monsi wrote: ..... football design. The description of the effects of roughness (from seams etc.) on flight distance interested me - "This agitation is essential for fast and reliable flight. A perfectly smooth ball experiences large amounts of drag and high aerodynamic forces." ....
a bluff body eg a ball experiences up to 5 different types of flow according to Reynolds No (speed for a given ball/fluid combination)
these flow types are characterised by very different drag coefficients
at a critical Re adding roughness can change the flow from a high Cd type to a lower Cd type
ie at some speeds a smooth ball will have the same or less drag than a rough ball
at some other speeds the rough ball will have less drag
http://www.grc.nasa.gov/WWW/k-12/airpla ... phere.html
(and a spinning ball will generate eg lift, this is a whole other ball game)
when cars had wooden-spoked wheels roughening the spokes would at race speeds have actually reduced drag
or increasing spoke diameter (so raising Re beyond the critical value) would have also worked
but the above science was not then known
there's very few situations with road vehicles where this method is applicable, eg (note to self) none on bicycles