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  #1  
Old November 24th 09, 02:25 PM posted to rec.bicycles.tech
Gennaro
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Posts: 43
Default 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  
Old November 24th 09, 05:05 PM posted to rec.bicycles.tech
Andy Coggan
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Posts: 296
Default 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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