269 research outputs found
Efficient Algorithm on a Non-staggered Mesh for Simulating Rayleigh-Benard Convection in a Box
An efficient semi-implicit second-order-accurate finite-difference method is
described for studying incompressible Rayleigh-Benard convection in a box, with
sidewalls that are periodic, thermally insulated, or thermally conducting.
Operator-splitting and a projection method reduce the algorithm at each time
step to the solution of four Helmholtz equations and one Poisson equation, and
these are are solved by fast direct methods. The method is numerically stable
even though all field values are placed on a single non-staggered mesh
commensurate with the boundaries. The efficiency and accuracy of the method are
characterized for several representative convection problems.Comment: REVTeX, 30 pages, 5 figure
Convective Motion in a Vibrated Granular Layer
Experimental results are presented for a vertically shaken granular layer. In
the range of accelerations explored, the layer develops a convective motion in
the form of one or more rolls. The velocity of the grains near the wall has
been measured. It grows linearly with the acceleration, then the growing rate
slows down. A rescaling with the amplitude of the wall velocity and the height
of the granular layer makes all data collapse in a single curve. This can
provide insights on the mechanism driving the motion.Comment: 10 pages, 5 figures submitted to Phys. Rev. Let
Kinetics of Ordering in Fluctuation-Driven First-Order Transitions: Simulations and Dynamical Renormalization
Many systems where interactions compete with each other or with constraints
are well described by a model first introduced by Brazovskii. Such systems
include block copolymers, alloys with modulated phases, Rayleigh-Benard Cells
and type-I superconductors. The hallmark of this model is that the fluctuation
spectrum is isotropic and has a minimum at a nonzero wave vector represented by
the surface of a d-dimensional hyper-sphere. It was shown by Brazovskii that
the fluctuations change the free energy structure from a to a
form with the disordered state metastable for all quench depths.
The transition from the disordered to the periodic, lamellar structure changes
from second order to first order and suggests that the dynamics is governed by
nucleation. Using numerical simulations we have confirmed that the equilibrium
free energy function is indeed of a form. A study of the dynamics,
however, shows that, following a deep quench, the dynamics is described by
unstable growth rather than nucleation. A dynamical calculation, based on a
generalization of the Brazovskii calculations shows that the disordered state
can remain unstable for a long time following the quench.Comment: 18 pages, 15 figures submitted to PR
Noise sensitivity of sub- and supercritically bifurcating patterns with group velocities close to the convective-absolute instability
The influence of small additive noise on structure formation near a forwards
and near an inverted bifurcation as described by a cubic and quintic Ginzburg
Landau amplitude equation, respectively, is studied numerically for group
velocities in the vicinity of the convective-absolute instability where the
deterministic front dynamics would empty the system.Comment: 16 pages, 7 Postscript figure
Selfsimilar Domain Growth, Localized Structures and Labyrinthine Patterns in Vectorial Kerr Resonators
We study domain growth in a nonlinear optical system useful to explore
different scenarios that might occur in systems which do not relax to
thermodynamic equilibrium. Domains correspond to equivalent states of different
circular polarization of light. We describe three dynamical regimes: a
coarsening regime in which dynamical scaling holds with a growth law dictated
by curvature effects, a regime in which localized structures form, and a regime
in which polarization domain walls are modulationally unstable and the system
freezes in a labyrinthine pattern.Comment: 13 pages, 6 figure
Inflammation decreases keratin level in ulcerative colitis; inadequate restoration associates with increased risk of colitis-associated cancer
Background Keratins are intermediate filament (IF) proteins, which form part of the epithelial cytoskeleton and which have been implicated pathology of inflammatory bowel diseases (IBD).
Methods In this study biopsies were obtained from IBD patients grouped by disease duration and subtype into eight categories based on cancer risk and inflammatory status: quiescent recent onset (<5 years) UC (ROUC); UC with primary sclerosing cholangitis; quiescent long-standing pancolitis (20–40 years) (LSPC); active colitis and non-inflamed proximal colonic mucosa; pancolitis with dysplasia-both dysplastic lesions (DT) and distal rectal mucosa (DR); control group without pathology. Alterations in IF protein composition across the groups were determined by quantitative proteomics. Key protein changes were validated by western immunoblotting and immunohistochemical analysis.
