Experimental investigation of the macroscopic flow of He II due to an oscillating grid in the zero temperature limit.

ABSTRACT

A systematic experimental investigation of the macroscopic flow properties of extremely pure He II in the zero temperature limit is reported, covering the pressure range 0.3nickel grid. With growing amplitude of oscillation, the flow changes character at a first critical threshold from pure inviscid superflow past a submerged body of hydrodynamically enhanced mass, to a flow regime that is believed to involve a boundary layer composed of quantized vortex loops. Here the oscillatory motion of the grid acquires strongly nonlinear features. These include double-valued (reentrant) resonance curves and a decrease in the resonant frequency with increasing drive amplitude, but without any appreciable increase in damping. On further increase of the drive level, a second critical threshold is attained here, the resonant frequency reaches a stable value, the response amplitude almost stops growing, but the linewidth increases. Finally, the flow acquires the character of fully developed classical turbulence, characterized by a square-root dependence of flow velocity on the driving force. Additional flow features attributable to the presence of remanent vorticity are observed and discussed.