Opposite whirlpools?

[malaka]

If you have a container (say a pasta pot) filled with water and you begin stirring the liquid clockwise just inside the outer edge, you will create a small "whirlpool". Then you take a second spoon (or whatever) and begin stirring the liquid counter-clockwise slightly more towards the middle at the same time. What happens?

(Random thoughts while driving to work this morning)

[Matabiri]

Just as a thought experiment, I might expect one vortex (the one near the centre of the container) to grow at the expense of the other - so the one closer to the edge might start orbiting the pot, shrinking as it went.

I'd go and try it, if I had any pots to hand.

[CurtC]

You might think that you could set up concentric, opposite-turning whirlpools, but you won't be able to. At the boundary where the two would have to slip past each other, the water would be turbulent and would quickly mess things up.

One night, I was washing out my iron skillet after making a batch of cornbread. There were still quite a few crumbs of corn meal on the bottom of the skillet, obviously heavier than water, and when I swirled the water in a circle, I noticed the surprising result that these heavier crumbs, instead of being thrown to the outside of this centrifuge, actually collected right in the middle of the bottom surface of the skillet.

The same effect causes piles of leaves to collect in one spot on my deck. The prevailing wind must swirl around a corner at that point, depositing the leaves in the center of the swirl. It's interesting to me that right out there in the middle of my deck, there will be a big pile of leaves.

[Matabiri]

Ah, I misunderstood what was being proposed.

I withdraw my thought-experiment.

[geni]

I have just done the experiment and you do not get two whirlpools. I migh be interwsting to try this with liquid helium as I suspect you would get two whirlpools.

[T'ai Chi]

Originally posted by malaka
If you have a container (say a pasta pot) filled with water and you begin stirring the liquid clockwise just inside the outer edge, you will create a small "whirlpool". Then you take a second spoon (or whatever) and begin stirring the liquid counter-clockwise slightly more towards the middle at the same time. What happens?

(Random thoughts while driving to work this morning)

Without experimentation, I'd hypothesize that their interference patterns 'cancel out', to completely disallow both whirlpools to continue. I've just personally never seen any of these nested whirlpools in nature, so that might be a hint. :)

[malaka]

To be a bit more clear, I was envisioning flows of water moving in opposite directions.

geni, how were you able to continue stirring in both direections without entangling your hands?

I tend to think that CurtC is correct. I think that the 2 oppositly moving water flows would cause turbulance. However, would this turbulance be restricted to the outer edge of the pot? Would the inner (counter-clockwise) moving flow continue to the middle of the pot?

Does the viscosity of the liquid matter?

What if there was a third "stirrer" moving clockwise within the other 2. Then there would be "outer" and "inner" flows moving clockwise with a "middle" flow moving counter-clockwise. Would the 2 clockwise flows "overpower" the counter-clockwise one?

[geni]

Originally posted by malaka

geni, how were you able to continue stirring in both direections without entangling your hands?

I got the water sping one way fast then relided on momentium to keep it moving while I swilled it the other way. not quite perfect I admit but close enough. I have seen this done properly. When I was at school there was a mixer for the glazes in pottry which involed two stires spining in opsite directions. You did not get to whirlpools

Does the viscosity of the liquid matter?

If the viscosity is above zero it dont think you are going to get two whirlpools. It migh work with liquid helium.

[Matabiri]

Originally posted by geni
I have just done the experiment and you do not get two whirlpools. I migh be interwsting to try this with liquid helium as I suspect you would get two whirlpools.

I think with helium you get a series of micro-vortices formed along the boundary between the flows, but I can't remember who did the work to look it up.