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Old 05-03-2020, 12:58 PM   #2 (permalink)
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This is great! I already have a Magnahelic that I've used to test pressure above and below the hood, but I think this summer I'll play around with it some more.

But tuft testing is still one of the best aero testing techniques, ever.
I would be careful with tuft testing, however--it doesn't always tell you what the lowest-drag solution is. I was reading some papers on Hucho's website and came across this one on fastback shapes:

"This pseudo-tail [used on the Tatra 87, VW Beetle, Volvo PV544, etc.] wasn't that fast [i.e. low drag]! On the contrary, it was characterized by a particularly high air resistance. And that met with incredulous amazement. Because if you looked at the flow around these cars, be it with woolen threads on the street or with the smoke probe in the wind tunnel, it looked as if the flow remained close to the back. How should there be a high resistance? This secret could only be revealed much later, during the development of the VW Golf I." (emphasis and notes added)

As he goes on to explain, the flow remained attached because a pair of inward-rotating vortices "pulled" the flow down to the surface but at the same time, the low pressure of the vortices induced lift and overall drag was increased--despite the tufts showing attached flow.

Tuft testing might be the best technique in conjunction with something like pressure testing, then, just to make sure one is getting a complete picture.

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