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, L0, 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 L0, 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−t0)−3/2, where t0 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
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