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The Holy Grail, part two. You are correct that VG's require controlled and intelligent placing. However, with a few tuff test and understanding of flow velocities and separation, you can, as a layman (not having a wind tunnel), get descent results with VG's. VG's are not designed for placement into relative wind as you see so many people doing on you tube and on their VG sales page. VG's are designed to take the energy from the free flow stream and impart the velocities into the boundary layer, thereby increasing the energy of the boundary layer for it to continue to do its job. This same energy that is imparted from the created vortex is the energy you wanted proof of above. The very design of a vortex generator is this proof of energy you desired, that is why they are used, they impart energy. As I stated, you can control this energy by the placement of VG's at the rear of a car to extend the "perceived" rear-of-car, thereby decreasing the biggest form of drag on most vehicles, the deep low created at the rear. Placement doesn't mean, stand the VG's in soldier like form all across the rear, not at all. The relative wind is already flowing that direction so keeping it flowing that direction does nothing. What is desired is the vortex energy that is created by off-setting the VG by approximately 30 degrees. When off-set like this, the VG now gives off a controlled vortex which extends into and over the deep low at the rear of the car. When the upper or above the car flow is controlled, the divergent flow now lessens because for a car, the road is the termination point of the free airflow, in essence, you close the gap of your profile. Just a basic set of VG's on my truck produced 2 mpg difference, that isn't using but on area of the drag profile. I sold the truck before I could finish the top row and do the sides, but I that was an immediate result do to tuff tests and my sons GoPro.
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