The Decay of Forced Turbulent Coflow of He II Past a Grid
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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 7×7 mm2 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)∝(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.
KeywordsQuantum turbulence Decay Superfluid 4He Second-sound Grid
We acknowledge the valuable help of M. Rotter and J. Šebek with the development of cryostat, flow source, and thermometry; of L. Doležal for the construction of grid and flow conditioner; and of F. Soukup for instrumentation. We also thank J. Boháč and P. Vacek for smooth supply of liquid helium. We acknowledge financial support of Charles University in Prague under GAUK No 366213 and SVV-2013-267303.
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