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Originally Posted by akashic
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?
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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.