Quote:
Originally Posted by vortex
Air Tabs work much like an open air NACA submerged duct inlet. They influence and mix (till if you will) the boundary layer by drawing faster high energy air toward the surface.
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Sure there are the counter-rotating vortices but these are a byproduct of function. The vortex filaments are not what delay downstream flow separation but rather the higher energy lower static pressure flow pushed against the surface making it stick longer before the boundary layer pressure gradient grows adverse. This is useful in extending Coanda flow attachment. That said aft facing Coanda attachment surface suction is generally bad for overall vehicle drag unless that pressure investment can be profitably applied saving a greater drag downstream, as a fairing costs drag but saves a greater drag as its return on investment.
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I thought the VG's delta-wing vortices were what produced the low pressure,which induced inviscid flow kinetic energy into the boundary layer to forestall the reverse flow rollup,which would trigger eddies,then full-blown separation and turbulence?
The Reynolds number is enough to guarantee attached flow all the way to the back of the trailer.With or without VGs,the separation line remains fixed.
Turbulence cannot ever be converted to static pressure.If the VG accelerates the flow leading up to the separation line,under the Bernoulli Theorem,the pressure at the modified separation point would by default be at an even lower pressure.If the wake pressure is governed by the separation line pressure,this would lead to an even lower base pressure behind the trailer,increasing the pressure drag.
When you use the Coanda language,it is typically associated with a selective geometry perimeter nozzle which injects a 'tuned' volume of air from an internally provided source.I don't see how that's germane to a discussion of VGs.
With respect to a fairing,I might say that it aggravates frontal area,however, the superior,separation-free flow more than compensates,ending in an overall lower drag reduction.