Abstract: Steady-state thermal counterflow of superfluid 4-He (He II) is experimentally investigated in a channel of square cross section, with a planar heater placed at its bottom. We focus on the flow region close to the heat source, which has received little attention to date. Relatively small particles of solid deuterium hydride, having a density comparable to that of He II, are suspended in the liquid and their flow-induced dynamics is studied by using the particle tracking velocimetry technique. The comparison with results obtained in similar conditions with solid deuterium particles, which are about 1.4 times denser than He II, confirms that, in the heater proximity, the mean distance between quantized vortices, representing the characteristic length scale of the flow, appears to be about one order of magnitude smaller than that expected in the bulk, at the same temperature and applied heat flux. Additionally, we find that the lighter particles seem to experience a slightly denser vortex tangle, supporting therefore the view that heavy particles tend to stay trapped on quantized vortices for longer times than light ones. In the range of investigated parameters, the heavier particles consequently appear to be more suitable to probe the occurrence of vortex reconnections, deemed to be crucial in explaining energy dissipation mechanisms in quantum flows.

Abstract: We investigate cryogenic flows of liquid 4He between two grids oscillating in phase, at temperatures ranging from approximately 1.3 to 2.5 K, resulting in suitably defined Reynolds numbers up to 10^5 . We specifically study the flow-induced motions of small particles suspended in the fluid by using the particle tracking velocimetry technique. We focus on turbulent flows of superfluid 4He that occur below approximately 2.2 K and are known to display, in certain conditions, features different from those observed in flows of classical viscous fluids, such as water. We find that, at large enough length scales, larger than the mean distance between quantized vortices, representing the quantum length scale of the flow, the shapes of the velocity and velocity increment statistical distributions are very similar to those obtained in turbulent flows of viscous fluids. The experimental outcome strongly supports the view that, in the range of investigated parameters, particles probing flows of superfluid 4He behave as if they were tracking classical flows.