Why does water spin in funnel?

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SUMMARY

The discussion centers on the phenomenon of water spinning in a funnel as it drains, driven by the principles of angular momentum and energy conservation. Initially, water spins slowly due to the conservation of angular momentum, but as the water level decreases, the speed of rotation increases significantly. This acceleration is attributed to the decrease in radius as water exits, which leads to an increase in velocity, demonstrating the relationship between mass, velocity, and radius in angular momentum calculations. The conversation also touches on the effects of viscous forces and torque exerted by the tub on the water.

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  • #31
This is a good problem. I would be satisfied, if it was mathematically shown that the fluid could accelerate and get pushed into a small tube from a large tank, with smaller action by spinning than by going in symmetric manner. By "action" I mean the action of the Hamilton's principle. I have no idea if this indeed is the case, but I hope it is, and that it could be shown with some elegant proof.

If the initial condition is perfectly symmetric, then the water should start get into the tube without spinning, but I am probably not wrong to guess, that this is an unstable flow. That means, a small disturbance will make the flow find another stable extrema of action. So I don't think the Coriolis force claim is unreasonable.
 
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  • #32
This is an interesting question / discussion. May I post the details of an experiment regarding this phenomenon and ask for advice on it?

Thanks,

-Arkham Angel-
 
  • #33
hey guys, i was just wondering if any of this angular rotation would occur if the Earth was not moving? because without a rotational 'nudge', the water would not begin to spin. this fact seems to be neglected here?
 
  • #34
meeeee5 said:
hey guys, i was just wondering if any of this angular rotation would occur if the Earth was not moving? because without a rotational 'nudge', the water would not begin to spin. this fact seems to be neglected here?

It has not been neglected. It has been ruled out. It is not a significant factor.
 
  • #35
The coriolis force seems to be unimportant, unless you're really careful to prevent any initial rotation. Ordinary bathtubs can rotate in the other direction as well, and also rotate on the equator.
It seems that apart from the direction, the speed of the vortex is independent of the amount of initial motion if you wait long enough.
Any torque driving the vortex can only come from the vessel that contains it. The only way that could happen, is if the direction of the flow near the bottom is in the opposite direction.
 

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