Quote:
Originally Posted by hat_man
Ok after re reading that maybe it's low pressure under the tonneau and a higher pressure (still low but not as low as under the half tonneau) behind the truck at the tailgate, causing the pressure differential and the idea of 'thrust"?
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*Without the tonneau cover,there's nothing but turbulence back there.All the kinetic energy is lost forever,and there's no chance for any re-attachment.A lot of drag and a lot of lift.
*The tonneau provides a surface of re-attachment for the separated flow,allowing for a much smaller wake,and it kills most of the rear lift.
*The half-tonneau is a peculiar animal.It allows a locked-vortex to be captured behind the rearward facing step of the cab.
*The inviscid (streamline flow) follows over the vortex as if it were more a solid body,strikes the tail end's upper surface,really killing lift.
*The extremely low pressure of the 'eye' of the vortex is transferred to the inner face of the tailgate.This inner face is at a lower pressure than behind the gate,creating a pressure differential which 'pushes' the gate forwards,something which doesn't happen with a full tonneau.
*If you cut ducting through the gate,you lose the differential,and the 'push.'
*Also,air would actually go through the hole.from the back to the front,tripping up the flow and creating drag.