Asymmetry of motion: vortex rings crossing a density gradient

TL;DR

This study investigates how vortex rings behave when crossing a density interface in stratified fluids, revealing asymmetries caused by baroclinic vorticity that affect their penetration and evolution.

Contribution

It demonstrates that vortex ring propagation across density gradients is asymmetric due to baroclinic effects, challenging previous assumptions of symmetry in stratified fluid dynamics.

Findings

Baroclinic vorticity significantly influences vortex ring propagation.

Symmetry between vortex rings traveling in opposite directions is broken.

Maximum penetration occurs against the density gradient due to pressure-density misalignment.

Abstract

Vortex rings are critical for thrust production underwater. In the ocean, self-propelled mesozooplankton generate vortices while swimming within a weakly stratified fluid. While large-scale biogenic transport has been observed during vertical migration in the wild and lab experiments, little focus has been given to the evolution of induced vortex rings as a function of their propagation direction relative to the density gradient. In this study, the evolution of an isolated vortex ring crossing the interface of a stable two-layer system is examined as a function of its translation direction with respect to gravity. The vortex ring size and position are visualized using Planar Induced Fluorescence (PLIF) and the induced vorticity field derived from Particle Image Velocimetry (PIV) is examined. It is found that the production of baroclinic vorticity significantly affects the propagation of vortex rings crossing the density interface. As a result, any expected symmetry between vortex rings traveling from dense to light fluids and from light to dense fluids breaks down. In turn, the maximum penetration depth of the vortex ring occurs in the case in which the vortex propagates against the density gradient due to the misalignment of the pressure and density gradients. Our results have far-reaching implications for the characterization of local ecosystems in marine environments.