2 MARKS UNIT I PRINCIPLES OF MODEL TESTING 1. Define Rayleigh method.
A functional power relation is assumed between the p arameters and then the values of indices are solved for to obtain the grouping. For example in t he problem in example 1 one can write f(π1, π 2) = Fa ρb Dc µd Ue The values of a, b, c, d , and e are obtained obtained by comparing the dimensions on both sides sides the dimensions on the L.H.S. being zero as π terms terms are dimensionless. This is also tedious and considerable expertise is needed to form these groups as the number of unknowns will be more than the number of available equations. This method is also called ‘‘indicial” method. 2. Define Buckingham Pi theorem method
The application of this theorem provides pro vides a fairly easy method to identify id entify dimensionless parameters (numbers). However identification of the influencing parameters is t he job of an expert rather than t han that of a novice. 3. Define THE PRINCIPLE OF DIMENSIONAL HOMOGENEITY
The principle is basic for the correctness of any equation. It states ‘‘ If an equation truly expresses a proper relationship between variables in a physical phenomenon, then each of the additive terms will have the same dimensions or these should be dimensionally homogeneous.’’ homogeneous.’’ 4. Define model.
In the engineering point of view model mod el can be defined as the representation o f physical system that may be used to predict the behavior of the system in the desired aspect. 5. Define prototype.
The system whose behavior is to be predicted by b y the model is called the protot ype. 6. Define Geometric Similarity
Some of the PI terms involve the ratio of length parameters. All the similar linear dimension of the model and prototype should have the same ratio. This is called geometric similarity. The ratio is generally d enoted by the scale or scale factor. 7. Define Dynamic Similarity. Similitude requires that PI terms terms like Reynolds number, Froude number, Weber number etc. be equal for the model and prototype. These numbers are ratios of inertia, viscous gravity and surface tension forces. This condition implies that the ratio of forces on fluid elements at corresponding points (homologous) (homologous) in the model and prototype should be the same. This requirement is called dyn d ynamic amic similarity. similarit y.
8. Define Kinematic Similarity. When both geometric and dynamic similarities exist, then velocity ratios and acceleration ratios will be the same throughout the flow field. This will mean that the streamline patterns will be the same in both cases of model and prototype. This is called k inematic similarly. 9. What are the Non- Dimensional Numbers? Reynold’s number, Froude”s number, Weber’s Number, Mach number, Euler’s Number.
10. Define Model Law. For the dynamic similarities between the model and prototype, the ratios of corresponding force acting on the corresponding points should be equal. The ratio of the forces of non-dimensional numbers it means that for the dynamic similarity between model and prototype , the dimensionless number should be same for model and prototype. The law on which the model are designed for dynamic similarity are called model laws or similarity laws.
11. Define Reynolds’s model law. It states that the Reynolds number for the model and prototype must be equal where the viscous force is predominant in addition addition to the inertia force. 12. Write the Froule’s model law equation.
13. What are the Types of Models? i). Undistorted model
ii). Distorted model
14. What is Undistorted model? An undistorted model is one which is geometrically similar with its prototype. The condition of similarities are completely satisfied for such models hence the results obtained from the model test and easily used to prevent the perform o f prototype. 15. What are Geometrical Distortion types? 1. Distortion of dimension- adoption of different scale for vertical and horizontal 2. Distortion of Configuration – Model does not be an resemblance of its Protot ype. 16. What is Material Distortion? Adoption of different material for model and prototype. 17. What is Distortion of Hydraulic Quantizes? Adoption of different uncontrollable hydraulic quantizes like Velocity, Discharge , Pressure etc… , for the model and prototype. p rototype. 18. What is Scale Effect? The behavior of the prototype as predicted b y two models with different scale ratio is generally not the same. Such an difference in the pre diction of behavior prototype is known as Scale Effect. 19. What is Distorted Model? The distorted model is the one geometrically similar with its prototype. p rototype. In such model, the different scale ratios for linear dimensions are adopted.
20. Write Mach’s model law with Expression? Its state that Mach number for the Model and prototype must be sam e when the elastic forces are predominant in addition to the inertia force.
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UNIT II WIND TUNNELS What are the two basic types of wind tunnels? i. Open Circuit wind tunnels ii. Closed circuit wind tunnels What are the advantages of Open circuit tunnels? i. Construction cost is less ii. There is no purging problem What are the disadvantages of open c ircuit wind tunnels? i. More energy to run ii. Tunnel is noisy iii.It Cause environmental problems and limits on hours o f operation. What are the advantages of closed circuit wind tunnels? i. Less Energy is required. ii. Less noise when operating What is variable density tunnels? In this type of tunnels, we can use different density working fluids for tunnel operations. Explain Propulsion Tunnels? These types of tunnels are used for testing aircraft engines, either recipro cating or jet; requires simulation of both flight velocity and the variation of atmospheric pressure and t emperature. What is smoke tunnels? Smoke tunnels are used primarily for flow visualization. Smoke tunnels used for research rather than demonstration purposes tend to have large contraction ratios. Explain automobile tunnels? These types of tunnels are used for test aerodynamic parameters that affects an automobiles performance and stability. What is meant by viscosity? The property which characterises the resistance that a fluid offers to applied shear force is termed as viscosity
10. Explain specific heat of a medium. The specific heat is defined as the amount of heat required to raise the temperature of a unit mass of a medium by one degree. 11. Which are the factors affecting the sur face tension of a fluid? i. The nature of the fluid ii. The nature of the substance with which it is in contact at the surface. iii.The temperature and pressure.
