Quote:
Originally Posted by sregord
Your patience is very much appreciated.
more from 8.5.4.5...The best geometry, with a cavity depth of 0.13d, reduced the drag coefficient by 5 per cent.
0.13 times 11'(tall) = 17.16"
0.13 times 8.5'(wide) = 13.26" 
gluing, shaping, epoxing 2 or 3" thick pink foam to the perimeter of the rear edge, adding 13+" to the overall length of Moby ...like building a foam tail on an Insight... maybe? |
I suppose, under an exact reading, Hucho's 'rear-end extension panels' qualify as a 'box-cavity', however, we can 'blow' by that performance, using Continuum Dynamics', aerodynamic backward's-facing step, captured-vortex technology.
It performs much like a tonneau cover on a pickup, except that, it travels down the rear sides too, and along the bottom, enclosing a four-sided 'step.'
And the 'length' and transverse 'inwards displacement of the 'step' coincides with the location a streamlined tail would intersect, as illustrated in Bilanin's patent.
Flow separates at the trailing edge of the body, but reattaches onto the end of the step, capturing a 'locked-vortex' in between.
Outer flow travels over this captured-vortex much as it would if the void were actually skinned over as a curved surface. It's a Lanchester/Prandtl surface of discontinuity in reverse.
As the outer flow moves rearwards, it also displaces 'inwards', along the 'curve' of the vortex, reducing the effective local cross-section, just like a boat-tail.
As it does this, it's decelerating, regaining pressure ( Bernoulli's constant total energy continuity ), and when it DOES separate at the 'new' trailing edge, it's at a higher static pressure, plus it's vectored towards the wake.
You end up with not only a 'smaller' wake, but a wake of higher base pressure, the goal of streamlining.
It's not as 'good' as a proper boat-tail, but it gets you 'towards' the performance, with less material, a lighter structure, and a few orders of magnitude easier to fabricate. All the reasons for Trailer Tail.