Quantum vortices leave a macroscopic signature in the normal fluid

TL;DR

This paper demonstrates through numerical modeling that quantum vortices in superfluid helium II create macroscopic wakes in the normal fluid, significantly affecting its flow and challenging the traditional passive view of the normal fluid in quantum turbulence.

Contribution

It introduces a fully coupled model showing vortex lines generate large wakes in the normal fluid, revealing their active role in quantum turbulence dynamics.

Findings

Vortex lines produce wakes larger than the average vortex spacing.

Normal fluid flow includes wakes, leading to non-classical velocity statistics.

Evidence suggests independent fluid structures in the normal fluid influence experimental results.

Abstract

Recent work has highlighted the remarkable properties of quantum turbulence in superfluid helium II, consisting of a disordered tangle of quantised vortex lines which interact with each other and reconnect when they collide. According to Landau's two-fluid theory, these vortex lines move in a surrounding of thermal excitations called the normal fluid. Until now, the normal fluid has often been considered a passive background which simply provides the vortex lines with a mechanism for dissipating their kinetic energy into heat. Using a model which fully takes into account the two-way interaction between the vortex lines and the normal fluid, here we show numerically that each vortex line creates a macroscopic wake in the normal fluid that can be larger than the average distance between vortex lines; this is surprising, given the microscopic size of the superfluid vortex cores which induce these wakes. We show that in heat transfer experiments, the flow of the normal fluid can therefore be described as the superposition of an imposed uniform flow and wakes generated by the vortex lines, leading to non-classical statistics of the normal fluid velocity. We also argue that this first evidence of independent fluid structures in the thermal excitations postulated by Landau may be at the root of recent, unaccounted for, experimental findings.