262 research outputs found
How to handle the inelastic collapse of a dissipative hard-sphere gas with the TC model
The inelastic hard sphere model of granular material is simple, easily
accessible to theory and simulation, and captures much of the physics of
granular media. It has three drawbacks, all related to the approximation that
collisions are instantaneous: 1) The number of collisions per unit time can
diverge, i.e. the ``inelastic collapse'' can occur. 2) All interactions are
binary, multiparticle contacts cannot occur and 3) no static limit exists. We
extend the inelastic hard sphere model by defining a duration of contact t_c
such that dissipation is allowed only if the time between contacts is larger
than t_c. We name this generalized model the ``TC model'' and discuss it using
examples of dynamic and static systems. The contact duration used here does not
change the instantaneous nature of the hard sphere contacts, but accounts for a
reduced dissipation during ``multiparticle contacts''. Kinetic and elastic
energies are defined as well as forces and stresses in the system. Finally, we
present event-driven numerical simulations of situations far beyond the
inelastic collapse, possible only with the TC model.Comment: 15 pages, Latex, 14 bw.ps figures + 2 col.ps figures, to be published
in Granular Matter 1(3) 199
Simulations of vibrated granular medium with impact velocity dependent restitution coefficient
We report numerical simulations of strongly vibrated granular materials
designed to mimic recent experiments performed both in presence or absence of
gravity. The coefficient of restitution used here depends on the impact
velocity by taking into account both the viscoelastic and plastic deformations
of particles, occurring at low and high velocities respectively. We show that
this model with impact velocity dependent restitution coefficient reproduce
results that agree with experiments. We measure the scaling exponents of the
granular temperature, collision frequency, impulse, and pressure with the
vibrating piston velocity as the particle number increases. As the system
changes from a homogeneous gas state at low density to a clustered state at
high density, these exponents are all found to decrease continuously with the
particle number. All these results differ significantly from classical
inelastic hard sphere kinetic theory and previous simulations, both based on a
constant restitution coefficient.Comment: to be published in Phys. Rev.
Eshelby inclusions in granular matter: theory and simulations
We present a numerical implementation of an active inclusion in a granular
material submitted to a biaxial test. We discuss the dependence of the response
to this perturbation on two parameters: the intra-granular friction coefficient
on one hand, the degree of the loading on the other hand. We compare the
numerical results to theoretical predictions taking into account the change of
volume of the inclusion as well as the anisotropy of the elastic matrix
Turning Down the Volume on Granular Materials
International audienceA reformulation of the statistical mechanics of granular materials replaces the volume of thematerial with a function related to its structure
On the relevance of numerical simulations to booming sand
We have performed a simulation study of 3D cohesionless granular flows down
an inclined chute. We find that the oscillations observed in [L.E. Silbert,
Phys. Rev. Lett., 94, 098002 (2005)] near the angle of repose are harmonic
vibrations of the lowest normal mode. Their frequencies depend on the contact
stiffness as well as on the depth of the flow. Could these oscillations account
for the phenomena of "booming sand"? We estimate an effective contact stiffness
from the Hertz law, but this leads to frequencies several times higher than
observed. However, the Hertz law also predicts interpenetrations of a few
nanometers, indicating that the oscillations frequencies are governed by the
surface stiffness, which can be much lower than the bulk one. This is in
agreement with previous studies ascribing the ability to sing to the presence
of a soft coating on the grain surface.Comment: accepted for publication in Physical Review E http://pre.aps.org;
Physical Review E (2012) to be publishe
Absorbing boundary conditions for granular acoustics
The boundary conditions of soft-sphere DEM are usually perfect reflectors of acoustic waves, leading to an unrealistic accumulation of energy. This situation is usually dealt with by global damping. In some situations, this solution is undesirable, so we present an alternative. If the grain-wall contact is made soft and dissipative, most acoustic energy incident on the boundary will be trapped and dissipated there. We show that these boundary conditions can efficiently damp both high and low frequency waves
Computer Simulation of Particle Suspensions
Particle suspensions are ubiquitous in our daily life, but are not well
understood due to their complexity. During the last twenty years, various
simulation methods have been developed in order to model these systems. Due to
varying properties of the solved particles and the solvents, one has to choose
the simulation method properly in order to use the available compute resources
most effectively with resolving the system as well as needed. Various
techniques for the simulation of particle suspensions have been implemented at
the Institute for Computational Physics allowing us to study the properties of
clay-like systems, where Brownian motion is important, more macroscopic
particles like glass spheres or fibers solved in liquids, or even the pneumatic
transport of powders in pipes. In this paper we will present the various
methods we applied and developed and discuss their individual advantages.Comment: 31 pages, 11 figures, to appear in Lecture Notes in Applied and
Computational Mechanics, Springer (2006
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