1,566 research outputs found
Segregation of an intruder in a heated granular dense gas
A recent segregation criterion [V. Garz\'o, Phys. Rev. E \textbf{78},
020301(R) (2008)] based on the thermal diffusion factor of an
intruder in a heated granular gas described by the inelastic Enskog equation is
revisited. The sign of provides a criterion for the transition
between the Brazil-nut effect (BNE) and the reverse Brazil-nut effect (RBNE).
The present theory incorporates two extra ingredients not accounted for by the
previous theoretical attempt. First, the theory is based upon the second Sonine
approximation to the transport coefficients of the mass flux of intruder.
Second, the dependence of the temperature ratio (intruder temperature over that
of the host granular gas) on the solid volume fraction is taken into account in
the first and second Sonine approximations. In order to check the accuracy of
the Sonine approximation considered, the Enskog equation is also numerically
solved by means of the direct simulation Monte Carlo (DSMC) method to get the
kinetic diffusion coefficient . The comparison between theory and
simulation shows that the second Sonine approximation to yields an
improvement over the first Sonine approximation when the intruder is lighter
than the gas particles in the range of large inelasticity. With respect to the
form of the phase diagrams for the BNE/RBNE transition, the kinetic theory
results for the factor indicate that while the form of these diagrams
depends sensitively on the order of the Sonine approximation considered when
gravity is absent, no significant differences between both Sonine solutions
appear in the opposite limit (gravity dominates the thermal gradient). In the
former case (no gravity), the first Sonine approximation overestimates both the
RBNE region and the influence of dissipation on thermal diffusion segregation.Comment: 9 figures; to be published in Phys. Rev.
Thermal diffusion segregation in granular binary mixtures described by the Enskog equation
Diffusion induced by a thermal gradient in a granular binary mixture is
analyzed in the context of the (inelastic) Enskog equation. Although the Enskog
equation neglects velocity correlations among particles which are about to
collide, it retains spatial correlations arising from volume exclusion effects
and thus it is expected to apply to moderate densities. In the steady state
with gradients only along a given direction, a segregation criterion is
obtained from the thermal diffusion factor measuring the amount of
segregation parallel to the thermal gradient. As expected, the sign of the
factor provides a criterion for the transition between the Brazil-nut
effect (BNE) and the reverse Brazil-nut effect (RBNE) by varying the parameters
of the mixture (masses, sizes, concentration, solid volume fraction, and
coefficients of restitution). The form of the phase diagrams for the BNE/RBNE
transition is illustrated in detail for several systems, with special emphasis
on the significant role played by the inelasticity of collisions. In
particular, an effect already found in dilute gases (segregation in a binary
mixture of identical masses and sizes {\em but} different coefficients of
restitution) is extended to dense systems. A comparison with recent computer
simulation results shows a good qualitative agreement at the level of the
thermal diffusion factor. The present analysis generalizes to arbitrary
concentration previous theoretical results derived in the tracer limit case.Comment: 7 figures, 1 table. To appear in New J. Phys., special issue on
"Granular Segregation
Diffusion in a multi-component Lattice Boltzmann Equation model
Diffusion phenomena in a multiple component lattice Boltzmann Equation (LBE)
model are discussed in detail. The mass fluxes associated with different
mechanical driving forces are obtained using a Chapman-Enskog analysis. This
model is found to have correct diffusion behavior and the multiple diffusion
coefficients are obtained analytically. The analytical results are further
confirmed by numerical simulations in a few solvable limiting cases. The LBE
model is established as a useful computational tool for the simulation of mass
transfer in fluid systems with external forces.Comment: To appear in Aug 1 issue of PR
Spinodal Decomposition in Binary Gases
We carried out three-dimensional simulations, with about 1.4 million
particles, of phase segregation in a low density binary fluid mixture,
described mesoscopically by energy and momentum conserving Boltzmann-Vlasov
equations. Using a combination of Direct Simulation Monte Carlo(DSMC) for the
short range collisions and a version of Particle-In-Cell(PIC) evolution for the
smooth long range interaction, we found dynamical scaling after the ratio of
the interface thickness(whose shape is described approximately by a hyperbolic
tangent profile) to the domain size is less than ~0.1. The scaling length R(t)
grows at late times like t^alpha, with alpha=1 for critical quenches and
alpha=1/3 for off-critical ones. We also measured the variation of temperature,
total particle density and hydrodynamic velocity during the segregation
process.Comment: 11 pages, Revtex, 4 Postscript figures, submitted to PR
Transport coefficients for inelastic Maxwell mixtures
The Boltzmann equation for inelastic Maxwell models is used to determine the
Navier-Stokes transport coefficients of a granular binary mixture in
dimensions. The Chapman-Enskog method is applied to solve the Boltzmann
equation for states near the (local) homogeneous cooling state. The mass, heat,
