4,200 research outputs found
Quantum turbulence at finite temperature: the two-fluids cascade
To model isotropic homogeneous quantum turbulence in superfluid helium, we
have performed Direct Numerical Simulations (DNS) of two fluids (the normal
fluid and the superfluid) coupled by mutual friction. We have found evidence of
strong locking of superfluid and normal fluid along the turbulent cascade, from
the large scale structures where only one fluid is forced down to the vorticity
structures at small scales. We have determined the residual slip velocity
between the two fluids, and, for each fluid, the relative balance of inertial,
viscous and friction forces along the scales. Our calculations show that the
classical relation between energy injection and dissipation scale is not valid
in quantum turbulence, but we have been able to derive a temperature--dependent
superfluid analogous relation. Finally, we discuss our DNS results in terms of
the current understanding of quantum turbulence, including the value of the
effective kinematic viscosity
Evolution of a Network of Vortex Loops in HeII. Exact Solution of the "Rate Equation"
Evolution of a network of vortex loops in HeII due to the fusion and
breakdown of vortex loops is studied. We perform investigation on the base of
the ''rate equation'' for the distribution function of number of loops
of length proposed by Copeland with coauthors. By using the special ansatz
in the ''collision'' integral we have found the exact power-like solution of
''kinetic equation'' in stationary case. That solution is the famous
equilibrium distribution obtained earlier in
numerical calculations. Our result, however, is not equilibrium, but on the
contrary, it describes the state with two mutual fluxes of the length (or
energy) in space of the vortex loop sizes. Analyzing this solution we drew
several results on the structure and dynamics of the vortex tangle in the
superfluid turbulent helium. In particular, we obtained that the mean radius of
the curvature is of order of interline space. We also obtain that the decay of
the vortex tangle obeys the Vinen equation, obtained earlier
phenomenologically. We evaluate also the full rate of reconnection events.
PACS-number 67.40Comment: 4 pages, submitted to PR
Reconnection and acoustic emission of quantized vortices in superfluid by the numerical analysis of the Gross-Pitaevskii equation
We study numerically the reconnection of quantized vortices and the
concurrent acoustic emission by the analysis of the Gross-Pitaevskii equation.
Two quantized vortices reconnect following the process similar to classical
vortices; they approach, twist themselves locally so that they become
anti-parallel at the closest place, reconnect and leave separately.The
investigation of the motion of the singular lines where the amplitude of the
wave function vanishes in the vortex cores confirms that they follow the above
scenario by reconnecting at a point. This reconnection is not contradictory to
the Kelvin's circulation theorem, because the potential of the superflow field
becomes undefined at the reconnection point. When the locally anti-parallel
part of the vortices becomes closer than the healing length, it moves with the
velocity comparable to the sound velocity, emits the sound waves and leads to
the pair annihilation or reconnection; this phenomena is concerned with the
Cherenkov resonance. The vortices are broken up to smaller vortex loops through
a series of reconnection, eventually disappearing with the acoustic emission.
This may correspond to the final stage of the vortex cascade process proposed
by Feynman. The change in energy components, such as the quantum, the
compressible and incompressible kinetic energy is analyzed for each dynamics.
The propagation of the sound waves not only appears in the profile of the
amplitude of the wave function but also affects the field of its phase,
transforming the quantum energy due to the vortex cores to the kinetic energy
of the phase field.Comment: 11 pages, 16 figures, LaTe
Unexpected impact of D waves in low-energy neutral pion photoproduction from the proton and the extraction of multipoles
Contributions of waves to physical observables for neutral pion
photoproduction from the proton in the near-threshold region are studied and
means to isolate them are proposed. Various approaches to describe the
multipoles are employed
--a phenomenological one, a unitary one, and heavy baryon chiral perturbation
theory. The results of these approaches are compared and found to yield
essentially the same answers. waves are seen to enter together with
waves in a way that any means which attempt to obtain the multipole
accurately must rely on knowledge of waves and that consequently the latter
cannot be dismissed in analyses of low-energy pion photoproduction. It is shown
that waves have a significant impact on double-polarization observables
that can be measured. This importance of waves is due to the soft nature of
the wave and is a direct consequence of chiral symmetry and the
Nambu--Goldstone nature of the pion. -wave contributions are shown to be
negligible in the near-threshold region.Comment: 38 pages, 13 figures, 19 tables. Version to be published in Physical
Review
Coherent laminar and turbulent motion of toroidal vortex bundles
Motivated by experiments performed in superfluid helium, we study numerically
the motion of toroidal bundles of vortex filaments in an inviscid fluid. We
find that the evolution of these large-scale vortex structures involves the
generalised leapfrogging of the constituent vortex rings. Despite three
dimensional perturbations in the form of Kelvin waves and vortex reconnections,
toroidal vortex bundles retain their coherence over a relatively large distance
(compared to their size), in agreement with experimental observations.Comment: 22 pages, 12 figure
A Kelvin-wave cascade on a vortex in superfluid He at a very low temperature
A study by computer simulation is reported of the behaviour of a quantized
vortex line at a very low temperature when there is continuous excitation of
low-frequency Kelvin waves. There is no dissipation except by phonon radiation
at a very high frequency. It is shown that non-linear coupling leads to a net
flow of energy to higher wavenumbers and to the development of a simple
spectrum of Kelvin waves that is insensitive to the strength and frequency of
the exciting drive. The results are likely to be relevant to the decay of
turbulence in superfluid He at very low temperatures
Vortex lattices in a stirred Bose-Einstein condensate
We stir with a focused laser beam a Bose-Einstein condensate of Rb
atoms confined in a magnetic trap. We observe the formation of a single vortex
for a stirring frequency exceeding a critical value. At larger rotation
frequencies we produce states of the condensate for which up to eleven vortices
are simultaneously present. We present measurements of the decay of a vortex
array once the stirring laser beam is removed
Energy-dependent effective interactions for dilute many-body systems
We address the issue of determining an effective two-body interaction for
mean-field calculations of energies of many-body systems. We show that the
effective interaction is proportional to the phase shift, and demonstrate this
result in the quasiclassical approximation when there is a trapping potential
in addition to the short-range interaction between a pair of particles. We
calculate numerically energy levels for the case of an interaction with a
short-range square-well and a harmonic trapping potential and show that the
numerical results agree well with the analytical expression. We derive a
generalized Gross--Pitaevskii equation which includes effective range
corrections and discuss the form of the electron--atom effective interaction to
be used in calculations of Rydberg atoms and molecules.Comment: 6 pages, 2 figure
Interferometric detection of a single vortex in a dilute Bose-Einstein condensate
Using two radio frequency pulses separated in time we perform an amplitude
division interference experiment on a rubidium Bose-Einstein condensate. The
presence of a quantized vortex, which is nucleated by stirring the condensate
with a laser beam, is revealed by a dislocation in the fringe pattern.Comment: 4 pages, 4 figure
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