12. What are the two methods of fluid analysis? i. Lagrangian method ii. Eulerian method. 13. Explain the pathline. Pathline may be defined as a line in the flow field describing the trajectory of a given fluid particle. 14. Explain the term streakline. Streakline may be defined as the instantaneous locus of all the fluid elemnts that have passed the point of injection at some earlier time. 15. Explain streamlines. Streamlines are imaginary lines in a fluid flow drawn in such a manner that the flow velocity is always tangential to it. 16. Explain the term simple vortex. A simple or free vortex is a flow in which the fluid elements simply move along co ncentric circles, without spinning about their own axis. 17. Explain the lower critical Reynolds number. The lower critical Reynolds number is t hat Reynolds number below which the entire flow is laminar. 18. Explain the streamlines and bluff bodies. Lowe drag shapes are called streamlined bodies and high drag shapes are termed bluff bodies. 19. Explain the term shear drag. Drag due to the friction between the body surface and the fluid is known as shear drag. 20. What is meant b y Fanno flow? Fanno flow is an adiabatic flow with no external work. UNIT III CALIBRATION OF WIND TUNNELS 1. What are the factors concerning selection and mounting of strain gauges in wind tunnels? i. Adequate strain must be provided in the measuring elements under the design loads ii. Match gauges for gauge factors and resistance. 2. How the internal strain gauge are balances are calibrated? Internal sting balances are generally calibrated outside the tunnel. The Calibrations use a calibration rig upon which the balance is mounted. 3. What are the purposes of Calibration? i. To proof load the balance ii. To determine calibration slopes for a ir component iii. To determine component sensitivity.
4. What are the importance observations made from pressure d istributions? i. The reaction of the minimum pressure point and its strength. ii. The load that the skin is to withstand and its distribution. 5. What are the factors considering in the flutter testing? i. Critical flutter speed ii. Flutter frequency iii. Wing Empennage interaction. 6. What is solid blockage? The presence of the model in wind tunnel testing reduces the area, so increase the velocity this effect is called the solid blockage. 7. What are the commonly used scattering instances? i. Micro polythene spheres ii. Diluted milk droplets iii. Aerosal iv. Fine alumina powder. 8. What is fringe system? In the fringe system the fringes obtained during interference of the two beams are made use of for the measurement of flow velocities. 9. What is vortex shedding technique? Measurement of low velocities, below 2m/s, are difficult to do with instrument like a pitot-static probe in conjunction with a manometer, for measuring such low velocities, vortex shedding techniques could be used. 10. What is fluid jet anemometer? The deflection of the jet strongly d epends on the ratio of flow velocity to jet velocity. This kind of instrument has been used successfully in various industrial processing plants. 11. What is Beckmann Thermometer? It is used for accurate determination of a small temperature changes, such as that encountered while using the bomb calorimeter. 12. What is net flap span ratio? It is the ratio between flap span and wing span 13. What is flap at out ratio? It is the ratio between total flap at out area and total wing area. 14. What is the limitations of Dimensional Analysis? i. Dimensional Analysis does not give any clue regarding the selection of Variables. ii. The complete information is not provided b y the dimensional analysis. 15. Name the different similarity laws? i. Reynolds Similarity law ii. Froude similarity law iii. Euler similarity law iv. Weber similarity law v. Mach similarity law 16. What is Weber similarity law? In a fluid system, where surface te nsion effects predominate in addition to inertia force, t he dynamic similarity is obtained by equating the weber number for the model and its prototype, which is known as weber model law. 17. What is laser velocimetry?
The most common laser Doppler velocimeter uses optics to split the laser beam into two parallel beams that are focused to cross at the point where measurements are to be made. 18. What are the advantages of forward scatter mode? i. It can use low power lasers, such as helium, neon output .5 to 3.00 mv. ii. The laser does not need cooling iii. The signal to noise ratio is large.
19. What is the diaphragm type pressure transducer? The most commonly used pressure transducers are of the diaphragm type. Generally they are of the differential pressure type and a preselected reference pressure is applied to the reference side. 20. When the fluorescence photography is u sed? Ultraviolet fluorescence photography is used when the medium used for visualizing the flow has been treated with a dye that radiates in the visual spectrum when excited by long wave ultraviolet light. The wavelength of the long wave ultraviolet light is 320-400 millimicrons and it is transmitted b y optical glass. UNIT IV WIND TUNNEL MEASUREMENTS 1. What are the fundamental qualities to b e measured in fluid flow experiments? Pressure, temperature, velocity and density. 2. What is the main group of pressure measuring devices? Manometers and pressure transducers. 3. Give the examples for manometers. i. U tube manometers ii. Multitube manometers iii. Betz type manometers. 4. Give the examples for pressure transducers. i. Electrical type transducers ii. Mechanical type transducers iii. Optical type transducers. 5. What is the basic principle of manometers? The pressure applied is balanced b y weight of a liquid column. The sensitivity of the instrument depends on the density of the manometric fluid used. 6. What is LDA? LDA is laser Doppler are mometry; it is used to measuring velocities in fluid flows. 7. What are the advantages of LDA? i. The measurement with LDA is absolute ii. It has high frequency response. 8. Explain the term “temperature”. Temperature is a measure of the intensity of hotness or coldness. 9. What are the factors co nsidering for the temperature measurements? i. Increase or decrease of size ii. Change of colour iii. Generation of electromotive force. 10. What is thermometer? Give examples for thermometer. A thermometer is a general name given to thermometric measuring devices. Some of the commonly used thermometers are the mercury in glass thermometer. Beckmann and gas thermometer.
11. Give the examples for Electrical thermometers/ i. Thermocouples ii. Resistance temperature detectors iii. Thermistors 12. What is the principle of pyrometers? When temperature of the metal increases, it emits different colours.
