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Historically, there is little faith in particle tracking velocimetry (PTV) asa tool to make quantitative measurements of thermal counterflow in He II, sincetracer particle motion is complicated by influences from the normal fluid,superfluid, and quantized vortex lines, or a combination thereof. Recently, weintroduced a scheme for differentiating particles trapped on vortices (G1) fromparticles entrained by the normal fluid (G2). In this paper, we apply thisscheme to demonstrate the utility of PTV for quantitative measurements ofvortex dynamics in He II counterflow. We estimate $\ell$, the mean vortex linespacing, using G2 velocity data, and $c_2$, a parameter related to the meancurvature radius of vortices and energy dissipation in quantum turbulence,using G1 velocity data. We find that both estimations show good agreement withexisting measurements that were obtained using traditional experimentalmethods. This is of particular consequence since these parameters likely varyin space, and PTV offers the advantage of spatial resolution. We also show adirect link between power-law tails in transverse particle velocity probabilitydensity functions (PDFs) and reconnection of vortex lines on which G1 particlesare trapped.

Characterizing vortex tangle properties in steady-state He II counterflow using particle tracking velocimetry