Fully Coupled Two-Fluid Dynamics in Superfluid He4 : Anomalous Anisotropic Velocity Fluctuations in Counterflow

Abstract

We investigate the thermal counterflow of the superfluid 4 He by numerically simulating threedimensional fully coupled dynamics of the two fluids, namely quantized vortices and a normal fluid. We analyze the velocity fluctuations of the laminar normal fluid arising from the mutual friction with the quantum turbulence of the superfluid component. The streamwise fluctuations exhibit higher intensity and longer-range autocorrelation, as compared to transverse fluctuations. Those anomalous fluctuations are consistent with the experiments. The fluctuations will trigger the transition of the normal fluid to turbulence unlike single-component flows.

Bathtub vortex in superfluid He4

“…Microscopically, this assumption does not hold. Recent studies [31][32][33][34] have shown that the normal fluid profile is nontrivially modulated by the presence of quantized vortices, through mutual friction on the scale of the intervortex distance. However, in the analysis below, we consider only the macroscopic vortex bundle structure that develops in the macroscopic steady normal flow; a study of the characteristic small-scale structures that emerge due to coupled dynamics remains future work to be dealt with.…”

Ubiquity of particle–vortex interactions in turbulent counterflow of superfluid helium

“…2000; Yui et al. 2020), in the close proximity of moving quantized vortices. However, these wake structures were observed at relatively small fluid velocities, that is, their topology in dense vortex tangles, which are specifically relevant for the present study, is currently unknown.…”

“…This might be related once more to the relatively small size of the data sets collected in the past and/or to the fact that these studies were performed at relatively small heat fluxes (Yui et al. 2020).…”

“…Based on this observation, they claimed that these particles behave as if the vortex tangle were absent, 10 -4 10 -2 but our results suggest a different physical picture, that is, both fast and slow particles appear to interact with the vortex tangle, at least in the range of investigated parametersseveral arguments supporting the close relation between velocity distribution heavy tails and particle-vortex interactions are presented by La . Additionally, the latter interactions could be influenced by the the normal fluid vortical structures observed in numerical simulations (Idowu et al 2000;Yui et al 2020), in the close proximity of moving quantized vortices. However, these wake structures were observed at relatively small fluid velocities, that is, their topology in dense vortex tangles, which are specifically relevant for the present study, is currently unknown.…”

“…They specifically found that horizontal and vertical velocity fluctuations of fast particles are qualitatively different, with the latter larger than the former, but this is not apparent from our data, that is, we do not see any consistent dependence of velocity fluctuations on experimental parameters. This might be related once more to the relatively small size of the data sets collected in the past and/or to the fact that these studies were performed at relatively small heat fluxes (Yui et al 2020).…”