13. What is wind tunnel balances? Wind tunnel balance is a device to measure the actual force and moments acting on a model placed in the test section stream. 14. What are the classifications of wind tunnel balances? i. Wire type balance ii. Strut type balance iii. Yoke type balance 15. What is wire type wind tunnel balances? In wire type wind tunnel balances o nly wires are used to support the model. All the load components are transmitted to the measuring device by these wires. 16. What is the classification of strut type wind tunnel balances? i. Yoke type iii. Platform type iv. Pyramid type 17. What are the advantages of strut type balances? i. The strut being rigid, their deflection can be kept at a very small value. ii. Weight of the support structure can be kept very low. 18. What is pyr4amid type wind tunnel balances? In pyramid type balances, the model mounting is a bout one point connecting the model to the platform by four arms. 19. What are the types of strain gauge wind tunnel balances? i. Internal strain gauge wind tunnel balance ii. Semi-Internal strain gauge wind tunnel balance iii. External strain gauge wind tunnel balance. 20. What is strain gauge wind tunnel balances? Balances with strain gauges as the load sensing element are t ermed strain gauge wind tunnel balances. UNIT V HIGH SPEED WIND TUNNELS 1. Explain high speed wind tunnels? High speed wind tunnels are those, with test section speed more than 650 kmph. 2. What are the advantages of blow down wind tunnels? i. They are simplest among the high speed tunnel types ii. Most Economical to build iii. Running time of considerable duration can be achieved. 3. What are the disadvantages of the b low down wind tunnels? i. Charging time to running time ratio will be very high ii. Starting load is high. 4. What are the advantages of Induction t ype tunnels? i. Stagnation pressure and stagnation temperature are constants.
ii. Starting and shut down operations are simple. 5. What are the disadvantages of Induction t ype wind tunnels? i. Vaccum tank size required is very large ii. Reynolds number is very low. 6. What are the advantages of continuous super sonic wind tunnels? i. Only a small capacity drier is required. ii. Better control over the Reynolds number is possible. 7. What are the disadvantages of continuous supersonic wind tunnels? i. Power required is very high ii. Requires large size cooler. 8. What are the losses in supersonic tunnels? i. Frictional losses ii. Expansion losses iii. Losses in cooling system iv. Losses in guide vanes. 9. What is the need for second throat in supersonic wind tunnels? The second throat is used to provide isentropic deceleration and highly efficient pressure recovery after the test section. 10. What is the pressure and temperature range in the hypersonic wind tunnels? The pressure range of hypersonic wind tunnels are 1 0 to 100 atmosphere and stagnation temperatures are 223K to 2273K. 11. Explain why heated and dry air is u sed in hypersonic wind tunnels. Use of dry and heated air is necessary for hypersonic operation to avoid condensation effects and liquefaction during expansion to the high Mach numbers and corresponding low temperatures. 12. Explain surging in wind tunnels. It is a low frequency vibration in velocity that may run as high as five percentage of the dynamic pressure. 13. What are the disadvantages of the surging? i. Surging makes the validity of assigning a Reynolds number to the test doubtful. ii. It usually makes the d ynamic testing impossible. 14. Give the examples for high speed wind tunnels. Hotshot tunnels, Plasma jets, shock tubes, shock tunnels and light gas guns. 15. What is hot shot tunnels? Hotshot tunnels are devices meant for the generation of high speed flows with high temp eratures and pressures for a short duration. 16. Explain plasma Arc tunnels. Plasma arc tunnels are devices capable of generating high speed flows wit h very high temperature. 17. What is shock tubes? The shock tubes is a device to produce high speed flow with high temperatures by traversing normal shock waves that are generated by these rupture of a diaphragm which separates a high pressure gas form a low pressure gas. 18. Explain shock tunnels. Shock tunnels are wind tunnels that op erate at Mach numbers of the order25 or higher for time intervals up to a few milliseconds by using air heated and compressed in a shock tube. 19. What is Gun tunnels?
The gun tunnels are similar to the shock tunnels in operation. It has a high pressure driver section and a low pressure driven section with a diaphragm separating the two. 20. What are the advantages of the Ludwig Tube tunnel? i. No regulation of pressure and temperature during the run is necessary. ii. It is suitable for transient heat transfer tests.
6 MARKS UNIT-I
PRINCIPLES OF MODEL TESTING 1. Discuss about Rayleigh’s method of analysis.
Explanation about Rayleigh’ method Formulation of relationship of variables Solving by using dimensional homogeneity Example
2. Explain the types of similarity.
Types of similarity- geometric, kinematic, d ynamic Geometric similarity- definition, formulae, explanation Kinematic similarity- definition, formulae, explanation Dynamic similarity- definition, formulae, explanation
3. Explain the types of models.
Model- definition Types of models- distorted, undistorted Distorted- definition, example, types of distortion Undistorted- definition, example.
4. Explain any two similarity laws.
Similarity law- definition Types of laws- Reynold’s , Froude’s, Euler’s, Weber’s , Mach Explanation for any two laws Definition of law Basic equation of law
5. Discuss about Scale effect.
Scale effect- definition Explanation Importance of scale effect
Advantages Disadvantages
UNIT II Wind Tunnels
1. Explain the types of wind tunnels.
Wind tunnel- definition Types- open circuit, closed circuit Classification according to speed Low speed and high speed tunnels- definition Types of high speed tunnels- transonic, sup ersonic, hypersonic
2. Explain the role of honey combs and guide vanes in wind tunnel design.
Honey combs- function and graph Different honeycomb structures and K values Guide vanes- function and its shape Different shapes with K values
3. Explain the general features of wind tunnel.
Wind tunnel- definition Features of wind tunnel- effuser, test section, diffuser, driving unit Effuser- function, contraction factor Test section- function, shapes Diffuser- function, kinetic energy conversion Driving unit- function, problems of starting a tu nnel
