Quote:
Originally Posted by stefaantje
I've read in at least one thread here, that Cd is said to be quasi-constant at legal road speeds.
However, this doesn't align with my gut-feeling. If I'd compare a relatively short teardrop shape with a very long one, I imagine they can perform equally well at low speeds.
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Going back to your original question--
I think you're also confusing Cd with drag force, when these are not the same; Cd is a factor in drag force and so is the square of velocity.
A short teardrop and long teardrop will probably see similar
drag force at a low speed, but not because Cd has changed or because the flow characteristics have changed. Assuming your "low speed" metric has Re large enough that the flow field is turbulent, there will be little difference between that and "high speed." But because of the v^2 factor, the velocity makes more difference than Cd anyway.
For example: Consider a short teardrop of cross-sectional area 1.0 m^2 and Cd 0.12, and another of the same area but Cd 0.06. At 60 mph (26.8 m/s) the teardrops will see 52.8 N and 26.3 N respectively. At 30 mph (13.4 m/s), the teardrops will see 13.2 N and 6.6 N respectively, a much smaller difference--but it isn't because their Cd has changed, it's because the velocity has.
Is that what you're asking?