Mystery of the Sink Vortex: Investigating Why Water Spins

[vin300]

I have observed that when water goes down the sink, a vortex forms. Why doesn't water just down, not in a vortex?

 

[trollcast]

http://science.howstuffworks.com/nature/climate-weather/storms/tornado1.htm

A couple of paragraphs into the page has quite a good explanation of what is going on:

"Why does the water start rotating? There are many explanations, but here's one way to think about it. Imagine yourself as a particle in the water, suddenly pulled toward the suction that the drain creates. At first, you'd find yourself accelerating toward the drain. Then, quite literally, there's a twist. Because of your previous momentum and the number of other particles rushing toward the drain at the same time, chances are that you're going to be pushed off to one side of the point of suction when you arrive. That deflection sets you on a spiraling path into the point of suction, like a moth spiraling in toward a light. Once the spiral has started in one direction, it tends to influence all the other particles as they arrive. A very strong spiraling tendency is created. Eventually, there's enough spiraling energy to create a vortex."

 

[Doc Al]

See: Shapiro’s Bathtub Experiment

 

[vin300]

So this is controversial stuff.

 

[0xDEADBEEF]

It is not really controversial. Even the fact that there is a Coriolis force is agreed upon. The only thing that is controversial is if its magnitude is enough to dominate the spinning direction under lab conditions. The main driver of the vortex is the conservation of angular momentum. Just like the figure skater who speeds up when she draws the arms close, the minute rotation of the water gets amplified by being pulled to the sink.

 

[vin300]

So now there are two explanations-1)Either the momentum entering into the sink is not same from all directions or 2)The Coriolis effect

 

[juniorcarty]

http://science.howstuffworks.com/nature/climate-weather/storms/tornado1.htm

' Once the spiral has started in one direction, it tends to influence all the other particles as they arrive. A very strong spiraling tendency is created. Eventually, there's enough spiraling energy to create a vortex."

If this was the case, the spiral would either take a clockwise or anti-clockwise rotation in a 50/50 split expectation. However, the spiral ALWAYS adopts a clockwise rotation in the northern hemisphere and alternatively, an anti-clockwise rotation in the southern. Illustrating that it is the rotation of the Earth which is the major influence on why the water adopts a spiral down the plug hole in the first place.

 

[etudiant]

If this was the case, the spiral would either take a clockwise or anti-clockwise rotation in a 50/50 split expectation. However, the spiral ALWAYS adopts a clockwise rotation in the northern hemisphere and alternatively, an anti-clockwise rotation in the southern. Illustrating that it is the rotation of the Earth which is the major influence on why the water adopts a spiral down the plug hole in the first place.

Is there a reference to that assertion?
I'd love to know who did the experiments and whether they were also done near the equator, there the effect should then be randomly clockwise or counterclockwise.

 

[juniorcarty]

Is there a reference to that assertion?
I'd love to know who did the experiments and whether they were also done near the equator, there the effect should then be randomly clockwise or counterclockwise.

This article in Scientific American explains the phenomenon
http://www.scientificamerican.com/article.cfm?id=can-somebody-finally-sett
It was also demonstrated as true by Ascher Shapiro, a researcher at MIT in 1962.

 

[Doc Al]

This article in Scientific American explains the phenomenon
http://www.scientificamerican.com/article.cfm?id=can-somebody-finally-sett
It was also demonstrated as true by Ascher Shapiro, a researcher at MIT in 1962.

Realize that this reference contradicts what you said earlier:

If this was the case, the spiral would either take a clockwise or anti-clockwise rotation in a 50/50 split expectation. However, the spiral ALWAYS adopts a clockwise rotation in the northern hemisphere and alternatively, an anti-clockwise rotation in the southern. Illustrating that it is the rotation of the Earth which is the major influence on why the water adopts a spiral down the plug hole in the first place.

Shapiro had to take great pains to demonstrate Coriolis in a draining tub.

 

[juniorcarty]

Yes. Got my clockwises and anti-clockwises in a twist. However, still confirms that the rotation of the Earth is an influence on the direction of the rotation.

 

[A.T.]

 

[davenn]

in anything other than well controlled lab conditions, the water vortex will be in either direction in either hemisphere

been there, tried that ... was the first thing ( well almost ) that I tried when getting to hotel room in LA, California some yrs ago

Dave

 

[Nugatory]

If this was the case, the spiral would either take a clockwise or anti-clockwise rotation in a 50/50 split expectation. However, the spiral ALWAYS adopts a clockwise rotation in the northern hemisphere and alternatively, an anti-clockwise rotation in the southern. Illustrating that it is the rotation of the Earth which is the major influence on why the water adopts a spiral down the plug hole in the first place.

It is simply not true that the spiral is always clockwise in the northern hemisphere and counterclockwise in the southern. This experiment has been done many times (I've done it myself, as an exercise in constructing proper controlled experiments in college) and the result is always the same: if you control all the random variables, of which the most important is what motion you impart to the water as you fill the tub, it is completely random which way the vortex rotates.

This is not a surprising result, as you can calculate the approximate magnitude of the Coriolis force on the opposite sides of a vortex a few centimeters across in water moving at a few centimeters per second, and it is nowhere near enough to influence the movement of the water. It's a good exercise to compare the magnitude of this force with the force produced if a few stray paramecia happen to be lashing their flagellae back and forth in the vicinity of the drain when you open the drain and the water starts flowing.

 

[A.T.]

This is not a surprising result, as you can calculate the approximate magnitude of the Coriolis force on the opposite sides of a vortex a few centimeters across in water moving at a few centimeters per second, and it is nowhere near enough to influence the movement of the water.

That's what I always think too. On the other hand these pools they used in the videos above are about the size of an common Foucault pendulum, which precesses noticeably during the time it takes to empty such a pool.

The video description says they repeated each experiment 3 times, but it's not clear if they rebuild the pools 3 times to randomize any bias that might come from the shape of the pool. Especially the geometry of the drain seems crucial here. I hope many people will try to reproduce it.