vortex ring

基本解释

The effect of the viscosity is mainly shown in the diffusion of vorticity and the generation of secondary vortex ring.
粘性的作用主要躰現在渦量的耗散和新的二次渦環的産生；
The results show that the interaction between a viscous vortex ring and a free surface is different from the inviscid one.
結果表明：由於粘性的存在，使得渦環與自由表麪的相互作用與無粘情形相比有許多不同。
Physically, at a special critical wave speed the flow may have a naturally periodical separated-reattached pattern, with each wave trough capturing a stable vortex ring.
物理上看，在一個特定的臨界行波速下會形成自發的周期性分離再附流，每個波穀的分離區捕獲一個穩定的渦環。

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