Quote:
Originally Posted by COcyclist
So is it safe to say that we cannot always rely on the tufts, at least as far as the back of the vehicle is concerned? I recall a thread on here about the aerodynamics of the New Beetle and people wondering about why the drag numbers were so bad, even though it had attached flow down to the bumper?? That is why we have the template, right? Exceed its angles at your own peril.
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The angle thing goes back to boundary layer theory and has to do with too rapid of a pressure rise in the presence of an unfavorable pressure gradient.
Beyond the max camber point of the roof or sides of the car,the air immediately adjacent to the skin of the car is at zero velocity because of viscosity effects of the air.
If the profile of the car is too radical,the boundary layer won't be able to transport kinetic energy in,the flow will be demanded to slow,but it's already at a standstill and it will just lift off the car with air flowing backwards into the void,beginning the eddies which will become full-blown turbulence downstream.
*The streamline body which the 'Template' is based upon has Cd 0.04
*When the body is brought into 'ground reflexion'(Rumpler/Prandtl) it jumps to Cd 0.08.
*When you cut away for ground clearance,you get a car body of Cd 0.08.
*When you add wheels and tires the drag jumps to Cd 0.12-13.
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*The 2.5:1 streamline body of the 'Template' has zero separation
* As a car body,the Template' will have no separation.
*If you make the aft-body steeper,it violates W.A.Mair's research on aft-bodies,which found 22-degrees to be the max angle air will follow without separation and vorticity.( Hucho allows 23-degrees ) (I'm conservative @22-degrees).
gotta go! hope I didn't muddy the waters.