[aerohead]

*A Clark Y airfoil will stall above 22-degrees AOA
*With VGs,stall is postponed to around 26-degrees AOA
*The VG requires the lower surface boundary in which to broadcast the vortex-induced kinetic energy to prevent stall/burble/recirculation/eddies/turbulence
*The base pressure of the wake is determined by the static pressure of the inviscid flow just outside the Turbulent boundary layer.
*The separation line determines the pressure of the wake.
*On a squareback vehicle,the VG does not alter the separation line,as the body falls away so rapidly that we lose the lower surface boundary;and with it,the turbulent boundary layer which might have received kinetic energy from the VGs.
*Since the separation line,with or without the VGs is essentially identical,there's no facility for pressure recovery,the only thing which could raise the base pressure,reducing pressure drag of the wake.
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*VGs in front of a 'fast' box-cavity or 'fast' boat tail WOULD provide for beneficial locked-vortices all around the perimeter.And this WOULD raise the base pressure,lowering the overall drag.
*Scientifically,I just don't see how VGs can benefit a squareback type aft-body.
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*In Hucho's chapter on commercial vehicles he comments that VGs,turning vanes,etc.,all fail to reduce drag on road vehicles with square backs.
*The box-cavity,next to the boat tail, is the only add-on device to register a drag reduction when subjected to rigorous wind tunnel testing.
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*Suction and blown slots DO show a drag reduction,however,the energy required to operate the pump exceeds the energy saved.
*This is why Hucho emphasizes the importance of 1922 technology body elongation through boat-tailing for meaningful drag reduction.