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Old 12-11-2008, 03:33 PM   #11 (permalink)
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significant drag

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Originally Posted by akashic View Post
Oh, really... Does there appear to be some rule of thumb regarding the size of such openings or other conditions to suggest when this will happen vs. when significant drag appears?
akashic,the importance of your question was such that I dug a little deeper.So here's some tidbits that may help. Hoerner refers to these local disturbances as "stationary vortices - formed in holes and gaps.More or less restricted to their place,they consume but little momentum which is furnished by the passing outer flow." A good graphic appears in Hucho's "Aerodynamics------------",Chapter-13,page 520,Fig.13.20 (a) showing a "backward-facing step",which is essentially the back of a pickup cab.The lower plain would represent a tonneau cover with the "stationary-vortex" whirling above it.Sorry,no image! Perhaps a member or lurker with the text can post that.Essentially,the forward half of the bed is vortex,and air follows over it as if it were a shell.Some of the members are using computational fluid dynamics software which will illustrate these kind of fields in two-dimensions.

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Old 12-11-2008, 09:23 PM   #12 (permalink)
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Right, that's the "bubble" theory to my untrained mind. So here's a question... suppose you extended side fairings all the way back to the tailgate so that the bed's stationary vortex was trapped between the walls... essentially like your aeroshell, but without the top... maybe the cab-top is faired down a little, but the bed is completely open, no tonneau... Would it behave nearly as well as the aeroshell?
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Old 12-13-2008, 02:33 PM   #13 (permalink)
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trapped

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Originally Posted by akashic View Post
Right, that's the "bubble" theory to my untrained mind. So here's a question... suppose you extended side fairings all the way back to the tailgate so that the bed's stationary vortex was trapped between the walls... essentially like your aeroshell, but without the top... maybe the cab-top is faired down a little, but the bed is completely open, no tonneau... Would it behave nearly as well as the aeroshell?
akashic,I think it would be a great experiment.My intuition is that it would provide an improvement,although as they say,"proof's in the puddin'" The only downside that I can think of off-hand,is that w/o the roof,the "fins" might move the aerodynamic center-of -pressure forward,depending on cargo loading,which could have adverse handling effects in strong cross-winds or gusts.That's a hard one to nail down without a windtunnel.----------- The Bertone "BAT" car kinda did this sorta thing,but the fins on that car increased in size towards the rear rather than reduce,which would honor stability issues.

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