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Old 05-12-2020, 08:50 PM   #14 (permalink)
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Originally Posted by aerohead View Post
* I believe that the lift issues that you believe to be the major reason for rear lift would actually be attributed to separated flow over a horizontal portion of the aft-body, ahead of the transom. Squarebacks are incapable of generating rear lift. Proper fastbacks don't typically generate dramatic lift. Notchbacks are historically the worst offenders, and responsible for the commercial development of the rear spoiler, by Kamai, in 1982,beginning with a product for the BMW 2002.That spoiler merely extended the tail surface up into the inviscid flow, to provide re-attachment on the boot, while sequestering the low pressure of the greenhouse turbulence away from the 'base' of the car's transom, which would otherwise contaminate the entire wake, lower the base pressure, and increase pressure drag.
*The Cayenne and I-Pace both will create this separation with, zero chance of re-attachment, as they violate the limits of the Mair/Buchheim departure slope angles, necessary to allow for re-attachment.The plan-view section of the roof exposed to the turbulence is at the lowest static pressure, compared to the suction peak at the windshield header,and this low, acting over the 'lever arm 'spanning the distance to the tail creates the 'moment' which lifts the tail. Simply extending the roof to the back of the vehicle cancels the moment, leaving a higher base pressure, and lower pressure drag. On the Jaguar, this is part of the reason for the Cd 0.29,vs the Tesla S 0.26. The Porsche Taycan is close to the 'template' and enjoys Cd 0.25.
*On Spirit, the aeroshell and boat-tail decelerates the flow, and as per the Bernoulli Theorem, imparts a higher static pressure, removing the original separated, high-vorticity-induced low pressure from any surface 'over' the aft-body. This would be the same if no belly pan were present, as per the 1988 Texas Tech, SAE published research results. By the way, Spirit's belly pan was compromised at the time of wind tunnel testing, and probably would have rendered the diffuser useless, along with compromised underbody flow.

I was thinking about this - how could someone have developed a theory about lift which is so comprehensively wrong? But I think I now have it. The clue was in the mention of notchbacks having highest lift, and the rear spoiler on the BMW 2002.

Old cars, like that BMW 2002, had flow that separated at the end of the roof. Therefore, yes, lift was in part created by the separated flow and the low pressure associated with it acting on the trunk lid / rear window. And in that case, the spoiler did in fact work in the way stated.

But of course, major flow separation at the rear of the roof of a sedan hasn't occurred in any aerodynamic cars made since, about, 1990. So for the last 30 years, this mechanism of lift production has been irrelevant.

To attempt to apply it to the Cayenne (etc) is as absurd as I previously stated. In fact, it is easy to show how absurd it it. The measured lift pressures on the upper surfaces of these cars is lower than the wake pressure...

So, there's a good example of the perils of not keeping up with aerodynamic understanding, and applying to current cars a conceptual model that works only with old-shape cars.

Last edited by JulianEdgar; 05-12-2020 at 10:29 PM.. Reason: typo
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