climbandpunishment wrote:Rick wrote:Reynolds Number for 40 kph in 20°C Air for a characteristic length of .5 meters (the bicycle overall ?) is about 3.7 X 10^5
Please correct if I'm mistaken, but from what I understand, I think it's probably most appropriate to use the horizontal length of the bike section of interest for the Reynolds number (though wheels and rotating parts might be different and I'm no expert on those). So the appropriate characteristic length would then be the horizontal length of that individual part of the bike. For bike tubes and speeds where you'd start to worry about aerodynamics, 3.7 X 10^5 is probably close to the upper limit of the Re #s you'd expect, with a lower limit of closer to roughly 5 X 10^4, depending of course on exact tube size and speed.
Both "characteristic lengths" are used in aerodynamic calculations, but for slightly different situations.
If you are measuring surface drag or boundary layer parameters, then the Re is typically length ALONG the unit being tested.
But for overall drag of a body, the Re is usually based on the "cross sectional area"; and since area is length squared, it is usually the diameter of a circle of equal X-sectional area. That is why it is tough to get too precise on complex shaped bodies. That is why emprical measurements still rule in areodynamics. The experimenter could really choose any characteristic length athat is reasonably associated with the body, so long as he clearly states what he is using.
rchung wrote:As for the Reynolds numbers, our empirical findings are that when doing field tests at speeds up to at least the current hour record speed, the CdA appears to be constant.
So that would indicate that the "complex body" is in the "turbulent flow regime", like in the flat section of the "flat disk" in the chart. That makes sense and it is a good thing to keep in mind.
So when different values are obtained for different rider positions are evaluated, are you measuring the actual change in cross-sectional frontal area, or assuming that is constant and the Cd value is what is changing....or are both being tracked ?
Just sort of a rhetorical question. I am not really hung up on the technicalities of the answer; but it seems like some people like to probe the intimate mathematical details.