4. Discuss about the term energy ratio.
Energy ratio- definition Power losses in wind tunnel Calculation of percentage of energy loss Estimation of energy ratio
5. Explain about hypersonic tunnel layout.
Hypersonic tunnel- definition Layout of hypersonic tunnel
Working of tunnel Hypersonic nozzle Advantages and disadvantages
UNIT III Calibration of wind tunnels
1. Explain about wire type balances.
Definition of wire type balance Diagram Arrangements of wire type balance Six component equations Advantages Disadvantages
2. Discuss about strut type balances.
Definition for strut type balance Diagram Arrangement explanation Types of strut t ype balance- platform type, yoke type, pyramid type Advantages Disadvantages
3. Explain about strain gauge balances.
Definition of strain gauge balance Types of strain gauge balance- internal, semi internal, external Operation theory Strain gauge diagram, installation procedure Basic equations
4. Explain about measurement of turbulence. Turbulence- definition Explanation Turbulence factor Turbulence equation Measuring methods- turbulence sphere, pressure sphere, hot wire anemometer 5. Explain the working principle of hotwire anemometer.
Function of hot wire anemometer Turbulence equation Equation of rate of heat dissipation Applications Advantages Limitations
UNIT IV WIND TUNNEL MEASUREMENTS 1.
Write note on balances. The Balance should be capable of measuring the various loads acting on the model with the very high degree of accuracy. The interaction between the different load components should be kept small. The balance should have provisions to vary the angle of incidence, pitch, yaw, roll and so on. The balance and the supporting structures should be designed for very high rigidity The deflection of the parts under the influence of maximum load is negligible. Damping devices should be incorporated in the measuring s ystem. Use of bearing should as far as possible be avoided in as much they cause large hysteresis and zero error. 2. What are the types of Balances. i. Wire Type Balance ii. Strut type Balance iii. Plat form type balance iv. Yoke type balance v. Strain Gauge type balance 3. Clasify the struct type balance. Based on the structural construction, strut type balances are broadly classified into
Yolk Type Platform Type
Pyramid Type Struts are used to support the model and transmit the loads to the measuring device. Models are mounted on the balance w ith 3 mounting mountings The strut being rigid, there deflection can be kept at a very small value
4. Clasify the types of balances Wind tunnels balances with strain gauges as the load sensing element are termed strain gauge wind tunnel balances. Based on strain gauge fixing on the mode, the train gauge balances are classified as,
Internal strain gauge wind tunnel balance
Semi Internal strain gauge wind tunnel balance
External Strain Gauge Wind Tunnel Balance.
5. Write a note about t he Manometer. i. ii. iii. iv. v.
The basic measuring principle of the liqu id manometer is that the pressure app lied is balanced by the weight of the liquid column. The sensitivity of the instrument depends on the density of the manometric fluid u sed. Some of the commonly used manometric liquids are water, alcohol and mercury. Pressure measurements in compressible flows with high sub sonic and supersonic Mach Numbers mercury is the suitable liquid. The reason is that fluids l ike water and alcohol are unmanageable in column heights for the pressure associated with such speeds. UNIT V HIGH SPEED WIND TUNNELS
1.Write note on Hypersonic Wind Tunnels 1 It operate with Mach Number above 5. 2 They operate with stagnation pressure in the range from 10-100 atmosphere 3 The Stagnation range from 223K to 2273K 4 Models that can be tested in hypersonic tunnel are usually larger than those meant for testing supersonic tunnels. 5 The model frontal area can go up to 10% of the test section cross sectional area. 2. Explain about chocking. i. The Phenomenon of chocking occurs when the local Mach No. in a high speed tunnel reaches unity. ii. This may happen at points where the effective area of cross section of the tunnel is reduced as a result of boundary layer thickening. iii. Induced separation or simply the presence of a model which is too thick or at too high incidence. iv. Chocking leads supersonic flow in the working section and fina lly breaks down. v. This condition sets a limit to the range of operation of the tunnel. vi. A limit which may vary according to the model under test and its altitude 3. Write about Transonic Wind Tunnel i. It is usually impossible to achieve flow at a Mach Number above 0.9 with ordinary parallel sided liners. ii. It is also difficult to achieve supersonic flow at a Mach Number below 1.1 iii. A Convergent divergent nozzle with solid walls becau se of chocking difficulties special liners are required to cover the transonic range of Mach Number .85 to 1.15. iv. The design of such liners is loosed on a working section whose walls are not sold but perforated. v. The area of the perforations increasing with distance along the length of the sections. 4. Write about blow through tunnels. i. Wind Tunnels for intermittent operation may be of the blow through or suction type. ii. In the flow through tunnel the arrangement is such as the one similar to the supersonic wind tunnels. iii. Air is compressed to a very high pressure in a reservoir
iv. It is subsequently released through a valve into the nozzle and is eventually discharged into the atmosphere v. Since the reservoir can be charged up relatively slowly by means of a compressor which does not have to be especially highly powered. vi. The running time is generally short, b ecause of the limited amount of air in t he reservoir.
5. Write note on Suction type Tunnel . In this type of tunnel, instead of air flowing out of a compressed air reservoir, it is sucked into a previously evacuated chamber. Through out the run the stagnation condition remain constant at atmo spheric value. The flow through tunnel originates in the atmosp here outside the tunnel High pressure ratios are easily achieved and this implies that high Mach Numbers can be obtained without the use of a chamber which can withstand high pressurisation. The advantage is that the running time likely to be even s horter than that of a flow through tunnel especially at high Mach Numbers. 10 MARKS UNIT I PRINCIPLES OF MODEL TESTING 1. Write about the Similitude and types of Similarities. Similitude is defined as t he similarities between the model and its prototype in every aspect which means that the model and prototype have similar properties. Three types of similarities must exist between the model and prototype. They are
Geometric Similarity
Kinematic Similarity
Dynamic Similarity Geometric Similarity i.
