Stability and Drag Analysis of Wheeled Amphibious Vehicle Using CFD

Applied Mechanics and Materials Vols. 592-594 (2014) pp 1210-1219 Online available since 2014/Jul/15 at www.scientific.net © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.592-594.1210

Stability and Drag Analysis of Wheeled Amphibious Amphibious Vehicle using CFD and Model Testing Techniques RR More, Scientist ‘D’1, Piyush Adhav2, K Senthilkumar, Scientist ‘F’3 &MW Trikande, Scientist 'G'4 1,3,4

Vehicle Research and Development Development Establishment, Establishment, Defence Research and Development Organisation Ahmednagar, Ahmednagar, Maharashtra, 414006 414006

2

*

M.Tech Student at Sardar Vallabhbhai Vallabhbhai NIT, Surat, Gujarat, India

[email protected], [email protected], [email protected], d [email protected]

Keywords : Amphibio : Amphibious us design, Model testing, Combat vehicle vehicle

Abstract

Amphibious design of combat vehicle has become a challenging task in the context of increase in Gross Vehicle weight(GVW) of present generation combat combat vehicles due to demand for increased armour protection and higher capacity engine and transmission, incorporation of multiple weapon systems, increased ammunition storage and larger addition of electrical and electronic items. Development of combat vehicles is complex and very expensive, and normally limited with less number of prototypes. The scale modeling technique and experimental model testing in conjunction with CFD analysis offer a viable solution to accomplish the amphibian design of a combat vehicle with adequate confidence before manufacturing the actual prototype. In the present work, an approach involving experimental towing test of scaled vehicle model and CFD simulation has been used to carry out the amphibious design of an 8X8, wheeled, combat vehicle with a GVW of 22 ton. In this work, a 1/5 th scaled model of the vehicle was manufactured and tested in a towing tank at different test speeds for drag and stability analysis. CFD analysis was carried out on the full scale model to gain adequate details about the dynamics of vehicle in the water in addition to drag estimation. Good correlation has been found in drag values and the flow patterns obtained from towing tank tests and CFD simulations. Introduction:

Effective crossing of canals and rivers, and capacity to perform limited amphibious maneuvers form an imperative prerequisite for amphibious armoured personnel carriers intended for battle in terrains whose characteristics are influenced by water obstacles. An amphibious combat vehicle is a vehicle that is a means of battle transport, viable on land as well as on water. It may be tracked or wheeled and may be propelled by a jet or by the action of its wheels or tracks. t racks. The present day combat vehicles are normally characterized by high level of protection together with ballistic and mine protection, high capacity engine/transmission, multiple weapons, high caliber armaments, increased ammunition quantity and increased electrical and electronic systems. Incorporation of these essential and desirable systems and features eventually results in higher Gross Vehicle Weight. As the weight of the floating object has a direct correlation with the volume of water displaced, accomplishment of amphibian design becomes much tougher within the set dimensional limit for a combat vehicle. The combat vehicle has to have low silhouette, should be compact and transportable by road, train and air. Further, aspects such as location of CG, Moment of Inertia, weight distribution should be judiciously worked out and tweaked such that the vehicle meets the mobility requirements on land as well as in water.

All rights reserved. reserved. No part of contents contents of this paper may be reproduced reproduced or transmitted transmitted in any form or by any means means without the written written permission of of TTP, www.ttp.net. www.ttp.net. (ID: 161.139.102.14, 161.139.102.14, Universiti Teknologi Malaysia UTM, Johor Bahru, Johor, Malaysia-21/07/14,06:42:32) Malaysia-21/07/14,06:42:32)

Applied Mechanics and Materials Vols. 592-594

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In order to meet the terms of the field forces, the behavior of the vehicle during the floatation should be known. This emphasizes the importance of the flow simulation of the body of the vehicle, the study of which plays an important role in achieving the desired effect. However, as the development of combat vehicles is very expensive and manufacturing of number of prototypes are limited, conduct of experimental flow simulation on a dedicated full scale prototype is not viable in design finalization phase. Hence, the situation demands for conduct of studies using less expensive scale model testing and computer simulations. An appropriately scaled model provides adequate insight in to the water dynamics around the vehicle and the effect of water current on the stability and motion characteristics of the actual vehicle. In the present work, attempt has been made to fabricate a 1/5th scale model of an amphibious, wheeled combat vehicle having Gross Vehicle Vehicle Weight of 22t and carry out towing tests at different vehicle speeds. To gain detailed insight, CFD analysis has also been been carried out for stability and drag analysis and compared with the experimental results. Model Testing:

