I was doing quite a bit of tuft testing this last couple of weeks. The back window is an ideal area for experimenting in general, because I can use the rearview mirror to get a real time look at what is happening.
It is very interesting. The flow along the top four inches of the window is very consistent, moving almost directly outward both ways from the centerline, then shifting to directly downward at the outermost three inches of the window. It is turbulent and not consistent enough to really comment about through most of the middle, with one exception. In the center of the window, about 6 inches down the glass, the tuft was always standing straight out from the glass.
If I had to guess what the airflow was doing, I'd say it was a pair of counterrotating vortices, flowing up the centerline, outward, and down the outside edges.
None of the locations of the little VG's that I built seemed to have any visible effect on the airflow down the back window, except for a slightly larger area at the outside edges where the flow shifted downward rather than being visibly turbulent.
The angle of the rear window is about 40 degrees down from horizontal, which is pretty bad. I've been wondering about a trailing 'hood' sort of thing attached at the top of the window, which would taper down at a more gentle angle, maybe 15 degrees or so, and let the airflow move a little more gently into the wake of the roof.
But, for this week, the only change I make before the next long highway trip is to remove the huge front mudflaps, and see if I get any noticeable difference. I've thought about using the rear mudflaps as an anchor point for a trailing aero dam, to smooth the airflow up from the bottom of the body into the rear bumper area. I want to do one thing at a time, so I can get a feel for which mods are the most valuable.
A friend gave me the old video camera and recorder out of a police car, and it might end up being mounted to watch tufts on top of the car and in places I can't see directly.
So many ideas, so little money...