The Geometric Similarity is said to exist between the model and prototype if the ratio of all corresponding linear dimension in the mod el and prototype are equal.
Kinematic Similarity i. It mean the similarity of motion between model and prototype. ii. Thus the kinematic similarity is said to be exist between the model and the prototype if the ratios of the velocities and acceleration at the corresponding points in the prototype are the same.
iii. Also the directions of motions in the model and prototype should be same. Dynamic Similarity i. ii.
iii.
It means the similarity of forces between the model and prototype. Thus dynamic similarity is said to exist between model and protot ype if the ratios of the corresponding forces acting at the corresponding points are equal Also the directions of the corresponding forces at the corresponding points should be same.
2. Write a note about the Dimensionless numbers. i. Dimensionless numbers are those numbers which are obtained by dividing inertia force by the pressure or gravity or viscous or surface tension or elastic force. ii. As this is a ratio of one force to another force, it will be a dimensionless number. iv. It has no units v. These dimensionless numbers are also called non-dimensional parameters. The non dimensionless numbers are the following,
Reynolds Number It is defined as the inertia force of a flowing fluid to the viscous force of the fluid.
Froude’s Number The Froude’s Number is defined as the square root of the ratio of inertia force of the flowing fluid to the gravity force.
Euler’s Number It is defined as the square root of the inertia force of the flowing fluid to the pressure force.
Weber’s Number It is defined as the square root of the ratio of the inertia force of the flowing fluid to the surface tension force. Mach Number Mach Number is defined as the square root of the ratio of the inertia force of the flowing fluid to the elastic force.
3. Write principle and the rules for selecting repeating variables in Bucking pi theorem. Principle of Bucking pi theorem It states that if t here are n variables in a physical phenomena and these variables contains m fundamental dimensions the variables could be arranged into (n-m) dimensionless terms.
Bucking pi theorem is more advantageous if there are more number of variables than the fundamental variables. The Number of repeating variables are equal to the Number of fundamental dimensions of the problem (M,L,T). The choice of repeating variables is governed b y the following considerations. i.
As far as possible, the dependent variable, the dependent variable should not be selected as repeating variable ii. The repeating variables should be chosen in such a way that one variable contains geometric property, other variable contains flow property and the third variable contains fluid property. iii. The repeating variables selected should not form a dimensionless group. iv. The repeating variables together must have the sa me no. of fundamental dimensions. v. No two repeating variables should have the same dimensions. 4. Write a detailed note about the shock tubes. i.
The shock tube is a d evice to produce high speed flow with high temperatures by traversing normal shock waves. ii. They are generated by the rupture of a d iaphragm which separates a high pressure gas from a low pressure gas. iii. The shock tube is a very useful research tool for investigating not only the s hock phenomena. iv. But also the behaviour of materials and bodies when subjected to at very high pressures and temperatures. v. The diaphragm on the high and low pressure section is ruptured and the high pressure drives gas rushes into the driven gas section setting up a shock wave that compresses and heats the driven gas. vi. The pressure variation through the shock tube at the instant of diaphragm rupture and at too short intervals. vii. When the shock wave reaches the end of the driven tube, all of the driven gas will have been compressed and will have a velocity in the direction of the s hock wave travel. viii. Upon striking the end of the tube the shock is reflected and starts travelling back upstream. ix. As it passes through the driven gas and brings it to rest, additional compression and heating are accomplished. x. The heated and compressed gas sample at the end of the shock tube will retains its state except for heat loses until the shock wave reflected from the end of the tube passes through the driver gas – driven gas interface and sends a reflected wave back through the stagnant gas sample. 5. Write about the Gun Tunnels. i.
The gun tunnel is quite similar to shock tunnel in operation.
ii. It has a high pressure driver section and a low p ressure driven section with a diaphragm separating the two. iii. A piston is placed in the driver section adjacent to the d iaphragm so that the diaphragm ruptures. iv. The piston is propelled through the driven tube, compressing the gas ahead of it. v. The piston used is so light that it can be accelerated as the velocities significantly above the speed of sound in the driven gas. vi. This causes a shock wave t o proceed the piston through the driven tube and heat the gas. vii. The shock wave will be reflected from the end of the driven tube to the piston, passing further heating of gas. viii. The piston comes to rest with equal pressures on its two sides, and the heated and compressed driven gas ruptures the diaphragm and flows t hrough the nozzle. ix. Gun tunnels are limited in the maximum temperature that can be achieved b y piston design. x. The maximum temperature normally achieved are about 2000K. xi. Run times of an order of magnitude higher than the shock tunnels are possible in gun tunnels. UNIT II WIND TUNNELS 1. Write the advantages and disadvantages of Open and Closed Circuit Wind Tunnels. OPEN CIRCUIT WIND TUNNELS Advantages: i. ii. iii.
Construction cost is less If one intends to do much flow visualization via. Smoke, there is no purging problem. If both inlet and exhaust are open to atmosphere. Because of low initial cost, an open circuit is often ideal for schools and universities where a tu nnel is required for class room work, for research and high utilization is not required.
Disadvantages: i.
If located in room, depending on the size o f tunnel to the room size, it may require extensive streaming at the inlet to get high quality flow. ii. The same may be true if the inlet or exhaust is open to the atmosphere, when wind and cold weather affect operation. iii. For a given size and speed the tunnel will require more energy to run. iv. In general a tunnel is noisy for large tunnel noise may cause environmental p roblems. CLOSED CIRCUIT WIND TUNNELS Advantages: i. ii.