and momentum fluxes are obtained to first order in the spatial gradients of the
hydrodynamic fields, and the corresponding transport coefficients are
identified. There are seven relevant transport coefficients: the mutual
diffusion, the pressure diffusion, the thermal diffusion, the shear viscosity,
the Dufour coefficient, the pressure energy coefficient, and the thermal
conductivity. All these coefficients are {\em exactly} obtained in terms of the
coefficients of restitution and the ratios of mass, concentration, and particle
sizes. The results are compared with known transport coefficients of inelastic
hard spheres obtained analytically in the leading Sonine approximation and by
means of Monte Carlo simulations. The comparison shows a reasonably good
agreement between both interaction models for not too strong dissipation,
especially in the case of the transport coefficients associated with the mass
flux.Comment: 9 figures, to be published in J. Stat. Phy
Fluctuations in granular gases
A driven granular material, e.g. a vibrated box full of sand, is a stationary
system which may be very far from equilibrium. The standard equilibrium
statistical mechanics is therefore inadequate to describe fluctuations in such
a system. Here we present numerical and analytical results concerning energy
and injected power fluctuations. In the first part we explain how the study of
the probability density function (pdf) of the fluctuations of total energy is
related to the characterization of velocity correlations. Two different regimes
are addressed: the gas driven at the boundaries and the homogeneously driven
gas. In a granular gas, due to non-Gaussianity of the velocity pdf or lack of
homogeneity in hydrodynamics profiles, even in the absence of velocity
correlations, the fluctuations of total energy are non-trivial and may lead to
erroneous conclusions about the role of correlations. In the second part of the
chapter we take into consideration the fluctuations of injected power in driven
granular gas models. Recently, real and numerical experiments have been
interpreted as evidence that the fluctuations of power injection seem to
satisfy the Gallavotti-Cohen Fluctuation Relation. We will discuss an
alternative interpretation of such results which invalidates the
Gallavotti-Cohen symmetry. Moreover, starting from the Liouville equation and
using techniques from large deviation theory, the general validity of a
Fluctuation Relation for power injection in driven granular gases is
questioned. Finally a functional is defined using the Lebowitz-Spohn approach
for Markov processes applied to the linear inelastic Boltzmann equation
relevant to describe the motion of a tracer particle. Such a functional results
to be different from injected power and to satisfy a Fluctuation Relation.Comment: 40 pages, 18 figure
Tamoxifen for prevention of breast cancer: extended long-term follow-up of the IBIS-I breast cancer prevention trial
© Cuzick et al. Open Access article distributed under the terms of CC BY.http://dx.doi.org/10.1016/S1470-2045(14)71171-
Kinetic Theory of Plasmas: Translational Energy
In the present contribution, we derive from kinetic theory a unified fluid
model for multicomponent plasmas by accounting for the electromagnetic field
influence. We deal with a possible thermal nonequilibrium of the translational
energy of the particles, neglecting their internal energy and the reactive
collisions. Given the strong disparity of mass between the electrons and heavy
particles, such as molecules, atoms, and ions, we conduct a dimensional
analysis of the Boltzmann equation. We then generalize the Chapman-Enskog
method, emphasizing the role of a multiscale perturbation parameter on the
collisional operator, the streaming operator, and the collisional invariants of
the Boltzmann equation. The system is examined at successive orders of
approximation, each of which corresponding to a physical time scale. The
multicomponent Navier-Stokes regime is reached for the heavy particles, which
follow a hyperbolic scaling, and is coupled to first order drift-diffusion
equations for the electrons, which follow a parabolic scaling. The transport
coefficients exhibit an anisotropic behavior when the magnetic field is strong
enough. We also give a complete description of the Kolesnikov effect, i.e., the
crossed contributions to the mass and energy transport fluxes coupling the
electrons and heavy particles. Finally, the first and second principles of
thermodynamics are proved to be satisfied by deriving a total energy equation
and an entropy equation. Moreover, the system of equations is shown to be
conservative and the purely convective system hyperbolic, thus leading to a
well-defined structure
The bashful and the boastful : prestigious leaders and social change in Mesolithic Societies
The creation and maintenance of influential leaders and authorities is one of the key themes of archaeological and historical enquiry. However the social dynamics of authorities and leaders in the Mesolithic remains a largely unexplored area of study. The role and influence of authorities can be remarkably different in different situations yet they exist in all societies and in almost all social contexts from playgrounds to parliaments. Here we explore the literature on the dynamics of authority creation, maintenance and contestation in egalitarian societies, and discuss the implications for our interpretation and understanding of the formation of authorities and leaders and changing social relationships within the Mesolithic
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