The model tests should be performed such that model and full-scale hull exhibit similar behavior 1,2. The expansion of the measured drag of model to the drag of the full scale hull is based on the r elationship elationship between the components of the resistance, Reynold’s number and Froude Froude number of model and full scale hull (vehicle body without driveline). The resistance of an amphibian vehicle is made up of following main components 3,4 The frictional resistance due to the motion of hull through a viscous fluid The eddy making resistance due to energy carried away in eddies shed from the hull, wheels and appendages. The wavemaking resistance due to energy must be supplied by the craft to the wave system created on the water surface. In the above mentioned components, frictional resistance due to the motion of hull through a viscous fluid will be very small at 10kmph vehicle speed and hence can be neglected. The resistance due to eddymaking and wavemaking are commonly known jointly as the residuary resistance. The residuary resistance coefficient CR will be same for a model and full size hull if they are operating at the same Froude number.  



Froude number for a vehicle is given by:

Where, V = Vehicle speed, g= Gravitational acceleration & l= Characteristic length length of vehicle Therefore, ------------ Eq. (1)

Where the subscripts s subscripts s and and m refer to full size and model respectively. At the same Froude number C Rs equals CRm and the ratio of V s to Vm is given by

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Dynamics of Machines and Mechanisms, Industrial Research

If Eq. 2 is submitted into Eq. 1 and ρ sis assumed equal to ρm ,then

Thus, the ratio of residuary resistance to displacement is constant between the full scale and the scaled model. The relationship among the linear scale ratio, the speeds, and resistance ratio of the full size craft and the model are given by

------------------------------------------------------------ ----------------- Eq. (4)

i.e. Resistance of Full Vehicle =

x Resistance of scaled model.

Model Manufacturing: Manufacturing:

A 1/5th scale, wooden model of the wheeled amphibian vehicle has been fabricated as per the dimensions given in Table-1 and the pictures of the scaled model model are shown shown Fig-1. All the details of wheeled vehicle are incorporated in the model except the drive line. Proper water sealing has been provided to avoid water ingress during testing. Table 1: Design Dimensions of actual vehicle and scaled Model of Hull (Scale 1:5) Dimensions Length Breadth Height Weight

Hull (Actual size) 7895 mm 2800 mm 1653 mm 22000 kg

th

1/5 Model 1579 mm 560 mm 330 mm 176 kg

th

Fig.1: 1/5 Scaled Model of vehicle Model Testing Setup:

The experiments were carried out at Current Meter Rating Trolley (CMRT) at CWPRS, Pune . The setup which is shown in Fig 2 includes a Rating Tank of 228 m long, 3.66 m wide and 2.13 m deep. It has an electrically driven rating carriage (trolley) that is equipped with precision measuring instruments. Fig. 3 shows the vehicle model positioned in towing tank. The salient features of CMRT are given below 5


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Salient features of CMRT: Speed range 0.01 m/s to 6.0 m/s i.e. 0.036 kmph to 21.6 kmph.  AC servo motors and drives with PLC for precise speed control.   Real time PC based data acquisition and processing system using specially developed software.  Accuracy of measurement of calibration parameters conforms to National / International Standards (IS 13371/ ISO 3455).

Fig. 2 Current Meter Rating Trolley (CMRT)

Fig. 3 Testing of Scaled Model

Model Testing:

The loading inside the model was adjusted such that it experiences the same trim angle like an actual vehicle. Subsequent to the verification of trim angle, towing testing was carried out at various model speed(s) i.e. 1.389, 1.667, 1.944, 2.222, 2.5 & 2.778 m/s which correspond to the vehicle speed(s) of 5, 6, 7, 8, 9 & 10 kmph respectively. A photograph, taken for model speed of 1.944 m/s (vehicle speed of 8 kmph) is shown in fig-4. The vehicle model was stable during the testing at this speed. At the test speed of 2.778 m/s (vehicle speed of 10 kmph), it was found that the water was flowing over the vehicle as shown in Fig-5. However, the vehicle was stable during the testing at this speed of 2.778 m/s.

