[Transonic]

Hi, how does Ansys Fluent calculate Cd and Cl?

If my Cd and Cl showed some errors compared to experimental, could it be down to mesh density?

Thanks in advance

[LuckyTran]

Firstly it calculates the drag and lift forces along the vectors specified. Then it normalizes the forces by the reference area (user specified) and dynamic head (user specified via reference velocity and density). Best way to check if you are actually wrong is to compare forces.


Nonetheless, your CFD could be wrong because it is wrong for any number of reasons. So yes, it could be only the mesh, or it could be everything.

[Transonic]

Hi LuckyTran, thanks for the explanation there!

Could shockwaves affect the Cd and Cl calculation please too? I don’t think they could then from how they are calculated?

I don’t really get how the mesh could effect the Cd and Cl then please either if it uses the user specified numbers?

[LuckyTran]

Of course shockwaves have an influence.

Mesh affects drag coefficient because it affects the drag force that you get.

You seem to be under the impression that CFD and CFD software are omnipotent, they are not. I recommend some light reading. The drag force is what Fluent gives you by solving the navier-stokes. All the other things references provided by you. So check your inputs for those reference quantities. Better yet, do the original recommendation and calculate the expected drag force so you can actually better understand how to calculate these things yourself.

[Transonic]

Hi LuckyTran thank you for the advice! Can you recommend a book or website for this please?

I was looking over Ansys user guide, but it explains things either in a too complicated manner with lots of formulae or nor enough explanation is provided