Why does water spin in funnel?

  • Thread starter jdg812
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  • #26
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Also the direction of the vortex rotation depends what hemisphere you're on, North or South. That's because the initial, very subtle, average rotation of the water has the same direction as the rotation of the earth.

So if you put the tank on a rotating carousel to begin with, and rotate the carousel very very slowly, you can cause the vortex to rotate accordingly.

If you match the rotation rate of the earth, then in theory no vortex would be formed.

In practise we have unstable dynamics here, so you'd need to control the carousel dynamically based on the feedback from your eyes looking at the water or something, and a human might not be responsive enough for such a highly non-linear system.
 
  • #28
Integral
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Try Focault's pendulum - coriolis effect.
The effect is due to the rotation of the earth on its axis (of rotation...). We always subconciously assume Earth is staionary when it simply isn't.
And it's caused by moving in toward that axis (ever so slightly with Focault pendulum as it falls), and likewise opposite effect when it swings away away from the Earth's axis. It's like trying to keep a straight line as you walk inwards towards the centre of a roundabout or carousel. The fact you're already rotating throws you to one side of the line that you're trying keep.
You could also demonstrate the effect by dropping something verticlly from 100m at the equator. I think it should hit the ground about 1mm to one side.

So basically I think there wouldn't be any torque if you were on a planet that doesn't spin.

The reason the vortex speeds up is probably the positive feedback effect that somebody earlier mentioned. ie it's because all the particles in the water are connected by intermolecular forces :-) and obviously a larger effect from surface tension. -So what happens at the center has a knock on effect on the water further out. Especially true for the water on the surface where moleculr forces are stronger..

my 2cents
If coriolis effect is your claim. please provide some calculations to show the magnitude of this force?
 
  • #29
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If coriolis effect is your claim. please provide some calculations to show the magnitude of this force?

Here's my little bit of help.

At the equator there is no rotational force. That's one way to tell where the equator is.

As for calculations, I think they are best done by using a non-inertial reference frame.
 
  • #30
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If coriolis effect is your claim. please provide some calculations to show the magnitude of this force?

I don't need to.
If you'd watched the first video I linked to, at the end the guys at MIT demonstrate the coriolis effect on water falling down a funnel under idealised conditions. It rotated counter-clockwise and they attribute it to coriolis.

The OP. asked the question in this frame - ie as if in an ideal situation, so that is my answer - coriolis.

Obviously from our own experience we know that the water is just as likely to spiral either direction, so it's fair to assume that when water starts to fall into the vortex the situation is unstable. It's most probably like balancing one football on top of the other, it can fall either way due to the tiniest of displacements. But once started in that direction it will continue and at increasing speed.. It's a positive feedback effect. a.k.a. Unstable equilibrium. Presumably the vortex reaches a final speed dependant on viscosity.

I wonder if there's a maths explanation why the vortex can change direction. I'm pretty sure I saw it change direction last time I emptied the bath... Maybe something like vortex trails off the end of an aerofoil will spin one way and then the other. Each successive vortex that leaves the wing is turning in opposite sense to it's predecessor.

p.s. I can't do the maths, just as you probably guessed, but I trust the experts at MIT could, even way back in 1961.
;-)
 
  • #31
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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.
 
  • #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
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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
DaveC426913
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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
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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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