CFD - General

I'll write this up while I wait for the drier to finish.

I'm sure as soon as I get into the meat of any CFD or FEA I'll get comments of criticism. Some out of genuine concern and interest (cool), others just so they sound like experts while really just being a dick (not cool).

The accuracy of such analysis always comes into question, and rightfully so. In my limited experience doing designer-level structural FEA in college, and just from watching some stuff at work, good results inevitably come down to the following:

1. Assumptions made. Could be assuming your upright material is isotropic and homogenous (more or less true on billet, not at all on a weldment!) or that flow around an airfoil is incompressible.
2. Boundary conditions / loads / constraints. Maybe it was just CU, but FSAE students seemed notoriously off in this regard, just from inexperience.
3. Refinement of mesh. Pretty self explanitory. Capturing fine details (be it a Gurney flap or a non-radiused corner for a stress riser) requires localized mesh refinement.
4. What you expect to get out of it.
   

As an example of 2 & 3, on one of our FSAE cars we had a highly-loaded structural part (wheel hub). Billet aluminum, with material properties well-defined. Design criteria was based on von Mises stress, with FOS=2. The part had a thin tube section of about 0.06" wall thickness. I believe the mesh used globally in the model was 0.10 - 0.12". Additionally when I looked at it years after, the constraints were off. As a result, the first time driving the car this part sheared as soon as the driver got on the brakes. FOS=2, not so much.

On the other hand, we designed the same part years later with a different method. Billet steel. I think it actually wound up being lighter than the aluminum one (steel rocks). Design criteria again based on von Mises stress, with FOS=1.2. May sound low, but 20% is a lot of overshoot! Very carefully looked at the constraints and conditions with a highly refined mesh. Part never failed.

Good examples of how changing a few small things has a big impact on results. For this 2-d CFD work I'm not looking for absolute numbers. I'm interested in directionality in how to increase negative lift, reduce drag, look at effects of Gurney's, etc. Hopefully I won't get in too much trouble.