Through the use of corner turning vanes and po ssibly screens the quality of the flow can b e easily controlled. Less Energy is required for a given test section size and velocity.
iii. iv.
This can be important for a tunnel used for developmental testing with high utilisation. Less noise when operating.
Disadvantages: i. ii.
Higher initial cost due to return ducts and corner vanes. Used extensively for smoke test or running of inertial combustion engine there must be a way to purge tunnels. If tunnel has high utilisation it may have to have an air exchange or some other method of cooling during hot summer months.
iii.
2. Write about the Straight through Subsonic Wind Tunnel. i.
The diffuser serves to slow down the air leaving the fan, so that it flows at low speed through a region called the settling chamber. ii. In this region, the slowly moving air settles to fairly uniform and low turbulence conditions, with the help of a series of “Honey Combs” iii. A flow is then contracted to give a fast jet in the wo rking section. iv. All the Kinetic Energy of the fast moving air in the working section is wasted and the consequent high power requirement makes such an arrangement impracticable, except on a fairly small scale. v. The air is sucked through by a fan downstream of the working section. vi. The entry to the tunnel should be clear off walls and o ther obstacles in the tunnel room. vii. There is a large contraction ratio and a small angle diffuser. viii. Such an arrangement gives reasonably low turbulence and constitute a useful general purpose tunnel. ix. There is still considerable power wastage because the Kinetic Energy of the air leaving the tunnel is not recovered. x. But it is clearly better since the speed at the exit is less. 3. Write about the Return Circuit Subsonic Wind Tunnel. i. ii. iii. iv. v. vi. vii. viii. ix.
In a return circuit tunnel the air leaving the diffuser is not simply discarded but collected It travels around the closed circuit to be passed through the working section again. The kinetic Energy of the jet is recovered, not wasted and the power required is reduced. It has all the features of the straight through tunnel with a n addition of corners. The diffuser is not immediately downstream of the working section, though it could easily be so. Here it serves to reduce the speed of the flow in part of the tunnel circuit Corner vanes are fitted to enable the air to flow smoothly round the cor ners It is difficult to obtain uniform, steady flow and the tunnel is b est suited to demonstration work and fairly rough measurement. There is a breather slot downstream of the working section which serves to main the pressure at or near the atmospheric values.
4. Write about the Annular Type Return Circuit Subsonic Wind Tu nnel. i. It has a section through the axis of the tunnel, which is a body of revolution. ii. It is difficult to achieve uniform steady flow without elaborate arrangements of corner vanes etc.
iii. This arrangement is therefor only used for variable density tunnels iv. It has an advantage when pressurisation is used or when the working fluid is other than air v. A variable density tunnel generally consists of an annular type tunnel enclosed in a large pressure vessel. vi. By increasing the pressure and hence the d ensity, it is possible to achieve relatively high Reynolds Number. vii. Pressurisation also results in power economy viii. Some tunnels are also designed to operate at much reduced pressures ix. Hence it can work at low density and at high speeds. 5. What are the types of forces acting in a moving fluid. There are six types of forces acting in a moving fluid. They are, i.
Inertia Force
Inertia force is equal to the product of mass and acceleration of the flowing fluid and act in the direction opposite to the direction of acceleration. It is always existing in the fluid flow problems. ii.
Viscous Force
Viscous force is equal to the product of shear stress due to viscosity and the surface area of the flow. It is present in the fluid flow problems where viscosity is having an important role. iii.
Gravity Force Gravity force is equal to the product of mass and acceleration due to gravity of the flowing fluid. It is present in case of open Surface flow.
iv.
Pressure Force Pressure force is equal to the product of pressure intensity and cross sectional area of the flowing fluid. It is present in the case of flow in a pipe.
v.
Surface Tension Force It is equal to the product of surface tension and length of the surface of the flowing fluid.
vi.
Elastic Force It is equal to the product of elastic stress and area of the flowing fluid.
UNIT III CALIBRATION OF WIND TUNNELS 1. Write about the optical methods of flow visualization. i. Several methods of flow visualization consist in utilizing the changes in the refractive index of air caused by changes in density. ii. They may be used to investigate the flow in boundary layers iii. But their main application is to the visualization of high speed flow. iv. When light passes through a region of a gas in which the densit y varies, the changes in the refractive index of the gas associated with these density changes causes the direction of t he rays to be altered. v. The amount of the deflection is proportional to the density gradient, and the sense o f the deflection is determined by the sign of this gradient. vi. In the high speed flow, the density of the fluid changes throughout the field of flow, and the above principle may be utilised in order to show up these variations. vii. There are three commonly used methods of doing so, * the shadowgraph, * schlieran and *interferometer systems. viii. All are based on the same principle, but, whereas, the first two are mainly useful simply for flow visualization, the third may also b e used for quantitative measurement, though it is much more difficult to interpret than the o ther two. ix. The Shadowgraph system is simplest, but the schlieran system is more effective, and is the most effective and is the most commonly used of all three methods 2. Write about the Shadowgraph system. i. The Shadowgraph system is the simplest of all the methods. ii. Light from a point source is collimated by a lens iii. A parallel beam passes through the working section of the wind tunnel onto a screen, where an image of the working section then appear s. iv. If there is no model on the tunnel, the screen will be uniformly illuminated v. If a model is introduced its shadow will appear on the screen. vi. If the tunnel is run, there will be density changes in the air in the working section associated with the flow past the model. vii. If light passes through a region of increasing density gradient, the rays will diverge more and more, and the corresponding region on the screen will b e lightened. viii. The area on the screen corresponding to r egions of reducing density gradient in the working section will be darkened.
ix. The sensitivity of the system is low x. The variations in intensity of illumination clearly depend on the second derivative of the density. xi. However, the system is adequate to show up the location of shock waves as dark lines on the screen, and it is cheap and easy to construct.
