Vinen's latest thoughts on the "bump'' puzzle in decaying He II counterflow turbulence

TL;DR

This paper discusses William Vinen's recent insights into the unresolved 'bump' phenomenon in the decay of quantum turbulence in superfluid helium-4, combining his latest thoughts with recent experimental and numerical findings.

Contribution

It presents Vinen's latest perspectives on the 'bump' in vortex-line density decay, challenging previous theoretical models and integrating recent experimental and numerical results.

Findings

Vinen's new insights question the adequacy of earlier theories.

Recent experiments and simulations provide new data on turbulence decay.

The paper highlights ongoing challenges in understanding quantum turbulence decay.

Abstract

The pioneering work of William F. Vinen (also known as Joe Vinen) on thermal counterflow turbulence in superfluid helium-4 largely inaugurated the research on quantum turbulence. But despite decades of research on this topic, there are still open questions remaining to be solved. One such question is related to the anomalous increase of the vortex-line density $L(t)$ during the decay of counterflow turbulence, which is often termed as the ``bump'' on the $L(t)$ curve. In 2016, Vinen and colleagues developed a theoretical model to explain this puzzling phenomenon (JETP Letters, \textbf{103}, 648-652 (2016)). However, he realized in the last a few years of his life that this theory must be at least inadequate. In remembrance of Joe, we discuss in this paper his latest thoughts on counterflow turbulence and its decay. We also briefly outline our recent experimental and numerical work on this topic.