Quote:
Originally Posted by csnyder
We did some testing of Air-tabs on a 35 foot travel trailer using an aircraft engine blowing air down the side of the trailer and both tufts and smoke. Without the airtabs the tufts or ribbons just snapped around everywhere, tying themselves in knots behind the trailer, and the smoke curled tightly around behind the trailer. With the airtabs about 6 inches from the back of the trailer the tufts/ribbons stood straight out behind the trailer with virtually no flapping, and the smoke went back about 15 feet before starting to swirl. Wind speed was in excess of 60MPH down the side of the trailer. (about 12 feet? from a wall) I'd say they do pretty much what the information from airtabs claims they do. They sure keep the back of the trailer clean.
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I don't know what their claim is.And I don't understand any mechanism which would allow for a drag reduction.The thickness of turbulent boundary layer would exceed the 'height' of the VGs,they would just be embedded in turbulence.The pressure at the same separation line would virtually be the same.That pressure determines the base pressure of the entire wake and basically the entire pressure drag of the truck.
I can see that they would alter the appearance of the wake,but uncertain that they'd have any effect on pressure.
If these are the 'Wheeler' wishbone VGs,Wheeler's original claim,was that on his HONDA Accord notchback sedan,that the VGs would allow reattachment onto the boot/trunklid of the car,which WOULD provide a drag reduction mechanism,as they do on the underside of a Lockheed C-130,preceding the tail boom.
Without any tail structure behind the trailer,there would be no facility for flow reattachment,locked-vortices,and pressure recovery,such that at the 'new' separation line,the pressure regime WOULD be increased.
And again,Hucho tested these sorts of things at Volkswagen,and he said that none of these 2-D palliatives worked on 3-D bodies.I don't know what else to add.