What Causes Whirlpool Formation in Draining Water?

[Raza]

Why does a whirlpool form in your bathtub, sink or toilet when the water is draining? And what factors are responsible for which way the whirlpool rotates?
Thanks

 

[G01]

As far as I know, the whirlpool forming in the drain depends only on the initial conditions of the water in the tub, such as the direction it was flowing when the drain was opened, and maybe the type of hardware involved, etc. In other words, only on local small scale parameters, not on atmospheric forces like the Coriolis Force, etc.

I do remember a post on this forum somewhere where someone showed that research was done to show that the Coriolis Force had nothing to do with the direction of rotation for draining water. So, if you were wondering about this specifically, you can rest assured, that this doesn't affect your toilet!

 

[arunma]

Indeed they say the Coriolis force is responsible for the flow of water in bathtubs, toilets, etc. I admit it would explain why they always flow in one direction in the northern hemisphere, but the opposite in the southern hemisphere; but then, I'm guessing that this too is a misconception, especially since the Coriolis force would make the water spin in the opposite direction than what happens in reality.

Angular momentum, however, does have a role to play. Every molecule of water in the bathtub has a certain "orbital" angular momentum about the drain. When you open it, conservation of angular momentum takes over. As each particle gets closer, its speed must increase to conserve angular momentum (since L = mvr). That's why the water speeds up closer to the drain. Eventually the surface of the water breaks, resulting in a vortex.

Well, I hope my rambling helps explain this!

 

[lpfr]

The explanation given by arunma is good, but is just the beginning. I think that there is a positive feedback due to water viscosity. As the water near the sink turns faster, it transfers some of its angular momentum to the neighbor outer water, which will turn faster than it would without the moment transfer. This cumulative process can be observed: at the beginning there is no much rotation. But rotation increases and ends forming a vortex.