Quote:
Originally Posted by California98Civic
Aerohead, how might the distinction between Coefficient of Lift and the Coefficient of Pressure affect this discussion? It seems to me, aerohead, your observation about fastback bodies was about pressure, not lift. How are they different? I have been reading this really interesting paper that reports pressure and lift as part of an aerodynamic comparison between fastback and notchback body types:
https://www.researchgate.net/publica...DrivAer_Models |
1) the first complication for us is,that neither model is a 'notchback' or 'fastback.'
2) they're both 'sportbacks.' One more aggressive than the other.
--------------------------------------------------------------------------------------
3) the thing about lift is, that you have pressure acting over a surface of some area.
4) on a car, the lowest pressure is just head of the windshield header. That's your minimum pressure. All air around the car would like to go there. It's where the highest lift occurs.
5) on a 'streamline car, the negative lift at nose and tail cancels this lift.
6) on a non-streamline car there is separation. And typically, it's at the rear of the car where it goes off 'template.'
7) the closer the separation line is to the windshield the lower the pressure. ( it's faster and MUST be of lower pressure for conservation of energy).
8) many, if not most 'fastback' cars are 'pseudo-Jaray' forms. While they're 'swoopy' looking, they violate the ground rules of fluid mechanics, as Hucho refers to them.
9) if a fastback roofline descends too rapidly ( Taycan), the deceleration due to the expansion creates a pressure rise which the turbulent boundary layer cannot withstand.
10) the air adjacent to the body is at rest. It's at zero velocity. In the back of the car, the only reason the air doesn't flow forwards, is that momentum from the clean air above the boundary layer is 'shooting' kinetic energy of this momentum right down to the 'wall', the surface of the car, literally pinning it in place.
11) without a streamline contour, a too-rapid divergence creates a magnitude of pressure rise which exceeds the energy balance at that location, requiring the air to 'slow down', when it's already at zero velocity.
12) that's impossible, because it's already at rest. The result is that kinetic energy which formerly attacked the boundary layer, now just skips over it.
13) without the momentum interchange, the air slips, begins to move forwards towards the windshield, rolling up into eddies, then full-blown turbulence, with all kinetic energy now lost, other than heating the atmosphere.
11) Where this separation occurs, determines the local pressure which will be translated to the wake as base pressure/ pressure drag/ aerodynamic drag.
12) any flow, breaking away from the body before reaching the end of the car will be a t a lower pressure than achievable if streamlined.
13) and the entire area under this region of low pressure experiences lift, just like a wing. This is exactly what originally happened on the Porsche 911.
14) in the far back of Hucho's text, he comments that something like the duckbill spoiler added to the 911, reached up out the region submerged in turbulence, allowing inviscid ( outer clean flow) to capture a locked-vortex of which the inviscid flow would travel over as if it were solid, plus, block this region from the new wake, created by the spoiler, so as not to allow low pressure air access to the wake. The air would expand out only as far as the spoiler would let it, arriving as 'slower', higher pressure ( Bernoulli Theorem) air as it reached the spoiler/ wake.
15) simply lofting the back of the car would have cut drag and lift even more, but would violated the 'look' of the car; something that would take decades for Porsche to resolve, increment by increment.
-------------------------------------------------------------------------------------
The original Mitsubishi Lancer would have been subject to extreme separation-induced drag/lift, partially mitigated by VGs and the rear wing of the 'Evo.'
------------------------------------------------------------------------------------
The Volkswagen XL1 would be a recent example of a streamline body. The main body is boat-tailed just like the greenhouse. With the speed limiter defeated, the car is capable of in excess of 100-mph. You don't see any splitters or spoilers on it, except for the diffuser. Smoke flow images reveal fully-attached flow all the way to the rear, then a clean Kamm truncation.It's dead-nuts on the 'template.' Just a coincidence.
So in summary, local pressure has everything to do with lift.And shape has everything to do with local pressure.
All wings have an angle-of-attack which produces zero lift. You can design cars that look like wing sections with zero lift. Pressures at nose and tail cancel any lift in between.