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Originally Posted by Engeu1
Was that in his 1987 book?
Your Ragatz and Thornton post is also interesting helpful.
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Yes, you'll find it in his chapter on commercial vehicles, if memory serves me.
The work on the Ahmed Body will also be of interest. There's a full development of drag and vortex drag as a function of angles, boat-tailing, and diffuser geometry, all combined into a single graphic.
Graduate student Mark Funderburk, along with Professor, Dr. Carvin, at Texas Tech University's College of Engineering picked up on the Ahmed body research, along with General Motors, who'd come up with an 'OPTIMUM TAIL' investigated in the Texas Tech 1/10-scale model wind tunnel.
GM's OPTIMUM was a spitting image of the 25-degree/10-degree/10-degree Ahmed tail, with all-edge radii added to kill any chance of vortex formation. This tail, however, was optimized for a 'box' or 'van' which already had upper edge radii, something not seen typically in commercial vehicles, due to the intrusion into the cargo volume. GM's tail is probably the best thing that can be fabricated without compound curves, as in Fachsenfeld's inflatable boat-tail, developed next door to Daimler-Benz at the FKFS, in Stuttgart.
Fachsenfeld's is the best I've ever seen. Hucho has a depiction of this in his bus section.
I tested an inflatable tail on my Toyota truck back in the 1990s, and along with partially stuffing the open truck bed, allowed a 4-mpg improvement on the highway. And it was deformed so much under inflation pressure, that it didn't even resemble what it was tailored for in static form. ( don't ever use expandable Naugahyde for inflatables!)