Finding a reconnection in a vortex-haystack

Abstract

Counter-rotating vortex reconnection is a topology-transforming event that leads to strong vorticity amplification and is often associated with the most irregular regions of the velocity field. Detecting such events directly in turbulent flows is a major challenge due to the ambiguity inherent in the definition of a vortex and the rapid proliferation of intense small scale vortices. We develop a novel methodology by combining two tools. The first is a local in scale, space and time regularity indicator that isolates spatio-temporal regions of interest. The second is a reconnection indicator that exploits a well-known property of vortex reconnection namely, that the enstrophy (volume integral of the square of vorticity) flows from the symmetry to the collision plane. To examine the flow of enstrophy in the correct reference frame, a new method inspired by orientational order parameters in liquid crystals is developed. We then show a proof of concept for our reconnection indicator by comparing it with diagnostic methods available in the literature. The method is validated on experimental datasets from a turbulent von Kármán flow at two different resolutions, one of which allows us to probe the flow below the Kolmogorov scale, $η$. Our findings suggest that counter-rotating vortex reconnection could be a potential mechanism for the creation of motions at scales below $η$.