Particle dynamics in wall-bounded thermal counterflow of superfluid helium

Abstract

The motions of relatively small particles in wall-bounded thermal counterflow of superfluid helium are experimentally investigated, above 1 K, by using the particle tracking velocimetry technique. The effect of a solid boundary on this quantum flow has received little attention to date, and the focus here is on the corresponding flow-induced particle dynamics. The velocity and velocity difference statistical distributions of the particles are computed at length scales straddling two orders of magnitude across the mean distance between quantized vortices, the quantum length scale of the flow. The imposed counterflow velocity ranges between about 2 and 7 mm/s, resulting in suitably defined Reynolds numbers up to 20 000. The distributions are found to be wider in the bulk than close to the solid boundary, at small enough scales, and this suggests that the mean distance between the vortices increases with the distance from the wall. The outcome reinforces the view, supported to date solely by numerical simulations, that in thermal counterflow quantized vortices are not homogenously distributed in the channel and that they preferentially concentrate close to its walls. Boundary layers might therefore also exist in quantum flows, although some of their features appear to be significantly different from those attributed to wall-bounded flows of viscous fluids, due to the presence of quantized vortices.

Strong anisotropy of superfluid He4 counterflow turbulence

“…The counterflow velocity U ns = U n −U s creates a random vortex tangle with an energy spectrum, peaking at the intervortex scale ℓ and with a close to Gaussian statistics, as demonstrated experimentally in Refs. [14][15][16] and rationalized theoretically in Refs. 18,33 .…”

“…The function D(k, θ) in Eqs. (16) describes the level of decorrelation of the normal-fluid and superfluid velocity components by the counterflow velocity. Within the approximation (16b), D(k, θ) defines the rate of energy dissipation caused by mutual friction:…”

“…Recent advances in the visualization techniques offer for the first time a direct access to the statistics of the velocity fluctuations of the normal fluid [11][12][13] and the superfluid [14][15][16] . It was shown that the large scale statistics of the normal fluid in the counterflow is very different [11][12][13] from the statistics of classical fluids.…”

Statistics of turbulence and intermittency enhancement in superfluid He4 counterflow

“…Other studies 15,16 measured the statistics of the superfluid component. Recent theoretical analysis 17 found that the energy spectra in counterflow turbulence are not scale-invariant and cannot be rigorously connected with apparent scaling exponents of the second-order velocity structure functions measured in Ref.…”

Theory of energy spectra in superfluid He4 counterflow turbulence

“…The statistics of turbulent fluctuations was inaccessible. Only few years ago, with the development of breakthrough experimental visualization techniques, the studies of the turbulent statistics of the normal [17][18][19] and superfluid 20,21 components in the 4 He counterflow become possible.…”