Vortex rings are commonly seen in nature as the smoke rings from a cigarette or the bubble rings from a whale. They are a unique fluid dynamical phenomenon that consists of a vortex in a closed loop; as the vortex entraps and circulates fluid, it propagates forward. When two dyed vortex rings are fired into each other head-on at high speed (i.e. at a high Reynolds number), they expand radially before suddenly exploding into a puff of “smoke.” Experimentally, we image the full three-dimensional dynamics of this collision through the use of high-speed scanning light sheet microscopy. We observe that as the rings expand, the cores of the vortices generate a circumferential array of vortex filaments before rapidly breaking down into a turbulent cloud.
In order to better understand how these flow structures emerge and break down, we also perform direct numerical simulations that examine the close-range interaction between two counter-rotating vortex tubes. As the vortex tubes interact, they develop a series of anti-symmetric, short-wavelength perturbations, indicative of an elliptical instability. Once these perturbations develop, they rapidly eject an array of high-vorticity secondary vortex filaments, which bridge the gap between the vortex tubes. Because each of these secondary filaments is antiparallel (i.e. counter-rotating) to its neighbors, adjacent filaments interact with each other in the same manner as the original vortex tubes except on a smaller scale and at a faster rate. If the initial Reynolds number of the vortex tubes is sufficiently high, these interacting secondary filaments form another generation of even smaller tertiary vortex filaments that are, themselves, perpendicular to the secondary filaments. This iterative breakdown mechanism captures how the head-on collision of vortex rings rapidly generates small-scale flow structures. More importantly, this mechanism could provide a deeper understanding of how highly inertial vortices interact and break down to small scales in turbulent flows.