23 research outputs found
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
Kolmogorov Spectrum of Quantum Turbulence
There is a growing interest in the relation between classical turbulence and
quantum turbulence. Classical turbulence arises from complicated dynamics of
eddies in a classical fluid. In contrast, quantum turbulence consists of a
tangle of stable topological defects called quantized vortices, and thus
quantum turbulence provides a simpler prototype of turbulence than classical
turbulence. In this paper, we investigate the dynamics and statistics of
quantized vortices in quantum turbulence by numerically solving a modified
Gross-Pitaevskii equation. First, to make decaying turbulence, we introduce a
dissipation term that works only at scales below the healing length. Second, to
obtain steady turbulence through the balance between injection and decay, we
add energy injection at large scales. The energy spectrum is quantitatively
consistent with the Kolmogorov law in both decaying and steady turbulence.
Consequently, this is the first study that confirms the inertial range of
quantum turbulence.Comment: 14pages, 24 figures and 1 table. Appeared in Journal of the Physical
Society of Japan, Vol.74, No.12, p.3248-325
From Coherent Modes to Turbulence and Granulation of Trapped Gases
The process of exciting the gas of trapped bosons from an equilibrium initial
state to strongly nonequilibrium states is described as a procedure of symmetry
restoration caused by external perturbations. Initially, the trapped gas is
cooled down to such low temperatures, when practically all atoms are in
Bose-Einstein condensed state, which implies the broken global gauge symmetry.
Excitations are realized either by imposing external alternating fields,
modulating the trapping potential and shaking the cloud of trapped atoms, or it
can be done by varying atomic interactions by means of Feshbach resonance
techniques. Gradually increasing the amount of energy pumped into the system,
which is realized either by strengthening the modulation amplitude or by
increasing the excitation time, produces a series of nonequilibrium states,
with the growing fraction of atoms for which the gauge symmetry is restored. In
this way, the initial equilibrium system, with the broken gauge symmetry and
all atoms condensed, can be excited to the state, where all atoms are in the
normal state, with completely restored gauge symmetry. In this process, the
system, starting from the regular superfluid state, passes through the states
of vortex superfluid, turbulent superfluid, heterophase granular fluid, to the
state of normal chaotic fluid in turbulent regime. Both theoretical and
experimental studies are presented.Comment: Latex file, 25 pages, 4 figure
Quantum Turbulence
The present article reviews the recent developments in the physics of quantum
turbulence. Quantum turbulence (QT) was discovered in superfluid He in the
1950s, and the research has tended toward a new direction since the mid 90s.
The similarities and differences between quantum and classical turbulence have
become an important area of research. QT is comprised of quantized vortices
that are definite topological defects, being expected to yield a model of
turbulence that is much simpler than the classical model. The general
introduction of the issue and a brief review on classical turbulence are
followed by a description of the dynamics of quantized vortices. Then, we
discuss the energy spectrum of QT at very low temperatures. At low wavenumbers,
the energy is transferred through the Richardson cascade of quantized vortices,
and the spectrum obeys the Kolmogorov law, which is the most important
statistical law in turbulence; this classical region shows the similarity to
conventional turbulence. At higher wavenumbers, the energy is transferred by
the Kelvin-wave cascade on each vortex. This quantum regime depends strongly on
the nature of each quantized vortex. The possible dissipation mechanism is
discussed. Finally, important new experimental studies, which include
investigations into temperature-dependent transition to QT, dissipation at very
low temperatures, QT created by vibrating structures, and visualization of QT,
are reviewed. The present article concludes with a brief look at QT in atomic
Bose-Einstein condensates.Comment: 13 pages, 5 figures, Review article to appear in J. Phys. Soc. Jp
Grid turbulence in superfluid He-3-B at low temperatures.
Quantum turbulence consists of a tangle of quantised vortex lines which interact via their self induced flow. At very low temperatures there is no normal fluid component and no associated viscosity. These are very simple conditions in which to study turbulence which might eventually lead to a better understanding of turbulence in general. There are a number of interesting questions, such as how closely does quantum turbulence resemble classical turbulence and how does it decay in the absence of the viscous dissipation. We have recently developed techniques for detecting quantum turbulence in superfluid He-3-B in the low temperature limit. Using a vibrating grid, we find an unexpected sharp transition to turbulence via the entanglement of emitted vortex rings. Measurements also suggest that the quantum turbulence produced by the grid decays in a manner similar to that expected for classical turbulence, but the decay rate appears to be governed by the circulation quantum rather than viscosity