Turbulence in Boundary Flow of Superfluid $^4$He Triggered by Free Vortex Rings

TL;DR

This study explores how free vortex rings can trigger and sustain turbulence in the boundary flow of superfluid helium-4, combining experimental and numerical approaches to understand vortex dynamics and turbulence onset.

Contribution

It demonstrates the role of free vortex rings in initiating and maintaining turbulence in superfluid boundary flow through combined experiments and simulations.

Findings

Vortex rings can attach and expand unstably on oscillating obstacles.

Turbulence persists even after vortex ring injection stops.

Numerical simulations confirm experimental observations.

Abstract

The transition to turbulence in the boundary flow of superfluid $^4$He is investigated using a vortex--free vibrating wire. At high wire vibration velocities, we found that stable alternating flow around the wire enters a turbulent phase triggered by free vortex rings. Numerical simulations of vortex dynamics demonstrate that vortex rings can attach to the surface of an oscillating obstacle and expand unstably due to the boundary flow of the superfluid, forming turbulence. Experimental investigations indicate that the turbulent phase continues even after stopping the injection of vortex rings, which is also confirmed by the simulations.