Post on 22-Jan-2018
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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.
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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
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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.
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The Base Case was set up and the view of the Mesh generated is shown below:
Fig 2
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Another view of the generated mesh in the front:
Fig 3
The mesh generated includes prism layers as well in the round sections.
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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.
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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.
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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.
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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.
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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.
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The Velocity Scene is shown below:
Fig 9
The scene shows a high amount of turbulence at the rear end of the car.
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The underbody has almost zero velocity as it is seen from the figure below.
Fig 10
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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.
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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
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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
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Fig 15
The front diffuser and spoiler is shown above.
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The mesh that was generated is shown below:
Fig 16
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A closer look at the modified Mesh from the front:
Fig 17
The mesh seems to be finer than the previous case.
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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.
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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.
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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.
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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.
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The Cd and Cl are shown below:
Fig 22
Fig 23
Both the plots show zero counts from the beginning to the end.
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The Pressure Scene is shown below:
Fig 24
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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.
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The velocity scene is shown below:
Fig 26
The velocity is the highest right at the engine hood and on top of the body.
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Fig 27
The velocity is almost zero at the rear end of the car.
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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.
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The Mesh scene corresponding to the number of cells is shown below:
Fig 30
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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.
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The Body Drag plot is shown below:
Fig 32
There is no change in the body drag from the previous case.
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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.
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The Residuals plot is shown below:
Fig 34
There is also no change in the flow of the residuals.
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The Cd and Cl plots are shown below:
Fig 35
Fig 36
There is no change in either plots from the previous case.
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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.
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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.
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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.
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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.