14,062 research outputs found
Particle simulation of vibrated gas-fluidized beds of cohesive fine powders
We use three-dimensional particle dynamics simulations, coupled with
volume-averaged gas phase hydrodynamics, to study vertically vibrated
gas-fluidized beds of fine, cohesive powders. The volume-averaged interstitial
gas flow is restricted to be one-dimensional (1D). This simplified model
captures the spontaneous development of 1D traveling waves, which corresponds
to bubble formation in real fluidized beds. We use this model to probe the
manner in which vibration and gas flow combine to influence the dynamics of
cohesive particles. We find that as the gas flow rate increases, cyclic
pressure pulsation produced by vibration becomes more and more significant than
direct impact, and in a fully fluidized bed this pulsation is virtually the
only relevant mechanism. We demonstrate that vibration assists fluidization by
creating large tensile stresses during transient periods, which helps break up
the cohesive assembly into agglomerates.Comment: to appear in I&EC Research, a special issue (Oct. 2006) in honor of
Prof. William B. Russe
Strong Correlation to Weak Correlation Phase Transition in Bilayer Quantum Hall Systems
At small layer separations, the ground state of a nu=1 bilayer quantum Hall
system exhibits spontaneous interlayer phase coherence and has a
charged-excitation gap E_g. The evolution of this state with increasing layer
separation d has been a matter of controversy. In this letter we report on
small system exact diagonalization calculations which suggest that a single
phase transition, likely of first order, separates coherent incompressible (E_g
>0) states with strong interlayer correlations from incoherent compressible
states with weak interlayer correlations. We find a dependence of the phase
boundary on d and interlayer tunneling amplitude that is in very good agreement
with recent experiments.Comment: 4 pages, 4 figures included, version to appear in Phys. Rev. Let
Compaction and dilation rate dependence of stresses in gas-fluidized beds
A particle dynamics-based hybrid model, consisting of monodisperse spherical
solid particles and volume-averaged gas hydrodynamics, is used to study
traveling planar waves (one-dimensional traveling waves) of voids formed in
gas-fluidized beds of narrow cross sectional areas. Through ensemble-averaging
in a co-traveling frame, we compute solid phase continuum variables (local
volume fraction, average velocity, stress tensor, and granular temperature)
across the waves, and examine the relations among them. We probe the
consistency between such computationally obtained relations and constitutive
models in the kinetic theory for granular materials which are widely used in
the two-fluid modeling approach to fluidized beds. We demonstrate that solid
phase continuum variables exhibit appreciable ``path dependence'', which is not
captured by the commonly used kinetic theory-based models. We show that this
path dependence is associated with the large rates of dilation and compaction
that occur in the wave. We also examine the relations among solid phase
continuum variables in beds of cohesive particles, which yield the same path
dependence. Our results both for beds of cohesive and non-cohesive particles
suggest that path-dependent constitutive models need to be developed.Comment: accepted for publication in Physics of Fluids (Burnett-order effect
analysis added
Electronic structures of layered perovskite Sr2MO4 (M=Ru, Rh, and Ir)
We investigated the electronic structures of the two-dimensional layered
perovskite Sr\textit{M}O (\textit{M}=4\textit{d} Ru, 4\textit{d}
Rh, and 5\textit{d} Ir) using optical spectroscopy and polarization-dependent O
1\textit{s} x-ray absorption spectroscopy. While the ground states of the
series of compounds are rather different, their optical conductivity spectra
exhibit similar interband transitions, indicative of the
common electronic structures of the 4\textit{d} and 5\textit{d} layered oxides.
The energy splittings between the two orbitals, ,
and , are about 2 eV, which is much larger
than those in the pseudocubic and 3\textit{d} layered perovskite oxides. The
electronic properties of the Sr\textit{M}O compounds are discussed
in terms of the crystal structure and the extended character of the 4\textit{d}
and 5\textit{d} orbitals
Optical observations of NEA 162173 (1999 JU3) during the 2011-2012 apparition
Near-Earth asteroid 162173 (1999 JU3) is a potential target of two asteroid
sample return missions, not only because of its accessibility but also because
of the first C-type asteroid for exploration missions. The lightcurve-related
physical properties of this object were investigated during the 2011-2012
apparition. We aim to confirm the physical parameters useful for JAXA's
Hayabusa 2 mission, such as rotational period, absolute magnitude, and phase
function. Our data complement previous studies that did not cover low phase
angles. With optical imagers and 1-2 m class telescopes, we acquired the
photometric data at different phase angles. We independently derived the
rotational lightcurve and the phase curve of the asteroid. We have analyzed the
lightcurve of 162173 (1999 JU3), and derived a synodic rotational period of
7.625 +/- 0.003 h, the axis ratio a/b = 1.12. The absolute magnitude H_R =
18.69 +/- 0.07 mag and the phase slope of G = -0.09 +/- 0.03 were also obtained
based on the observations made during the 2011-2012 apparition.Comment: 4 pages, 3 figure
Single-mode approximation and effective Chern-Simons theories for quantum Hall systems
A unified description of elementary and collective excitations in quantum
Hall systems is presented within the single-mode approximation (SMA) framework,
with emphasis on revealing an intimate link with Chern-Simons theories. It is
shown that for a wide class of quantum Hall systems the SMA in general yields,
as an effective theory, a variant of the bosonic Chern-Simons theory. For
single-layer systems the effective theory agrees with the standard Chern-Simons
theory at long wavelengths whereas substantial deviations arise for collective
excitations in bilayer systems. It is suggested, in particular, that Hall-drag
experiments would be a good place to detect out-of-phase collective excitations
inherent to bilayer systems. It is also shown that the intra-Landau-level modes
bear a similarity in structure (though not in scale) to the inter-Landau-level
modes, and its implications on the composite-fermion and composite-boson
theories are discussed.Comment: 9 pages, Revtex
Coarse-grained computations of demixing in dense gas-fluidized beds
We use an "equation-free", coarse-grained computational approach to
accelerate molecular dynamics-based computations of demixing (segregation) of
dissimilar particles subject to an upward gas flow (gas-fluidized beds). We
explore the coarse-grained dynamics of these phenomena in gently fluidized beds
of solid mixtures of different densities, typically a slow process for which
reasonable continuum models are currently unavailable
PseudoSkyrmion Effects on Tunneling Conductivity in Coherent Bilayer Quantum Hall States at
We present a mechamism why interlayer tunneling conductivity in coherent
bilayer quantum Hall states at is anomalously large, but finite in the
recent experiment. According to the mechanism, pseudoSkyrmions causes the
finite conductivity, although there exists an expectation that dissipationless
tunneling current arises in the state. PseudoSkyrmions have an intrinsic
polarization field perpendicular to the layers, which causes the dissipation.
Using the mechanism we show that the large peak in the conductivity remains for
weak parallel magnetic field, but decay rapidly after its strength is beyond a
critical one, Tesla.Comment: 6 pages, no figure
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