Abstrakt
We present an experimental study of the decay of He II turbulence created mechanically, by a bellows-induced flow past a stationary grid in a 7x7 mm(2) superfluid wind tunnel. The temporal decay L(t) originating from various steady-states of vortex line length per unit volume, L (0), has been observed based on measurements of the attenuation of second-sound, in the temperature range 1.17 K < T < 1.95 K.
Each presented decay curve is the average of up to 150 single decay events. We find that, independently of T and L (0), within seconds past the sudden stop of the drive, all the decay curves show a universal behavior lasting up to 200 s, of the form L(t)ae(t-t (0))(-3/2), where t (0) is the virtual origin time.
From this decay process we deduce the effective kinematic viscosity of turbulent He II. We compare our results with the bench-mark Oregon towed grid experiments and, despite our turbulence being non-homogeneous, find strong similarities.