1,801 research outputs found

### Breakdown of hydrodynamics in the inelastic Maxwell model of granular gases

Both the right and left eigenfunctions and eigenvalues of the linearized
homogeneous Boltzmann equation for inelastic Maxwell molecules corresponding to
the hydrodynamic modes are calculated. Also, some non-hydrodynamic modes are
identified. It is shown that below a critical value of the parameter
characterizing the inelasticity, one of the kinetic modes decays slower than
one of the hydrodynamic ones. As a consequence, a closed hydrodynamic
description does not exist in that regime. Some implications of this behavior
on the formally computed Navier-Stokes transport coefficients are discussed.Comment: Submitted to PRL (13/04/10

### Glass-like dynamical behavior in hierarchical models submitted to continuous cooling and heating processes

The dynamical behavior of a kind of models with hierarchically constrained
dynamics is investigated. The models exhibit many properties resembling real
structural glasses. In particular, we focus on the study of time-dependent
temperature processes. In cooling processes, a phenomenon analogous to the
laboratory glass transition appears. The residual properties are analytically
evaluated, and the concept of fictive temperature is discussed on a physical
base. The evolution of the system in heating processes is governed by the
existence of a normal solution of the evolution equations, which is approached
by all the other solutions. This trend of the system is directly related to the
glassy hysteresis effects shown by these systems. The existence of the normal
solution is not restricted to the linear regime around equilibrium, but it is
defined for any arbitrary, far from equilibrium, situation.Comment: 20 pages, 7 figures; minor changes, accepted in Phys. Rev.

### Memory effects in vibrated granular systems

Granular materials present memory effects when submitted to tapping
processes. These effects have been observed experimentally and are discussed
here in the context of a general kind of model systems for compaction
formulated at a mesoscopic level. The theoretical predictions qualitatively
agree with the experimental results. As an example, a particular simple model
is used for detailed calculations.Comment: 12 pages, 5 figures; to appear in Journal of Physics: Condensed
Matter (Special Issue: Proceedings of ESF SPHINX Workshop on ``Glassy
behaviour of kinetically constrained models.''

### Closed model for granular compaction under weak tapping

A one dimensional lattice model is formulated to study tapping dynamics and
the long time steady distribution in granular media. The dynamics conserves the
number of particles in the system, and density changes are associated to the
creation and destruction of empty sites. The model is shown to be consistent
with Edwards thermodynamics theory of powders. The relationship with lattice
models in which the number of particles is not conserved is discussed.Comment: 18 pages in revtex preprint style, 4 figures; Phys. Rev. E (in press

### Scaling and universality of critical fluctuations in granular gases

The global energy fluctuations of a low density gas granular gas in the
homogeneous cooling state near its clustering instability are studied by means
of molecular dynamics simulations. The relative dispersion of the fluctuations
is shown to exhibit a power-law divergent behavior. Moreover, the probability
distribution of the fluctuations presents data collapse as the system
approaches the instability, for different values of the inelasticity. The
function describing the collapse turns out to be the same as the one found in
several molecular equilibrium and non-equilibrium systems, except for the
change in the sign of the fluctuations

### Critical Behavior of a Heavy Particle in a Granular Fluid

Behavior analogous to a second order phase transition is observed for the
homogeneous cooling state of a heavy impurity particle in a granular fluid. The
order parameter $\phi$ is the ratio of impurity mean square velocity to that
of the fluid, with a conjugate field $h$ proportional to the mass ratio. A
parameter $\beta$, measuring the fluid cooling rate relative to the
impurity--fluid collision rate, is the analogue of the inverse temperature. For
$\beta <1$ the fluid is ``normal'' with $\phi =0$ at $h=0$, as in the case of a
system with elastic collisions. For $\beta >1$ an ``ordered'' state with $\phi
\neq 0$ occurs at $h=0$, representing an extreme breakdown of equipartition.
Critical slowing and qualitative changes in the velocity distribution function
for the impurity particle near the transition are notedComment: 4 pages (4 figures included

### Hydrodynamic profiles for an impurity in a open vibrated granular gas

The hydrodynamic state of an impurity immersed in a low density granular gas
is analyzed. Explicit expressions for the temperature and density fields of the
impurity in terms of the hydrodynamic fields of the gas are derived. It is
shown that the ratio between the temperatures of the two components, measuring
the departure from energy equipartition, only depends on the mechanical
properties of the particles, being therefore constant in the bulk of the
system. This ratio plays an important role in determining the density profile
of the intruder and its position with respect to the gas, since it determines
the sign of the pressure diffusion coefficient. The theoretical predictions are
compared with molecular dynamics simulation results for the particular case of
the steady state of an open vibrated granular system in absence of macroscopic
fluxes, and a satisfactory agreement is found

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