Originally Posted by auv-ee
Generally agreed with your post, except this statement. Perhaps you meant something else. The density of water changes very little (some, but not much) with increasing pressure. The form-drag (in the applicable regime) is proportional to the density of the media, and no other property of it. Thus drag is not a significant function of depth.
Well, you might think that, but I couldn't possibly comment...
But will, anyway.
If the viscosity changes significantly with depth (x 2), then Reynolds number does too (for a given free-fall speed).
Form (profile) drag definitely changes with Re, as Re determines the flow regime* (hint: dimples in golf ball make large difference, they
simulate a much higher Re)
Thus, I'd say, even profile drag is depth dependent, although one would have to iterate to get the terminal speed (Re not invariant with speed), which may go up
or down with >> Viscosity. Not going to predict that here..
Agree that profile drag (form drag) is an order of magnitude more than skin friction, but this would still change somewhat
* e.g. Laminar separation bubble; vortex wake; fully attached (inviscid); fully separated flow, etc
NB. This effect is either negligible, small
or if Re near a critical flow regime change (Re crit) > then could make quite a big difference to drag.
Streamlined shapes (auvs en vol de ligne!) at a particular (low) speed range could well, as you say, have effectively depth independent drag.
But certainly not every object at any speed...