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Old 08-01-2008, 02:14 PM   #131 (permalink)
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vortex generation

Originally Posted by 95badbird View Post
so what exactly is causing a vortex?
As the air moves rearward along the cap,the air atop, and aside the cap are moving at different velocities, and at different pressures,mingling only at the top intersection,and held in place by the positive pressure gradient ahead of,and around the truck.When the air reaches the back,there's no longer any surface left to support the flow,and pressure differential will cause the slower,higher pressure side- flow,to head for the faster,lower pressure top- flow,spiraling into the vortex as the two dissimilar flow fields attempt to mingle and blend.------------------------------------ From Hucho's research at Volkswagen, a backlite angle of 28-degrees produces the most violent vortex,and this kinetic energy of the "spin" cannot be converted back to static pressure,for perhaps hundreds of feet behind the vehicle,where viscous attrition from colliding with surrounding air,finally brings the vortex to rest.I think this is what MetroMPG and Bicycle Bob are referring to.-------------------------------- 28-degrees is big trouble!

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