A Unified Wake Topology Map for He II Counterflow Past a Cylinder

TL;DR

This paper develops a comprehensive model for superfluid helium's complex wake patterns behind a cylinder, revealing how mutual friction and vortex interactions lead to multistable states and upstream eddies, with a predictive phase diagram.

Contribution

It introduces a unified numerical framework capturing all observed wake topologies and uncovers the role of mutual friction in shaping superfluid wake structures.

Findings

Normal-fluid wake states are fully captured by the model.

Superfluid component can develop upstream eddies, previously unreported.

A phase diagram delineates wake regimes based on key parameters.

Abstract

Thermal counterflow of superfluid $^4$He past a cylinder produces quasi-steady eddies not only downstream but also anomalously upstream. However, the mechanism and organizing principles behind the observed multistable wake topologies (0-, 2-, 4-, and 6-vortex states) have remained unclear. We show that the full spectrum of reported normal-fluid wake states is captured numerically with a two-fluid model coupled to Vinen's vortex-line-density equation. Our simulations further reveal that the superfluid component can also develop anomalous upstream eddies, a feature not previously reported. We trace these behaviors to a self-organized zone of enhanced mutual-friction dissipation near the cylinder shoulders that reshapes the effective obstacle, drives upstream eddies in both components, and suppresses intrinsic wake oscillations in the normal fluid. Guided by this mechanism, we perform systematic parameter scans and construct a unified phase diagram in terms of the normal-fluid Reynolds number $Re_n$ and a dimensionless interaction number $N$, separating inertia- and mutual-friction-controlled transitions and delineating the parameter windows for the discrete wake topologies. These results turn a striking phenomenology into a predictive map and establish mutual-friction feedback as a robust route to unusual wake structures in quantum fluids.