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I've played with VGs on the roof, and I know I can make the airflow stick to the back window with two VGs near the centerline, 8" ahead of the back window (normally airflow separates on the center 1/4 of the back window, top to bottom). They change the airflow to an inward direction on the bottom outside corners of the rear window and trunk lid.
Anyone care to guess whether this would be good or bad for drag reduction? I think it might be evidence of outward-turning vortices forming at the rear corners of the roof, like the 30-degree fastback example in Hucho but opposite direction spin. Might just be converging flow, though.
Also, a VG about a foot inward and forward of the back corner of the hood kept airflow attached to the side windows (eliminating the whorl at the upper front corner of the front window), but that was with mirrors in place, so I'll have to try that one again.
Hopefully I can get time to do some tuft or grease-blob testing and some A-B-A to sort it out.
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