Quote:
Originally Posted by ChazInMT
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Anyway, when looking at louvers through the lens of this idea, it is less then optimum. As always, I have to say that filling the wake will always yield a better result than nothing. But at the end of the day, we want the air to return it's potential energy to the car and in my mind, the air cascading down the louvers does not have the angled surface to push against.
So the louvered Kamm will help by eliminating the potential for large vortices to form at the back of the car, but it does not allow the air to push on the car.
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Maybe louvers don't break your idea. So long as there is dead air between the louvers (need to ensure it's sufficiently closed along the azimuth so air doesn't slip in through the sides), the pressure in the dead zone will be equal to the pressure applied by the descending stream. - Just because it isn't a solid surface doesn't mean you can't push on it - The right question to ask is whether there is a substantial difference in the flow characteristics around a solid smooth surface vs a discreet chopped louver-tip surface. I expect the answer to be that the solutions approach each other as the louver spacing is reduced. How small? I almost called it the boundary layer thickness, but I think I'm grasping for a term I can't name - more like the "characteristic detachment length", which must be somewhere between 0.5" and 6".
Anyway, there's one way to find out. Build a louvered rear-window cover and perform A-B-C-A testing, with plastic wrap over the louvers for C.
I agree that louvers are unlikely to be aerodynamically as good as a smooth surface (in addition to being more challenging to design and fab). I'm disputing whether it's likely to be aerodynamically "almost as good" or "not nearly as good". If I'm correct that it's "almost as good" (which we can talk forever about until ABCA
), then I think the other benefits I listed are worth considering. MPG can't be the only figure of merit. The M is the value we get out of that equation, but I also get value out of comfort and convenience.
(Sorry for the threadjacking.)