1,653 research outputs found
Analytic solutions for nonlinear waves in coupled reacting systems
We analyze a system of reacting elements harmonically coupled to nearest
neighbors in the continuum limit. An analytic solution is found for traveling
waves. The procedure is used to find oscillatory as well as solitary waves. A
comparison is made between exact solutions and solutions of the piecewise
linearized system, showing how the linearization affects the amplitude and
frequency of the solutions.Comment: 5 pages, 5 figures, RevTeX 4 styl
Outflows at the Edges of an Active Region in a Coronal Hole: A Signature of Active Region Expansion?
Outflows of plasma at the edges of active regions surrounded by quiet Sun are
now a common observation with the Hinode satellite. While there is
observational evidence to suggest that the outflows are originating in the
magnetic field surrounding the active regions, there is no conclusive evidence
that reveals how they are driven. Motivated by observations of outflows at the
periphery of a mature active region embedded in a coronal hole, we have used a
three-dimensional simulation to emulate the active region's development in
order to investigate the origin and driver of these outflows. We find outflows
are accelerated from a site in the coronal hole magnetic field immediately
surrounding the active region and are channelled along the coronal hole field
as they rise through the atmosphere. The plasma is accelerated simply as a
result of the active region expanding horizontally as it develops. Many of the
characteristics of the outflows generated in the simulation are consistent with
those of observed outflows: velocities up to 45 km per sec, properties akin to
the coronal hole, proximity to the active region's draining loops, expansion
with height, and projection over monopolar photospheric magnetic
concentrations. Although the horizontal expansion occurs as a consequence of
the active region's development in the simulation, expansion is also a general
feature of established active regions. Hence, it is entirely possible and
plausible that the expansion acceleration mechanism displayed in the simulation
is occurring in active regions on the Sun and, in addition to reconnection, is
driving the outflows observed at their edges.Comment: 19 pages, 9 figure
On the Emergence of Unstable Modes in an Expanding Domain for Energy-Conserving Wave Equations
Motivated by recent work on instabilities in expanding domains in
reaction-diffusion settings, we propose an analog of such mechanisms in
energy-conserving wave equations. In particular, we consider a nonlinear
Schr{\"o}dinger equation in a finite domain and show how the expansion or
contraction of the domain, under appropriate conditions, can destabilize its
originally stable solutions through the modulational instability mechanism.
Using both real and Fourier spacediagnostics, we monitor and control the
crossing of the instability threshold and, hence, the activation of the
instability. We also consider how the manifestation of this mechanism is
modified in a spatially inhomogeneous setting, namely in the presence of an
external parabolic potential, which is relevant to trapped Bose-Einstein
condensates
A Family of Exact, Analytic Time Dependent Wave Packet Solutions to a Nonlinear Schroedinger Equation
We obtain time dependent -Gaussian wave-packet solutions to a non linear
Schr\"odinger equation recently advanced by Nobre, Rego-Montero and Tsallis
(NRT) [Phys. Rev. Lett. 106 (2011) 10601]. The NRT non-linear equation admits
plane wave-like solutions (-plane waves) compatible with the celebrated de
Broglie relations connecting wave number and frequency, respectively, with
energy and momentum. The NRT equation, inspired in the -generalized
thermostatistical formalism, is characterized by a parameter , and in the
limit reduces to the standard, linear Schr\"odinger equation. The
-Gaussian solutions to the NRT equation investigated here admit as a
particular instance the previously known -plane wave solutions. The present
work thus extends the range of possible processes yielded by the NRT dynamics
that admit an analytical, exact treatment. In the limit the
-Gaussian solutions correspond to the Gaussian wave packet solutions to the
free particle linear Schr\"odinger equation. In the present work we also show
that there are other families of nonlinear Schr\"odinger-like equations,
besides the NRT one, exhibiting a dynamics compatible with the de Broglie
relations. Remarkably, however, the existence of time dependent Gaussian-like
wave packet solutions is a unique feature of the NRT equation not shared by the
aforementioned, more general, families of nonlinear evolution equations
Anomalous diffusion with absorption: Exact time-dependent solutions
Recently, analytical solutions of a nonlinear Fokker-Planck equation
describing anomalous diffusion with an external linear force were found using a
non extensive thermostatistical Ansatz. We have extended these solutions to the
case when an homogeneous absorption process is also present. Some peculiar
aspects of the interrelation between the deterministic force, the nonlinear
diffusion and the absorption process are discussed.Comment: RevTex, 16 pgs, 4 figures. Accepted in Physical Review
Singularly Perturbed Monotone Systems and an Application to Double Phosphorylation Cycles
The theory of monotone dynamical systems has been found very useful in the
modeling of some gene, protein, and signaling networks. In monotone systems,
every net feedback loop is positive. On the other hand, negative feedback loops
are important features of many systems, since they are required for adaptation
and precision. This paper shows that, provided that these negative loops act at
a comparatively fast time scale, the main dynamical property of (strongly)
monotone systems, convergence to steady states, is still valid. An application
is worked out to a double-phosphorylation ``futile cycle'' motif which plays a
central role in eukaryotic cell signaling.Comment: 21 pages, 3 figures, corrected typos, references remove
Population Dynamics and Non-Hermitian Localization
We review localization with non-Hermitian time evolution as applied to simple
models of population biology with spatially varying growth profiles and
convection. Convection leads to a constant imaginary vector potential in the
Schroedinger-like operator which appears in linearized growth models. We
illustrate the basic ideas by reviewing how convection affects the evolution of
a population influenced by a simple square well growth profile. Results from
discrete lattice growth models in both one and two dimensions are presented. A
set of similarity transformations which lead to exact results for the spectrum
and winding numbers of eigenfunctions for random growth rates in one dimension
is described in detail. We discuss the influence of boundary conditions, and
argue that periodic boundary conditions lead to results which are in fact
typical of a broad class of growth problems with convection.Comment: 19 pages, 11 figure
γ-Cyclodextrin Metal-Organic Frameworks: Do Solvents Make a Difference?
