Features of quasistable laminar flows of He II and an additional dissipative process

Abstract

Quasistable laminar flow of He II at a temperature of 140 mK is studied experimentally. The liquid flow was excited by a vibrating quartz tuning fork with a resonance frequency of about 24 kHz. It was found that for velocities of the tuning fork oscillations from 0.046 to 0.16 m/s, the He II flow can be both quasistable laminar and turbulent. Transitions between these flow regimes were observed. When the velocity of the tuning fork oscillations increases more rapidly, the velocity at which the quasistable flow becomes unstable and undergoes a transition to a turbulent flow is higher. Mechanisms for the dissipation of the energy of the oscillating tines of the tuning fork in the quasistable laminar flow regime are analyzed. It is found that there is an additional mechanism for dissipation of the energy of the oscillating tuning fork beyond internal friction in the quartz. This mechanism is associated with mutual friction owing to scattering of thermal excitations of He II on quantized vortices and leads to a cubic dependence of the exciting force on the fluid velocity.