Quote:
Originally Posted by mwilliamshs
the horizontal fins on the side skirts...
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Yep,you've got a good handle on it.
For the spanwise flow,here's a better explanation:
If the pressure below the car was higher than the pressure besides the car,the high pressure air would bleed up into the lower pressure air alongside the car,moving transversly,or,sideways (spanwise if on a wing).
This transverse flow spins up into vortices,as a fast-moving tributary meeting a slow-moving river,or as the jet stream spins warm,rising,moist gulf air over Colorado into supercell tornadoes.
The vortices lockup kinetic energy which can never be converted to pressure,ultimately raising the pressure drag.
If the fence can keep the two pressure regimes apart,we can skip the vorticity.
Winglets on commercial airliner wingtips are a form of exaggerated fence which helps spanwise vorticity from forming.