11,237 research outputs found
Employing per-component time step in DSMC simulations of disparate mass and cross-section gas mixtures
A new approach to simulation of stationary flows by Direct Simulation Monte
Carlo method is proposed. The idea is to specify an individual time step for
each component of a gas mixture. The approach consists of modifications mainly
to collision phase and recommendation on choosing time step ratios. It allows
softening the demands on the computational resources for cases of disparate
collision diameters of molecules and/or disparate molecular masses. These are
the cases important in vacuum deposition technologies. Few tests of the new
approach are made. Finally, the usage of new approach is demonstrated on a
problem of silver nanocluster diffusion in carrier gas argon in conditions of
silver deposition experiments.Comment: The goal of submission is to find native English speaker willing to
help me polish the paper. This is paper draft sent to Communications in
Computational Physics. It is recommended to publication. The need of
polishing was one of editors decision. See Additional data for MS Word source
fil
INTEGRAL discovery of unusually long broad-band X-ray activity from the Supergiant Fast X-ray Transient IGR J18483-0311
We report on a broad-band X-ray study (0.5-250 keV) of the Supergiant Fast
X-ray Transient IGR J18483-0311 using archival INTEGRAL data and a new targeted
XMM-Newton observation. Our INTEGRAL investigation discovered for the first
time an unusually long X-ray activity (3-60 keV) which continuously lasted for
at least 11 days, i.e. a significant fraction (about 60%) of the entire orbital
period, and spanned orbital phases corresponding to both periastron and
apastron passages. This prolongated X-ray activity is at odds with the much
shorter durations marking outbursts from classical SFXTs especially above 20
keV, as such it represents a departure from their nominal behavior and it adds
a further extreme characteristic to the already extreme SFXT IGR J18483-0311.
Our IBIS/ISGRI high energy investigation (100-250 keV) of archival outbursts
activity from the source showed that the recently reported hint of a possible
hard X-ray tail is not real and it is likely due to noisy background. The new
XMM-Newton targeted observation did not detect any sign of strong X-ray
outburst activity from the source despite being performed close to its
periastron passage, on the contrary IGR J18483-0311 was caught during the
common intermediate X-ray state with a low luminosity value of 3x10^33 erg s^-1
(0.5-10 keV). We discuss all the reported results in the framework of both
spherically symmetric clumpy wind scenario and quasi-spherical settling
accretion model.Comment: Accepted for publication on MNRAS. 10 pages, 7 figures, 1 tabl
Electron Dynamics in a Coupled Quantum Point Contact Structure with a Local Magnetic Moment
We develop a theoretical model for the description of electron dynamics in
coupled quantum wires when the local magnetic moment is formed in one of the
wires. We employ a single-particle Hamiltonian that takes account of the
specific geometry of potentials defining the structure as well as electron
scattering on the local magnetic moment. The equations for the wave functions
in both wires are derived and the approach for their solution is discussed. We
determine the transmission coefficient and conductance of the wire having the
local magnetic moment and show that our description reproduces the
experimentally observed features.Comment: Based on work presented at 2004 IEEE NTC Quantum Device Technology
Worksho
Interfacial friction between semiflexible polymers and crystalline surfaces
The results obtained from molecular dynamics simulations of the friction at
an interface between polymer melts and weakly attractive crystalline surfaces
are reported. We consider a coarse-grained bead-spring model of linear chains
with adjustable intrinsic stiffness. The structure and relaxation dynamics of
polymer chains near interfaces are quantified by the radius of gyration and
decay of the time autocorrelation function of the first normal mode. We found
that the friction coefficient at small slip velocities exhibits a distinct
maximum which appears due to shear-induced alignment of semiflexible chain
segments in contact with solid walls. At large slip velocities the decay of the
friction coefficient is independent of the chain stiffness. The data for the
friction coefficient and shear viscosity are used to elucidate main trends in
the nonlinear shear rate dependence of the slip length. The influence of chain
stiffness on the relationship between the friction coefficient and the
structure factor in the first fluid layer is discussed.Comment: 31 pages, 12 figure
The relationship between induced fluid structure and boundary slip in nanoscale polymer films
The molecular mechanism of slip at the interface between polymer melts and
