Why does water spin in funnel?

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Discussion Overview

The discussion centers on the phenomenon of water spinning in a funnel as it drains, exploring the underlying physics, particularly the roles of angular momentum and energy conservation. Participants examine the behavior of water immediately after a hole is opened compared to later stages of draining, considering both theoretical and practical implications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that angular momentum is preserved as water drains, similar to a skater pulling in their arms to spin faster.
  • Others argue that the increase in spinning speed over time is due to the decrease in the effective radius as water exits, leading to an increase in velocity.
  • A numerical example is presented to illustrate the changes in speed at different distances from the funnel, questioning why speeds increase significantly after a minute despite a small decrease in water mass.
  • Some participants note that angular momentum may not be precisely conserved due to external torques, such as friction from the tub and gravity, which could affect the system.
  • There is discussion about the conservation of energy, suggesting that potential energy lost as water drains could convert to kinetic energy, contributing to the observed increase in speed.
  • Concerns are raised about whether the entire volume of water participates in the spinning motion, with some suggesting that only a smaller volume near the drain should be considered due to inertia and friction.
  • Viscous forces are mentioned as a factor that connects the water near the drain to the rest of the water, influencing the dynamics of the system over time.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the mechanisms behind the spinning of water in a funnel, particularly about the conservation of angular momentum and energy. The discussion remains unresolved, with no consensus on the primary factors influencing the speed increase over time.

Contextual Notes

Limitations include the complexity of defining the boundaries of the system and the interactions between different volumes of water, as well as the potential impact of external forces like friction and gravity on angular momentum.

  • #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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