vortex ring

noun

a stable perturbation in a fluid that takes the form of a torus in which the flow rotates in the section of the torus so that the pressure difference between the inside and outside of the torus balances body forces. The best-known vortex ring is a smoke ring

He must will the imagined vortex-ring to move forward as if it were actually boring a tunnel through the atmosphere.

A vortex-ring produced in the air behaves in the most surprising manner.

Such a vortex-ring is made by the smoker who purses his lips into a round hole and sends out a puff of smoke.

Now this is just what a vortex-ring would do in an incompressible frictionless fluid.

As though it is foolish to follow a vortex ring into a vacuum, and wise to whack at a dirty golf ball on a suburban railway bank.

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