Quote:
Originally Posted by kach22i
The underbelly angle must be kept very shallow to retain attachment (2 or 3 degrees if I recall correctly), otherwise turbulence results.
The diffuser fences/strakes allows a roughly doubling of inclination angle. The 10 degree angle I drew was considered undo-able.
Going off memory of sharing a coffee with a professional aerodynamicist about five years ago that had work experience with several automakers.
The fences work because air attaches to the vertical surface much the same way as the near horizontal surfaces, I suspect as parasitic drag hence people's words of caution about assuming a rear diffuser will always lower drag.
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These are of course generalisations and not
necessarily applicable to every car type/groud clearance.
"im going to keep it fairly flat and a mild angle like the eqxx also. pretty much just extending the existing underbody shape out another foot or so"
That is rarely a bad thing.
Generally, high diffuser angles (greater than 18 degrees) cause seperation and vortices and increase drag. Low diffuser angles (less than 12 degrees) allow attached flow and can reduce drag.
What that exact optimum (for low drag) angle is on your specific car? Who knows, likely somewhere between 0-12 degrees.
If you are talking about just extending out the diffuser a long way, past the end of the car then there is a paper on this. In their tests, on their car, they saw around 9 degrees was the optimum angle.
Paper name: Drag and lift reduction of a 3D bluff body using flaps