1 INTRODUCTION

Before manufacturing any product basically its model should be tested under the similar conditions and with some assumptions. Modelling and testing is the basic step towards manufacturing any product and analysis of the results will gives idea about the product. Now a day’s numerical simulation is widely used for predication of performance of the product which is to be modelled. Ahmed body is generally a computational model used to replicate a passenger car on computer simulation.

IMPORTANCE OF AHMED BODY

Aerodynamics of car is an important criterion which is to be analysed before manufacturing a car body. Ahmed body is oldest and frequently used model for passenger car which is used for testing numerically as well as experimentally. Optimization of external parameters like forces, pressure is very important to have a good product while Ahmed body have all basic property to replicate as an aerodynamics car

2 GEOMETRY & MESHING

2.1 Geometry

The dimensions of model used for simulation on ANSYS Fluent is shown in figure 1. The model (Ahmed body) is enclosed in large domain as shown in figure 1 to have a good effect of simulation. Due to having a symmetric model along the horizontal axis only half part of the model (Ahmed body) is considered for the simulation and a symmetry boundary conditions is used in the setup part for simulation.

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2.2 MESHING

The Ahmed body is enclosed by the two enclosure of different dimensions. The first enclosure near by the Ahmed body is created to have a refinement zone (i.e. region of refinement) for meshing while outer enclosure act as a whole domain for simulation. The multizone option in automatic messing is used to have a uniform and regular meshing for outer enclosure as shown in the figure 2. While for the inner enclosure (nearer to the Ahmed body) body sizing with an element size of 50mm is used for meshing. Also an inflation layer is also created near the Ahmed body to capture the results with a good accuracy.

Total number of nodes and element for the whole domain is 33202 and 103910 respectively for mentioned messing details

3 SETUP & PHYSICS

This is a steady state simulation with a density based solver is used. Density based solver is used because of the reason that compressibility effect cant ignored at or below a Mach number equal to 0.3. The flow will shows a turbulence effect so turbulence model like k-ε method is used with energy equation is turned on. The simulation has been done with a standard fluid air having inlet velocity 25m/s. Symmetry boundary conditions is used to ensure the results for a complete Ahmed body as half of the Ahmed body is used for the simulation.

4 RESULTS & DISCUSSIONS

The Drag and lift coefficient is calculated for the different cases by changing the number of nodes and elements. Velocity and pressure contours has been created to have an observation about flow pattern as well as pressure distribution across the wall of the Ahmed body.

4.1 RESIDUAL

Following plot shows the residual for the 33202 number of nodes

4.2 VELOCITY CONTOUR

Following plot shows the velocity contour over the body

4.3 PRESSURE CONTOUR

Following plot shows the pressure contour around the body

4.4 PRESSURE DISTRIBUTION -REAR END

Following plot shows the pressure at rear end. Almost there is negative pressure everywhere. Only red color region has positive pressure.

It is clearly observed from the above contour that the highest pressure is acting at the rear end of the Ahmed body and also it has some area with a negative pressure.

4.5 VELOCITY VECTOR

We can see the wake region clearly in front of body shadded by blue colour.

4.6 DRAG & LIFT COEFFICIENT

4.6.1 DRAG COEFFICIENT

The value of drag coefficient was found to be nearly 0.36. It was fluctuating around this value for all type of mesh

4.6.2 LIFT COEFFICIENT

The value of lift coefficient was found to be nearly 0.25. It was fluctuating around this value for all type of mesh

For a number of nodes 33202 and number of elements 103910 the drag coefficient and lift coefficient is observed as 0.36 and 0.24 respectively

5 CONCLUSION

5.1 GRID INDEPENDENCE TEST

We have conducted grid independence test for ahmed body. Following table shows the value of lift & drag for different mesh size. The value of lift & drag was almost constant in all 4 cases simulated for grid independent test.

Cases	Number of nodes	Number of elements	Drag coefficient	Lift coefficient
Case 1	33202	103910	0.36	0.24
Case 2	34403	105045	0.35	0.25
Case 3	37112	111567	0.37	0.27
Case 4	48974	172203	0.35	0.28

5.2 SEPARATION POINT

Separation point plays an important role for flow over any body. We can find out this point with the help of pressure gradient. Negative pressure gradient will cause a faborable pressure gradient & there will be smooth flow over body.

If pressure gradient becomes positive it will give rise to unfavorable pressure gradient . This is starting of adverse pressure gradient & effect can be felt more dominantely in near wall region due to lesser momentum as compared to free stream velocity region.Shear stress becomes zero due to continious retardation. After zero shear stress point, there is negative shear stress. This causes the flow to reverse its direction & devlopment of recirculating flow region occurs.

In above figure there was a negative gradient up to point S. At S, pressure gradient becomes zero & this is the point where flow start seprating. As fluid moves forward pressure gradient becomes positive & flow get seprated. Now flow will not remain in contact with blue color boundary. It will create a new boundary shown by red color.

5.3 WAKE-LOW PRESSURE REGION

The maximum value of pressure has been found at stagnation point & gradually decreases in one half of the body. As disscused in section 5.3, flow will remain attached in this region.At other hand, pressure start increasing at other side & flow sepration occurs. It creates a high turbulent region around the body & known as wake region. The pressure inside the wake region remains low as the flow separates and a net pressure force (pressure drag) is produced.