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Old 12-02-2020, 02:20 PM   #174 (permalink)
aerohead
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body of revolution near the ground

Quote:
Originally Posted by AeroMcAeroFace View Post
All I can reasonably take from that is, the template is low drag, especially without wheels, and even more so nowhere near the ground.

But actually, reading through the bits of that book I can access, it says "Klemperer (1922) recognized that flow over a body of revolution... changed drastically and lost symmetry when the body came close to the ground"

Seems pointless to follow something like that to me when designing cars, which live on the ground.

"Despite their extreme length, flow separates from the rear of streamlined cars. By truncating the rear shortly upstream of the location where separation would take place, shapes of acceptable length were generated with no drag penalty"

So that says to me that using a template may actually increase drag, a bit like extending a kamm-back can be pointless.
1) We're not discussing a body of revolution near the ground with the 'template.'
2) We're discussing a 'half-body' derived from a streamlined body of revolution, which completes the streamlined body in mirror-image of its ground reflection.
3) The streamlined body from which the 'template' is derived has the lowest drag measured, at supercritical Reynolds number, and capability for forward outward vision from the driver's position.
4) If you watched Julian Edgar's video, and have access to Hucho's 2nd-Edition, you'll know that :
* The VW-Blunt Body ( solid border schematic ) is the AST-II contour.
* The VW-Blunt Body ( dashed line schematic ) is Wolfgang Klemperer's 1922 'minivan', of which it's contour is co-opted by Hucho in 1978 for his half-body, and you'll see it used also on the 1937, Jean Andreau' Peugeot 402, and 1971 Renault ALPINE A 110 1600S. Klemperer's body can be thought of as a 'curved' version of the Ahmed body, used in vortex-drag research.
* The VW-Drop Shape ( dotted line border ) has the AST-I roofline.
* The VW-Flow Body ( short-tail ) is very close to the AST-I contour.
* The VW-Flow Body ( long-tail ) Cd 0.14, shown opposite Hucho's nested 5-car image, is also AST-I-esque.
* The AVA ( Gottingen ) Streamline Shape is from the 1938 Schl'o'rwagen. developed under Ludwig Prandtl's perview, developer of the 'Lange' car, of which Porsche borrowed from. Cd 0.186, as with the VW XL1, and Renault Vesta-II of 1987.
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1) Of course, when the body runs out, the flow no longer has a surface of attachment.
2) When the flow does leave a streamline half-body, it's at maximum deceleration, highest pressure, with a drag penalty strictly due to surface friction. Lift should be lowest as well.
3) I've yet to see a body truncation which did not result in a measurable drag increase. I'll ignore the knife-edged Morelli shape of 1976. What they cut off would even allow enough room for a European license plate.
4) The long-tail Kamm car was Cd 0.12- 0.13.
5) The 'length' issue of the long-tail was addressed by Fachsenfeld with his 'extensible' tail technology of the 1930s. Hucho shows it in the chapter on commercial vehicles.
6) On the T-100, along with a half-tonneau cover, and rooftop cargo carrier in the bed of the truck, this inflatable tail helped pushed the Toyota to over 30-mpg on the open road.
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To get drag lower than the 'template' requires very small vehicles, going so slow, ( 15-mph average ) that they remain at subcritical Reynolds number.
The lowest drag car reported so far, the 2015, ECORUNNER- V, by Delft Technical University, Cd 0.0512, even uses the AST-II for most of it's roofline.
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Last edited by aerohead; 12-02-2020 at 02:22 PM.. Reason: typo
 
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