Whirlpools and Vortices

Abstract

AT the Friday evening discourse at the Royal Institution on May 22, Prof. E. N. da C. Andradel discussed “Whirlpools and Vortices”. The best example of what is ordinarily thought of as a vortex is the whirlpool that is formed where the water runs out of a bath or basin. There is a widespread belief that this always spins one way, clockwise or counter-clockwise, and that this is due to the rotation of the earth. Actually, however, if the water is allowed to come to complete rest in a vessel where the hole is accurately in the middle, and if the plug is carefully withdrawn, no whirlpool is formed. The whirlpool that is generally observed is due to some slight rotation given to the water near the edge of the vessel, which, by the laws of fluid motion, becomes very much accentuated where the water runs out. The direction of the spin depends on the direction of this slight motion. If the spin is actually always in the same direction in a particular bath, it is because of some chance influence, such as the position of the taps. The spin of the earth should actually give rise to a rotation of the water where it runs out, but the effect is far too small to be observed with a bath or basin. Where a large still sheet of water runs out through a vertical pipe, as at the intake of a water turbine, the effect should, however, be large enough to observe. It is, in fact, found that under these conditions whirlpools form, and that the spin of the water is in an opposite direction in the northern and in the southern hemisphere, as it should be. The consideration of vortices is extremely important for aircraft or other bodies moving through the air, and the whole object of stream-lining is to avoid the formation of vortices, which fritter away energy. The sensitive flame, which ducks in response to sounds, is, it turns out, an example of vortex motion, the vortices being produced as a consequence of the sound vibration.