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Name: Katheryn F.
Status: other
Age: 30s
Location: N/A
Country: N/A
Date: 7/14/2003

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 vortex forms.

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.

Vince Calder

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