Investigations of turbulence in a liquid helium II counterflow jet

Using the phase information in a high-frequency second-sound beam, it was possible to investigate the temperature and velocity in a counterflow jet a few diameters downstream of the jet exit. At the heat fluxes investigated, the jet was characterized by turbulent velocity fluctuations. No temperature difference could be measured between the jet and the surrounding fluid, within the experimental accuracy (∼ ${10}^{-5\mathrm{}}$ K). Velocity-sensitive phase measurements suggest that the normal-fluid jet entrains the surrounding superfluid to minimize any relative velocity and mutual friction between the two fluids. To our knowledge these observations constitute the first direct measurements of liquid-helium fluid velocities, as they enter the equations of motion.