Reconnections of hairpin vortices

A dense collection of hairpin vortices are seen in visualizations of turbulent flow near walls, and isolated ones are observed upstream during transition. They also appear in the breakdown of round jets as a product of vortex ring instability and transition. The evolution of isolated and interacting hairpin vortices were studied via Direct Numerical Simulations to understand such processes in round jet transition to turbulence. The evolution of an isolated hairpin vortex begins with its tip deflecting due to the large curvature and self-induction, forming a nearly circular head for the hairpin and bringing its limbs together. Locally, two antiparallel vortices have come together and, as in previous studies, these parts merge, forming two bridges connected by threads, and then pull away---described as vortex re-connection. The tip of the original hairpin detaches as a nearly circular ring from a new tip, and the process repeats. This development could have been anticipated from the previous studies of vortex reconnection of antiparallel line vortices. The second set of studies is quite novel and begins with a circular array of hairpins in

the wake of a vortex ring---a model of structures observed in jet breakdown. A very complex evolution of this configuration was seen, driven by several vortex reconnection events. When the hairpins were offset axially, the development was even more complex and indicative of the complex structures that appear during the last, nonlinear stages of rapid breakdown of round jets.