i want to know how to make sure that result from solver and post prossesing is correct?

i'm doing my aerodynamic simulation at subsonic speed and apply 3 different velocity,but the different in Cd value is to large where its support to be constant at speed below speed of sound..why this thing happen and how to minimize this problem?

drag always varies. No commercial package is accurate enough in predicting drag. There has been some CFD drag workshop the past few years. All those "close" results all require lots of computers to solve the problem due to high numbers of mesh counts in the entire model. If you are only using one PC, really don't expect your drag to come out even close to the experimental data.

Plus, there are so many other issues to look at. What is the type of problem you are solving? your turbulence model selection, use any wall function or not etc. just to name a few.

Having said that, lift on the other hand would be really close if you take care of the above issues. I have done some 2d airfiol analysis, my lift is around +/- 5%, but my drag is always 500% of experimental result. So don't waste your time on trying to match your Cd. Move on (Kambate ^_^ )!!

Ken, I am not sure I agree with you. I have had some success simulating Cd, Cl etc. I think a lot depends on meshing, boundary conditions and the physics solutions. There is a write up on the Ahmed on the CFX community site that Sham81 can take a look at. While it may be a challenge, it is not impossible.

Hi,

I agree with Meri. It is not impossible to get accurate drag predictions. For instance drag in laminar flow should be very accurately predicted with a good simulation, and as long as the model is not too complicated should fit on even a modest PC. Certainly drag on complicated bodies such as a car or aircraft bodies at high Reynolds numbers with seperations is a very difficult model and would need serious computing power to get a reliable drag solution.

But even at high Reynolds numbers, if there are no major seperations you should be able to get a good drag number on modest computing resources.

Glenn