Quote:
Originally Posted by sregord
Aerohead..the Mair drawing/graph shows a proper (truncated) boattail of a "torpedo/bullet", but we're only trying to duplicate the top half ... correct?
The attached had me going in a different direction.
Now I'm thinking rotate 90*?
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Exactly! Mair's model was a long, body of revolution. It CAN be morphed into a square or rectangular section ,as ASRAE engineers do with ductwork ( square-to-round, and visa versa ) as long as the sectional density is maintained per unit length.
The underbody flow has very little energy and cannot withstand a sudden change in cross-section like above it. For the best aero performance we'd want a diffuser. That would require full belly panning upstream.
Tractor trailers may be constrained by weird, DOT regulations about DOT bumpers. Don't know. I believe that we're not held to the same metrics. Personally, if I were going to rear end an eighteen-wheeler, I'd prefer to drive into an 'air bag' rather than a steel jungle-gym.
In the photos of Orbywan's MH, you can see how he modified his diffuser over time, to 'relax' its angle.
The 10-degree SAE 'departure' angle, up from the back of the rear tires ,does need to be respected or there'll be a ground-strike.
With Moby's long overhang, you may already have to ascend ramps at some oblique angle.
Your attached thumbnail photograph depicts a tail which would in all probability, suffer separation like NASA's. In plan-view and side elevation. Bottom for sure.
The thing would be to transition inwards and downwards to the 'safe' 22-degree downslope, and then, just hold a constant 22-degrees from there, for as long as your willing to go, knowing that there's a high confidence for fully-attached flow.
Simultaneously, your 'square' roof/sides corner intersection would begin to morph to a gradually-increasing corner radius.
General Motors 'Optimum' tail had this feature. ( 'Tailwind Project', Texas Tech University, circa 1994, Professor Carver and Scott Funderburk ).
This kills any propensity for vortex formation ( one of the highest sources of drag ).