A superfluid boundary layer

TL;DR

This paper models superfluid flow over a rough surface, revealing vortex nucleation and turbulence formation, which provides new insights into superflow boundary layers unlike classical viscous ones.

Contribution

It introduces a numerical model of superfluid boundary layers over rough surfaces, highlighting vortex dynamics and turbulence mechanisms unique to superfluids.

Findings

Rough surface features induce vortex nucleation both intrinsically and extrinsically.

A dense turbulent vortex layer forms with a non-classical velocity profile.

The turbulent layer sheds vortex rings into the bulk flow.

Abstract

We model the superfluid flow of liquid helium over the rough surface of a wire (used to experimentally generate turbulence) profiled by atomic force microscopy. Numerical simulations of the Gross-Pitaevskii equation reveal that the sharpest features in the surface induce vortex nucleation both intrinsically (due to the raised local fluid velocity) and extrinsically (providing pinning sites to vortex lines aligned with the flow). Vortex interactions and reconnections contribute to form a dense turbulent layer of vortices with a non-classical average velocity profile which continually sheds small vortex rings into the bulk. We characterise this layer for various imposed flows. As boundary layers conventionally arise from viscous forces, this result opens up new insight into the nature of superflows.