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When you fill the sink with water and then allow the water to be drained, the water forms a vortex.. And then it starts to follow a curved path downwards by effects of gravity.

Why this phenomena occurs while rain follows a straight line path (in perfect conditions) towards the ground.

I would guess that when the water molecules closest to the drain hole go through first, they create a temporary void causing other water molecules nearby (above and on the sides) to rush in and take their place each having an equal chance to fill that void (since pressure is equal in all directions at a certain point in liquids). So I was thinking more of cone like figure with water collapsing in, equally from each direction. Why the circular path??

Qmechanic
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Force
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2 Answers2

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The difference between rain and water in the sink is that rain is simply falling, while water in the sink is being drawn into a center from a distance away, and the water in the sink is not perfectly still. It is rotating, if only a little bit. As it is drawn to the center, the rotation becomes more rapid.

The principle is Conservation of Angular Momentum.

Here, Ms. Kawaguti speeds up her rotation by pulling in her arms:

enter image description here

Mike Dunlavey
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  • Water in a raindrop is rotating as well. it's overly-simplifying to describe a raindrop as static. In fact, such circulation can influence aerosol dynamics in a significant way, as I recall from related lectures. – New Alexandria Apr 08 '13 at 13:37
  • @New: I'm sure you're right. This is a very intro-level question, where the object is understanding, rather than hitting the fine points. But thanks for that observation. – Mike Dunlavey Apr 08 '13 at 16:11
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    @Mike But what's causing this rotation in the first place, Water in the sink should be still , if not affected by any force to invent the angular velocity. Molecules move randomly in all directions and cancel out each other, but the whole liquid remains there. If you drop a colored liquid in such a sink, before you unblock the drain, it will disperse in every direction, but not rotate until the drain hole opens. So why this direction in particular? – Force Apr 08 '13 at 22:59
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    @Jim: It's almost impossible for it to have absolutely zero rotation. At the very least, it's turning with the earth at about 1/4 degree per minute. If you put in a drop of ink, of course it spreads out, but does the center of the ink blob stay in exactly one place? You can make the drain swirl clockwise or counterclockwise by just slightly stirring it with a spoon before hand, or by putting in some kind of baffle so that as it flows toward the center it is nudged in one direction or the other. You can perceive that the water has no rotation, but it only needs a tiny tiny bit. – Mike Dunlavey Apr 09 '13 at 01:42
  • @Mike Ok so I would sum it all up that the rotational motion gets it's energy from the downward motion, and the direction is triggered by the smallest irregularities in the sink's floor and wall ? – Force Apr 09 '13 at 03:04
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    @Jim: I wouldn't have paid much attention to the sink's floor and wall, but yeah, I think that's right. You always get a vortex, but its size depends on how much angular momentum is in the water. If the water is really really still, then the vortex is really thin, but it's there. – Mike Dunlavey Apr 09 '13 at 12:54
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In basic principle, both could do the same thing.

Pragmatically, water in a drain has the resistance of the sink/drain walls to influence the effect. (This is a hairpin vortex regime.) Basically, vortices differ per sink.

Surface tension of a rain drop exceeds wind friction. Coriolis forces still exist within the rain drop, and could produce a toroidal-like vortex flow therein.

The vortex is a cascade phenomenon influenced by

  1. molecular dynamics,
  2. boundary conditions, and
  3. environmental forces
New Alexandria
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  • How would the sink walls resistance exert such circular motion? – Force Apr 08 '13 at 03:37
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    You have to realize that it concerns angular momentum conservation. Suppose the water starts the flow as a cone, as you say. Friction with the sides may give a perpendicular direction to the cone flow. This induces an angular momentum that is conserved for this flow and is added too as the flow continuous. The angular momentum can be induced from the beginning, in the way the water starts falling, air currents etc. Any asymmetries in forces will induce angular momentum,which will then be conserved and built up . – anna v Apr 08 '13 at 04:23
  • Yes, but the flow I was talking about is radial, inwards which will not stimulate friction with the walls, it will in fact recede from the wall towards the center of the vortex then downwards. If the friction from the walls caused an angular momentum in any direct wouldn't that -if I understand correctly- be canceled out by friction itself in the opposite direction at the same instant stopping the whole process from going any further. Besides this phenomenon occurs also in large sinks where the fluid close the walls have little effect on that near the center, so what am I missing? – Force Apr 08 '13 at 04:41
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    @Jim It starts, imo, mainly where the liquid adheres on the bottom of the sink. Any irregularity at the bottom can induce a force perpendicular to the radial. It will probably start at the surface of the lip of the drain and build up, The angular momentum of the surface is absorbed by the whole sink and will end up as vibrations of the mass of the sink. – anna v Apr 08 '13 at 12:18
  • @Jim you'll enjoy to see this search on hairpin vortex dynamics. Particularly look for the work by Dr. Ron Adrian. – New Alexandria Apr 08 '13 at 13:33
  • @anna Well it seems that the slightest irregularity might cause the whole liquid to rotate all in one direction, but I'm a little skeptical because if the smallest amount of liquid followed that direction started the chain reaction, which sound perfectly logical, in a perfect world where no other forces in the opposite direction cancel out this effect. So this particular sink will always have anticlock wise rotation all the time, am I correct? (I want to ignore the Coriolis effect, of the earth's rotation, this is another matter) – Force Apr 08 '13 at 23:13
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    A lot of factors might influence the original rotational direction to start with, depending on how one opens the hole for the flow, for example when pulling a chain, the direction will not be perfectly vertical but random as to rotational start. I believe if one does a completely controlled experiment : water at rest, hole opened from below smoothly (an iris mechanism opening at the same time radially, no drain underneath to start directional sucking) yes, the individual sink will show the same direction rotational propensity. – anna v Apr 09 '13 at 04:05
  • @Jim you cannot ignore coriolis force, but given the imaginative scenario then, yes, a sink could possibly be predisposed to initiate a certain direction of motion. – New Alexandria Apr 09 '13 at 13:34