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Old 04-14-2022, 12:36 PM   #117 (permalink)
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40-mph vs 80-mph

Originally Posted by Drifter View Post
Of course. I was being a bit pedantic in pointing out that the coefficient of drag, which we usually assume is one static number, it isn't really static. It varies with the air's speed, density, & viscosity. A rear diffuser shape/angle steep enough to be optimized for 40mph would begin to have some flow separation at 80mph.

Again, the differences are minute so you can just ignore it unless you're super OCD or trying to win a very close competition
I'm gonna get anal-retentive and submit that, at sub-sonic ( below 250-mph ), incompressible flow, and above critical Reynolds number ( about 20-mph for automobiles ), that the Cd will be 'fixed' at all 20-250mph velocities, and separation points will also be fixed.
The context is:
1) testing occurs within a short enough window that air density ( hence viscosity ) doesn't alter in any significant manner. Or any change is monitored, recorded, and calculated for in the data reduction.
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