title: 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 0.3<P<24.8 bar . The flow is generated by electrostatically driven oscillations of a thin, tightly stretched, circular, square-mesh nickel 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.
authors.affiliation: Department of Physics, Lancaster University, Lancaster LA1 4YB, United Kingdom.
authors.forename: H A
authors.initials: HA
authors.lastname: Nichol
authors.forename: L
authors.initials: L
authors.lastname: Skrbek
authors.forename: P C
authors.initials: PC
authors.lastname: Hendry
authors.forename: P V E
authors.initials: PV
authors.lastname: McClintock