Noun
vortex ring (plural vortex rings)
A toroidal vortex in a fluid or gas, i.e. a region where the fluid mostly spins around an imaginary axis line that forms a closed loop.
Vortex rings are rarely noticed unless the motion is revealed by suspended particles, as in smoke rings.
By 2012, changes had been made to the V-22's hardware, software, and procedures in response to hydraulic fires in the nacelles, vortex ring state control issues, and opposed landings. Source: Internet
The disturbance in the jet is a symmetric vortex ring that moves at some speed slower than the mean jet speed until it encounters the hole and some fluid is forced through it, resulting in a monopole-like sound field in the half space outside. Source: Internet
By doing so, the second vortex ring rolls under it and begins to spin faster. Source: Internet