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.
粘性的作用主要体现在涡量的耗散和新的二次涡环的产生；
The results show that the interaction between a viscous vortex ring and a free surface is different from the inviscid one.
结果表明：由于粘性的存在，使得涡环与自由表面的相互作用与无粘情形相比有许多不同。
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，在一个特定的临界行波速下会形成自发的周期性分离再附流，每个波谷的分离区捕获一个稳定的涡环。

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