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Old 09-08-2011, 01:03 PM   #213 (permalink)
ChazInMT
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Join Date: Jan 2010
Location: Vero Beach, FL
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MagMetalCivic - '04 Honda Civic Sedan EX
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I will say this blasphemous statement once again, Attached Flow Is Not The Goal. There are plenty of times that flow is attached while far short of the design template. Just because it is attached, it does not mean the shape is optimal. There is NO DOUBT the flow is attached till the back portion of the roof on any pickup, it is the highest point of the truck, and the air density is very high here.

So what is the goal? I say it is to let the air which has been pushed up by the front of the truck, thus building density (not pressure, but density), re-expand or return to normal density while pressing down on the skin on the back of the vehicle, thus returning its energy.

We optimize this reclaiming of energy when we follow the template. If we drop a little quicker in the back, then the energy return (pressure) is not as great. If we drop quicker yet, there is still attached flow, but essentially no pressure at all returned. If we drop way too fast, then all hope is lost and a lot of turbulence is generated. Conversely, if we make the shape longer, then skin drag starts to become an issue.

Think of the air as a spring that gets compressed as we drive under it, and as we go past it, it is capable of releasing its energy back into the car. But if the shape is too fast, the spring is not able to keep up with it, and the energy is wasted.

Look at my thread here and I explain it a bit better (with diagrams). Let me know what you think.
Up & Down Aerodynamics
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