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Old 08-19-2008, 05:25 PM   #7 (permalink)
aerohead
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curve or straight chop

Hey Darin,Al let me use his wife's computer so I could hear the audio.And here's my shot at your question.-------------- The air wants to follow that ideal pathway we talk about with the teardrop.Without a flow support surface(the back of the car) the air is going to break away.If you round off the trailing edges,the flow will follow to the point,which is 15-degrees of rotation behind the initial origin of the curve,given any radius of curvature ( just like the back side of a golfball ).With this curve,air can enter the wake with more vorticity,which requires more time(and money) to dissipate.------------------------ If you chop the tail off sharply,the air will flow longitudinally,and it has been observed that this increases a pumping action within the wake,creating and strengthening the locked -vortices within the wake,which are associated with the"phantom-tail" as reported by Morrelli and others,which cuts skin friction,while providing form drag reduction approaching that of the full boat tail.In yachting they call it an "embryonic transom." In artillery its called the Aberdeen Projectile.Chrysler aerodynamacists say the flow will "burst" with the hard cut,and that it is preferable,as Kamm and Hoerner and others have commented.There may be advantages unrelated to aero,as progressive crumpling during explosive deformation in a wreck.Also from the video is a clue with respect to the rear spoiler.The little 5mm upkick may direct an upflow,which,when it collides with the flow coming off the roof,blasts any vorticity into a more homogeneous turbulent wake.Tarzan say turbulence"Good!",Frankenstein say vorticity"Bad!".Morrelli said that during the development of the CNR "banana"car,in the windtunnel,they literally sliced pieces off the back of the car like a loaf of bread,and as Kamm had predicted,flow remained attached right up to the chop,and the wake behaved much like a solid surface,with the air skimming along it.Hope I didn't talk this thing to death.
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