3. Write about the Schlieran System i. The Schlieran system is the most effective method of optical visualization. ii. Light from a point source is collimated by a lens iii. A parallel beam passes through the working section of the wind tunnel onto a screen, where an image of the working section then appears. iv. It is collected by another lens and brought to a focus at B, then passes through yet another lens to project an image of the working section onto a screen. v. At the focus position B, a knife edge is introduced, which can be moved into or out of the beam to cut out some or all of the light. vi. When the tunnel is empt y, the intensity of illumination on the screen ma y be varied by moving the knife edge, but it remains uniform over the whole image area. vii. When a model is introduced into the working section appears on the screen and if the tunnel is now run, variations in density produce refraction of the light ra ys. viii. Light passing through regions of positive density grad ient normal to the knife edge are deflected towards the knife edge are d eflected toward the knife edge, and the corresponding area on the screen are lightened. ix. Thus shock waves appear as d ark lines, expansion regions as light regions on the screen. 4. Write about the interferometer. i. A interferometer is a flow visualization device concentrating on the quantitative measurement. ii. Light from a source S is split into two beams b y a semi-permeable mirror at A. iii. Some light travels on to a totally reflecting mirror at B iv. The remaining is reflected to another totally reflecting mirror at C. v. Both these beams are then reflected to another semi-permeable mirror at D vi. One of the two beams has passed through the working section. vii. Its speed changes by virtue of the changes in refractive index associated with the density changes in the tunnel. viii. As a result it arrives out of phase with the other b eam, and so interference fringes appear on the screen. ix. These fringes may be photographed x. These can be measured to give detailed and quantitative information concerning the density distribution in the working section. xi. The interpretation of this information is difficult. xii. A visualization ot the flow, the pictures obtained are much harder to assimilate t han are schlieran pictures. xiii. The system is very sensitive to even slight mechanical vibrations and it is essential to mount the optical components on a rigid frame independent of the tunnel mounting.
5. Write about the smoke Techniques. i. This technique simply consists in introducing into the working section of a wind tunnel a number of fine some filaments ii. It is done usually in the plane of the section of the model wing under test. iii. As the particles of smoke move downward the filaments define the particle paths. iv. By this a stream line pattern may be observed. v. Many different phenomena may be demonstrated such as trailing vortices, or the breakdown of the streamline flow in a region of separated flow. vi. The principle requirements of a smoke tunnels for uniform flow with low turbulence. vii. If this is not achieved, the smoke will quickly become diffused. viii. The speed must not be too low or gravity will affect the p aths of the smoke particles, curving these paths downwards. ix. If the speed is too high, the smoke filaments may become attenuated, even when large quantities of smoke are introduced. x. Good lightning is needed if te smoke filaments are to be clearly seen, and an open circuits tunnel is required in order to get rid of smoke. xi. The main disadvantage is that is usually only isolated span wise positions can be examined, and not the flow over the whole wing UNIT IV WIND TUNNEL MEASUREMENTS 1. Write about the Wind Tunnel Balances Basically a wind tunnel balance is a device to measure the actual forces and moment, acting on a model placed in the test section. Based on the construction details, the wind tunnel balances are broadly classified into i. Wire Type Balance ii. Strut type Balance iii. Plat form type balance iv. Yoke type balance v. Strain Gauge type balance Irrespective of its type of construction, a wind tunnel balance should have certain basic features and characteristics for proper measurements They are the following
The Balance should be capable of measuring the various loads acting on the model with the very high degree of accuracy. The interaction between the different load components should be kept small. The balance should have provisions to vary the angle o f incidence, pitch, yaw, roll and so on.
The balance and the supporting structures should be designed for very high rigidity
The deflection of the parts under the influence of maximum load is negligible.
Damping devices should be incorporated in the measuring system.
Use of bearing should as far as possible be avoided in as much they cause large hysteresis and zero error.
2. Write about the Wire type wind Tunnel Balances. i. ii. iii. iv. v. vi. vii. viii. ix.
In wire type wind tunnel balances only wires are used to support the mode. All the load components are transmitted to the measuring device by this wires. Wire type balances are probably the simplest and easiest to build. But they have several disadvantages due to the use of too many bearings and b ello crank system, friction of the wires a nd high damping requirements. They are extremely bulky because the support s ystem should be very rigid. Large tare drag because of the exposed wires. It is difficult to streamline the wires and hence it is difficult to determine the magnitude of tare drag accurately. Bearings and linkages cause zero error. Wires have the tendency to crystallize and break
3. Write about the Strut Type Balance. i. The strut type balances are proved to be suitable for such tunnels. ii. Based on the structural construction, strut t ype balances are broadly classified into
Yolk Type Platform Type
Pyramid Type iii. Struts are used to support the model and transmit the loads to the measuring device. iv. Models are mounted on the balance with 3 mounting mountings v. The strut being rigid, there deflection can be kept at a very small value vi. The arrangement enables the strut to be fair so that the tare and inference drag are minimised. vii. By using a cross spring pivot instead of knife edge, ball bearings, a the like in the beam the zero error can be minimised. viii. By choosing proper linkages, interaction loads can be reduced to a small value. ix. Weight of the support structure can be kept very low, because efficient members can be easily adopted
