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Old 07-28-2012, 12:00 PM   #30 (permalink)
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Originally Posted by NeilBlanchard View Post
Jack, you may have that backwards -- if you shrink the vehicle (but don't change the shape) then the Cd will improve, I think. Punching a bigger hole in the air cannot improve the Cd.
Sure it can, and usually does. If you double the frontal area of a vehicle, but only increase its drag by 1.98, you’ve dropped the Cd by 1%. Cd varies with Reynolds number (in general, as speed and/or size increase, drag increases at a slightly lower rate than the Reynolds number, which is why Reynolds number matters when designing for low drag) and if we’re comparing vehicles at a given speed, the bigger vehicle has the higher Reynolds number.
Originally Posted by Diesel_Dave View Post
size doesn't affect Cd at all. That's the whole point. Same shape = same Cd. A 1x1x1 cube in theory has exactly the same Cd as a 2x2x2 cube. The frontal area of the 2x2x2 cube is 4 times larger so the total drag is 4 times larger, but the Cd is the same.
In theory.
Originally Posted by Diesel_Dave View Post
Note how different shapes are given different Cd's, with not mention of thier size.
That link does mention size, though indirectly. It gives the Reynolds number.
Originally Posted by NeilBlanchard View Post
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.
Right you are. In theory, Cd is a constant regardless of size or speed. Using Reynolds number in drag calculations gets the theoretical results closer to observed results. So in theory you’re right, Neil and Dave, but there’s a difference* between theory and practice, and though Cd by itself is a useful cocktail-napkin-calculation tool, it misses out on some subtleties that show up in wind tunnels (and the real world), such as how increasing size (at automobile-scale size and speed) reduces Cd.

*The difference between theory and practice is: in theory there is no difference, and in practice, there is.
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