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WIND
"Jobst Brandt" wrote...
Andy Coggan wrote: It might appear that side winds create more or less drag than direct headwinds or tailwinds as a result of a different rider and bicycle profile that determines a drag coefficient. Practically the aerodynamic profile is the same from any direction Because all parts of the rider, including arms, legs, torso and head, as well as all major bicycle parts, are round. Using a round model called a bluff body to compute drag power from any wind directions was verified in the wind tunnel (Fig 2), and has been used for these calculations. The "bluff body" assumption is a gross oversimplification. I think deciphering what is meant by bluff body is important in this discussion. A bluff body (the rider and his bicycle) are essentially round cross sections who's drag is governed more by cross sectional shape rather than orientation to the wind, having no airfoil tear-drop streamlining. Therefore, the bluff body has similar drag in all directions (in line-, cross-, and tailwinds.) http://www.efluids.com/efluids/pages/bicycle.htmop http://www.efluids.com/efluids/pages/bicycle.htm Even if a cylinder is indeed a bluff body, a bluff body is by no menas whatsoever bound to have round cross section (a brick is a bluff body!), as properly explained in the web pages that you yourself linked above. Furthermore, a bluff body does not necessarily have a drag coefficient independent of flow direction - see for example cube and angled cube he http://en.wikipedia.org/wiki/Drag_coefficient It would be good if you strived to avoid such gross inaccuracies in your "didactic" posts. I do understand that it is tempting to model a cyclist+bike as a series of vertical cylinders so that they all look the same from any direction of the air, wrap this assumption under a (misleading) "technical" term, and give it as an explanation why cd for a bike is essentially the same in any direction, but this shouldn't be done. (I don't know if this is the case, but it might actually vary only slightly with direction).. Jobst Brandt Gennaro |
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#2
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WIND
On Nov 24, 7:25*am, "Gennaro" wrote:
I do understand that it is tempting to model a cyclist+bike as a series of vertical cylinders so that they all look the same from any direction of the air, wrap this assumption under a (misleading) "technical" term, and give it as an explanation why cd for a bike is essentially the same in any direction, but this shouldn't be done. (I don't know if this is the case, but it might actually vary only slightly with direction).. It had long been known that the Cd of a cyclist varies with wind direction. Jobst just refuses to accept it. Andy Coggan |
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