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Originally Posted by JulianEdgar
Yes, you're right - my mistake. Otherwise, my point remains - flow wrapping around those upper curves create lift.
That diagram is for cars (Type 31 Volkswagen fastback, squareback and notchback) that are 59 years old!
Flow patterns on modern car shapes (eg notchbacks) are quite different. This is a primary cause for your confusion - car shapes (and their flow patterns) have changed, but you act as if they have not.
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So let's consider 'flow-wrapping'.
1) lift is a function of pressure distribution.
2) pressure distribution is a function of velocity
3) velocity is a function of streamlines
4) streamlines are a function of body shape
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* the lowest pressure on the upper body of a contemporary automobile is at the suction peak, just ahead of the windshield header. The streamlines are closest together, the air is at maximum velocity, and pressure at it's minimum.
* if the body cross-section contracts rearwards of the roof apex, then by default, as the flow travels further aft, it gets slower, and its static pressure increases.
* the further the flow gets away from the suction peak, the slower, and higher pressure it becomes.
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A) if the flow can remain attached all the way to the rear, it will be at it's highest pressure, compared to the forward stagnation point, dependent upon aft-body length ( 'Verjungungsverhaltnis' as per 'Kamm's' research )
B) adding a spoiler would only force the flow into a faster, lower pressure regime. A spoiler's deflection, or wing's negative lift 'will ' develop downforce, but at a drag penalty.
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* 'Downwash' does not qualify technically as 'attached' flow. Downwash is an artifact of separation-induced attached longitudinal vortices, which generate the highest drag. The small wake, which is also an artifact of flow separation is not the same as a small wake generated from a progressively diminishing body cross-section which produces only a moderate pressure increase, incapable of triggering separation of the boundary layer.
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* modern notchbacks demonstrate architectures that are all over the map. There are Hyundai Accents, Kia Rios, Nissan Sentras, and the like which don't improve on some notchbacks of 1986.
* Hucho addressed in 1986 all the caveats of windshield- to- roof transition, roof camber, aft-body length percentage, onset flow off the roof, boot height, and especially C-Pillar shape, which would govern notchback performance.
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1) I could not find a single instance in Hucho's 2nd-Edition where a spoiler wasn't in separated flow.
2) In his section on high-performance sports cars he recommended zero lift, or if you were going to introduce negative lift, then provide rear bias.
3) Schenkel's rear spoiler introduced front lift where none had existed.
4) beyond 17.8 mm height, Schenkel's spoiler increased drag, at the expense of downforce.
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* Kamm's research predicted Cd 0.303 for the 1964 Porsche 911 with a 'template' roofline, vs Cd 0.40.
* Kamm's research predicted Cd 0.29 for the VW New Beetle with a 'template' roofline, vs Cd 0.39.
* a Volkswagen wind tunnel photograph of the VW Beetle, which appears in the book, ' the new VW Beetle', by Jonathan Wood, page 28, shows smoke-flow imaging, with the boundary layer coming off the car on a 19-degree angle, exactly as predicted by the 'fast template.'
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Longitudinal centerline pressures on the upper body of a vehicle are meaningless without the underbody pressure profile, especially if there's an active diffuser.
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Any information derived from tuft testing is extremely problematic. They clearly misrepresent the flow on the Beetle, New Beetle, and 911.