Quote:
Originally Posted by aerohead
At:
forums.pelicanparts.com/porsche-911-technical-forum/740377-anyone-have-911-cfd-analysis.html
there are some additional wind tunnel tuft study images of the 911 Carrera, shot from a higher vantage point.
There is clearly separation behind the backwards facing step at the top of the recessed backlight, where vents are positioned. An inverted pyramidal area of disturbed tufts extend halfway down the glass, illustrating the reversed flow. This area is sloped at 26-degrees to the horizon, three degrees steeper than what can support attached flow.
The angling of the C-Pillar tufts also suggest high pressure body side flow directed towards the low pressure, fast-moving flow above the roof, rolling up into the tornadic vortices as these two regions' attempt at pressure equalization fails at the intersection.
Further down the rear surface, the tuft orientation is 'corrected', presumably,now caught in the vortex-induced fastback downwash, strongest of all body forms.
According to Hucho, page 281,in his Second Edition, by simply lofting the 911's rear contour ,up to the thickness of the flow separation,you'd kill both the lift and drag, and better than with the spoiler.(page 175). This WOULD, however, alter the 'look' of the car, and potential for a ' risk to sales.' ( page 188).
It would take until after 1994,and the end of the air-cooled 911s, before Porsche would begin this lofting of the rear body contour. Parked side-by-side, the aft-bodies of the early and current 911s would show little resemblance. A good thing!
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Well, your reference starts off with a picture that I commissioned from an artist (Dave Heinrich), drawn on the basis of wool tuft testing that I carried out on the road in 2001 - see
https://autospeed.com.au/cms/a_1065/article. So thanks for the compliment. (However, it is of a different 911 model, so it doesn't match the rest of the pics in your cited thread.)
So let's look at
Anyone have 911 CFD Analysis - Pelican Parts Forums, the thread you cited.
The pics show, without the spoiler, a separation bubble on the upper part of the slightly recessed rear glass, with flow attachment then re-occurring.
With the spoiler fitted, the separation is much more widespread on the rear glass, and flow attachment does not re-occur.
We don't need to read all your theories to see exactly what is happening: the spoiler creates flow separation that reduces lift, exactly as would occur over any lifting shape.
That's the reality, that is obvious to any observer looking at the story of the tufts - and I encourage people to go look for themselves at these pics.
(And, as you often do, you're misquoting Hucho. Neither page reference supports what you claim of it.)