weakly attractive smooth surfaces is investigated using molecular dynamics
simulations. In agreement with our previous studies on slip flow of
shear-thinning fluids, it is shown that the slip length passes through a local
minimum at low shear rates and then increases rapidly at higher shear rates. We
found that at sufficiently high shear rates, the slip flow over atomically flat
crystalline surfaces is anisotropic. It is demonstrated numerically that the
friction coefficient at the liquid-solid interface (the ratio of viscosity and
slip length) undergoes a transition from a constant value to the power-law
decay as a function of the slip velocity. The characteristic velocity of the
transition correlates well with the diffusion velocity of fluid monomers in the
first fluid layer near the solid wall at equilibrium. We also show that in the
linear regime, the friction coefficient is well described by a function of a
single variable, which is a product of the magnitude of surface-induced peak in
the structure factor and the contact density of the adjacent fluid layer. The
universal relationship between the friction coefficient and induced fluid
structure holds for a number of material parameters of the interface: fluid
density, chain length, wall-fluid interaction energy, wall density, lattice
type and orientation, thermal or solid walls.Comment: 33 pages, 14 figure
Droplet evaporation in one-component fluids: Dynamic van der Waals theory
In a one-component fluid, we investigate evaporation of a small axysymmetric
liquid droplet in the partial wetting condition on a heated wall at . In the dynamic van der Waals theory (Phys. Rev. E {\bf 75}, 036304
(2007)), we take into account the latent heat transport from liquid to gas upon
evaporation. Along the gas-liquid interface, the temperature is nearly equal to
the equilibrium coexisting temperature away from the substrate, but it rises
sharply to the wall temperature close to the substrate. On an isothermal
substrate, evaporation takes place mostly on a narrow interface region near the
contact line in a late stage, which is a characteristic feature in
one-component fluids.Comment: 6 pages, 6 figure
Supersymmetry solution for finitely extensible dumbbell model
Exact relaxation times and eigenfunctions for a simple mechanical model of
polymer dynamics are obtained using supersymmetry methods of quantum mechanics.
The model includes the finite extensibility of the molecule and does not make
use of the self-consistently averaging approximation. The finite extensibility
reduces the relaxation times when compared to a linear force. The linear
viscoelastic behaviour is obtained in the form of the ``generalized Maxwell
model''. Using these results, a numerical integration scheme is proposed in the
presence of a given flow kinematics.Comment: 5 pages, 2 figure
Influence of Magnetic Moment Formation on the Conductance of Coupled Quantum Wires
In this report, we develop a model for the resonant interaction between a
pair of coupled quantum wires, under conditions where self-consistent effects
lead to the formation of a local magnetic moment in one of the wires. Our
analysis is motivated by the experimental results of Morimoto et al. [Appl.
Phys. Lett. \bf{82}, 3952 (2003)], who showed that the conductance of one of
the quantum wires exhibits a resonant peak at low temperatures, whenever the
other wire is swept into the regime where local-moment formation is expected.
In order to account for these observations, we develop a theoretical model for
the inter-wire interaction that calculated the transmission properties of one
(the fixed) wire when the device potential is modified by the presence of an
extra scattering term, arising from the presence of the local moment in the
swept wire. To determine the transmission coefficients in this system, we
derive equations describing the dynamics of electrons in the swept and fixed
wires of the coupled-wire geometry. Our analysis clearly shows that the
observation of a resonant peak in the conductance of the fixed wire is
correlated to the appearance of additional structure (near or
) in the conductance of the swept wire, in agreement with the
experimental results of Morimoto et al
Impact of high-energy tails on granular gas properties
The velocity distribution function of granular gases in the homogeneous
cooling state as well as some heated granular gases decays for large velocities
as . That is, its high-energy tail is
overpopulated as compared with the Maxwell distribution. At the present time,
there is no theory to describe the influence of the tail on the kinetic
characteristics of granular gases. We develop an approach to quantify the
overpopulated tail and analyze its impact on granular gas properties, in
particular on the cooling coefficient. We observe and explain anomalously slow
relaxation of the velocity distribution function to its steady state.Comment: 5 pages, 5 figure
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