A study of the reconnection of antiparallel vortices in the infinitely thin case and in the finite thickness case

TL;DR

This paper investigates the vortex reconnection process for antiparallel vortices, comparing the idealized infinitely thin case with finite thickness vortices, revealing quasi-periodic post-reconnection behavior and dominant frequencies in the Fourier spectrum.

Contribution

It introduces an analysis of vortex reconnection using an infinitely thin vortex approximation and compares it with finite-thickness vortex simulations, highlighting the quasi-periodic nature of post-reconnection dynamics.

Findings

Post-reconnection behavior is quasi-periodic in the thin vortex approximation.

Fourier analysis shows frequencies related to squares of integers dominate.

Finite thickness vortices exhibit reconnection characteristics consistent with Navier-Stokes solutions.

Abstract

The simplest case is the reconnection of a pair of antiparallel line vortices, e.g., condensation trails of an aircraft. The vortices first undergo long wave deformation (Crow waves), and then reconnect to form coherent structures. Although the behavior of the vortices before and after the reconnection can be clearly observed, what happens during the reconnection still needs to be explained. One of the challenges is related to the fact that the vortices have finite thickness, and therefore, the time and the point of the reconnection cannot be determined. Moreover, the smallest scale of coherent structures that can be observed also depends on the vortex thickness. In this paper, we consider an infinitely thin vortex approximation to study the reconnection process. We show that, in this case, the behavior after the reconnection is quasi-periodic, with the quasi-period being independent of the angle between the vortices at the time of the reconnection. We also show that, in the Fourier transform of the trajectory of the reconnection point, the frequencies that correspond to squares of integers are dominating in a similar way as in the evolution of a polygonal vortex under the localized induction approximation. At the end, we compare the results with a solution of the Navier-Stokes equations for the reconnection of a pair of antiparallel vortices with finite thickness. We use the fluid impulse to determine the reconnection time, the reconnection point, and the quasi-period for this case.