Decaying counterflow turbulence in HeII

Systematic investigations of the counterflow turbulence and its decay in three channels of different geometry have been performed using the second sound attenuation technique. The steady-state counterflow turbulence is generated by applying power to the heater placed in the closed end of the channel; when switched-off, the measured temperature difference across the channel quickly decays, in accord with the developed simple model. At all investigated temperatures, after the fast initial decay the second sound amplitude indicates a net increase of the observed vortex line density, which can be understood in terms of depolarization of the vortex tangle. After saturation time, the decay closely follows the classical -3/2 power law and for high enough initial vortex line density the decaying turbulence can be approximated here as homogeneous and isotropic and characterized by an effective kinematic viscosity. The influence of the channel geometry on the form of the observed decay is discussed.