Rcmutz
Cleared for Takeoff
I have had the discussion with a few aerodynamic engineers. Depending on the airfoil, the placement of the VGs and angle of attack, there may be a net decrease in drag. Now let's see if I can explain it correctly.
VGs effectively add energy to the boundary layer by creating a turbulent flow above the lowest layer of the boundary space. This does two things, delays the separation of the boundary layer, and two reduces the drag in the area past the where separation normally occurs. So, if in cruise, the boundary layer normally separated at 50% of the wing, the VGs may push that down to 75%. So the energy used by the VGs can decrease the total drag on the wing. On the other hand, if the boundary layer normally made it the end of the wing in cruise, then VGs will cost cruise speed because there is no decrease in drag. The rule of thumb I was told was the closer in cruise you fly to L/D, the less VGs have an effect on cruise performance.
Hopefully I explained it well enough....
Anyway, it is very airplane specific (at least airplane model specific).
Tim
As an Aeronautical Engineer with a BS and MS, you got it correct. VGs are used to delay separation by energizing the boundary layer. But, by tripping the BL turbulent, you create additional drag. So the use of VGs is highly dependent on the airfoil section being used, the chord length, and what you are trying to do with the 3 dimensional wing lift/drag/moment characteristics. Wing fences are used to cut down on spa wise flow and prevent/reduce the strength of wingtip vortices. Fences gave rise to winglets when aero engineers realized that if designed properly, winglets could add additional lifting force on swept wings.