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Dave, you may well be right. But, I do know that if one tests a quarter scale model of a car, you have to increase the velocity and/or the air pressure to get an accurate Cd of what that shape would be full size. This has to do with the Reynolds number; but that is as far as my knowledge goes.
So, for a given shape the size would seem to change the Cd? Or, maybe this is just the CdA that is changing, and that affects the calculation of the Cd?
And I know that Cd is a dimensionless ratio comparing the tested shape with a square flat plate broadside to the air flow.
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