SimScale CAE Forum

Drag Coefficient off


#1

Hello,

I am trying to reduce the drag coefficient of a CAD model by adding holes in the wheelhouse.
The CAD model is inspired from the Audi A7, there are three model, the original one with no modifications, a first modified model with holes and a second model with holes located at different positions. The projects are named respectively Original CAD Assembly, CAD Assembly 1 and CAD Assembly 2.

The drag coefficient of an actual Audi A7 is 0.27 according to the manufacturer. With the two modified CAD model I manage to get 0.24-0.25 which is not too far from the actual model.
The problem is that my original model, which is supposed to be my source of comparison has a drag coefficient of 0.19, which is not too off from the real one but compared to the other two CADs it is a bit far off.
The three models have the same mesh and same settings (apart from the last runs since I tried to find the source of the offset)

However, the models do not have exactly the same dimensions even after scaling, but they are still quite close to each other.
I was wondering where this offset could come from.

Thank you,

Vynith

https://www.simscale.com/workbench/?pid=6431837002836102068&mi=spec%3Af280f189-e56a-4604-b7e1-bf9f0d0146ee%2Cservice%3ASIMULATION%2Cstrategy%3A3

https://www.simscale.com/workbench/?pid=1939620470572827615&mi=spec%3A7ed8dec5-423c-47b2-aa48-0ae796042f95%2Cservice%3ASIMULATION%2Cstrategy%3A15

https://www.simscale.com/workbench/?pid=6916631508946711761&mi=spec%3A5c061c0a-0ba8-4333-add8-78ddd8cf39aa%2Cservice%3ASIMULATION%2Cstrategy%3A3


#2

Hi @vwirth!

I saw that the scaling is still not correct though, you have illegal cells in your mesh and the geometry looks kind of odd to me (wavy structure). @CFD-SQUAD - can you comment on the setup of our user?

Thanks!

Jousef


#3

Hi @vwirth ,

Your calculation of Cd 0.19 has not converged, so the result is not correct.
https://www.simscale.com/workbench/?pid=6431837002836102068&rru=f0b9b878-bad0-437f-8f4e-92753fbd809c&ci=3eec8080-038b-40e0-99af-7b39c705b917&ct=CONVERGENCE_PLOT&mt=SIMULATION_RESULT

In addition, your setting of “Lift direction” should be [ 0, 0, 1 ], not [ 0, 1, 0 ] .

Best,
Yosuke