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Old 11-01-2009, 10:24 PM   #30 (permalink)
Moderate your Moderation.
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Originally Posted by brucepick View Post

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?
This one is in a separate post so the information doesn't get confused.

Yes, anything with a flat back is going to be worse than something that is smaller and has even a virtually tapered rear end, such as a sedan. If the roof isn't tapered down as it leads rearward, it's going to create a larger wake than one that does taper downward.

In theory, something that would allow flow to stay attached around those rearmost edges would also reduce wake area, but not necessarily reduce drag, as I understand it.

In essence, while you're technically still decreasing the area that the wake occupies, you're increasing drag versus clean separation, so the decreased wake will probably be canceled out by the increased drag.

If one were to use a "duckbill" type spoiler, which is mid-mounted just below the windshield, and it didn't quite stick out far enough to make the 12* virtual slope, one could probably use dimples to train the flow downward, like a vortex generator. That's all they really are.
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