ok i did a little more reading on wikipedia, as their info is usually fairly easy to digest and not overly complicated.
one interesting thing is this:
Quote:
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Vortices readily deflect and attach themselves to a solid surface...
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this propperty makes them usefull in aviation appli
" "cations, where one wants to delay wing stall by keeping the airflow attachted with vortices where it would otherwise separate.
however
www.airtab.com - The smart way to cut the cost of your highway fuel costs manifacturer of the airtabs state in their instalation guidelines:
Quote:
Location On Vehicles
In general terms, and for maximum effect, AirtabsTM should be installed as close to the back (trailing) edge of the vehicle
as possible while ensuring proper adhesion surfaces. They are effective in shielding gaps or reducing turbulent air from
entering undesirable areas. Any part of a vehicle that presents an edge that is at right angles to the airflow is a possible
Airtab® location. This includes forward of wheel wells etc. In order to accommodate rivets etc during installation,
AirtabsTM can be mounted so that the leading edge of the Airtab® is 14 or less inches from the trailing edge of the vehicle.
Mounting further forward will reduce performance.
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this indicates that their purpous is not to reattach air to the surface but that a different purous is intended.
if we look back at the naca intake :
Quote:
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This design is believed to work because the combination of the gentle ramp angle and the curvature profile of the walls creates counter-rotating vortices which deflect the boundary layer away from the intake and draws in the faster moving air,
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than perhaps the VG's like airtabs seen on cars work the same as a naca duct by drawing air into the wake of the car, and thereby increasing the pressure slightly. since this negative pressure behind the car actually pulls it back an increase in pressure would be a very good thing.
further the presence of of the vortices might cause the wake to form in a different way wich is more beneficial.
tuft testing is obviously fairly useless in this case as all the action goes on behind the car... (unless one uses very long strings, but that could lead to hairy situations). pressure measurements behind the vehicle might reveal a difference in pressure tough.
sometimes i really wich i had a windtunnel