Mykhailenko K. Dissipative processes at resonant excitation of a weakly turbulent flow in superfluid helium

The dissertation is devoted to the experimental study of dissipative processes in laminar and weakly turbulent flow of superfluid helium at low temperatures. The features of the occurrence of a weakly turbulent helium flow are reveale. In a weakly turbulent flow the effect of mutual friction is observed, namely scattering of phonons on quantized vortices. It is shown that the transition from the laminar regime to the turbulent occurs at resonance excitation of the oscillating body of the current of helium and at reaching a critical velocity. It is established that at the same time, the friction force of helium is proportional to the cube of the maximum velocity of the oscillating body. In the framework of the nonlinear Duffing equation, taking into account the action of external force, dissipative phenomena in superfluid helium with an oscillating body are described, in particular, the nonlinearity of the dependence of the velocity of its oscillations on the excitatory force is investigated . The existence of two frequency dependent regimes of viscous friction in the laminar flow of helium in the region of hydrodynamic scattering of thermal excitations has been found. A connection between the mass of helium displaced by the oscillating body and its resonant frequency of oscillations has been established, and it has been shown that in this region the coefficient of the connected mass of helium depends on the frequency and temperature.