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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??

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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
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How would the sink walls resistance exert such circular motion? –  Force Apr 8 '13 at 3:37
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 8 '13 at 4:23
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 8 '13 at 4:41
@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 8 '13 at 12:18
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 9 '13 at 4:05
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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

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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 8 '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 8 '13 at 16:11
@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 8 '13 at 22:59
@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 9 '13 at 1:42
@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 9 '13 at 12:54
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