Ba is what I'm talking about
Here, I drew a picture. this is what Phil calls "attached vorticity"
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Originally Posted by aerohead
Attached-vorticity is a different animal.
The front of a vehicle defines how the atmosphere will be distributed as it passes the vehicles body.
In order to comply with conservation of mass,the windshield and roof cause the air to move at a greater velocity than down the sides of a vehicle,or underneath,due to the greater distance traveled by the air in the same time.
To comply with Daniel Bournouli's (sp?) Theorem,this accelerated flow must exist at a lower pressure than elsewhere around the vehicles body.
If the body designer is competent,the roof curvature,tumblehome, and upper radius of the greenhouse/C-pillar region will be such that when the flow off the roof rejoins the slower flow off the sides,these flows are at comparable velocity and pressure,the they simply continue on in a laminar flow fashion.
If the flows should meet beyond a critical velocity or pressure differential ( delta-V/delta-P) the slower,higher pressure flow will seek out the faster,lower pressure flow,and the two streams will spin up into a vortex as they attempt to blend together.
The vacuum created at the center of the vortex creates tremendous drag,and the kinetic energy required to feed the vortex robs precious kinetic energy from the flow,which,again,can never be converted to useful static pressure,hence the high drag and lower mpg.
If you've ever had a window seat on an airliner you may have seen wingtip vortices form during landing,caused when the higher pressure air below the wing is bleeding over the wingtip into the lower pressure air above,spinning it into a vortex.The pressure drop is extreme enough to create the refrigeration effect which brings the temperature of the water vapor in the airstream below dew-point,creating the clearly visible water fog condensing at the center.
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So in effect, the air traveling along the top of the Beetle's body side detaches sooner and has to go somewhere. It pushes inward because of the atmospheric pressure around the moving vehicle. The air coming around near the beltline detaches about a foot later and starts the same pattern. The offset rhythm of these two areas creates a rotating mass of air on each side of the New Beetle. The car's specific shape is geometric enough that the top can be treated almost as a half cylinder. There probably aren't counter rotating vortexes coming off the bottom side to even them out. Herein lies the problem. Large, unbalanced vortexes.
Or, that's what I figure