Testing the performance of a cryogenic visualization system on thermal counterflow by using hydrogen and deuterium solid tracers.

An experimental apparatus has been designed to analyze by visualization cryogenic flows of liquid (4)He and consequently address unresolved problems of quantum turbulence. The newly implemented flow visualization setup is described and its specific features discussed. Thermal counterflow experiments have been performed and the motion of solid hydrogen and deuterium tracers studied by using the particle tracking velocimetry technique in order to probe the system capabilities. It is shown that the obtained results are consistent with the two-fluid model describing the behavior of superfluid (4)He. A number of technical and fundamental issues, such as particles' aggregation, role of rotating particles in counterflow and evidence of non-Gaussian distribution of tracers' velocities, are also discussed. The apparatus appears to be well-suited to the task of analyzing cryogenic flows and potentially capable of obtaining new results stimulating further understanding of the underlying physics.

Steady and decaying flow of He II in a channel with ends blocked by superleaks.

We report a new thermally generated steady-state flow of He II and its decay. An upward superflow through vertical channels of a square cross section with ends blocked by sintered silver superleaks is induced by a fountain pump. On increasing the heat power Q[over ] a vortex-free superflow and, above an intrinsic critical velocity of about 1.5 cm/s, two distinctly different turbulent steady-state flows characterized by the vortex line density L proportional, variantQ[over ];{2} and L proportional, variantQ[over ] are observed using the second sound attenuation technique. When Q[over ] is switched off, after a fast initial decay an exponential decay law is observed. A simple phenomenological model is introduced, capturing the observed features.

Effective kinematic viscosity of turbulent He II.

The temperature dependence of the effective kinematic viscosity of turbulent He II, nu(eff)(T) , is deduced from second sound attenuation data using the late stage of decay of thermally induced counterflow He II turbulence in two channels of square cross section. It is shown to qualitatively agree with the published data for nu(eff)(T) calculated based on experiments on decaying-grid-generated He II turbulence [Niemela, J. Low Temp. Phys. 138, 537 (2005)]. Corrections to these data due to the "sine squared" law that describes attenuation of the second sound wave propagating along an arbitrary direction with respect to the direction of the core of a quantized vortex in turbulent He II are discussed and applied.