Physics of solid particles in thermal counterflow

The average properties of Lagrangian motion of test solid particles in helium II counterflow are analyzed. We consider the case where the flow of normal fluid is laminar and uniform, and the turbulence in the superfluid component manifests itself as a tangle of quantized vortices. The model employed in this paper has certain limitations: It assumes that particles do not disturb superfluid vortices and neglects the possibility of trapping particles by vortices. We estimate the time and length scales of the particle motion, and calculate the statistical properties of the particle motion as well as the statistical properties of superfluid turbulence along particle trajectories. We analyze the alignment between particle velocities and the superfluid velocity induced by the vortex tangle, and calculate the statistical properties of proximity between particles and quantized vortices. We expect these statistical properties to be important in the context of experimental PIV measurements in the thermal counterflow