stability and drag analysis of wheeled amphibious vehicle using cfd

Stability and Drag Analysis of Wheeled 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

1,3,4Vehicle Research and Development Establishment, Defence Research and Development Organisation

Ahmednagar, Maharashtra, 414006

2M.Tech Student at Sardar Vallabhbhai NIT, Surat, Gujarat, India

*[email protected], [email protected], [email protected], [email protected]

Keywords : Amphibious design, Model testing, Combat 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 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 1/5th

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.

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.

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

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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 Weight of 22t and carry out towing

tests at different vehicle speeds. To gain detailed insight, CFD analysis has also 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

behavior1,2

. The expansion of the measured drag of model to the drag of the full scale hull is based

on the relationship between the components of the resistance, Reynold’s number and Froude

number of model and full scale hull (vehicle body without driveline).

The resistance of an amphibian vehicle is made up of following main components3,4

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 of

vehicle

Therefore,

------------ Eq. (1)

Where the subscripts s and m refer to full size and model respectively. At the same Froude

number CRs equals CRm and the ratio of Vs to Vm is given by

Applied Mechanics and Materials Vols. 592-594 1211

If Eq. 2 is submitted into Eq. 1 and ρsis 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:

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 are 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 Hull (Actual size) 1/5th

Model

Length 7895 mm 1579 mm

Breadth 2800 mm 560 mm

Height 1653 mm 330 mm

Weight 22000 kg 176 kg

Fig.1: 1/5th

Scaled Model of vehicle

Model Testing Setup:

The experiments were carried out at Current Meter Rating Trolley (CMRT) at CWPRS,

Pune5. 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

1212 Dynamics of Machines and Mechanisms, Industrial Research

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.


Table 2: Experimental Drag Forces at

(kmph) different Model speeds

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 calculation 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. The solid model of the bare hull and final HULL 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

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 air-

water 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.

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. 9 Surface Grid Fig. 10 Symmetry plane grid alongwith hull

Fig. 11 Grid in Cut plane Fig. 12 Zoomed view of grid near anti-surge vane

Flow Analysis:

For analysis purpose, following solver models have been used:

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 – Flat VOF Wave

Dynamic Fluid Body Interaction framework (DFBI) model used for 6-DOF

Hull properties used for DFBI are given below:

Fig. 13 C.G location of hull


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) mm

Y - 150.0 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 have been captured well. This 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 clearly shows the 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 unsteady and was run till 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


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

Fig.18 Drag Force vs Vehicle Speed

Comparison between model testing & CFD:

Comparison of experimental drag values and the results obtained from 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 from experimental testing 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 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 =

=

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. 19 Comparison of Drag Force vs Vehicle Speed

Table 4: % Error in Experimental and Analytical Values

Sr. No. Actual Speed Expt. Drag Analytical Drag % Error

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


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 1/5th

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 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 – Wheeled Amphibian, January-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

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stability and drag analysis of wheeled amphibious vehicle using cfd

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