Southern Hemisphere

Why does water run down a bathtub drain in a swirl?

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2011-01-13 03:46:22

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


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

speeds up.

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


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


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


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


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


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


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

flow'. Over a

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


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


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

a secondary

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


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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