[neilep (OP)]

This is probably one of the stupidest queries I've posted but I've just got out the bath so Im all nice and squeaky clean []..now, I did not have one of thise 'Eureka' moments but as I was watching the whirlpool manifest itself in it's own happy spinning way, I wondered on a couple of things....How fast does the whirlpool rotate ? and how fast does the water enter the plughole ?......are they the same speed ?.....I'm talking about the average size bath here with the average amount of water in it........Actually, that's just made me think of something else, the whirlpool thing only happens when the water drops to a certain level. I presume the water goes down the hole a lot quicker when the plug is first pulled, so, an answer for that speed would also be appreciated. Now I don't expect any french people to answer cos we all know they don't bathe[][][], but, any answers out there from bathing orientated people with experience of fluid dynamics would be most appreciated.

my last question ?...Why on a Saturday night am I doing this ?[]

'Men are the same as women...just inside out !'

[Ylide]

I'm not sure about the speed of the whirlpool, but the rate at which the water drains is going to be a function of only the drain size, hydrostatic pressure, and atmospheric pressure.  

It's common belief that it's coriolis effect due to the earth's rotation, but it that doesn't much of enough of an effect unless you're dealing with a large quantity of water.  It's usually the initial conditions of the container that cause the swirling effect.  (i.e. a flow is established when filling the tub or flushing the toilet)  Here's a couple of snippets from an urban legends website that deals with some science topics:

>>>As has been pointed out earlier, the observed swirl is due to residual rotation from when the bath is filled, which can take a LONG time to completely die out. It's easy to rig the results, particularly in a bath with two separate outlets (for hot and cold). To make the earth's rotation noticeable you would need to make the bath larger by a factor of 500 or so, by extending the linear dimensions by 20 or so AND leaving the whole thing to settle for a long time, as they do for the experiments. <<<

>>>This reminds me of a letter to the editor of Science News, when there was a real furor over how the Coriolis force interacts with this "sensitive dependence on initial conditions" water drainage thing. Some guy who was a tourist in Africa wrote that he was at the equator. One of the native folk had a pan full of water, and some leaves floating on the top. He would hold the pan in the air WITH HIS HANDS and let the water drain out the bottom. When he did this twenty feet from the equator, the leaves swirled clockwise, and twenty feet on the other side, it swirled counterclockwise, much to the astonishment of the tourists. When he stood directly on the equator, the leaves and water did not swirl around but just flowed radially into the hole. Someone who responded to this letter made the calculation that unless the guy held it to within one millionth of an arc second to the horizontal, the Coriolis force could not possibly be responsible for the direction of the drainage. He speculated that he just imperceptibly twisted the pan one way or the other, and let the water do the rest. (Anything for the tourists.)<<<




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[chris]

Interesting points cannabinoid but I'm going to disagree !

I refuse to believe that the residual movement of the bathwater (after I've been splashing about in it for 15 minutes) always makes the water go the same way down the plug hole.

Furthermore, as a child, I was fascinated by the whirlpool effect as water went down the drain and I wondered why it always seems to go the same way. So I manually reversed the direction of it with my hand (by making circles in the bath water in the opposite direction).

For a few seconds the whirlpool would rotate in the new direction I had jsut given it, but then it would slow, collapse and splutter a bit, and then resume in the 'normal' direction.

Storms also rotate in opposite directions in the northern and southern hemispheres.

However, i completely agree with your scepticism regarding the tourist-pleaser and his bowl of water at the equator !

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[neilep (OP)]

Jay, like Chris I used to be fascinated by the whirlpool effect, (still am), I once heard that it was a myth that where you were on the planet would effect the rotation direction of the whirlpool.With your hand it is easy to change the flow, well, that's certainly true. I  agree  that during the early stages of the whirlpool when it is weak , and I had changed the direction, it would collapse and then commence in the other direction, but as the water gets lower and the whirlpool larger, then once it has been made to change direction it would stay that way. But, I'm sure you're right Jay that an average bath is just not big enough to accurately empiricise (is there such a word as empiricise? but you know what I mean)the data required.

But I still want to know the actual speed, just an average size bath with an average amount of water...I'm sure with empirical study of this important subject then we could be well on our way to discovering the sought after 'theory of everything'[][][]

'Men are the same as women...just inside out !'

[Ylide]

Chris, here's why the flow changes back:  When you fill the tub, the flow of water into it establishes a net radial flow of all the water in on direction around the tub.  When it's draining, the whirlpool will appear when the water level gets low enough and will be in the direction of that initial flow. Your hand swirling around the drain will locally change the flow...the reason it changes back is because of the force of the net flow....more water molecules have a net angular momentum in that direction and will "overpower" your smaller counterflow.  