Fig. 4 Vehicle Speed - 8 kmph

Fig. 5 Vehicle speed - 10 kmph

After completion of tests at all speeds, drag forces measured at different speeds have been converted to corresponding full scale vehicle drag values. The drag values computed are given Table-2, and a graph depicting drag values against the test vehicle speed(s) is shown in Fig-6. The linear correlation between the drag and the speed, and the increase in drag with increase in vehicle speed are clearly evident from the graph.

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Table 2: Experimental Drag Forces at (kmph) different Model speeds Sr. No.

Actual Speed

Model Speed

Model Drag

Actual Drag

Km /hr

m/s

Km /hr

m/s

kg

kg

1

5

1.39

2.24

0.621

1.52

190

2

6

1.67

2.68

0.745

2.36

285

3

7

1.94

3.13

0.870

3

365

4

8

2.24

3.58

0.994

528

528

5

9

2.50

4.03

1.118

5. 144

643

Fig. 6 Drag Force(kg) vs Vehicle Speed CFD Analysis:

CFD analysis was carried out using commercial CFD software STAR-CCM+ 6 to find out the hydrodynamic forces acting on vehicle for the given flow conditions and dynamic stability of vehicle at different vehicle speeds. CFD modeling and analysis involved surface model preparation, discretized grid generation, application of boundary condition, analysis and post processing of results. Modeling & Grid Generation:

Initially, half symmetry boundary condition was considered for drag calculati on and stability analysis in longitudinal direction to reduce the computational time. The solid modeling was carried out in software Solid Works. Then the geometry was imported to STAR-CCM+ CFD software and geometry clean-up was carried out. out. The solid model model of the bare bare hull and final HULL geometry geometry considered for CFD analysis are shown in fig 7 & 8.

Fig. 7 Solid Model of bare hull

Fig. 8 Isometric view of Hull geometry geometry

After the geometry clean-up, triangular surface grid was generated along with proper clustering near air-water interface as shown in Fig. 9. Then volume grid with trimmed cells was generated for better simulation. The grid size consists of 6, 55,000 cells. Grid in symmetry plane along with the hull is shown in Fig 10, and a cut plane taken in X -direction is shown in Fig 11. Clustering has been carried out at locations wherever needed including area nearer to airwater interface as shown in Fig-11. Fig. 12 shows a zoomed view near anti-surge vane wherein the boundary layer grid generated to capture the boundary layer separation along the hull is clearly seen.


Fig. 9 Surface Grid

Fig. 11 Grid in Cut plane

Fig. 10 Symmetry plane grid alongwith hull

Fig. 12 Zoomed view of grid near anti-surge vane

Flow Analysis:

For analysis purpose, following solver models have been used:           

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Three-dimensional, Implicit Unsteady, Turbulent, Gravity Reynolds Averaged Navier Stokes K-Epsilon Turbulence Realizable K-Epsilon with Two-Layer All y+ Wall Treatment Segregated Flow Eulerian Multiphase = Eulerian Phases = Air and Normal-Water Multiphase mixture Multiphase equation of state Volume of Fluid (VOF) VOF Waves – Waves – Flat Flat VOF Wave Dynamic Fluid Body Interaction Interaction framework ( DFBI) model used for 6-DOF Hull properties used for DFBI are given below:

Fig. 13 C.G location of hull

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Hull Mass = 9,500 kg [i.e. 93100 N] (as only half-Hull is being modeled) Moment of Inertia = 50000.0 kg-m2(about Y-Axis) (for half-Hull) Location of CG from Reference Point X(4100.0,3900.0,3700.0) (4100.0,3900.0,3700.0) mm Y150.0 mm Z-191.5mm Free-motion: Z-translation and Y-rotation Flow Simulation:

For the above configuration, flow simulations were carried out for various vehicle speeds i.e. 5, 6, 7, 8, 9 &10 kmph. From the flow simulation carried out, it is observed that all flow characteristics such as wave generated generated have been been captured well. This detail detail is clearly evident from Fig 14 which shows the air-water interface contours (Wave Height in vertical direction ) obtained for vehicle speed of 5 kmph. Fig 15 15 clearly shows the amount amount of vehicle body submerged inside water, an essential verification required for safe operation of hydrojets. Simulation was carried out by considering the flow initially initiall y unsteady and was run till ti ll it reached a steady state. This can be seen in the simulation results of drag shown Fig 16 wherein the simulation was run till it converges to a steady drag. Fig. 17 shows the air-water contours (Wave Height in vertical direction ) obtained for the vehicle speed of 10 kmph. These contours have been found to have higher degree of resemblance with the images captured during model testing including the water flow over the hull body.

