The Flow of Water or other Fluid through a Small Orifice, or the Plughole Effect

Recent investigators have suggested that the direction of the vortex produced by the flow of a fluid through a small orifice, depends on the hemisphere of the globe on which the said flow takes place. That is to say it depends on Buy's Ballot Law as does the Geostrophic Wind.

It must be pointed out however that the rotation of the earth does not effect the rotation of a fluid passing through a small orifice. The factor governing the rotation is the magnetohydrodynamic effect, caused by the action of the earth's magnetic field on the pressure gradient across the boundary layer of the moving fluid.

One recent investigator in dealing with water going down the bath plugholes, suggests that the amount of 'crut' in the water has a considerable effect on the circulation. (Halley Bay Express, No.1, Vol. 1, June 21st, 1962). In the opinion of this writer this could well be true and furthermore if this 'crut' is of a ferrous material it could well upset the whole circulation.

The effect of none ferrous 'crut' is to change the density and viscosity (and thus the Kinematic viscosity) of the fluid, and in doing has a considerable effect on the preessure gradient across the boundary layer. In some cases the extent of reversing the direction of rotation of the vortex. It has been noticed by previous investigators that sometimes the circulation will start off in one direction and will then completely reverse. This is because of the thick dirty water is always the last to go out of the bath and there is so much 'crut' in this water that it completely reverses the direction of rotation.

An important point revealed by the magnetohydrodynamic theory is that a magnetic storm can have a devastating effect on the plug hole, and in fact during investigations at Halley Bay has even been known to completely freeze up the orifice thus stopping any kind of circulation. It is therefore advisable that, before having a bath, base members should check with Doug to make sure that there is not a magnettc storm in progress. It should be noted that the vortex motion of a certain bird which is reputed to fly around in ever decreasing circles has nothing to do with the magnetohydrodynamic theory mentioned above.