Water Rotation and Drains
Name: Katheryn F.
Why does water circle down the drain? I remember studying this in high school
physics but can not remember the reason and my young son is now asking me.
There is no "single cause" for the formation of a vortex in a drain. It is due to "turbulent flow",
which really only puts a name on it. As the water moves along the drain and into the sink trap, the
boundary of the water at the pipe walls is slower than in the center of the pipe. The water in the
sink/basin is not stagnant -- that is it has eddy currents either clockwise or counter clockwise
depending upon irregularities in the sink, the location of the faucet with respect to the drain,
temperature gradients,...and a list of other origins. Experiments under controlled conditions have
actually shown that it is VERY difficult to maintain water stagnant in a vessel --
and certainly not in any configuration you might have around the house. For example, you need
baffles in the X,Y,and Z directions. Once the water with a small angular component enters the
mouth of the drain, the velocity gradient in the down pipe accelerates the angular motion and a
The "explanation" you probably heard in high school physics is that the rotation of the earth
produces Coriolis forces that make the vortex flow anti-clockwise in the northern hemisphere
and clockwise in the Southern hemisphere. This is an error that has a life of its own, and
just will not die. A quick calculation of the magnitude of the Coriolis effect on the mass
and time scale of the draining of a sink are WAY too small to account for vortex formation.
The fact that a given sink always may flow in the same direction is most likely due to some
asymmetry in the geometry of the sink and the drain.
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Update: June 2012