Vortex

Vortex, in the ordinary meaning of the term, is a whirlpool. We find illustrations of it, on a large scale, in tornadoes, waterspouts, and whirlpools, and, on a smaller scale, in eddies of air or water produced by conflicting currents. If we draw the half-immersed blade of a teaspoon gently along the surface of tea or coffee we shall see two small eddies following in the wake of the edges. These eddies are the visible ends of a half ring of whirling liquid, which is formed originally as a fringe to the blade of the spoon. Similar effects may be observed in rowing as the oar is lifted out of the water at the end of the stroke. The general characteristic of such eddies is a rotational motion of the smallest visible portion in the centre. From this we readily pass to the scientific conception of vortex motion in fluids. Imagine a continuous frictionless fluid, which cannot therefore be molecular, for that would imply Viscosity (q.v.); and suppose any small spherical element to become solidified. If this elementary sphere is found to be rotating about any axis, there is vortex motion in the fluid at the region occupied by the element. If there is no vortex motion, the elementary sphere will have no rotation however complicated the motion of the fluid may be. Von Helmholtz, who first investigated the properties of vortex motion, showed that where vortex motion once exists in a frictionless fluid it must always exist, and where at any instant it does not exist it never can exist. Moreover a vortex with ends cannot exist in the fluid. Either it must form a closed ring or it must continue through the fluid until it stops at the boundary. These properties mathematically proved for the perfect frictionless fluid cannot be fully realised in practice. Indeed the possibility of forming vortex rings depends on the existence of viscosity; and the same viscosity finally destroys the vortex motion so produced. Vortex smoke-rings are often formed at the firing of a gun or mortar or even from the funnel of an engine, or on a smaller scale by a puff of tobacco smoke emitted from the mouth of a skilled smoker. Vortex rings may be produced with great ease by projecting them through an orifice in one side of a closed box whose opposite side is made of cloth or sheet india-rubber or other elastic material. To make the rings visible we may produce inside the box a cloud say of sal-ammoniac; and at every tap on the elastic side of the box a very evident vortex ring will be ejected. When one such vortex ring is following another, the one in front will open out and let the other shoot through it. This one in its turn will open out and slacken speed, while the one that is now behind will contract and accelerate its pace and pass through the other and again get in front. Von Helmholtz showed that this action would go on indefinitely in the perfect fluid. The laws governing the collision and vibration of vortices may also be illustrated by means of smoke rings in air. Lord Kelvin's conception of the material atom as a vortex ring of simple form, or of any degree of beknottedness, is one of the brilliant conceptions of the present era. Such a vortex atom in a frictionless plenum is as indestructible as the hard Lucretian atom, and yet is capable of vibration. See ATOM, HYDRODYNAMICS.