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'Ideal' shape for low drag
The 'ideal' shape for 2-D flow of struts/symmetrical wing sections was obtained by S. Goldstein,in 1938, and is presented in Fig. 2.28, page 244, of 'AERO-HYDRODYNAMICS OF SAILING, INTERNATIONAL MARINE PUBLISHING,Camden,Maine, copyright 1979, 1988, by C.A. Marchaj.
The drag is governed by surface friction and pressure drag. There exists a single iteration of the struts which embodies the least drag.If it is lengthened,the surface drag increases,along with an overall drag increase. If the strut is shortened,pressure drag increases,increasing the overall drag. This one geometry of struts is in the bottom of the drag 'bucket.' -------------------------------------------------------------------------------------- The same holds true for 3-D flow streamline bodies of revolution. Both surface friction and pressure drag bottom out with a body of Length-divided by Diameter = 2.5. Yielding Cd 0.04. As a half-body,in ground reflection,this body produces Cd 0.08. Adding wheels increases drag to Cd 0.13. According to Hucho,this drag can be reduced by integrating the wheels into the body. According to Goro Tamai of M.I.T.,a proper set of wheel fairings can reduce the wheel drag approximately 70%, allowing in the neighborhood of Cd 0.095. Honda achieved Cd 0.10 in 1994 with their 'Dream' solar racer. The 2013 Cambridge University Eco Racer CUER solar car was measured at Cd 0.11. ----------------------------------------------------------------------------------- The 'template' is intentionally fashioned after the Cd 0.04,2.5 L/D, streamline body of revolution. Hucho says aerodynamics is about pressure drag. Hucho says pressure drag is about separation. The 'template' is incapable of producing separation,by default. Without separation it achieves the drag minimum recognizable,as Hucho puts it.With tighter wheel fairings it's quite likely that she would have mimic'd CUER's Cd 0.11. ================================================ If you want to believe that aerodynamics is a 'black art',I can't stop any of you. The evidence suggests that it's quite easy to construct really low drag automobiles with out any further research or expenditure. Off - the -shelf technology if you will. And this argument would date to 1922. |
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- use rules of thumb (eg wheel fairings can reduce drag by 70 per cent and then applying that figure to another body) - make conceptual jumps from theory to real road cars (eg solar race cars are not practical road cars) - develop strawman arguments (who said aerodynamics was a black art?). |
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Guess what? Hucho is still alive and working. If you want to know what he says, you can ask him! I decided to do that a couple months ago, and he was gracious enough to reply. When I asked him specifically about using his textbook as a guide to reduce the drag of our cars--and remember, as Ecomodders our focus is necessarily limited to modification of existing cars since we aren't OEMs--he wrote back: Quote:
He did not mention boat-tailing; he cautioned that "a posteriory attached details" were not the focus of his work. He advised that I would find the papers published on his website, aerowolf.de, more useful. He also said that "cD = 0.20 seems to be possible for cars manufactured in large volume. And that’s it." That's what Hucho says. If you want to argue that we should all follow the template, fine. Go right ahead, but don't couch it in an appeal to authority. Otherwise you are putting words in someone else's mouth who isn't here to defend him or herself. Own your own words unless you proffer a direct quote with a proper citation. Rant over. |
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Not such a good...........
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Since the 2013,Cambridge University CUER Eco Racer is a 'template' car,and incorporates a full suite of swept wheel fairings,it's Cd 0.11 represents the fruition of this line of investigation/technology transfer. The CUER would make a fine production vehicle. Much of popular automotive journalism espouses that aerodynamics falls within the realm of the 'black arts.' Fed by public relations men from U.S. automakers. The 'conceptual leap' to Spirit allowed for up to 39.9-mpg,up from 26-mpg. This is why aerodynamicists publish. This is what the SAE exists for. |
citations
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https://ecomodder.com/forum/attachme...1&d=1590161983 As for Hucho pages, I think your citation here answers the question, which was only about this post/thread. I will dig out my partial copy (photocopier edition) and see what I see. Cheers. |
sure is
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PS Notice also,that nearly the entire belly of CUER is 'diffuser.' This is something Cybertruck will be able to incorporate,with it's all-wheel air-suspension. |
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In order to understand automotive air dynamics one must throw out most of what know of airplane aerodynamics and only consider WIG(Wing In Ground effect) aerodynamics
This team took the NACA 4412 "best in Ground effect" and optimized it for just above road going vehicle altitudes of 20% cord ground clearance ratio. EG 100" bumper to bumper = belly pan 20" off the deck Shape Optimization of an Airfoil in Ground Effect for Application to WIG Craft Yilei He,1 Qiulin Qu,2,3 and Ramesh K. Agarwal1 1 Department of Mechanical Engineering and Materials Science, Washington University in St. Louis, St. Louis, MO 63130, USA 2 Institute of Fluid Mechanics, Beijing Institute of Aeronautics and Astronautics, Beijing 100191, China 3Washington University in St. Louis, St. Louis, MO 63130, USA Correspondence should be addressed to Ramesh K. Agarwal; rka@wustl.edu Received 31 May 2014; Accepted 19 October 2014; Published 8 December 2014 Academic Editor: Jian-han Liang Even at 80% cord ground clearance ground effect must be considered in design Eg 100" bumper to bumper @80" belly pan clearance is still in full ground effect and free stream aerodynamics cannot apply. https://ecomodder.com/forum/attachme...1&d=1590530532 Free streem optimization of NACA 4412 versus wig optimization of the NACA 4412https://ecomodder.com/forum/attachme...1&d=1590531923 Quote:
Dream big chisel down to reality |
To be honest, I am not really sure how this is relevant to people modifying their road cars. If we were designing solar race cars, maybe. (And even then, I'd look at successful solar race cars, rather than wing in ground effect studies.)
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