Density functional theory modeling of vortex shedding in superfluid He-4

TL;DR

This study uses density functional theory to model vortex shedding in superfluid He-4, revealing the microscopic mechanisms and thresholds for vortex emission, and comparing results with experimental data and turbulence phenomena.

Contribution

It introduces a detailed DFT-based simulation approach to analyze vortex ring formation and thresholds in superfluid He-4, highlighting vortex emission over roton emission as the dissipation mechanism.

Findings

Vortex emission threshold decreases with sphere radius.

Threshold increases with pressure, indicating vortex emission as the dissipation mechanism.

Complex vortex dynamics and turbulence are observed at high emission rates.

Abstract

Formation of vortex rings around moving spherical objects in superfluid He-4 at 0 K is modeled by time-dependent density functional theory. The simulations provide detailed information of the microscopic events that lead to vortex ring emission through characteristic observables such as liquid current circulation, drag force, and hydrodynamic mass. A series of simulations were performed to determine velocity thresholds for the onset of dissipation as a function of the sphere radius up to 1.8 nm and at external pressures of zero and 1 bar. The threshold was observed to decrease with the sphere radius and increase with pressure thus showing that the onset of dissipation does not involve roton emission events (Landau critical velocity), but rather vortex emission (Feynman critical velocity), which is also confirmed by the observed periodic response of the hydrodynamic observables as well as visualization of the liquid current circulation. An empirical model, which considers the ratio between the boundary layer kinetic and vortex ring formation energies, is presented for extrapolating the current results to larger length scales. The calculated critical velocity value at zero pressure for a sphere that mimics an electron bubble is in good agreement with the previous experimental observations at low temperatures. The stability of the system against symmetry breaking was linked to its ability to excite quantized Kelvin waves around the vortex rings during the vortex shedding process. At high vortex ring emission rates, the downstream dynamics showed complex vortex ring fission and reconnection events that appear similar to those seen in previous Gross-Pitaevskii theory-based calculations, and which mark the onset of turbulent behavior.