Research done by Ascher Shapiro in the early s in Boston, USA did report a tendency for water to swirl anti-clockwise viewed from above. Scientists in Sydney, Australia copied his method and described seeing clockwise swirling.
The experimental baths were perfectly circular and nearly 2m in diameter. The water was 15cm deep and Shapiro allowed it to stand for 24 hours so that any currents from filling would die down.
The plug was on the outside and could be removed without disturbing the water. A small outlet meant the water took about half an hour to drain away. Search term:. Read more. This page is best viewed in an up-to-date web browser with style sheets CSS enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets CSS if you are able to do so.
This page has been archived and is no longer updated. Find out more about page archiving. Home Hands-on Science. Plugholes Down the drain. Step by step Yan's video guide In order to see this content you need to have both Javascript enabled and Flash installed. Dr Yan shows you how to investigate Plugholes scientifically. A draining bathtub is somewhat different because the water flows out at a rate that is approximately determined by the diameter of the drain, and there is also an open surface at the top, which can develop a conical indentation when there's rotation.
The open surface complicates things, so let's start by considering the case where there is no open surface. Then I think the same considerations apply, at least qualitatively, as in the case of the stream of water from the faucet. The bathtub doesn't have a shape that constricts strongly at the bottom.
It has a nearly constant horizontal cross-sectional area. Presumably there is some very special shape for a bathtub that would allow the water to flow freely downward while satisfying constraints For any other shape, the water has to violate 3 by rotating rapidly.
This would suggest that the vortex forms spontaneously in all cases, and that it has a fixed speed of rotation regardless of the initial conditions. A possible loophole in all this is that according to the experiments and calculations in Andersen , there is a layer of water near the drain hole that goes up and then comes back down. This allows us to have more kinetic energy for a fixed rate of flow. In the case where there's an open surface, this gets even more complicated.
The depth of this indentation is a variable that classifies different solutions. These different solutions have different rates of rotation, as predicted by Andersen. I don't know whether there is a solution for zero rotation. As discussed in the answers to another question , sufficiently careful experiments are able to see the tiny Coriolis effect, which suggests that there is a zero-rotation solution, but it's unstable.
In this experiment , they used two holes and found that only one whirlpool formed. This would seem consistent with my analysis in terms of conservation laws, since conservation laws are additive; only the total has to be conserved.
Andersen gives a complete mathematical analysis and comparison with experiment in the case where a cylindrical container with a hole at the bottom is intentionally rotated. The main effect is angular momentum rotational inertia in the water set up by various movements before you start observing, such as getting out of your bath.
This results in the water level being lower near the centre of rotation than further away, setting up centripital forces which maintain the rotation.
When the difference in levels is significant relative to the average water level, you notice the typical whirl effect. You can think about it like this: It takes one day for the earth to perform a full rotation about 86k seconds , on the other hand, it takes a few seconds for your sink to drain lets say 10 seconds.
So it takes times longer for the earth to do a full rotation than it takes the water to drain down the sink. It is not too hard to imagine that the earth's rotation can have no influence on the process of draining a sink.
However, if the sink was the size of lake Michigan and you were to drain it, Coriolis would play a role. Here is Schapiro's paper but I feel you will need academic access via a university or library to read the full PDF:. The Coriolis effect works for big things like cyclones, but for a bathtub, the slightest asymmetry of the tub, air movement on top, convection current from uneven temperature etc.
Once there is the slightest sideways movement of the water any where around the hole, it will deflect the incoming water as in the diagram. That will cause the incoming water to push the water around causing it to rotate. It's like standing on a wheel. If you are right in the middle, where your weight is straight down towards the shaft, it will not rotate. The Coriolis effect is so weak that it simply cannot measure up to the forces at play in a toilet, tub or sink, where the shape of the container and the effects of residual currents — which can persist for up to a day after filling — tend to dominate.
Shapiro and his colleagues in the Southern Hemisphere demonstrated in that Coriolis could, quite slowly and weakly, affect drainage direction. Did that settle the issue? Decker, professor emeritus of oceanic and atmospheric science at Oregon State University notes, however, that the Coriolis effect may actually have little to do with the behavior of real-world sinks and tubs:. The local irregularities of motion are so dominant that the Coriolis effect is not likely to be revealed.
An empirical test could help. If you had a specially prepared bathtub, the answer would be yes. For any normal bathtub you are likely to encounter in the home, however, the answer is no. Imagine a cannon fired southward from any latitude above the equator.
Its initial eastward motion is the same as that at a point on the spinning earth. This initial eastward velocity is less than that at a point later in its trajectory, because points closer to the equator travel in a bigger circle as the earth rotates. Therefore, the cannon shell is deflected westward to the right , from the perspective of a person standing on the earth. A gunner firing a cannon northward would find that the shell is also deflected toward the right.
These sideways deflections are attributed to the Coriolis force, although there really is no force involved--it is just an effect of being in a rotating reference frame. The circulation directions result from interactions between moving masses of air and air masses moving with the rotating earth.
The effects of the rotation of the earth are, of course, much more pronounced when the circulation covers a larger area than would occur inside your bathtub.
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