A lot of the time when I'm at the pool I like to make whirlpools with my hand, like this:
From my limited understanding of fluid mechanics, vortices/vortex filaments/vorticity tubes cannot end within the fluid in a perfect fluid, it can only close in on itself or end at a boundary. Vortices in a real fluid are similar, but not quite due to viscosity, etc.
But find it hard to imagine the vortex I create having another end on the water surface, since part of it should (I think) be inside the circle I make when my hand spins my hand would repeatedly interrupt the vortex.
I can think of a handful of explanations for this. Maybe the vortex really just ends in the fluid, but this doesn't seem right. The whirlpool keeps spinning for a few seconds after I stop moving my hand and I feel like an 'incomplete vortex' would not last that long.
Maybe it misses my hand entirely and connects back to the surface unbroken, possibly splitting into several smaller vortices.
Maybe my hand really does cut through the vortex, since my skin is a boundary, and the vortex reconnects after my hand passes through.
Probably some combination of these.
