https://ecomodder.com/forum/showthre...rt-c-9287.html
Quote:
Originally Posted by ERTW
I have a question about which template is correct; the web app, or the jpeg in this thread? Refer to the Aston Martin Vantage pics. Web app in white, forum template in green.
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Designers have been scaling the template to the leading edge of the bumper to good effect. the GM Precept concept had 0.175 in "production" state, and was a bit lower with some cleaned up details (around 0.157). It only contends with the air that's being pushed up over the car - it's not exactly in ground effect.
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The Model Y does this as well. I think a box extending from the back would stop any downwash from the back screen and clean up the flow.
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1) Both the 'green' and 'white' contour comparisons for the Aston Martin are 'correct', as far as proper use of the templates go.
2) As there are AST- Parts A, B, and C, they're only useful if among the three, at least one of them happens to coincide with the transition zone of some OEM roofline contour / silhouette, from the roof apex rearwards.
3) The full vehicle height alignment, from the road surface to the roof apex, as with the Vantage, is the only 'proper' application of the tool.
4) We're striving to establish the effective fineness ratio of the vehicle, in the context of the ground reflection technique, developed by Ludwig Prandtl and Edmund Rumpler, as this is related to the drag minimum possible at this ratio.
5) Mechanical Engineering did an article on the development of the GM, PNGV 'Precept' concept, contributed by the team members who developed the car. They attribute Cd 0.163 to the actual 1:1- scale car.
6) I don't have the Tesla Model Y blueprint with me today. With the optional, carbon-fiber rear spoiler installed, the spoiler's tearing edge happens to intersect the contour of the AST, version 'C', which has the most aggressive down-slope of the three.
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I've lost my source for posting images, and awaiting an acquaintance's finishing her Masters in Design. She's going to help me come back up to speed on sharing the dimensional analysis materials.
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Here is the original AST, which illustrates how a vehicle is analyzed in mirror image with ground reflection.
The image ended up at the top of the page.
If you'll look at the bottom row of illustrations, the table, that's 4th from the left depicts what the air 'sees' when your passing by. The above ground, and 'underground' bodies.
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The drag table to the left of the 'mirrored' VW Beetle is from Sighard Hoerner's 1965 book, AERODYNAMIC DRAG. You can see that the lowest drag known occurs at a fineness-ratio of 2.5:1, and Cd 0.04.
All three ASTs are based on the 2.5:1 length divided by diameter streamline body of revolution.
The 'half-bodies' generated from the 'full body' all have Cd 0.08, with ground clearance. When wheels are added, the drag climbs to Cd 0.12.
When extremely 'tight', full, swept wheel fairings are added, the drag falls to below Cd 0.09.
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As to why those who use ASTs to compare only the a vehicles' 'greenhouse', I'd be speculating.
On a LeMans Prototype ( LMP ) type race car, or supercar, in which the width of the greenhouse constitutes only a fraction of the vehicle's overall body width, the designer may be treating this structure merely as a 'blister canopy', as if it originated from a much 'smaller' car, and is then just glommed onto the main body, conserving frontal projected area. Don't know.