighten the plug in this position using the screw on the tapping, so that it supports the top of the plate. Fit blanking plugs to the remaining roof tappings. The small hatch cover should be secured in position over the small hatch. The special flattened head Pitot static tube should be secured in position in the large hatch, with the opening of the tube facing upstream. The tunnel static pressure sensor should be connected to the 1-way quick-release fitting on the black box fitted to the tunnel frame. If using the C15-11 inclined manometer bank, check that the manometer has been filled and primed, with a convenient water level and no trapped air bubbles. If using the C15-12 electronic manometer, check that the manometer is connected to a suitable PC with the USB cable. Connect the Pitot tube should be connected to the manometer. Fit the static head connection to socket 11 on the manometer, and the total head connection to socket 12. Check the surroundings to see that there is no obstruction at the inlet or outlet of the tunnel and that there are no loose objects nearby which could cause a hazard. Check that the IFD7 is connected to a suitable mains electrical supply and to the USB socket of a suitable PC. The PC should be switched on and the appropriate software version run (C15-11 version or C15-12 version depending on the manometer used). Select ‘Exercise H’ and ensure that ‘IFD: OK’ is displayed in the bottom right-hand corner. Switch on the IFD7 using the mains switch on the front.

Procedure Check that the fan is set to 0%, then switch it out of standby mode by selecting the ‘Fan On’ button on the mimic diagram. Check that the manometer readings are all the same at zero velocity. Measure the ambient temperature in Celsius and pressure of the laboratory in Pascals and enter the results in the appropriate boxes on the mimic diagram. 105

Armfield Instruction Manual Set the ‘Plate Type’ to ‘Smooth Plate’ on the mimic diagram. Loosen the thumb nuts securing the locating pins for the flat plate. Loosen the screw securing the blanking plug above the plate. Gently slide the plate along its slot towards the outlet of the working section using the locating pins, until the plate is at the limit of its travel. Tighten the locating pins and the locking screw for the blanking plug. On the underside of the working section, block the slot with tape (the tape is optional, but will give better results as it prevents leakage of air through the floor of the tunnel which would cause pressure reduction). Use the ‘Notes’ facility to enter the relative position of the Pitot tube to the plate (‘Leading Edge’). Gradually set the fan to 20% by using the up arrows. Check that all fittings on the tunnel remain secure and that there is no safety hazard due to the inlet and outlet air streams. Allow time for the fan to stabilise. Adjust the Pitot tube so that it is just touching the plate: wind the screw until the ratchet mechanism clicks, then stop. Enter a ‘Vertical Distance’ of 0mm on the mimic diagram. If using the C15-11 inclined manometer, take a reading for the water level in columns 11 and 12 and enter the results on the mimic diagram. Log the sensor readings by selecting the

icon.

Move the Pitot tube 1mm away from the plate. Enter a Distance of 1mm. If using the C15-11 inclined manometer, take a reading for the water level in columns 11 and 12 and enter the results on the mimic diagram. Log the sensor readings by selecting the

icon.

Move the Pitot tube another 1mm. Enter a Distance of 2mm, and record the corresponding Pitot readings. Repeat, moving the Pitot tube in 1mm increments, recording the distance and sensor readings each time, until two identical readings are obtained indicating that the boundary layer has been crossed. Save the results sheet by selecting ‘Save As…’ from the ‘File’ menu, so that the data is not lost in the event of a computer failure. Give the file a suitable name so that it can be found easily at a later date, for example the equipment code, experiment letter and date. Create a new results sheet using the

icon.

Remove the tape. Loosen the locating pin thumb nuts and blanking plug screw, and slide the plate towards the tunnel inlet until the Pitot tube is approximately central to the plate. Secure the plate in position. Apply tape again. In ‘Notes’, enter ‘Middle’. Position the Pitot tube so it is touching the plate. Take a second set of readings, moving the Pitot tube in 1mm increments as before. Save the results using ‘Save’this will add the new data to the existing file.

