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Old 05-03-2020, 05:51 PM   #4 (permalink)
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Originally Posted by Vman455 View Post
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.

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.
None of that casts a negative light on tuft testing - you just need to know enough to make a judgement on what you are seeing. For those who have my Veloce aero book, attached flow on the rear part of the car being associated with higher drag and lift is shown on page 65 (new Beetle), page 134 (rear roof extension angle on the Roomster) and page 195 (Porsche with ducktail).
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