143 research outputs found
Application of the Gillespie algorithm to a granular intruder particle
We show how the Gillespie algorithm, originally developed to describe coupled
chemical reactions, can be used to perform numerical simulations of a granular
intruder particle colliding with thermalized bath particles. The algorithm
generates a sequence of collision ``events'' separated by variable time
intervals. As input, it requires the position-dependent flux of bath particles
at each point on the surface of the intruder particle. We validate the method
by applying it to a one-dimensional system for which the exact solution of the
homogeneous Boltzmann equation is known and investigate the case where the bath
particle velocity distribution has algebraic tails. We also present an
application to a granular needle in bath of point particles where we
demonstrate the presence of correlations between the translational and
rotational degrees of freedom of the intruder particle. The relationship
between the Gillespie algorithm and the commonly used Direct Simulation Monte
Carlo (DSMC) method is also discussed.Comment: 13 pages, 8 figures, to be published in J. Phys. A Math. Ge
Fluctuating lattice Boltzmann
The lattice Boltzmann algorithm efficiently simulates the Navier Stokes
equation of isothermal fluid flow, but ignores thermal fluctuations of the
fluid, important in mesoscopic flows. We show how to adapt the algorithm to
include noise, satisfying a fluctuation-dissipation theorem (FDT) directly at
lattice level: this gives correct fluctuations for mass and momentum densities,
and for stresses, at all wavevectors . Unlike previous work, which recovers
FDT only as , our algorithm offers full statistical mechanical
consistency in mesoscale simulations of, e.g., fluctuating colloidal
hydrodynamics.Comment: 7 pages, 3 figures, to appear in Europhysics Letter
Gaussian density fluctuations and Mode Coupling Theory for supercooled liquids
The equations of motion for the density modes of a fluid, derived from
Newton's equations, are written as a linear generalized Langevin equation. The
constraint imposed by the fluctuation-dissipation theorem is used to derive an
exact form for the memory function. The resulting equations, solved under the
assumption that the noise, and consequently density fluctuations, of the liquid
are gaussian distributed, are equivalent to the random-phase-approximation for
the static structure factor and to the well known ideal mode coupling theory
(MCT) equations for the dynamics. This finding suggests that MCT is the
canonical mean-field theory of the fluid dynamics.Comment: 4 pages, REVTE
On the velocity distributions of the one-dimensional inelastic gas
We consider the single-particle velocity distribution of a one-dimensional
fluid of inelastic particles. Both the freely evolving (cooling) system and the
non-equilibrium stationary state obtained in the presence of random forcing are
investigated, and special emphasis is paid to the small inelasticity limit. The
results are obtained from analytical arguments applied to the Boltzmann
equation along with three complementary numerical techniques (Molecular
Dynamics, Direct Monte Carlo Simulation Methods and iterative solutions of
integro-differential kinetic equations). For the freely cooling fluid, we
investigate in detail the scaling properties of the bimodal velocity
distribution emerging close to elasticity and calculate the scaling function
associated with the distribution function. In the heated steady state, we find
that, depending on the inelasticity, the distribution function may display two
different stretched exponential tails at large velocities. The inelasticity
dependence of the crossover velocity is determined and it is found that the
extremely high velocity tail may not be observable at ``experimentally
relevant'' inelasticities.Comment: Latex, 14 pages, 12 eps figure
Enskog Theory for Polydisperse Granular Mixtures. I. Navier-Stokes order Transport
A hydrodynamic description for an -component mixture of inelastic, smooth
hard disks (two dimensions) or spheres (three dimensions) is derived based on
the revised Enskog theory for the single-particle velocity distribution
functions. In this first portion of the two-part series, the macroscopic
balance equations for mass, momentum, and energy are derived. Constitutive
equations are calculated from exact expressions for the fluxes by a
Chapman-Enskog expansion carried out to first order in spatial gradients,
thereby resulting in a Navier-Stokes order theory. Within this context of small
gradients, the theory is applicable to a wide range of restitution coefficients
and densities. The resulting integral-differential equations for the zeroth-
and first-order approximations of the distribution functions are given in exact
form. An approximate solution to these equations is required for practical
purposes in order to cast the constitutive quantities as algebraic functions of
the macroscopic variables; this task is described in the companion paper.Comment: 36 pages, to be published in Phys. Rev.
