When you pull the stopper....

[Ridseard]

....does your water swirl clockwise or counterclockwise as it goes down the drain? It used to be thought to be consistently in opposite directions in the northern and southern hemispheres, due to the Coriolis force (or some such weirdness).

[amar]

yeah, the coriolis force. but it actually is not true, that's what i read, it doesn't matter on which hemisphere you are, the water will twirl any way it likes.

[Walden]

yeah, the coriolis force. but it actually is not true, that's what i read, it doesn't matter on which hemisphere you are, the water will twirl any way it likes.

You'd think people'd've noticed that before they went around saying the hemisphere thing.

[Ridseard]

Amar and Walden: I don't trust everything I read.

[amar]

yeah, i guess people would want it to be true, because it would be a cool thing, the theoretical basis is quite solid too, only the coriolis force just isn't strong enough to have an effect on the water's inertia.
but, the question is: why does the water go down in a whirpool in the first place? why does it twirl?




well.....who knows....

http://madsci.wustl.edu/posts/archives/ ... .Ph.r.html
http://www.badastronomy.com/phpBB/viewt ... highlight=

[Ridseard]

but, the question is: why does the water go down in a whirpool in the first place? why does it twirl?

Good question. Water normally follows the way of least resistance (the watercourse way = Tao), which should be straight down the drain.

[Walden]

Water normally follows the way of least resistance (the watercourse way = Tao), which should be straight down the drain.

Or, in the case of the toilet, it depends on the angle of the openings where the flush water comes.

[IDAwHOa]

I have done a simple study on this regarding water twirling after hearing the hemisphere theory. One time (big sample eh?) I let the water twirl in the direction it desired. I then forced it to go in the opposite direction. After a short time (hey, I said it was a simple study!!!) it made itself go back to its original desired direction, whatever that was.

Steven - in ABQ, NM for a couple more hours

[burnsbyrne]

In the northeastern part of Ohio the water generally runs to the left. In the rest of the state water runs strongly to the right.
Mike

[Darwin]

but, the question is: why does the water go down in a whirpool in the first place? why does it twirl?

Good question. Water normally follows the way of least resistance (the watercourse way = Tao), which should be straight down the drain.

I don't actually know anything about this, but it appears to involve a kind of symmetry-breaking, due to instability.

Water is not a particularly stable material, so the slightest irregularity in the drain will cause what "should" be a clean column of water going down the drain to deform. One part of the surface of the column will move faster than another, and internal friction will disrupt the flow still further.

So the question then becomes, why a spiral rather than a totally irregular turbulent flow?

According to Stewart and Golubitsky (Fearful Symmetry: Is God a Geometer?, chapter 9):

"In a different way, time-period spiral vortices form from time-independent Couette flow by a breakdown of both spatial and temporal symmettry. ... This kind of pattern-formation has its own pattern: structure is gained as symmetry is lost. Indeed, the general picture that's emerged so far is that, as a system is put under greater stress, the observed states lose symmetry and gain in complexity (both spatial and temporal). ... Indeed, the prevailing theory is that turbulence is a physical manifestation of chaotic dynamics. ... Can symmetry, or pattern, coexist with chaos, or disorder? The two requirements seem contradictory. Yet the answer is 'yes', and it leads to a topic that's well worth explaining in its own right."

The stress that the water in the draining sink is under is the effect of gravity. My guess is that the spiral flow provides the most efficient non-chaotic structure for the water to get down the drain. In the midst of turbulence, bits of spiral structure "randomly" appear, but when they do, their relative efficiency (higher speed) will tend to drag along adjacent areas, resulting in the eventual imposition of the spiral structure on that part of the water nearest the drain--almost like natural selection of competing structures.

The direction of the flow in real systems should be most heavily influenced by the structural details of the container and drain. We have one bathroom sink that is only symmetrical left-to-right. On top of that, the drain plug (one of those linked to a rod that comes up behind the faucet) has four flat blades that no doubt further inhibit the formation of a spiral flow. Here's a rather grainy shot:



Note the tiny tornado-like spiral flow at the two-o'clock position. It took a moment to focus, so the water was about gone by the time the shutter clicked, but this skinny whirlpool formed early and wandered around like a real tornado, except that its base was anchored to that one point. During other tests, this didn't show up at all. No whirlpool formed. The irregularities in the system prevented it.

In order to perform a scientific test, you would have to start with the most perfectly circularly-symmetrical equipment you could manage--and that still might not be sufficient. The water should sit pefectly still before starting the draining, and the opening of the drain shoudn't prejudice the initial flow.

Still, it would be interesting to know if there really is a statistically significant tendency toward a counterclockwise flow under near-ideal conditions. I haven't seen any rigorous attempt to test this, including the one described on the Bad Coriolis site. The counter-example given there is no more scientific than NorCal's (no offense, Steven, it was an interesting result).

[Ridseard]

.....My guess is that the spiral flow provides the most efficient non-chaotic structure for the water to get down the drain.....

Sounds plausible. Good explanation. Thanks, Mike.

[jbarter]

Amar and Walden: I don't trust everything I read.

Hmmm..... I'm not sure I believe this.

[chas]

.....My guess is that the spiral flow provides the most efficient non-chaotic structure for the water to get down the drain.....

Sounds plausible. Good explanation. Thanks, Mike.

The vortex is often the lowest-energy (most stable) state for a system. Coincidentally, a few years ago someone showed that the reason the shower curtain bellows inward is due to vortices formed by the interaction of the shower water, steam, etc. in the shower. He won an Ig Nobel prize for his work.

Some work was done on the bathtub problem in the (I think) '70's using a climate-controlled room, a large symmetric tub with the drain in the middle, and a whole lot of time and patience. In that case, under most conditions the drain water didn't form a vortex. They could get it to swirl either way by applying a small perturbation. They had done a previous analysis and found that the northern-vs-southern hemisphere had no bearing on the direction of swirl in a large sample of bathtubs.

[FJohnSharp]

Straight from The Straight Dope, which is a pretty good debunker.


http://www.straightdope.com/classics/a1_161.html

[amar]

.....My guess is that the spiral flow provides the most efficient non-chaotic structure for the water to get down the drain.....

Sounds plausible. Good explanation. Thanks, Mike.

The vortex is often the lowest-energy (most stable) state for a system. Coincidentally, a few years ago someone showed that the reason the shower curtain bellows inward is due to vortices formed by the interaction of the shower water, steam, etc. in the shower. He won an Ig Nobel prize for his work.

Some work was done on the bathtub problem in the (I think) '70's using a climate-controlled room, a large symmetric tub with the drain in the middle, and a whole lot of time and patience. In that case, under most conditions the drain water didn't form a vortex. They could get it to swirl either way by applying a small perturbation. They had done a previous analysis and found that the northern-vs-southern hemisphere had no bearing on the direction of swirl in a large sample of bathtubs.

true, the vortex does seem to be the most stable and energy-preserving way for water to drain. try out the following, take an empty bottle and fill it with water, turn it upside down to drain it, you'll see total irregularity in how the water drains, making way for the air to enter the bottle, total chaos. now, do the same, but rotate the bottle vigorously and then hold it steady, see now how orderly the water drains. It'll drain faster this way too.

an other riddle just entered my head, but i think i just found the answer to it: why don't you see a vortex/whirlpool when you empty a bottle (in the standard way)?
My best guess: the incoming air while draining a bottle prevents the vortex from forming.
AHHH, Sweden, here I come.