Instability and reconnection in the head-on collision of two vortex rings

ONE mechanism by which fluid flows increase their complexity is through the instability of vortex filaments. When an instability brings vortex filaments of opposite circulation together, the filaments may break and rejoin in a process known as reconnection. This process of instability and reconnection leads to some fundamental changes in the topology of flows. Here we present experimental observations of a special type of instability in which two colliding vortex rings become unstable and reconnect to form a series of smaller rings. Although this phenomenon was briefly noted more than a decade ago¹, no detailed observations were made, and little is known about the mechanisms involved. We have used coloured dyes to reveal the detailed structure of the small rings and many other features, including a short-wavelength instability around the circumference of the colliding rings. At high Reynolds number, collision leads to a turbulent cloud, with the occasional appearance of small rings.