1,005 research outputs found
Phase Separation of a Fast Rotating Boson-Fermion Mixture in the Lowest-Landau-Level Regime
By minimizing the coupled mean-field energy functionals, we investigate the
ground-state properties of a rotating atomic boson-fermion mixture in a
two-dimensional parabolic trap. At high angular frequencies in the
mean-field-lowest-Landau-level regime, quantized vortices enter the bosonic
condensate, and a finite number of degenerate fermions form the
maximum-density-droplet state. As the boson-fermion coupling constant
increases, the maximum density droplet develops into a lower-density state
associated with the phase separation, revealing characteristics of a
Landau-level structure
Dynamics of vortex tangle without mutual friction in superfluid He
A recent experiment has shown that a tangle of quantized vortices in
superfluid He decayed even at mK temperatures where the normal fluid was
negligible and no mutual friction worked. Motivated by this experiment, this
work studies numerically the dynamics of the vortex tangle without the mutual
friction, thus showing that a self-similar cascade process, whereby large
vortex loops break up to smaller ones, proceeds in the vortex tangle and is
closely related with its free decay. This cascade process which may be covered
with the mutual friction at higher temperatures is just the one at zero
temperature Feynman proposed long ago. The full Biot-Savart calculation is made
for dilute vortices, while the localized induction approximation is used for a
dense tangle. The former finds the elementary scenario: the reconnection of the
vortices excites vortex waves along them and makes them kinked, which could be
suppressed if the mutual friction worked. The kinked parts reconnect with the
vortex they belong to, dividing into small loops. The latter simulation under
the localized induction approximation shows that such cascade process actually
proceeds self-similarly in a dense tangle and continues to make small vortices.
Considering that the vortices of the interatomic size no longer keep the
picture of vortex, the cascade process leads to the decay of the vortex line
density. The presence of the cascade process is supported also by investigating
the classification of the reconnection type and the size distribution of
vortices. The decay of the vortex line density is consistent with the solution
of the Vinen's equation which was originally derived on the basis of the idea
of homogeneous turbulence with the cascade process. The obtained result is
compared with the recent Vinen's theory.Comment: 16 pages, 16 figures, submitted to PR
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
Crossover between Kelvin-Helmholtz and counter-superflow instabilities in two-component Bose-Einstein condensates
Dynamical instabilities at the interface between two Bose--Einstein
condensates that are moving relative to each other are investigated using
mean-field and Bogoliubov analyses. Kelvin--Helmholtz instability is dominant
when the interface thickness is much smaller than the wavelength of the
unstable interface mode, whereas the counter-superflow instability becomes
dominant in the opposite case. These instabilities emerge not only in an
immiscible system but also in a miscible system where an interface is produced
by external potential. Dynamics caused by these instabilities are numerically
demonstrated in rotating trapped condensates.Comment: 10 pages, 9 figure
Statistics for comparison of simulations and experiments of flow of blood cells
In this article we propose statistical method for comparison of simulation and real biological experiments of elastic objects moving in fluid. Our work is focused on future optimization of microfluidic devices used for capture of circulating tumor cells from blood samples. Since the design optimization using biological experiments is both time consuming and expensive, in silico experiments with a broad spectrum of complex and computationally simulations are intensely performed. Necessary verification if simulation models, hitherto mainly realised by comparision of individual cells properties must be extended to more complex simulations. We present our first results with characteristics designed for this purpose
Vortex Multiplication in Applied Flow: the Precursor to Superfluid Turbulence
The dynamics of quantized vortices in rotating He-B is investigated in
the low density (single-vortex) regime as a function of temperature. An abrupt
transition is observed at . Above this temperature the number of
vortex lines remains constant, as they evolve to their equilibrium positions.
Below this temperature the number of vortices increases linearly in time until
the vortex density has grown sufficiently for turbulence to switch on. On the
basis of numerical calculations we suggest a mechanism responsible for vortex
formation at low temperatures and identify the mutual friction parameter which
governs its abrupt temperature dependence.Comment: 5 pages, 4 figures; version submitted to Phys. Rev. Let
A particle method computer simulation: applications to the study of blood flow
The need to analyse the microscopic mechanical behaviour of blood flow was one of the main
reasons to develop a new computer simulation using a particle method. This new mesh free
method is based on a moving-particle semi-implicit (MPS) method, which has been developed
to simulate incompressible fluids based on the Navier-Stokes equations. The simulation region
was discretized by particles that moves in Lagrangian coordinates, where the plasma and
platelets were modelled as fluid particles, red blood cells (RBC) as elastic particles and vessel
wall as rigid particles. In this paper, some applications of the MPS method to study the blood
flow are briefly analysed, such as the motion and deformation of red blood cells (RBC) in
plasma flow and the platelet aggregation process in blood flow. Some preliminary studies
suggest that there is evidence that the proposed method enables the analysis of the RBC
motion and deformation in the plasma flow and also the initial thrombogenesis, growth and
destruction of thrombus
Theory of vortex-lattice melting in a one-dimensional optical lattice
We investigate quantum and temperature fluctuations of a vortex lattice in a
one-dimensional optical lattice. We discuss in particular the Bloch bands of
the Tkachenko modes and calculate the correlation function of the vortex
positions along the direction of the optical lattice. Because of the small
number of particles in the pancake Bose-Einstein condensates at every site of
the optical lattice, finite-size effects become very important. Moreover, the
fluctuations in the vortex positions are inhomogeneous due to the inhomogeneous
density. As a result, the melting of the lattice occurs from the outside
inwards. However, tunneling between neighboring pancakes substantially reduces
the inhomogeneity as well as the size of the fluctuations. On the other hand,
nonzero temperatures increase the size of the fluctuations dramatically. We
calculate the crossover temperature from quantum melting to classical melting.
We also investigate melting in the presence of a quartic radial potential,
where a liquid can form in the center instead of at the outer edge of the
pancake Bose-Einstein condensates.Comment: 17 pages, 17 figures, submitted to Phys. Rev. A, references update
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
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