Poor mans wind tunnel results 2010 honda insight
 

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I had a previous discussion with a colleague regarding the areas where the dust sticks and the conclusion was that the dust sticks in the turbulent boundary layer.
In your video was obvious that the she source of the dust is the road therefore you have dust only in the area where the dust raised because of the car passing. For a more general situation, try to drive behind another car lifting the dust up to the car height. Than you might see also the influence of the external mirrors....
Regarding the vortex generators I expect no change on dust accumulation the area where you want to generate vortex is already turbulent...but I would like to see the results because I might be wrong. I supposed better result will be covering the rear wheel passages and smoothing the wheel rim. On the back side on car sides and bottom, to set a clear separation line you can add some spoilers that creates a sharp edge as the trunk edge or sharper.

Pretty cool!! The AC/DC bumper sticker is priceless.

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