Visualization of the normal-fluid turbulence in counterflowing superfluidHe4

Abstract

We describe a new technique, using thin lines of triplet-state He * 2 molecular tracers created by femtosecond-laser field-ionization of helium atoms, for visualizing the flow of the normal fluid in superfluid 4 He, 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.

Numerical Studies of Quantum Turbulence

“…For example, Marakov et al have observed the flow profiles of normal fluid in thermal counterflow in a square channel by following the motion of seeded metastable He * 2 molecules using a laser-induced fluorescence technique [35]. Figure 3 shows how the normal fluid profile changes as the heat flux is increased.…”

“…The visualization experiment of thermal counterflow by Marakov et al showed that the normal fluid velocity distribution becomes a Poiseuille profile, a tail-flattened profile, and a turbulent profile [35], as explained above. Recall that the Melotte-Barenghi assumption argued that the laminar normal flow corresponds to the T1 state of QT [20,27].…”

“…This study investigated a mean velocity profile in a superfluid boundary layer, motivated by numerical studies for spatially inhomogeneous thermal counterflow [74,75,78,91] and visualization experiments [35,93,94,95,96,97,98]. As a result, they found that the logarithmic velocity profile could appear in wall-bounded QT.…”

“…The second reason is based on the recent visualization experiments. Marakov et al [35] recently observed that the normal fluid profile showed different states such as Poisueille, tail-flattened, and turbulent states. Understanding these states also requires numerical studies of the system in a channel.…”

“…Furthermore, they studied the transient state from the initial state to the steady state and found that the normal fluid velocity distribution becomes quasiparabolic profile in the transient state, which is approximately uniform in the central region. Galantucci et al concluded that their numerical model predicts the shape of the profile of normal fluid that has been experimentally observed in channels using laser-induced fluorescence of metastable helium molecules [35]. However, in the 2D simulation, we must inject quantized vortices into the system, and the results of simulation will change based on the choice of method of injection.…”