Why does water run down a bathtub drain in a swirl?
Often in sinks, toilets, bathtubs, etc., the water is already
maybe very slowly. As the water drains, that slow rotation
visible. In the case of a sink or bathtub, where the width of
decreases as it drains, the rotation increases speed as it
drains. This is
because angular momentum is conserved (think of a figure skater
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
remaining water with more momentum takes its place. If an object
angular momentum moves closer to the center of its rotation, it
So, the water gradually picks up speed, and you see
Even if there is *no* angular momentum to start with, fluid can
rotate. This is called a "secondary flow". To understand why
develop, you have to understand the nature of viscosity. When
molecules of a
liquid are attracted to each other, they resist being pulled
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
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
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
the point of the drain, some of the molecules go down the drain,
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,
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
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.
opposite occurs: a rotational flow causes vertical motion. If
green tea, watch the leaves as you stir it. Even though you are
tea in a rotational direction, you can see the leaves are pushed
upward motion, caused by viscosity in response to the rotation,
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
flow. Sometimes secondary flows are hard to see (they can be
small/slow), but they are there! This is the underlying reason
'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.