CFD is extremely useful in that you can easily visualize flow, something that is a bit more difficult in the wind tunnel, and you can get numbers without having a physical model. However, it takes a long time to run (and thus take a long time to see the effects of each change) and it's not very good for absolute numbers, especially with the limited computing power I have on hand. But where CFD can be extremely useful is relative results, in a sense optimization- as long as your CFD setup doesn't change.
|RP model in the wind tunnel|
|CAD representation of the wind tunnel model|
Maybe we'll move to another package someday, but for this year, I think I'm more or less done with aero design.
One issue I'm having is model size. On the simplified full car, I can't really run more than a few million cells (and that's after refinement) without running out of RAM. I need a dedicated CFD machine... at one point this summer I had 5 computers cranking out CFD simulations 24/7 just to get through the simulations faster. One thing I will probably end up doing is a simulating a half car instead of a full car.
The way I model the car is on a moving ground plane with rotating wheels (at least a rotating reference boundary condition on the wheels). I've done my tests at a range of speeds up to 60mph and found the results to be more or less Reynolds number insensitive.
Eventually we'll have to translate these CFD results to the real world. I tried setting aero balance in CFD, but we'll have to see how well that translates to real world. Luckily, that's something that's relatively easily measured through suspension travel, so I'm not too worried about it. I have a little more faith in the downforce numbers from CFD since the body itself isn't going to be a huge generator of lift/downforce - the wings and undertray will dominate the total downforce, and those aren't too geometrically complicated for CFD to give us reasonable results.