Quote:
Originally Posted by Frank Lee
I did explain, right? I don't think it'll interact with the free stream air and I think to be effective it should. I'm not seeing how it works on the Audi either, and when I load the pdf the stupid pics land on top of the cd charts so I can't read them. How effective is the box by itself?
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Hucho and Emmelmann researched these at VW around 1978.In his chapter on commercial vehicles he says they're good for a 5% drag reduction.And that's as a simple extension,straight back.
Around 1991,Continuum Dynamics won an SBA DARPA research grant for a staggered box cavity which,as you say,had its free end intersecting along an imaginary line where Mair might have had his boat tail contour.
The air immediately separates at the beginning of the cavity but does re-attach at the back,capturing a vortex on top,both sides,and bottom.
As with the tonneau cover on a pickup,the outer flow skips over the stationary vortices and as it exits the cavity,its flow direction is now cambered inwards,vectored into the wake,running slower and at a higher static pressure,which increases the base pressure,lowering the pressure drag.
This 'staggered' configuration showed a 10% drag reduction on an 18-wheeler inside NASA's Ame's Research Center wind tunnel at Palo Alto,California.
Magazines like Pop Sci carried photos of the rig and also a colorized pressure histogram of the wake flow.
They're 'quick and dirty' although now,probably eclipsed by the 'Trailer Tail' like Mesilla Valley Trucking and others are running.
In order for the cavity to function it must be airtight.