Conventionally, methanol is the solvent of choice in the synthesis of gamma-cyclodextrin metal-organic frameworks (γ-CD-MOFs), but using ethanol as a replacement could allow for a more food-grade synthesis condition. Therefore, the aim of the study was to compare the γ-CD-MOFs synthesised with both methanol and ethanol. The γ-CD-MOFs were characterised by scanning electron microscopy (SEM), surface area and pore measurement, Fourier transform infrared spectroscopy (FTIR) and powder X-ray diffraction (PXRD). The encapsulation efficiency (EE) and loading capacity (LC) of the γ-CD-MOFs were also determined for curcumin, using methanol, ethanol and a mixture of the two as encapsulation solvent. It was found that γ-CD-MOFs synthesised by methanol and ethanol do not differ greatly, the most significant difference being the larger crystal size of γ-CD-MOFs crystallised from ethanol. However, the change in solvent significantly influenced the EE and LC of the crystals. The higher solubility of curcumin in ethanol reduced interactions with the γ-CD-MOFs and resulted in lowered EE and LC. This suggests that different solvents should be used to deliberately manipulate the EE and LC of target compounds for better use of γ-CD-MOFs as their encapsulating and delivery agents
Patient Outcomes at Twelve Months after Early Decompressive Craniectomy for Diffuse Traumatic Brain Injury in the Randomized DECRA Clinical Trial
Functional outcomes at 12 months were a secondary outcome of the randomized DECRA trial of early decompressive craniectomy for severe diffuse traumatic brain injury (TBI) and refractory intracranial hypertension. In the DECRA trial, patients were randomly allocated 1:1 to either early decompressive craniectomy or intensive medical therapies (standard care). We conducted planned secondary analyses of the DECRA trial outcomes at 6 and 12 months, including all 155 patients. We measured functional outcome using the Glasgow Outcome Scale-Extended (GOS-E). We used ordered logistic regression, and dichotomized the GOS-E using logistic regression, to assess outcomes in patients overall and in survivors. We adjusted analyses for injury severity using the International Mission for Prognosis and Analysis of Clinical Trials in TBI (IMPACT) model. At 12 months, the odds ratio (OR) for worse functional outcomes in the craniectomy group (OR 1.68; 95% confidence interval [CI]: 0.96-2.93; p = 0.07) was no longer significant. Unfavorable functional outcomes after craniectomy were 11% higher (59% compared with 48%), but were not significantly different from standard care (OR 1.58; 95% CI: 0.84-2.99; p = 0.16). Among survivors after craniectomy, there were fewer good (OR 0.33; 95% CI: 0.12-0.91; p = 0.03) and more vegetative (OR 5.12; 95% CI: 1.04-25.2; p = 0.04) outcomes. Similar outcomes in survivors were found at 6 months after injury. Vegetative (OR 5.85; 95% CI: 1.21-28.30; p = 0.03) and severely disabled outcomes (OR 2.49; 95% CI: 1.21-5.11; p = 0.01) were increased. Twelve months after severe diffuse TBI and early refractory intracranial hypertension, decompressive craniectomy did not improve outcomes and increased vegetative survivors
Nucleation of Al3Zr and Al3Sc in aluminum alloys: from kinetic Monte Carlo simulations to classical theory
Zr and Sc precipitate in aluminum alloys to form the compounds Al3Zr and
Al3Sc which for low supersaturations of the solid solution have the L12
structure. The aim of the present study is to model at an atomic scale this
kinetics of precipitation and to build a mesoscopic model based on classical
nucleation theory so as to extend the field of supersaturations and annealing
times that can be simulated. We use some ab-initio calculations and
experimental data to fit an Ising model describing thermodynamics of the Al-Zr
and Al-Sc systems. Kinetic behavior is described by means of an atom-vacancy
exchange mechanism. This allows us to simulate with a kinetic Monte Carlo
algorithm kinetics of precipitation of Al3Zr and Al3Sc. These kinetics are then
used to test the classical nucleation theory. In this purpose, we deduce from
our atomic model an isotropic interface free energy which is consistent with
the one deduced from experimental kinetics and a nucleation free energy. We
test di erent mean-field approximations (Bragg-Williams approximation as well
as Cluster Variation Method) for these parameters. The classical nucleation
theory is coherent with the kinetic Monte Carlo simulations only when CVM is
used: it manages to reproduce the cluster size distribution in the metastable
solid solution and its evolution as well as the steady-state nucleation rate.
We also find that the capillary approximation used in the classical nucleation
theory works surprisingly well when compared to a direct calculation of the
free energy of formation for small L12 clusters.Comment: submitted to Physical Review B (2004
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