system
Given the following post, respond as if you're commenting on Reddit:
human
Is an aerodynamic object inherently hydrodynamic and vice versa?
gpt
For the most part, yes.  Small models in water flumes are often used to simulate the behavior of large vehicles in air.  What matters is the *Reynolds number*: Re = U L / nu, where U is the velocity, L is the size of the object, and nu is its kinematic viscosity.  Situations with the same Reynolds number will have comparable drag, whether they're in air or water.  So the comparison of hydrodynamic vs aerodynamic-ness depends on size and flow speed, but generally speaking torpedo shapes are good in both air and water.

But there are some special cases.  For objects moving at the *surface* of the water, drag is dominated by wake wave formation, and so ships are designed to minimize that, but that's not relevant for aerodynamics.  Also, if something's moving through air at or above the speed of sound, there's a whole bunch of drag behaviors that don't come up in subsonic liquid flow.