Effect of acoustic emission on the critical velocity for the transition to turbulent flow in He II

The conditions for the transition from laminar to turbulent flow in superfluid 4He are investigated experimentally, and the effect of acoustic emission with variable power on the critical velocity for the transition is studied. The quartz tuning fork method is used at temperatures of 2–0.3 K. The experiments are done over a wide range of pressures, from the saturated vapor pressure to 24.8 atm. It is found that at high temperatures (T > 0.9 K) the critical velocity is determined by viscous friction and at low temperatures (T < 0.5 K) by the effect of acoustic emission, which leads to a significant increase in the critical velocity for the transition to the turbulent state. The critical velocity depends on the power of the acoustic emission and the transition to the turbulent state of the superfluid is similar to that in ordinary liquids or gases. In the absence of any effects of acoustic emission, the critical transition velocity is essentially independent of temperature and the driving power is mainly de...