Fig.14 Wave Height in vertical direction

Fig.15 Water-Air Interface view

Fig.16 Drag Force vs Time


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Fig.17 Wave Height in vertical direction

Table 3 shows vehicle speeds and corresponding drag value results of CFD analysis and Figure 18 shows a plot of drag force for different vehicle speeds. Table. 3 CFD Drag Force for different vehicle speeds Sr. No.

Actual Speed

Actual Drag

Km/hr

m/s

kg

1

5

1.389

173.30

2

6

1.667

254.85

3

7

1.944

329.26

4

8

2.222

484.2

5

9

2.500

611.62 Fig.18 Drag Force vs Vehicle Speed

Comparison between model testing & CFD:

Comparison of experimental drag values and the results obtained obtained from CFD CFD analysis is shown in Fig-19. It is observed that CFD drag results are marginally less in magnitude compared to the experimental values. The error in CFD analysis, obtained by assuming experimental testing as reference is shown in table 4. The flow patterns at 9 kmph obtained obtained from experimental testing and and CFD simulation are shown in Fig 20 and Fig 21 for comparison. It is quite evident from the figures that both the contours match closely with each e ach other. Sample Error Calculation: Experimental Drag value for 8kmph is 528 kg and the value obtained from CFD simulation is 484.2 kg. Therefore, %Error =

=

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Fig. 19 Comparison of Drag Force vs Vehicle Speed Table 4: % Error in Experimental Experimental and Analytical Values Actual Speed

Expt. Drag Analytical Analytical Drag

% Error

Sr. No. km/hr

m/s

kg

kg

1

5

1.389

190

173.3

8.789

2

6

1.667

285

254.85

10.578

3

7

1.944

367

329.26

10.283

4

8

2.222

528

484.2

8.2954

5

9

2.500

643

611.62

4.8802

Fig. 20 CFD Wave Contour

Fig. 21 Model testing Wave Pattern


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Conclusion:

Model testing and CFD analysis have been carried out for stability and drag analysis of an amphibious, combat wheeled vehicle. Model testing combined with CFD analysis proved to be an effective approach in accomplishing the amphibious design of the wheeled combat vehicle. The experiments and simulation carried out provided adequate insight about the amphibian capability of the vehicle. It has been observed from both the towing test and CFD analysis that the vehicle is able to successfully move at a speed of 9 kmph without any stability issues. The correlation obtained, in drag estimation, between the CFD analysis and towing tank tests is encouraging. Further, a very close similarity of wave patterns obtained in both the results indicates that flow features have been captured accurately. Error between CFD and experiments in drag estimation is around 10%. This error may be due to minor inaccuracies in position of loads in experimental scale model and consideration of smooth surface in CFD analysis against a rough external surface of the 1/5 th scaled model. The smoother surface in CFD simulation might have resulted in generation of lesser viscous drag. The drag results, obtained from present methodology which is primarily intended for finalisation of amphibious design in terms of vehicle shaping and position of systems in the vehicle, can also be used for proper selection sel ection or custom development of water jets for amphibian operation. References:

[1] Sebnem Helvacioglu, IsmailHakkiHelvacioglu, BurakTuncer, Improving the river crossing capability of an amphibious vehicle, J. Ocean Engineering. 38 (2011) 2201-2207. [2] Chun, H.H., Ahn, B.H. and Cha, S.M., 2003, Self-Propulsion Test and Analysis of an Amphibious Tracked Vehicle with Waterjet, in: Proceeding of World Maritime Technology Conference and SNAME Annual Meeting, Paper No. D6(D-133), USA. Reference to a book: [3]

AMCP 706-350, Engineering Design Handbook – Handbook – Wheeled Wheeled Amphibian, January-1971 Januar y-1971

Reference to a chapter in a book: [4] J S Carlton, Marine propellers & propulsion, Second Edition, Elsevier publication, 2007, pp. 285-316. [5]

Information on http://www.cwprs.gov.in

[6]

STAR-CCM+, CD-adapco India Private Ltd., Bangalore, India

Dynamics of Machines and Mechanisms, Industrial Research 10.4028/www.scientific.net/AMM.592-594

Stability and Drag Analysis of Wheeled Amphibious Vehicle Using CFD and Model Testing Techniques 10.4028/www.scientific.net/AMM.592-594.1210

Stability and Drag Analysis of Wheeled Amphibious Vehicle Using CFD

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