Quote:
Originally Posted by NeilBlanchard
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.
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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.
Quote:
Originally Posted by Diesel_Dave
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.
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In theory.
Quote:
Originally Posted by Diesel_Dave
Note how different shapes are given different Cd's, with not mention of thier size.
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That link does mention size, though indirectly. It gives the Reynolds number.
Quote:
Originally Posted by NeilBlanchard
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.
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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.