4. Write about the Strain Gauge Wind Balance i.
Wind tunnels balances with strain gauges as the load sensing element are termed strain gauge wind tunnel balances. ii. Based on strain gauge fixing on the mode, the train gauge balances are classified as,
Internal strain gauge wind tunnel balance
Semi Internal strain gauge wind tunnel balance
External Strain Gauge Wind Tunnel Balance. iii. In an internal balances, all the measuring elements are located inside the model. iv. In an semi internal balance the measuring elements are located partially inside and partially outside iv. In an external balance all the measuring elements are located outside the model
v. The gauges consists of a grid of very fine wire or very thin foil embedded in a seat of Bakelite having a thickness comparable to that of a thick paper vi. The grid material is usually Nichrome wit h small addition of iron and aluminium. vii. The grid length varies about 1mm to several centimetres. 5. Write a note about t he Manometer. vi. The basic measuring principle of the liqu id manometer is that the pressure app lied is balanced by the weight of the liquid column. vii. The sensitivity of the instrument depends on the density of the manometric fluid used. viii. Some of the commonly used manometric liquids are water, alcohol and mercury. ix. Pressure measurements in compressible flows with high sub sonic and supersonic Mach Numbers mercury is the suitable liquid. x. The reason is that fluids l ike water and alcohol are unmanageable in column heights fo r the pressure associated with such speeds. xi. In addition to these manometers, an accurate b arometer is essential for pressure measurements xii. The pressures are invariably measured in terms o f a difference in pressure from some known reference. xiii. Most common reference is the local atmospheric pressure. xiv. Therefore for pressures measured with reference to atmosp heric pressures conversions to absolute pressure requires the local atmosphere to be known. UNIT V HIGH SPEED WIND TUNNELS 1. Write about the Blow through Intermittent Operation. i. Wind Tunnels for intermittent operation may be of the blow through or suction type. ii. In the flow through tunnel the arrangement is such as the one similar to the supersonic wind tunnels. iii. Air is compressed to a very high pressure in a reservoir iv. It is subsequently released through a valve into the nozzle and is eventually discharged into the atmosphere v. Since the reservoir can be charged up relatively slowly by means of a compressor which does not have to be especially highly powered. vi. The running time is generally short, b ecause of the limited amount of air in the reservoir. vii. Once the pressure in the reservoir drops below the certain value the pressure retain across the tunnel will no longer be sufficient to give supersonic flow in the working se ction. viii. It is then necessary to stop the tunnel and recharge the reservoir ix. The changing time between the runs ma y be considerable. x. A further dis advantage is that the total temperature and total pressure, velocity may vary during run. 2. Write about the Suction type Intermittent Operation. i. ii.
In this type of tunnel, instead of air flowing out of a compressed air reservoir, it is sucked into a previously evacuated chamber. Through out the run the stagnation condition remain constant at atmo spheric value.
iii. iv. v. vi. vii. viii.
The flow through tunnel originates in the atmo sphere outside the tunnel High pressure ratios are easily achieved and this implies that high Mach Numbers can be obtained without the use of a chamber which can withstand high pressurisation. The advantage is that the running time likely to be even shorter than that of a flow through tunnel especially at high Mach Numbers. Air is compressed to a very high pressure in a reservoir It is subsequently released through a valve into the nozzle and is eventually discharged into the atmosphere Since the reservoir can be charged up relatively slowly by means of a compressor which does not have to be especially highly powered.
3. Write about the Induced Flow Tunnels. i. ii.
The induced flow usually occurs in Supersonic Wind Tu nnels. In this type of tunnel a high speed jet is forced through a narrow annular slot downstream of the working section. iii. This jet induces a flow through the working section which if the flowing pressure i s high enough will be supersonic iv. The jet through the slots is provided either by means of a compressor or more usually from a compressed air reservoir which as in the case of blow through tunnel is t he effective power supply. v. The pressure ratio is maintained with a compressed variable throttle valve. vi. These tunnels may be operated subsonically or supersonically vii. It is generally impossible because of the difficulties involved in the mixing of two jets to achieve Mach Number much above the value 1.8 viii. Their main advantage is that while they have the advantage of the blow through over the suction type tunnels in term of running time. ix. They don’t suffer from the disadvantage of varying stagnation condition. x. The air which flows through the Wind Tunnel comes out from the reservoir but from the atmosphere 4. Write about the Transonic Wind Tunnel. i. ii. iii. iv. v. vi. vii. viii. ix.
It is usually impossible to achieve flow at a Mach Number above 0.9 with ordinary parallel sided liners. It is also difficult to achieve supersonic flow at a Mach Number below 1.1 A Convergent divergent nozzle with solid walls bec ause of chocking difficulties special liners are required to cover the transonic range of Mach Number .85 to 1.15. The design of such liners is loosed on a working section whose walls are not sold but perforated. The area of the perforations increasing with distance along the length of the sections. The principle is that the excess air associated with boundary layer growth or the presence of the model can escape through these perforations. Thus eliminating the efforts of an narrowing channel and preventing chocking. The detail positioning, size and shape of the holes or slots is often crucial to the successful operation of the tunnel It is largely a method of trial and error.
5. What is Chocking in High Speed Wind Tunnels and say about Hypersonic Wind Tunnels
Chocking in High Speed Tunnels i. ii. iii. iv. v. vi.
The Phenomenon of chocking occurs when the local Mach No. in a high speed tunnel reaches unity. This may happen at points where the effective area of cross section of the tunnel is reduced as a result of boundary layer thickening. Induced separation or simply the presence of a model which is too thick or at too high incidence. Chocking leads supersonic flow in the working section and fina lly breaks down. This condition sets a limit to the range of operation of the tunnel. A limit which may vary according to the model under test and its altitude
Hypersonic Wind Tunnels i. ii. iii. iv. v.
It operate with Mach Number above 5. They operate with stagnation pressure in t he range from 10-100 atmosphere The Stagnation range from 223K to 2273K Models that can be tested in hypersonic tunnel are usually larger than those m eant for testing supersonic tunnels. The model frontal area can go up to 10% of the test section cross sectional area.
