Quote:
Originally Posted by aerohead
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.  |
I was taught low speed Cd was "fixed" in my undergrad fluid dynamics course 20 years ago, but I've subsequently learned it is more complicated than that. For time trial cyclists, it is not uncommon to see Cd change 0.0015 between 30mph and 20mph. An elite time trialist might have a CdA of 0.180 so we are talking about less than a 1% difference, but in high level competition that can make the difference between 1st and 2nd.
And that is just for 0-yaw in the wind tunnel. Out in the real world there are crosswinds and the slower you go, the higher the yaw angles you will encounter:
The ideal shape of your vehicle/object changes depending on the expected yaw angles. About 15 years ago Freightliner and International each released totally redesigned semi's. Freightliner boasted that its Cascadia was ~8% more aerodynamic than International's Prostar, which it apparently was in a 0 degree yaw wind tunnel. Out on actual highway loops in Colorado & New Mexico cross winds, the Prostar was ~5% more aerodynamic.
Freightliner Cascadia (8% better aerodynamics in a straight line wind tunnel):
International Prostar (~5% better aerodynamics in real world cross winds):
