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Analysis of 2D Car Drag coefficient and Lift coefficient

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Old   August 9, 2018, 00:15
Default Analysis of 2D Car Drag coefficient and Lift coefficient
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Chao
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I'm a beginner in the field. Now I am confused about some settings.
I'm doing a analysis job to compare the difference between a car with rear wing and the other one without rear wing.

First, I don't sure which car (the one with rear wing or the one without rear wing)should have a bigger Cd value, somebody says that cars with rear wings will increase drag as well as downforce, is that right ?

Second, I always get Cl values about -2.xxx, but the values seem to be too large. As I google some similar papers to check the results, their Cl values are about -0.2xxx.

So, plz help me to check if there are any mistakes settings that need to be corrected.Thanks!

There are my settings below: (I change these settings only, the others are default)

-I use 1:1 2D cars, created by SpaceClaim. I set up the cars with 4875mm 1780mm 1435mm, So should I change the unit in Ansys Workbench into mm ?

-viscous model: k-e

-inlet 30 m/s (velocity inlet)

-reference values:
Area : 2.09 m^2 (frontal area of the car)
Depth:1.58 m (car width)
length:1.78 m (car length)
velocity:30 m/s (should I give the velocity again here? I have given velocity inlet 30 m/s already)

after these I run 500 iterations
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Old   August 10, 2018, 13:10
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Check that your model is 4875mm and not 4875m.


The reference values are needed to calculate the coefficients. Yes you need to put a reference velocity. The inlet velocity is the inlet velocity as a boundary condition. You still need a reference velocity, which can be different than the inlet velocity (more obvious if you have more than 1 inlet).


Check that the reference values you supplied are consistent with the definition of drag coefficient. In 2D you have another problem to worry about. Make sure the reference area and reference depth are consistent. In 3D, the reference area is the frontal area, but in 2D there is no more a frontal area only the (height of the car front). Using the actual frontal area of the the 3D car in 2D might be incorrect because Fluent takes the pressure load on the "2D front of the car" and multiplies it by the reference depth to get the force and then normalizes it by the reference area. A lot of people set the reference depth to 1m so that all their results are per meter basis (i.e. force / meter depth) rather than using the width of the car.
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Old   August 11, 2018, 03:30
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thanks for reply~ I'll try that again)
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