1

CFD Approach to Evaluate External

Aerodynamic Evaluation on a Race Car

This project is done to understand the Aerodynamics at play on a

Race Car. A base case was set up and run. The number of cells in

the volume mesh for the base case was 4.5 million. Then

modifications were done in the design to achieve a higher cell

number and to see the external effect when the same physics

models and conditions were used.

2

Objectives:

>Body Drag and Downforce.

>Subtraction Operation.

>External Flow Analysis.

>Surface Wrapper and Meshing Operation.

>Prism Layer Quality.

>Mass Flow

>Drag and Lift Coefficients

Specifications were given to us to design a Race Car. The required specifications were taken into

account while designing. In the first design, I used a front diffuser even though the underbody was flat.

Then I changed the design completely but drew as per the specifications. This time I used a front

diffuser as well as a spoiler but did not use a wing. The third design was the same as the previous one

but a rear diffuser was placed in it.

The Physics set up models that were used are:

All y+ Wall Treatment

Cell Quality Remediation

Constant Density

Air was used as the Gas

Gradients (Hybrid Gauss LSQ and Venkatakrishnan)

K-Omega Turbulence

RANS

Segregated Flow

Steady State

Three Dimensional

Turbulent Flow

3

The first Race Car model is shown below:

Fig 1

It was drawn a little slanted to check if the front diffuser would show higher amounts of air pressure

and if it would produce greater lift. A spoiler is also added at the rear.

4

The Base Case was set up and the view of the Mesh generated is shown below:

Fig 2

5

Another view of the generated mesh in the front:

Fig 3

The mesh generated includes prism layers as well in the round sections.

6

The Body Downforce Plot is shown below:

Fig 4

The Body Downforce rises after negative values to a little more than 600 counts and after a little fall

below 600 counts remain constant although there is regular rise and fall within the same parameters.

The Downforce may be high because of the slant design of the car.

7

The Body Drag Plot is shown below:

Fig 5

The Body Drag rises sharply from around 100 counts and reaches more than 600 counts and then falls

below 400 and remains almost constant.

The drag is very high because the downforce is very high. This kind of high drag does not give high fuel

efficiency.

8

The Mass Flow Monitor is shown below:

Fig 6

The Mass Flow starts at a negative value and falls further. This maybe because of less flow of air in the

underbody coupled with high drag and downforce.

9

The Residuals Plot is shown below:

Fig 7

The residuals do not go to the negative. The Turbulent Kinetic Energy is also low although it rises to a

certain level, falls and then remains constant.

10

The Body Lift is shown below:

Fig 8

As I had stated earlier that the drag and downforce is very high, so the lift is very low. It starts at

negative count, rises up and then falls sharply to less than negative 400 counts.

11

The Velocity Scene is shown below:

Fig 9

The scene shows a high amount of turbulence at the rear end of the car.

12

The underbody has almost zero velocity as it is seen from the figure below.

Fig 10

13

The scalar scene is shown below:

Fig 11

As I had suspected earlier, the pressure at the front of the diffuser is very high. The pressure is also

negative at the windshield.

14

Upon closer look at front diffuser, it shows that the pressure is the highest and right below the diffuser

where the underbody starts the pressure is negative as shown in the figure below:

Fig 12

15

The Base case has been shown above. Then changes were made to the overall design.

The Volume Mesh Properties for the modified case are given below:

Fig 13

The new design drawn as per specification is shown below. A front diffuser and a spoiler were added to the

design:

Fig 14

16

Fig 15

The front diffuser and spoiler is shown above.

17

The mesh that was generated is shown below:

Fig 16

18

A closer look at the modified Mesh from the front:

Fig 17

The mesh seems to be finer than the previous case.

19

The Body Downforce Plot is shown below:

Fig 18

The downforce reached a negative count and then after a certain period of time reached zero and

stayed constant. Compared to the base case the downforce is negligible.

20

The Body Drag Plot is shown below:

Fig 19

The Body Drag is also lower than the previous case. At a certain stage it reaches a negative value too. Without

down force and too much drag, the car would not be able to reach top speed and the fuel economy would

be very low.

21

The Mass Flow monitor is shown below:

Fig 20

The Mass Flow starts at zero and stays as it is till the end. This maybe because of high drag and low

downforce.

22

The Residuals Plot is given below:

Fig 21

The residuals are lower than the base case. But like the previous case it does not reach negative value.

23

The Cd and Cl are shown below:

Fig 22

Fig 23

Both the plots show zero counts from the beginning to the end.

24

The Pressure Scene is shown below:

Fig 24

25

Fig 25

The front spoiler is supposed to take in the high pressure air and distribute it. From the figure we can

see that the pressure is the highest at the front. But on closer look at the underbody we could see that

the pressure is negative.

26

The velocity scene is shown below:

Fig 26

The velocity is the highest right at the engine hood and on top of the body.

27

Fig 27

The velocity is almost zero at the rear end of the car.

28

The third case is performed with a rear diffuser. The geometry scene is shown below:

Fig 28

The volume mesh properties are shown below:

Fig 29

There is not much change in the number of cells. Because of the rear diffuser the number of cells has

decreased.

29

The Mesh scene corresponding to the number of cells is shown below:

Fig 30

30

The Body Downforce plot is shown below:

Fig 31

There is not much change in the downforce expect that it does not reach zero and stays the same till

the end. But it reached negative values in the beginning.

31

The Body Drag plot is shown below:

Fig 32

There is no change in the body drag from the previous case.

32

The Mass Flow plot is shown below:

Fig 33

The mass flow is also the same as the previous case. It stays at zero throughout.

33

The Residuals plot is shown below:

Fig 34

There is also no change in the flow of the residuals.

34

The Cd and Cl plots are shown below:

Fig 35

Fig 36

There is no change in either plots from the previous case.

35

The Pressure scene is shown below:

Fig 37

The pressure scene shows a little variable pressure at the rear end where the diffuser is placed.

36

Fig 38

On a closer look we can see that pressure is negative just before the diffuser and then it rises but does

not reach a high count at the rear end of the car.

37

The velocity scene is given below:

Fig 39

There is not much change in the scene than the previous one except that the velocity at the engine

hood is lower than the previous one.

38

Fig 40

But if we look closely at the rear end, the pattern of flow has changed from the previous case

Conclusion: From the study of the race car above, a conclusion can be made that design changes are

very necessary in the car. This is because the downforce is low but the drag is very high. No race car

wants high drag. If the down force is more, then the handling and the cornering ability of the vehicle

improves. Some other attachments could also be added to the car like a rear spoiler or wing to

improve the performance. In certain cases, rear spoiler is much better than wing. This is because it

increases the downforce with a small penalty of drag.