contribution of base-drag to total drag budget

[Michael]

I just came across an interesting technical paper on automotive aerodynamics; it’s in the journal, “Experiments in Fluidsâ€, Vol. 37, 2004, pp. 763-768. its a good review of car aerodynamics in general.

 

Evidently, the large majority (quoted as 75% in some cases) of automobile drag is due to flow separation off the aft end – the hatch, the base, the rear bumper, and underneath. This means that the front end – the hood, the grill, the windshield – contribute relatively little.

 

For cars with a shallow hatch slope angle, such as the Z, a major contribution to drag (and to unwanted lift) is a pair of counter-rotating vortices emanating off the roof edges, where they pass into the rear hatch. These are like the wingtip vortices of airplanes.

 

The paper talks about measurements in a large water tunnel, at Reynolds numbers not too far from real life. This is reminiscent of my experiments with a wooden Z model in my water tunnel, but at much lower Reynolds numbers, where laminar separations dominate and the wake is too thick (so the flow separation is worse than in real life).

[A. G. Olphart]

I found a net site which hosts this article, but they want money.

 

Have you tried to lessen the twin vortex problem with small vertical fins, or maybe cute curved things like on airplane wing tips?

 

Any idea how far off the tail end of a Z with a BRE spoiler is from acting like a Kamm (SP?) back?