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Engineering
Aerospace Engineering
Ideal car rear roofline angle and shape for low drag?
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[QUOTE="jack action, post: 6820161, member: 240508"] Flow separation creates a low pressure zone that is pulling the car backward. You are mixing two concepts: Lift and drag. With lift, you speed up the flow on one side of a vehicle to reduce its static pressure, and this creates a net force [I]perpendicular to the flow[/I] that pulls the vehicle on the high-speed flow side. With drag, you have to imagine the vehicle moving instantly in the surrounding air which, for an instant, compresses the air in front of it (high pressure zone) and leaves an empty spot of vacuum behind it (low pressure zone). This creates a net force[I] parallel to the flow[/I] that pulls the vehicle backward. Of course, nature hates emptiness, so the surrounding air will try to fill the rear end vacuum as fast as possible, leading to chaotic turbulences. So with drag, the objective is to reduce this rear end low pressure by forcing the flow to follow the contour of the vehicle, thus no flow separation, no turbulences, and no low pressure pulling backward. You just want to "split" the air as you go in, not "break it apart". If there is flow separation behind the vehicle (behind the rear bumper), it acts on the vertical area of the rear end, but it has no horizontal surfaces to act upon so it doesn't affect the lift/downforce of the vehicle. But if the flow separation is on the upper surface of the vehicle (behind the rear window, over the trunk, for example), the low pressure zone will act on the vertical surface (of the rear window), but also on the horizontal surface (of the trunk) and increase the lift at the rear end of the vehicle. [/QUOTE]
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Ideal car rear roofline angle and shape for low drag?
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