Visualization of the normal-fluid turbulence in counterflowing superfluid 4He

TL;DR

This paper introduces a novel visualization technique using triplet-state He2 molecular tracers to study normal-fluid turbulence in superfluid helium-4, revealing laminar and turbulent flow regimes and their characteristics.

Contribution

The paper presents a new femtosecond-laser-based method for visualizing normal-fluid flow in superfluid helium-4, enabling detailed analysis of turbulence transition and flow profiles.

Findings

Normal fluid remains laminar at low velocities with distorted profiles

Transition to turbulence occurs at higher velocities

Turbulent state exhibits a distinct structure function from conventional turbulence

Abstract

We describe a new technique, using thin lines of triplet-state He2 molecular tracers created by femtosecond-laser field-ionization of helium atoms, for visualizing the flow of the normal fluid in superfluid 4He, together with its application to thermal counterflow in a channel. We show that, at relatively small velocities, where the superfluid is already turbulent, the flow of the normal fluid remains laminar, but with a distorted velocity profile, while at a higher velocity there is a transition to turbulence. The form of the structure function in this turbulent state differs significantly from that found in types of conventional turbulence. This visualization technique also promises to be applicable to other fluid dynamical problems involving cryogenic helium.