Mechanically Versus Thermally Generated Quantum Turbulence of 4He Superflow

An experiment on thermally driven He-4 superflow (Chagovets and Skrbek in Phys. Rev. Lett. 100:215302, 2008; J. Low Temp. Phys. 153:162, 2008) yielded very different steady state turbulence properties from a nominally identical recent realisation where the helium fountain drive was replaced by bellows. Here we argue that differences are most likely not due to fundamental reasons, but to incorrect velocity calculation in the fountain drive version. We show by a combination of direct velocity measurements and indirect analysis that the real velocity is significantly lower than predicted, and one main reason for this is unaccounted heat spent to enhance helium evaporation. The velocity correction brings the two experiments in reasonable agreement. We also suggest that the transition to a new state of superfluid turbulence claimed in Refs. (Chagovets and Skrbek in Phys. Rev. Lett. 100:215302, 2008; J. Low Temp. Phys. 153:162, 2008) may be a spurious effect of an observed nonlinearity occurring between fountain heat current and superflow velocity.