1,193 research outputs found
Dynamics of the vortex line density in superfluid counterflow turbulence
Describing superfluid turbulence at intermediate scales between the
inter-vortex distance and the macroscale requires an acceptable equation of
motion for the density of quantized vortex lines . The closure of such
an equation for superfluid inhomogeneous flows requires additional inputs
besides and the normal and superfluid velocity fields. In this paper
we offer a minimal closure using one additional anisotropy parameter .
Using the example of counterflow superfluid turbulence we derive two coupled
closure equations for the vortex line density and the anisotropy parameter
with an input of the normal and superfluid velocity fields. The
various closure assumptions and the predictions of the resulting theory are
tested against numerical simulations.Comment: 7 pages, 5 figure
Multifractal analysis of stress time series during ultrathin lubricant film melting
Melting of an ultrathin lubricant film confined between two atomically flat
surfaces is we studied using the rheological model for viscoelastic matter
approximation. Phase diagram with domains, corresponding to sliding, dry, and
two types of friction regimes has been built taking into account
additive noises of stress, strain, and temperature of the lubricant. The stress
time series have been obtained for all regimes of friction using the
Stratonovich interpretation. It has been shown that self-similar regime of
lubricant melting is observed when intensity of temperature noise is much
larger than intensities of strain and stress noises. This regime is defined by
homogenous distribution, at which characteristic stress scale is absent. We
study stress time series obtained for all friction regimes using multifractal
detrended fluctuation analysis. It has been shown that multifractality of these
series is caused by different correlations that are present in the system and
also by a power-law distribution. Since the power-law distribution is related
to small stresses, this case corresponds to self-similar solid-like lubricant.Comment: 22 pages, 10 figures, 41 reference
Superfluid Helium in Three-Dimensional Counterflow Differs Strongly from Classical Flows : Anisotropy on Small Scales
Three-dimensional anisotropic turbulence in classical fluids tends towards isotropy and homogeneity with decreasing scales, allowing-eventually-the abstract model of homogeneous and isotropic turbulence to be relevant. We show here that the opposite is true for superfluid He-4 turbulence in three-dimensional counterflow channel geometry. This flow becomes less isotropic upon decreasing scales, becoming eventually quasi-two-dimensional. The physical reason for this unusual phenomenon is elucidated and supported by theory and simulations.Peer reviewe
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