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Originally Posted by Nerys
how steep an angle can you go before losing laminar flow?
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Note the difference between attached flow and laminar flow. Very few automobiles (with the exception of solar cars, and maybe the Aptera) would have laminar flow over a significant portion of the body. For a discussion of the different flow types, see this thread:
A Crash Course in Aerodynamics and an Introduction to Turbulence
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I've seen figures ranging from 10-15 degrees as the highest initial change you can do while retaining attached flow, and up to and beyond 20 if the transition is gradual (think curve, not sharp angle).
Quote:
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it should be JUST on the verge of going unstable to be at optimum.
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Yup, exactly.
FYI, the top of my boat tail curves gradually from the roof to ~15.x degrees from horizontal. For the sides, I aimed for a transition of ~10 degrees, also taking advantage of the OEM curvature at the rear corners. (But since I accidentally made it off kilter, one side is actually less, the other more.)
I aimed for a smaller angle on the sides because I figure the boundary layer there (compared to the top of the car) is already more turbulent - from windows, mirror, front wheels, panel gaps, trim - and less likely to stay attached on a more aggressive taper. Only a guess though. I'm just making this up as I go along
It would take a fair amount of effort to keep refining the boat tail to find the "verge of going unstable" for a particular car. I'm going to be satisfied with my boat tail if the air likes it the way it is with these angles. I could keep refining until the cows come home, or make a best guess, and be happy with it if it appears to work (which it does so far).