Fyzikální ústav Akademie věd ČR

The thermally activated quantum turbulence in He II has been studied, in a canal with superfluid holes not allowing the normal component of a finite viscosity to flow through. Thus, the turbulence in the canal was generated by a longitudinal flow of the pure superfluid component. Besides the already in literature described stationary state of the flow, we discovered a new turbulent state in which the density of quantum vortices is proportional to the transport superfluid velocity, with a parabolic velocity profile like in a classical viscous liquid flow. The internal friction generates at the same time an internal toroidal flow of the normal component. Its existence can be recognized in decay experiments where after the primary fast damping the presence of this flow regime is manifested by exponential decay law. The most complex combination of flow regimes under the study was the flow of the classical viscous fluid and of superfluid, activated by oscillations of an immersed body. The most suitable tool for generating and at the same time detecting such classical and quantum flows proved to be an adapted oscillating quartz crystal, commercially used as a frequency standard. It proved to be an outstanding tool for studying cryogenic fluid flow dynamics. With its help we studied the cavitation process in liquid He I and He II and measured the cavitation threshold in dependence of temperature and pressure. We proved the cavitation optically and explained the striking shift of the cavitation threshold under temperature drop in vicinity of the superfluid transition.
The most important result of this study is the discovery and explanation of the transition from laminar to turbulent regime of the medium resistance in classical and quantum fluids. In classical fluids (gaseous and liquid normal helium) we measured in situ the critical velocities of the transition within three orders of magnitude of the kinematical viscosity and it was found that the critical velocity of the transition scales as square root of the product of the kinematical viscosity and frequency. We also explained this effect theoretically. For a more descriptive understanding of this transition we visualized it by means of Baker’s technique in the geometrically similar conditions in water. We proceeded then to the study of this transition in superfluid He II [A7, B9]. We conclude that the transition to the turbulent regime of the medium resistance consists of two steps - first a dense cloud of arbitrary oriented quantum vortices is formed. It behaves as a quasi-classical fluid of an effective kinematical viscosity. In the second step only the transition to the turbulent regime of the medium resistance occurs, similarly as in classical fluids.