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Exercise H Create a new results sheet and take a third set of results after moving the plate to the far upstream end of its travel. Enter ‘Trailing Edge’ in ‘Notes’. Block the slot with tape before taking results and use ‘Save’ to ensure the data is kept. Set the fan to 100%. Allow time for the fan to stabilise. Reposition the plate at the downstream end of the slot. Repeat the procedure used for a fan setting of 20% to take three sets of results, for the plate in the upstream, central, and downstream positions. Stop the fan gradually: set the fan to 50% and allow it to reach that setting. Reduce the setting in 10% increments, allowing it to reach the setting each time. Use the arrow keys to reduce the fan gradually to 0%. Ensure the fan has stopped before proceeding. Remove the floor and replace the smooth plate with the rough plate. Fit this in place in the working section. In the software, create a new results sheet using the Plate’ from the Plate Type selection box.

icon, and select ‘Rough

Repeat the procedures as before, first at a fan setting of 20%, then at a setting of 100%. Save the results. If time permits, it is possible to investigate methods to ‘trip’ the boundary layer, replacing a normally laminar boundary layer with a turbulent layer: Using a wire: Remove the tunnel floor and fit the smooth plate. Cut a section of wire long enough so that when bent it will extend down both sides of the plate. Bend this using pliers, allowing sufficient thickness in the bend to fit over the plate, and flattening the top of the bend so that it fits neatly against the edge of the plate. Do not use the plate itself to bend the wire, as this will damage the plate. Use a small piece of tape over the very top part of the plate near the leading edge to help protect the plate. Secure the wire in place vertically near the leading (upstream) edge of the plate, with the bent section slotting over the protective tape. Use small pieces of strong tape at the top and back of the plate to secure the wire, with another small section of tape at the base of the wire on the front for extra security. The wire must be fixed so that it does not become detached during use. Now repeat the same procedure as before. Using sandpaper or glasspaper: Remove the tunnel floor and fit the smooth plate. Cut a narrow strip of sandpaper of a length equal to the height of the plate. Roll a narrow string of bluetack of the same length, and use this to firmly fix the sandpaper strip to the front face of the plate, close to and parallel with the leading edge. For additional security, use small pieces of strong tape at the top and bottom of the plate to secure the sandpaper. The paper must be fixed so that it does not become detached during use. Now repeat the same procedure as before. Different grades of sandpaper and glasspaper may be used to investigate the effect that the degree of roughness has on the boundary layer; a strip of smooth paper or thin card may also be used for comparison. Shut down the fan and it to Standby by selecting the ‘Fan On button in the software.

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Armfield Instruction Manual Switch off the mains switch on the IFD7.


For each position of each plate, plot a graph of Total Pressure against distance from the plate. On each plate, locate the edge of the boundary layer from the change in pressure gradient and note the distance of the edge from the plate. Use the distance values obtained to sketch the shape of the boundary layer for each plate at low flow (20% fan setting) and high flow (100% fan setting), assuming that the boundary layer has a thickness of zero at the leading edge. For each plate position, plot a graph of distance from plate against air velocity.

Conclusion From the shape of the pressure gradients obtained in the graphs, determine whether the boundary layer was likely to have been laminar or turbulent at that point. For each plate, use the three graphs at each velocity to analyse whether the type of flow in the boundary layer remained the same along the length of the plate, and if it changed then whether this occurred towards the leading or trailing edge (i.e. before or after the central set of readings). Describe the shapes obtained for the boundary layer at different Reynolds numbers and with different plate finishes. Indicate any differences and similarities. If a trip wire was used, discuss the effect this had on the results, as compared to the results for the smooth plate without a wire. Were there any similarities between the results obtained for the wire and/or sandpaper and the rough plate?

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Exercise I - Project Work Objective To evaluate models or instruments of the student’s own design and/or manufacture.

