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Often in sinks, toilets, bathtubs, etc., the water is already moving, though

maybe very slowly. As the water drains, that slow rotation becomes more

visible. In the case of a sink or bathtub, where the width of the pool

decreases as it drains, the rotation increases speed as it drains. This is

because angular momentum is conserved (think of a figure skater speeding up

as she pulls her arms into her body -- it is the same effect). In a draining

tank, the water by the drain has the least momentum. As it leaves, the

remaining water with more momentum takes its place. If an object with

angular momentum moves closer to the center of its rotation, it speeds up.

So, the water gradually picks up speed, and you see rotation.

Even if there is *no* angular momentum to start with, fluid can start to

rotate. This is called a "secondary flow". To understand why secondary flows

develop, you have to understand the nature of viscosity. When molecules of a

liquid are attracted to each other, they resist being pulled apart. For

example, when you move your hand through water, you are dragging molecules

of water with your hand. Some of the resistance is simply the inertia of the

water, but much of it is viscosity. The molecules that you accelerate pull

on the ones next to them, and those next to them, etc. Moving water

molecules across each other, a 'sideways' force, is called 'shear'.

Resistance to shear is called viscosity.

Imagine a large cylindrical vessel full of water with a small drain at the

bottom. When you open the drain, water starts to flow down the hole. Of

course you have taken great care to ensure the water is completely

stationary first, and that opening the drain does not perturb it. As the

water flows downward, it drags the molecules around it due to viscosity. At

the point of the drain, some of the molecules go down the drain, but other

ones cannot fit. Yet, they have still gained some energy by being dragged by

the ones that did go down the drain. They have to go somewhere, and since

they cannot go down, and gravity makes it hard to go up, they go sideways.

Over time, they start a rotational flow, called 'secondary flow'. Over a

short period of time, viscosity, caused by the molecules' mutual attraction

to each other, ensures that they move together in the same direction. In

time, a vessel with a drain full of stationary water will develop a quite

noticeable rotation due to secondary flow.

Another place where secondary flows commonly occur is in tea. Here, the

opposite occurs: a rotational flow causes vertical motion. If you drink

green tea, watch the leaves as you stir it. Even though you are stirring the

tea in a rotational direction, you can see the leaves are pushed upward. The

upward motion, caused by viscosity in response to the rotation, is another

example of secondary flow. Of course, depending on how you stir, it could be

your spoon moving them up, not secondary flow. So be careful. :)

A lot of research has been performed to understand how and when secondary

flows occur. It turns out that any viscosity gradient can cause a secondary

flow. Sometimes secondary flows are hard to see (they can be very

small/slow), but they are there! This is the underlying reason for the

'swirl' you see.

If you travel from the northern hemisphere to the southern, you will notice that water 'swirls' counterclockwise. Simply enough, it is because of the rotation of the earth acting on the water in conjunction with gravity.

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โˆ™ 2011-01-13 03:46:22
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Q: Why does water run down a bathtub drain in a swirl?
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Related Questions

Why does water run down the bathtub drain like a swirl?

I think there are several factors that can cause the swirl. First, it is not, as many people erroneously think, the Coriolis forces. A tub is much too small for the Coriolis force to have any effect on the motion of the water. The location of the faucets and the shape of the tub probably have the the most effect. If the water already has some motion, it will cause the water to swirl in that direction.

What part of speech is the word Bathtub in the sentence Did my smelly soap slip down the bathtub drain?

In this usage, "bathtub" is an adjective, modifying the noun "drain."

Will running the hot water kill a spider in the bathtub?

Yes. If you scald him or flush him down the drain.

How do i unclog a bathtub that has dried paint stuck down in it to let the water drain?

via push it

Why dont people fall out og roller coasters at the amusement park?

Water swirls down a drain because the hole that the water is sucked down into is small to create a tornato. Water swirls down a drain because of gravity force. Water swirls down a drain because the world rotates all the time, creating a round or swirl movement in a drain.

What is wrong when a commode is flushed and water backs up in the bathtub?

The drain is blocked further down than both. You need to 'snake' the drain.

What are some ways to unclog a bathtub drain?

The first step to unclog a bathtub drain is to remove the drain cover and clean under the surface of the drain. Next, use a wire hanger or purchase a special drain claw and use it to clear the clog. If the drain is still clogged up, pour some boiling water down the drain.

When you flush the toilet fills with water to the top then gushes down but water comes up in the sink and bathtub what causes this?

The sewer line is plugged or collasped. The sink and bathtub may drain because they drain at a slower rate. Toilet is all at once and the drain can't handle it. Water has to go somewhere so it backs up in the sink and tub.

My son put a plastic shampoo cap down the bathtub drain It was working fine but now no water will go down at all I know that chemicals and hot water won't break down the plastic what will work?

Calling a licensed plumber to remove it

How many Ethiopians can you fit in a bathtub?

This is a joke and it is answered as; None, since they keep slipping down the drain.

How does a bathtub overflow work?

A quick way would be to drop into a plumbing supply store. They often have tubs on display with the overflow drain hooked up. You will be able to see where the overflow water goes down the drain. Des Perado

How do you thaw a bathtub drain?

Pour hot salt water down it, or alcohol if that fails. You can also just try putting a space heater next to it to thaw it externally if you can get access to the pipe.

When water drains out of the shower bathtub why do you hear a really loud ping ping ping noise in the wall across from the shower bathtub as it goes down the drain fast at first slower as it empties?

You are probably hearing the hot water hitting the cold pipe which causes expansion thus the pinging sound. Drain speed could probably be clogged vent stacks try cleaning them out.

In what country does the water swirl down the bathtub drain in a clockwise motion?

Well anything in the southern Hemisphere will be going In a Counter-Clockwise motion and anything in the northern Hemisphere will be going clockwise. All this due to the inward centripetal force the earth exerts while spinning. The southern Hemisphere has seasons opposite to ours... so while our Christmas is cold and white, Down under it's probably hot. Australia I think

What is the purpose for a bathtub overflow drain?

To make sure the bathtub does not overflow. There is a long discussion of this for another question and I do not intend to repeat that here. A properly installed unit will drain some water from an overfilled tub down through the waste water drain. This is done under gravity and is not guaranteed to keep up with the pressurized water supply that fills the tub. This contradicts the assumption that it prevents overflow. Any four-year-old can demonstrate this empirically, fifteen years before learning about proof by contradiction in an undergraduate mathematics class.

Why do drains make noise when the water is going down?

Drains make noise when the water is going down because the water hits the drain as the water goes down. The water also is going down at a rapid pace, that makes the same sound as a waterfall. When the water is falling to the direct drain, the water beats on the drain.

What does the idiom 'go down the drain' mean?

It's as if you were washing something away and it drained down the sink or bathtub drain. It's gone, it's not coming back, and there's no sense trying to get it back.

How pour the water down the drain in runescape?

Right click the water then select use then click the drain.

What do bathtubs and sinks have for the water to go down?

The water goes down the plughole/drain and then down the pipes.

Which way does water drain?

down hill

Where water goes down in a sink?

the drain.

What is the noun form of drain?

The word drain is a noun (drain, drains) or a verb(drain, drains, draining, drained). Examples:noun: The water ran down the drain.verb: I pulled the plug to drain the water.

What does water run down?

In a conventional household, water from a kitchen or bathroom sink flows down a drain. This drain leads to a storage compartment that is periodically emptied.

What Bathtubs and sinks have for the water to go down 5 letter word?

It is the drain.

Where will faucet water go after you use it?

Down the drain