Quote:
Originally Posted by JohnForde
Thanks JSH!
I do plan on keeping the boundary flow smooth. The width at the hinges is ~72"W. The hinges are 8 " behind the cuff which is 78"W. I will tape coroplast to the van's body to maintain this edge across the gap.
The hinges need to be 8" further aft to allow sight line to the tail lights.
The van's black plastic cuff has a 3" "rise" over 10" "run" for a slope of 16.7 degrees.
The red line in the photo is an even steeper angle to meet the orange line. The orange line is 3" below the cuff but ideal because it is in plane with the vans back up camera.
Photo in my previous post shows a lot more wood in the field of vision than is necessary.
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It won't be an aspect of keeping the boundary-layer 'smooth', as it would be keeping it 'attached.'
* We know that drag is primarily as function of base pressure.
* We know that base pressure is a function of pressure recovery.
* We know that pressure recovery is a function of flow deceleration.
* We know that flow deceleration is a function of flow attachment.
* AND THE MOST IMPORTANT THING WE KNOW ABOUT AERODYNAMICS IS THAT FLOW ATTACHMENT IS A FUNCTION OF THE time-rate-of-change-in-deceleration-of-flow ( TROCIDOF ) along the aft-body contour **********!
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Anything other than a 'streamlined' contour in the aft-body will trigger a 'super-deceleration' which is responsible for an 'adverse pressure gradient' which is the mother of all flow separation.
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The air, immediately adjacent to the body surface is 'at rest'. It has 'zero velocity'.
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The 'lowest' pressure on the body is at the windshield header, at the 'front' of the vehicle.
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The only reason flow can remain flowing 'rearwards', is that a horizontal vector of the kinetic energy from beyond the boundary-layer is being transported all the way 'through' the boundary-layer, literally 'pinning' the 'dead-air' against the body's surface.
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If the contour of the aft-body 'converges' too rapidly in cross-section, the kinetic flow vector will simply ricochet off the top of the dead air.
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Under the Bernoulli Theorem, should this happen, the flow would be required to 'decelerate', but it can't, as, it's 'always' been at 'zero' velocity.
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The consequence is that, without being held by the flow vector, the flow begins to move 'forwards' ( high-pressure -to- low-pressure ) towards the lowest-pressure windshield header ( reversion ), rolling up into eddies, and finally full-blown 'turbulence' as it attempts to get there, detaching from the body as the origin the 'wake', as there's no longer anything there to prevent it.
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All possibility of converting kinetic energy of accelerated flow back to static pressure at the 'base' of the body is lost forever.
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As a compromise to build simplicity, you can intentionally design 'separation' into the tail, using 'straight', rather than 'curvilinear' surfaces, as long as the 'tearing edges' downstream occur where a 'streamlined' tail's surface would reside in the more complex construction. Box-cavities and Trailer Tail depend on this technology of captured-vortex, phantom surfaces. They can't perform as well as the more ideal curved surfaces, but they require only a fraction of the man-hours necessary to create the more favorable shape. You just need to be able to stomach the difference in performance.