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

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.

Figures (4)

• Figure 1: (a) Schematic of the computational domain. (b)--(e) Comparison between experimentally reconstructed streamline patterns from particle tracks (top row) Zhang2005Chagovets2013 and simulated normal-fluid streamlines (bottom row) at matched $T$, $q$, and blockage ratio $B\equiv D/H$. Parameters are (b) $T=1.94$ K, $q=50$ mW/cm$^2$, $B=10\%$; (c) $T=1.94$ K, $q=170$ mW/cm$^2$, $B=10\%$; (d) $T=2.03$ K, $q=1120$ mW/cm$^2$, $B=31.75\%$; (e) $T=2.10$ K, $q=167$ mW/cm$^2$, $B=10\%$.
• Figure 2: Simulated superfluid streamlines overlaid on a colormap of the vortex-line density $L(x,y)$. Panels (a)-(d) use the same parameters $T$, $q$, and $B$ as Fig. \ref{['fig:domain']}(b)-(e), respectively. The eddies in (c) are upstream relative to the superfluid flow direction.
• Figure 3: Wake-topology phase diagrams in the $(q,B)$ plane obtained from systematic sweeps at (a) $T=2.04$ K, (b) $T=2.00$ K, and (c) $T=1.94$ K. Shaded backgrounds indicate the dominant physical mechanism, separating viscosity-dominated (gray), inertia-dominated (red), and mutual-friction-dominated (blue) regimes. Symbols mark the observed wake topologies.
• Figure 4: Stratified dimensionless phase diagram compiling all wake-topology points from Fig. \ref{['fig:phase_qB']} together with the experimental data. Bottom: separation onset from the 0-vortex to 2-vortex state plotted in the $(Re_n,B)$ plane. Top: wake topology plotted as the interaction number $N$ versus $B$, showing the 2-vortex region below $N_c\simeq 33$, the 4-vortex region above $N_c$ at larger $B$, and the 6-vortex region above $N_c$ at small $B$. Open and filled symbols denote experimental and numerical results, respectively, and symbol opacity encodes the bath temperature $T$ (colorbar).