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Old 05-12-2020, 12:47 PM   #11 (permalink)
aerohead
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Quote:
Originally Posted by JulianEdgar View Post
I am sorry, but that completely ignores the major reason that lift occurs on cars. The lift - especially rear lift - in most cars doesn't come from the wake (ie separated flow). It comes from the accelerating flows over the curved upper surfaces.



I am sorry, but that is wrong. An aerofoil has attached flow and can develop lift. You may choose to orientate a streamlined body (like an aerofoil) so it doesn't generate lift, but that is not the same as saying "streamline bodies are incapable of generating lift", as you previously wrote. In fact - of course - the best lifting bodies are streamlined.




I am sorry, but you are confusing cause and effect. You can be certain the upper surfaces of your car were developing lift. However, if the car in fact didn't develop overall lift, it would have been because of the low pressures developed under the car by its bellypan.

Note: all of this can be directly measured on real cars using a surface pressure disc (easy to make yourself), a Magnehelic gauge and a sealed reservoir.
*Firstly, Hucho appears to downplay apprehensions about 'lift' in passenger cars as related to stability. Below 100 km/h he says it's not an issue, even in crosswind. Hucho does emphasize the importance of lift in high speed sports cars and racing cars.
* 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.

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