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What you have there is an image of a vortex generator test of some sort. It does not mean the wing is helping the drag coefficient!
Naturally there is dirt on the flat rear surface. As in your image above, the blue area is all turbulence. As you can see the presence of vgs and/or wing do not affect the turbulent zone on the rear i.e. none of that rear surface is or will have attached flow.
Check out the mods and tips up top on the header bar.
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