Quote:
Originally Posted by freebeard
This is the basic question I would like answered. Vekke was no help. Dimples work, why not raked edges (cough! Cybertruck cough!)
|
This is an answerable question. It depends on the Reynolds number regime we are in and the size of the boundary layer. At 65 mph, the boundary layer near the front of the vehicle is around 1/4" and near the back is 1"+ (depending on location and how good the aero is). The general rule of thumb is that for subsonic, subcritical flow any imperfection
less than half the boundary layer isn't significant, and that flow will reattach right over it. That's why panel gaps of 1/8" are fine for passenger cars, but high speed racers actually care about them.
As long as the angle of curvature of the geodesic is under the constraints above, flow will have no problems remaining attached. At slightly greater angles it might reattach later on the surface, the same way flow reattaches to the side of a car after being disrupted by the front wheels / mirrors. At larger angles, there is too much shadowing and the flow doesn't to reattach. At that point, being a geodesic / sharp shape has nothing to do with things; the airflow is being deflected too much.
Anyhoo, there's already been CFD done on the Cybertruck. It's decent. Crap compared to fully aero vehicles, but great comparted to regular trucks. Because the general "silhouette" is vaguely aerofoil-like, the angularness doesn't count against it too much.