Think of it this way:  You're holding a pinwheel on a slightly windy day, facing into the wind.  The wind blows the pinwheel clockwise since it's coming from the back.  You blow on the pinwheel from 6 inches strongly and it changes direction, but stops and turns the other when when you cease blowing.  This is from the net flow of air that you slightly altered in a localized area that changes back to its original state as the larger flow exerts force on it again.

Storms are another story....that large quantity of air and water IS subject to the Coriolis effect, and will certainly rotate with respect to the earth's rotation.  The water in your bathtub will not.

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[tweener]

I'll have to agree with Chris that the Coriolis effect is the causative factor in the whirlpool.  Studying atmospheric physics, we calculated the forces due to the rotation of the earth as a function of latitude (quite extensively) and the effect is small but decisive in everything from hurricanes to bath water.

----
John - The Eternal Pessimist.

[neilep (OP)]

Could one use a very fine length of string or silk or cotton etc , that we know the length of and time it when it starts to enter the plughole until it's all gone ?...or could something with stroboscopic lights help ?, or I suppose a waterwheel type speedy thing that the water rotates.......[|)]

'Men are the same as women...just inside out !'

[Ylide]

http://geography.about.com/gi/dynamic/offsite.htm?site=http%3A%2F%2Fwww.ems.psu.edu%2F%7Efraser%2FBad%2FBadCoriolis.html

http://geography.about.com/gi/dynamic/offsite.htm?site=http%3A%2F%2Fmath.ucr.edu%2Fhome%2Fbaez%2Fphysics%2FGeneral%2Fbathtub.html

To quickly quote the second article:  But this effect is VERY weak for bathtub-scale fluid motions.  The order of magnitude of the Coriolis acceleration can be estimated from size of the "Rossby number" (1) (see below).  The effect of the Coriolis acceleration on your bathtub vortex is SMALL.  To detect its effect on your bathtub, you would have to get out and wait until the motion in the water is far less than one rotation per day.  This would require removing thermal currents, vibration, and any other sources of noise.  Under such conditions, never occurring in the typical home, you WOULD see an effect.

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[neilep (OP)]

Sorry to re-hash this one back, back after just having a bath I couldn't help but be hypnotized by the whirlpool. First it started all scraggly and thin (err..the whirlpool that is !!!) and rotated clockwise when the water was about a foot deep, it continued like this until quite suddenly, at about only 3-4 inches left it stopped and immediately began spinning anti-clockwise where it opened up much wider too...now I know Jay, you have already answered this one about the the change of direction (above somewhere)but in this case, the change of direction was with no intervention from me....any idea how this happenned ?

Cheers



'Men are the same as women...just inside out !'

[gsmollin]

I've seen the counter-clockwise drain whirlpool too. It is always counter-clockwise. That can't be random. But knowing, intuitively, that the coriolis effect has to be tiny, I tried to reverse the flow in a wash-tub. This is a large, deep sink that is square. I made a clockwise whirlpool with the water, then when the whole sink was moving well, I opened the drain. The CW rotation slowed as the water drained, and then reversed just as the water reached a few inches deep. I repeated this experiment numerous times over a period of years with the same effect. I live at 42 degrees N. latitude.

I have always wanted to take that sink to the equator, then to the southern hemisphere, and repeat the experiments.

[DrN]

i saw Michael Palin and which ever program he was doing that took him to the equator - Pole to Pole maybe? they were demonstrating this effect of the direction of the whirlpool with a bucket of water with some small twigs in it (because the effect was very small being only a few metres either side of the equator you needed something to see the flow of the water). it was really interesting, teh water really did change direction depending on which side of the equator they stood. and when they were dead on the line the water just fell through without spinning.

they also had this on the simpsons! remember the really long phone call to australia while the kid checked his water flow or something?  

[Monox D. I-Fly]

Now I don't expect any french people to answer cos we all know they don't bathe[][][],

What, seriously? I did hear that water is rare in French, but still...

[chris]

did hear that water is rare in French

not "rare", eau...

[alancalverd]

Time for some numbers.

The W > E speed of the earth at the equator is about 1000 mph, 1700 ft/sec. At the poles, it is zero. So the gradient is 1700/48,000,000 ft/sec per foot of latitude.

If you bath is 5 ft long and oriented north-south, the coriolis vector from one end to the other is thus 1.8 x 10^-4 ft/sec: two thousandths of an inch per second. Measurable in principle, but insignificant compared with all the other stuff happening in a real bath.