Diffusion in a Granular Fluid - Theory
Many important properties of granular fluids can be represented by a system
of hard spheres with inelastic collisions. Traditional methods of
nonequilibrium statistical mechanics are effective for analysis and description
of the inelastic case as well. This is illustrated here for diffusion of an
impurity particle in a fluid undergoing homogeneous cooling. An appropriate
scaling of the Liouville equation is described such that the homogeneous
cooling ensemble and associated time correlation functions map to those of a
stationary state. In this form the familiar methods of linear response can be
applied, leading to Green - Kubo and Einstein representations of diffusion in
terms of the velocity and mean square displacement correlation functions. These
correlation functions are evaluated approximately using a cumulant expansion
and from kinetic theory, providing the diffusion coefficient as a function of
the density and the restitution coefficients. Comparisons with results from
molecular dynamics simulation are given in the following companion paper
Electroviscous effects of simple electrolytes under shear
On the basis of a hydrodynamical model analogous to that in critical fluids,
we investigate the influences of shear flow upon the electrostatic contribution
to the viscosity of binary electrolyte solutions in the Debye-H\"{u}ckel
approximation. Within the linear-response theory, we reproduce the classical
limiting law that the excess viscosity is proportional to the square root of
the concentration of the electrolyte. We also extend this result for finite
shear. An analytic expression of the anisotropic structure factor of the charge
density under shear is obtained, and its deformation at large shear rates is
discussed. A non-Newtonian effect caused by deformations of the ionic
atmosphere is also elucidated for . This finding
concludes that the maximum shear stress that the ionic atmosphere can support
is proportional to , where , and
are, respectively, the shear rate, the Debye screening
length and the Debye relaxation time with being the relative diffusivity at
the infinite dilution limit of the electrolyte.Comment: 13pages, 2figure
Hydrodynamic modes, Green-Kubo relations, and velocity correlations in dilute granular gases
It is shown that the hydrodynamic modes of a dilute granular gas of inelastic
hard spheres can be identified, and calculated in the long wavelength limit.
Assuming they dominate at long times, formal expressions for the Navier-Stokes
transport coefficients are derived. They can be expressed in a form that
generalizes the Green-Kubo relations for molecular systems, and it is shown
that they can also be evaluated by means of -particle simulation methods.
The form of the hydrodynamic modes to zeroth order in the gradients is used to
detect the presence of inherent velocity correlations in the homogeneous
cooling state, even in the low density limit. They manifest themselves in the
fluctuations of the total energy of the system. The theoretical predictions are
shown to be in agreement with molecular dynamics simulations. Relevant related
questions deserving further attention are pointed out
Temperature-dependent quantum pair potentials and their application to dense partially ionized hydrogen plasmas
Extending our previous work \cite{filinov-etal.jpa03ik} we present a detailed
discussion of accuracy and practical applications of finite-temperature
pseudopotentials for two-component Coulomb systems. Different pseudopotentials
are discussed: i) the diagonal Kelbg potential, ii) the off-diagonal Kelbg
potential iii) the {\em improved} diagonal Kelbg potential, iv) an effective
potential obtained with the Feynman-Kleinert variational principle v) the
``exact'' quantum pair potential derived from the two-particle density matrix.
For the {\em improved} diagonal Kelbg potential a simple temperature dependent
fit is derived which accurately reproduces the ``exact'' pair potential in the
whole temperature range. The derived pseudopotentials are then used in path
integral Monte Carlo (PIMC) and molecular dynamics (MD) simulations to obtain
thermodynamical properties of strongly coupled hydrogen. It is demonstrated
that classical MD simulations with spin-dependent interaction potentials for
the electrons allow for an accurate description of the internal energy of
hydrogen in the difficult regime of partial ionization down to the temperatures
of about K. Finally, we point out an interesting relation between the
quantum potentials and effective potentials used in density functional theory.Comment: 18 pages, 11 figure
Interaction of photons with plasmas and liquid metals: photoabsorption and scattering
Formulas to describe the photoabsorption and the photon scattering by a
plasma or a liquid metal are derived in a unified manner with each other. It is
shown how the nuclear motion, the free-electron motion and the core-electron
behaviour in each ion in the system determine the structure of photoabsorption
and scattering in an electron-ion mixture. The absorption cross section in the
dipole approximation consists of three terms which represent the absorption
caused by the nuclear motion, the absorption owing to the free-electron motion
producing optical conductivity or inverse Bremsstrahlung, and the absorption
ascribed to the core-electron behaviour in each ion with the Doppler
correction. Also, the photon scattering formula provides an analysis method for
experiments observing the ion-ion dynamical structure factor (DSF), the
electron-electron DSF giving plasma oscillations, and the core-electron DSF
yielding the X-ray Raman (Compton) scattering with a clear definition of the
background scattering for each experiment, in a unified manner. A formula for
anomalous X-ray scattering is also derived for a liquid metal. At the same
time, Thomson scattering in plasma physics is discussed from this general point
of view.Comment: LaTeX file: 18 pages without figur
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