Dynamics of Vortex Clusters on a Torus

TL;DR

This paper explores the complex dynamics of vortex clusters on a torus, revealing how their collective behavior depends on topology, vortex sign, and initial configuration, with implications for fluid models involving harmonic velocities.

Contribution

It introduces a Hamiltonian framework for vortex dynamics on a torus using special functions and classifies vortex motion, including geodesics and collective behaviors, in multiply connected domains.

Findings

Vortex clusters exhibit confinement or scattering depending on their sign and configuration.

Fast and slow vortices within a cluster tend to occupy different regions, causing collective drift.

Impurities in vortex clusters can lead to jet-like ejections and re-entries.

Abstract

We investigate the collective dynamics of multivortex assemblies in a two dimensional (2D) toroidal fluid film of distinct curvature and topology. The incompressible and inviscid nature of the fluid allows a Hamiltonian description of the vortices, along with a self-force of geometric origin, arising from the standard Kirchhoff-Routh regularization procedure. The Hamiltonian dynamics is constructed in terms of $q$-digamma functions $\Psi_q(z)$, closely related to the Schottky-Klein prime function known to arise in multiply connected domains. We show the fundamental motion of the two-vortex system and identify five classes of geodesics on the torus for the special case of a vortex dipole, along with subtle distinctions from vortices in quantum superfluids. In multivortex assemblies, we observe that a randomly initialized cluster of vortices of the same sign and strength (chiral cluster) remains geometrically confined on the torus, while undergoing an overall drift along the toroidal direction, exhibiting collective dynamics. A cluster of fast and slow vortices also show the collective toroidal drift, with the fast ones predominantly occupying the core region and the slow ones expelled to the periphery of the revolving cluster. Vortex clusters of mixed sign but zero net circulation (achiral cluster) show unconfined dynamics and scatter all over the surface of the torus. A chiral cluster with an impurity in the form of a single vortex of opposite sign also show similar behavior as a pure chiral cluster, with occasional ``jets" of dipoles leaving and re-entering the revolving cluster. The work serves as a step towards analysis of vortex clusters in models that incorporate harmonic velocities in the Hodge decomposition.