Aero Experts - Radius for roofline/Kamm back curve?

[trebuchet03]

Here is what I would do..... Get some tape, and some red string (for contrast). Put a whole bunch of tufts just before the kamm, on the kamm, and on the rear windscreen. Then, drive at a variety of speeds and have someone take photos from another car.

Then, do the same thing without the modification. If the strings on your rear windscreen suddenly point up, you've found a less desirable trait of your design.
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Now, from research for the HPV - a teammate found a whole lot of information on diffusers and optimal angles. We found 8 degrees off a flat plane to be the best for minimizing wake.

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Unless you do a CFD model, the best way is by guess and check (considering the low cost of a prototype). Try several iterations and see what shows the least amount of disturbance.

All of that said - I've always wanted to try taking big PVC, quartering it and seeing the results. I recall one boat tail design (for trailers) that had rounded trailing edges.