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Old 11-01-2009, 10:19 PM   #29 (permalink)
Moderate your Moderation.
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Originally Posted by brucepick View Post
Thanks, majestic.

OK, so based on this understanding you could cover the rear glass with dimples and the separation would be delayed,
with a reduction in net c/d. Nice.

However -
1) You need to be able to see through the glass.
So we need researchers to develop 'glass' with an index of refraction identical to air (zero??) so the bubbles won't affect your view.

2) The worst offenders in the rear wake department generally are the vehicles with a non-sloping roof: vans, wagons or hatches with a near-vertical rear panel. It looks like dimples won't help them much. You could put the dimples along the rear edge, like vortex generators, but you're not delaying the separation to occur at a point with a smaller cross section to get a smaller wake.

Does that seem right?
No, Sir. Since flow is already detached at the glass, there is nothing there to create the vortices to keep attached flow, so they'd be most pointless there. Instead, the dimples would have to be placed at the rearmost point where flow is still attached, which would be the upper rear window frame on any vehicle.

1) Dimples don't necessarily have to be concave. They can be convex, and in either event, there are window shades that are perfectly legal in most states that you can only see through on one side. A lot of money was spent to figure out that it's pretty easy to see through a black mesh, but much more difficult to see through any other color or shade. Make sure the dimples/pimples are solid, and black on the side that sticks to the window, the driver only loses as much visibility through the rear window as those stupid full-window vinyl coverings.
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