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Originally Posted by MetroMPG
I've realized aero is a much more complicated subject since I decided to make an effort to go beyond "lay" or "pop" aerodynamics. Perhaps time to track down an expert and ask directly.
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I hope you succeed, since the car industry is probably not sharing its cutting edge knowledge on this subject. And besides, most companies don't consider it to be terribly important.
A question, and a theory for the discussion:
1. the rear angle of the EV1 (as for the Probe V) is rather close to the 30 degrees forbidden by theory. Why is that?
2. My gut feeling (not tested, sorry) is that the aerodynamic qualities of a car vary according to speed. The theory goes like this: air has mass; mass is slow (or how do you say this in English). The mass is pushed aside by the moving car, and once the car has passed, the mass comes back, attracted by the low pressure area behind the car. My point is this: because of the slowness of mass, the airflow picture will be different for a car depending on whether it's going 50 or 150. With 150, the wake will be longer, and the 'bow wave' will be stumper in shape. If you take one individual air molecule, and a car at 150, (as compared to car travelling 50) it goes like this: the molecule gets closer to the car before it starts being pushed to the side (by other molecules), it probably travels further outward since it hits a higher pressure area, and takes longer to travel back in the wake of the car - or would the lower pressure (stronger vacuum) cancel out this last effect?
In other words, the air behind an Insight travelling at 50 would be back to normal pressure at a smaller distance behind it, than with an Insight travelling at 150.
Or: does it make a difference for the Cd figure which wind speed is used in the wind tunnel that tests it?
There might be a link between the answer to my two points: namely, that at low speeds (say up to 50 MpH), the rear third of the EV1/Probe is perfect, since air flow remains well attached to the car, from start to finish. But the rear third would not be perfect if the EV1/Probe were to travel at a speed of 130 MpH. At that speed, the air flow would not remain attached to the rear third since the car would be going to fast: and a smaller angle would be needed, like 10 or 15 degrees.
This stuff is not necessarily based on existing (known) theory, but there may be a parallel (in the water, not in the air) that I know of: in windsurfing, the rear fin slices through the water, and prevents the tail from gliding sidewards. There is more pressure on one side of the fin than on the other. At low speeds, no problem. At high speeds however, you may encounter spinout: the pressure difference causes the angle of the fin (in comparison to the direction) to become too large, the water can no longer remain attached to the low pressure side (the wind side) and air bubbles start to form. The back of the board slides away to the leeside.
Now of course the properties of water and air are much, much different, but I see no reason why a small similar effect does not exist for air.
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