173 research outputs found
The Generation of Turbulence by Oscillating Structures in Superfluid Helium at Very Low Temperatures
The paper is concerned with the interpretation of many experiments that have
been reported recently on the production of quantum turbulence by oscillating
spheres, wires and grids in both 4He and 3He-B at temperatures so low that
there is a negligible fraction of normal fluid. The experimental results are
compared with those obtained in analogous experiments with classical fluids and
with preliminary simulations of the quantum turbulence. Particular attention is
paid to observed values of drag coefficients and to the very different critical
velocities observed in 4He and 3He. It is tentatively concluded that in the
case of 4He behaviour may well be similar to that observed in the classical
analogues, with relatively small changes when the characteristic size of the
oscillating structure is not large compared with the quantized vortex spacing,
but that in the case of 3He behaviour is very different and due perhaps to very
rapid intrinsic nucleation of the quantized vortices.Comment: 13 pages, 9 figure
Kolmogorov spectrum of superfluid turbulence: numerical analysis of the Gross-Pitaevskii equation with the small scale dissipation
The energy spectrum of superfluid turbulence is studied numerically by
solving the Gross-Pitaevskii equation. We introduce the dissipation term which
works only in the scale smaller than the healing length, to remove short
wavelength excitations which may hinder the cascade process of quantized
vortices in the inertial range. The obtained energy spectrum is consistent with
the Kolmogorov law.Comment: 4 pages, 4 figures and 1 table. Submitted to American Journal of
Physic
Quantum Turbulence in a Trapped Bose-Einstein Condensate
We study quantum turbulence in trapped Bose-Einstein condensates by
numerically solving the Gross-Pitaevskii equation. Combining rotations around
two axes, we successfully induce quantum turbulent state in which quantized
vortices are not crystallized but tangled. The obtained spectrum of the
incompressible kinetic energy is consistent with the Kolmogorov law, the most
important statistical law in turbulence.Comment: 4 pages, 4 figures, Physical Review A 76, 045603 (2007
Thermal dissipation in quantum turbulence
The microscopic mechanism of thermal dissipation in quantum turbulence has
been numerically studied by solving the coupled system involving the
Gross-Pitaevskii equation and the Bogoliubov-de Gennes equation. At low
temperatures, the obtained dissipation does not work at scales greater than the
vortex core size. However, as the temperature increases, dissipation works at
large scales and it affects the vortex dynamics. We successfully obtained the
mutual friction coefficients of the vortex dynamics as functions of
temperature, which can be applied to the vortex dynamics in dilute
Bose-Einstein condensates.Comment: 4 pages, 6 figures, submitted to AP
Classical and quantum regimes of the superfluid turbulence
We argue that turbulence in superfluids is governed by two dimensionless
parameters. One of them is the intrinsic parameter q which characterizes the
friction forces acting on a vortex moving with respect to the heat bath, with
1/q playing the same role as the Reynolds number Re=UR/\nu in classical
hydrodynamics. It marks the transition between the "laminar" and turbulent
regimes of vortex dynamics. The developed turbulence described by Kolmogorov
cascade occurs when Re >> 1 in classical hydrodynamics, and q << 1 in the
superfluid hydrodynamics. Another parameter of the superfluid turbulence is the
superfluid Reynolds number Re_s=UR/\kappa, which contains the circulation
quantum \kappa characterizing quantized vorticity in superfluids. This
parameter may regulate the crossover or transition between two classes of
superfluid turbulence: (i) the classical regime of Kolmogorov cascade where
vortices are locally polarized and the quantization of vorticity is not
important; and (ii) the quantum Vinen turbulence whose properties are
determined by the quantization of vorticity. The phase diagram of the dynamical
vortex states is suggested.Comment: 12 pages, 1 figure, version accepted in JETP Letter
Specific heat of the Kelvin modes in low temperature superfluid turbulence
It is pointed out that the specific heat of helical vortex line excitations,
in low temperature superfluid turbulence experiments carried out in helium II,
can be of the same order as the specific heat of the phononic quasiparticles.
The ratio of Kelvin mode and phonon specific heats scales with L_0 T^{-5/2},
where L_0 represents the smoothed line length per volume within the vortex
tangle, such that the contribution of the vortex mode specific heat should be
observable for L_0 = 10^6-10^8 cm^{-2}, and at temperatures which are of order
1-10 mK.Comment: 3 pages, 1 figur
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