Method By using the C15-10 wind tunnel, accessories and software to provide the required test facilities and data logging.

Equipment Required C15-10 Wind Tunnel with IFD7 PC (not supplied) running C15-304 software Other accessories according to requirements

Theory, Set Up, Procedure and Results The experiments presented earlier in this manual may be followed to perform investigations on the student’s own models. Further investigations may be devised using these experiments as a guide to correct working procedure when using the wind tunnel. The Project Work software exercise has the facility to log all sensor outputs, including all manometer readings (the correct version of the software should be selected according to the manometer used, either the C15-11 or C15-12). If the previous exercises do not properly cover the calculations required then the Project Work exercise may be used instead, and the results saved so that calculations can be performed with an alternative spread sheet package (not supplied). Mounting user created models via the circular hatch Alternative shapes can be mounted on the circular hatch and rotated to investigate the effect of wind direction on the shape. Different shapes can be investigated such as cylinders of different diameter, different surface finish, aerodynamic shapes such as aerofoil or teardrop, and alternative bluff shapes such as triangular or square, asymmetric aerofoils. The use of devices such as wires to alter boundary layer characteristics is another possible exercise. Experiments will be limited to flow visualisation only unless the user has the machining abilities to create surface tappings on the model that can be connected to one of the optional manometers C15-11 or C15-12. It is recommended that the model be securely and permanently attached to the hatch using a fixing screw through a drilled hole in the hatch, as damage to the model or the fan may occur should the model become detached in operation. A spare hatch and quick release connectors are supplied with the Project kit C15-26. Mounting user created models via the removable floor Basic streamlined bodies such as model cars or bluff bodies such as bridges, model buildings or groups of model buildings can be mounted on the removable floor. Experiments will be limited to flow visualisation only unless the user has the machining abilities to create surface tappings on the model 109

Armfield Instruction Manual that can be connected to one of the optional manometers C15-11 or C15-12. It is recommended that the model be permanently attached to the floor using a fixing screw through a drilled hole in the floor to prevent damage to the model or the fan should the model become detached in operation. A spare floor panel quick release connectors are supplied with the Project kit C15-26. Mounting user created models on the optional C15-13 Lift & Drag balance Alternative lift / drag shapes can be constructed and mounted on the spare rod supplied with C15-13. The weight of the model must not exceed 350g acting at the centreline of the model, to operate within the range of the balance. The use of the lift and drag balance allows more detailed investigation than a simple hatch mounted model. Examples include the effect of slats and flaps on an aerofoil, or a detailed analysis of the use of boundary layer control techniques. Mounting user created instruments via the roof tappings Alternative instruments can be constructed and inserted via one of the tappings in the roof of the test section. A simple support plug will allow the instrument to be secured using the plastic thumbscrew. Larger instruments could be installed via the circular hatch or removable floor as described above. Suggested Project Work In addition to the suggestions already made, other possibilities include but are not limited to: Alternative designs of Pitot tube such as forward and rearward facing tappings Yaw probes to determine flow direction by comparing two or more total head readings Alternative flow visualisation techniques Investigation of wind turbine designs, e.g. propeller-style versus vertical blade designs Investigation of the effect of dirt and/or ice on aerofoil performance Determining the lift and drag characteristics of an aircraft model of the student’s own design. Flow over / around bluff bodies. Flow over / around alternative shapes e.g. model buildings, model cars etc. Flow through multiple structures e.g. groups of buildings, bundles of tubes etc.

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Contact Details for Further Information Main Office:

Armfield Limited Bridge House West Street Ringwood Hampshire England BH24 1DY Tel: +44 (0)1425 478781 Fax: +44 (0)1425 470916 Email: [email protected] [email protected] Web: http://www.armfield.co.uk

US Office:

Armfield Inc. 436 West Commodore Blvd (#2) Jackson, NJ 08527 Tel: (732) 928 3332 Fax: (732) 928 3542 Email: [email protected]

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