Result Acute inflammation resulted in reduced K8, K18, K19 and VIM (all p<0.05) compared to controls and non inflamed mucosa; reduced levels of if– associated proteins were also seen in DT and DR. Increased levels of keratins in LSPC was noted relative to controls or ROUC (K8, K18, K19 and VIM, p<0.05). Multiple K8 forms were noted on immunoblotting, with K8 phosphorylation reduced in progressive disease along with an increase in VIM:K8 ratio. K8 levels and phosphorylation are reduced in acute inflammation but appear restored or elevated in subjects with clinical and endoscopic remission (LSPC) but not apparent in subjects with elevated risk of cancer.
Conclusions These data suggest that keratin regulation in remission may influence subsequent cancer risk
Heat kernel regularization of the effective action for stochastic reaction-diffusion equations
The presence of fluctuations and non-linear interactions can lead to scale
dependence in the parameters appearing in stochastic differential equations.
Stochastic dynamics can be formulated in terms of functional integrals. In this
paper we apply the heat kernel method to study the short distance
renormalizability of a stochastic (polynomial) reaction-diffusion equation with
real additive noise. We calculate the one-loop {\emph{effective action}} and
its ultraviolet scale dependent divergences. We show that for white noise a
polynomial reaction-diffusion equation is one-loop {\emph{finite}} in and
, and is one-loop renormalizable in and space dimensions. We
obtain the one-loop renormalization group equations and find they run with
scale only in .Comment: 21 pages, uses ReV-TeX 3.
From Gapped Excitons to Gapless Triplons in One Dimension
Often, exotic phases appear in the phase diagrams between conventional
phases. Their elementary excitations are of particular interest. Here, we
consider the example of the ionic Hubbard model in one dimension. This model is
a band insulator (BI) for weak interaction and a Mott insulator (MI) for strong
interaction. Inbetween, a spontaneously dimerized insulator (SDI) occurs which
is governed by energetically low-lying charge and spin degrees of freedom.
Applying a systematically controlled version of the continuous unitary
transformations (CUTs) we are able to determine the dispersions of the
elementary charge and spin excitations and of their most relevant bound states
on equal footing. The key idea is to start from an externally dimerized system
using the relative weak interdimer coupling as small expansion parameter which
finally is set to unity to recover the original model.Comment: 18 pages, 10 figure
Dynamics of fluctuations in a fluid below the onset of Rayleigh-B\'enard convection
We present experimental data and their theoretical interpretation for the
decay rates of temperature fluctuations in a thin layer of a fluid heated from
below and confined between parallel horizontal plates. The measurements were
made with the mean temperature of the layer corresponding to the critical
isochore of sulfur hexafluoride above but near the critical point where
fluctuations are exceptionally strong. They cover a wide range of temperature
gradients below the onset of Rayleigh-B\'enard convection, and span wave
numbers on both sides of the critical value for this onset. The decay rates
were determined from experimental shadowgraph images of the fluctuations at
several camera exposure times. We present a theoretical expression for an
exposure-time-dependent structure factor which is needed for the data analysis.
As the onset of convection is approached, the data reveal the critical
slowing-down associated with the bifurcation. Theoretical predictions for the
decay rates as a function of the wave number and temperature gradient are
presented and compared with the experimental data. Quantitative agreement is
obtained if allowance is made for some uncertainty in the small spacing between
the plates, and when an empirical estimate is employed for the influence of
symmetric deviations from the Oberbeck-Boussinesq approximation which are to be
expected in a fluid with its density at the mean temperature located on the
critical isochore.Comment: 13 pages, 10 figures, 52 reference
Colossal dielectric constants in transition-metal oxides
Many transition-metal oxides show very large ("colossal") magnitudes of the
dielectric constant and thus have immense potential for applications in modern
microelectronics and for the development of new capacitance-based
energy-storage devices. In the present work, we thoroughly discuss the
mechanisms that can lead to colossal values of the dielectric constant,
especially emphasising effects generated by external and internal interfaces,
including electronic phase separation. In addition, we provide a detailed
overview and discussion of the dielectric properties of CaCu3Ti4O12 and related
systems, which is today's most investigated material with colossal dielectric
constant. Also a variety of further transition-metal oxides with large
dielectric constants are treated in detail, among them the system La2-xSrxNiO4
where electronic phase separation may play a role in the generation of a
colossal dielectric constant.Comment: 31 pages, 18 figures, submitted to Eur. Phys. J. for publication in
the Special Topics volume "Cooperative Phenomena in Solids: Metal-Insulator
Transitions and Ordering of Microscopic Degrees of Freedom
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