Editing Vortex ring (section)

===Instabilities===
A kind of azimuthal radiant-symmetric structure was observed by Maxworthy<ref>Maxworthy, T. J.  (1972) ''The structure and stability of vortex ring'', Fluid Mech. Vol. 51, p. 15</ref> when the vortex ring traveled around a critical velocity, which is between the turbulence and laminar states.  Later Huang and Chan<ref>Huang, J., Chan,  K.T. (2007)  ''Dual-Wavelike Instability in Vortex Rings'', Proc. 5th IASME/WSEAS  Int. Conf. Fluid Mech. & Aerodyn., Greece</ref> reported that if the initial state of the vortex ring is not perfectly circular, another kind of instability would occur. An elliptical vortex ring undergoes an oscillation in which it is first stretched in the vertical direction and squeezed in the horizontal direction, then passes through an intermediate state where it is circular, then is deformed in the opposite way (stretched in the horizontal direction and squeezed in the vertical) before reversing the process and returning to the original state.{{Citation needed|date=June 2021}}