Quantum turbulence visualized by particle dynamics

Abstract

The Lagrangian dynamics of micrometer-sized solid particles of hydrogen and deuterium is investigated in thermal counterflow of superfluid $^{4}\mathrm{He}$ at length scales ${\ensuremath{\ell}}_{\mathrm{exp}}$ straddling about two orders of magnitude across the average distance $\ensuremath{\ell}$ between quantized vortices by using the particle tracking velocimetry technique. The normalized probability distribution functions of the particle velocities and accelerations change from the shapes typical of quantum turbulence, characterized by power-law tails, at length scales ${\ensuremath{\ell}}_{\mathrm{exp}}\ensuremath{\lesssim}\ensuremath{\ell}$, to forms similar to those obtained in classical turbulent flows, at ${\ensuremath{\ell}}_{\mathrm{exp}}\ensuremath{\gtrsim}\ensuremath{\ell}$, although the power-law behavior of the acceleration distribution tails is less clear than that observed for the particle velocities. Moreover, the acceleration distribution follows a nearly log-normal, classical-like shape, at $\ensuremath{\ell}\ensuremath{\lesssim}{\ensuremath{\ell}}_{\mathrm{exp}}\ensuremath{\lesssim}{L}_{\mathrm{int}}$, where ${L}_{\mathrm{int}}$ denotes the integral length scale, providing thus, within the just defined inertial range, experimental evidence of the existence of classical-like, macroscopic vortical structures in thermal counterflow of superfluid $^{4}\mathrm{He}$, which is traditionally regarded as a quantum flow with no obvious classical analog. Additionally, we report our observations of the added mass effect in quantum turbulence and discuss them in the framework of a developed model of particle dynamics.