383 research outputs found

### Sand stirred by chaotic advection

We study the spatial structure of a granular material, N particles subject to
inelastic mutual collisions, when it is stirred by a bidimensional smooth
chaotic flow. A simple dynamical model is introduced where four different time
scales are explicitly considered: i) the Stokes time, accounting for the
inertia of the particles, ii) the mean collision time among the grains, iii)
the typical time scale of the flow, and iv) the inverse of the Lyapunov
exponent of the chaotic flow, which gives a typical time for the separation of
two initially close parcels of fluid. Depending on the relative values of these
different times a complex scenario appears for the long-time steady spatial
distribution of particles, where clusters of particles may or not appear.Comment: 4 pages, 3 figure

### Mean Field Theory of Sandpile Avalanches: from the Intermittent to the Continuous Flow Regime

We model the dynamics of avalanches in granular assemblies in partly filled
rotating cylinders using a mean-field approach. We show that, upon varying the
cylinder angular velocity $\omega$, the system undergoes a hysteresis cycle
between an intermittent and a continuous flow regimes. In the intermittent flow
regime, and approaching the transition, the avalanche duration exhibits
critical slowing down with a temporal power-law divergence. Upon adding a white
noise term, and close to the transition, the distribution of avalanche
durations is also a power-law. The hysteresis, as well as the statistics of
avalanche durations, are in good qualitative agreement with recent experiments
in partly filled rotating cylinders.Comment: 4 pages, RevTeX 3.0, postscript figures 1, 3 and 4 appended

### Generation of Porous Particle Structures using the Void Expansion Method

The newly developed "void expansion method" allows for an efficient
generation of porous packings of spherical particles over a wide range of
volume fractions using the discrete element method. Particles are randomly
placed under addition of much smaller "void-particles". Then, the void-particle
radius is increased repeatedly, thereby rearranging the structural particles
until formation of a dense particle packing.
The structural particles' mean coordination number was used to characterize
the evolving microstructures. At some void radius, a transition from an
initially low to a higher mean coordination number is found, which was used to
characterize the influence of the various simulation parameters. For structural
and void-particle stiffnesses of the same order of magnitude, the transition is
found at constant total volume fraction slightly below the random close packing
limit. For decreasing void-particle stiffness the transition is shifted towards
a smaller void-particle radius and becomes smoother.Comment: 9 pages, 8 figure

### Projectile-shape dependence of impact craters in loose granular media

We report on the penetration of cylindrical projectiles dropped from rest
into a dry, noncohesive granular medium. The cylinder length, diameter,
density, and tip shape are all explicitly varied. For deep penetrations, as
compared to the cylinder diameter, the data collapse onto a single scaling law
that varies as the 1/3 power of the total drop distance, the 1/2 power of
cylinder length, and the 1/6 power of cylinder diameter. For shallow
penetrations, the projectile shape plays a crucial role with sharper objects
penetrating deeper.Comment: 3 pages, 3 figures; experimen

### A Model for Granular Texture with Steric Exclusion

We propose a new method to characterize the geometrical texture of a granular
packing at the particle scale including the steric hindrance effect. This
method is based on the assumption of a maximum disorder (entropy) compatible
both with strain-induced anisotropy of the contact network and steric
exclusions. We show that the predicted statistics for the local configurations
is in a fairly agreement with our numerical data.Comment: 9 pages, 5 figure

### Slow relaxation in granular compaction

Experimental studies show that the density of a vibrated granular material
evolves from a low density initial state into a higher density final steady
state. The relaxation towards the final density value follows an inverse
logarithmic law. We propose a simple stochastic adsorption-desorption process
which captures the essential mechanism underlying this remarkably slow
relaxation. As the system approaches its final state, a growing number of beads
have to be rearranged to enable a local density increase. In one dimension,
this number grows as $N=\rho/(1-\rho)$, and the density increase rate is
drastically reduced by a factor $e^{-N}$. Consequently, a logarithmically slow
approach to the final state is found $\rho_{\infty}-\rho(t)\cong 1/\ln t$.Comment: revtex, 4 pages, 3 figures, also available from
http://arnold.uchicago.edu/~ebn

### Velocity Fluctuations in Electrostatically Driven Granular Media

We study experimentally the particle velocity fluctuations in an
electrostatically driven dilute granular gas. The experimentally obtained
velocity distribution functions have strong deviations from Maxwellian form in
a wide range of parameters. We have found that the tails of the distribution
functions are consistent with a stretched exponential law with typical
exponents of the order 3/2. Molecular dynamic simulations shows qualitative
agreement with experimental data. Our results suggest that this non-Gaussian
behavior is typical for most inelastic gases with both short and long range
interactions.Comment: 4 pages, 4 figure

### Density waves in dry granular media falling through a vertical pipe

We report experimental measurements of density waves in granular materials
flowing down in a capillary tube. The density wave regime occurs at
intermediate flow rates between a low density free fall regime and a high
compactness slower flow.Comment: LaTeX file, 17 pages, 6 EPS figures, Phys.Rev.E (Feb.1996

### Symmetry-breaking instability in a prototypical driven granular gas

Symmetry-breaking instability of a laterally uniform granular cluster (strip
state) in a prototypical driven granular gas is investigated. The system
consists of smooth hard disks in a two-dimensional box, colliding inelastically
with each other and driven, at zero gravity, by a "thermal" wall. The limit of
nearly elastic particle collisions is considered, and granular hydrodynamics
with the Jenkins-Richman constitutive relations is employed. The hydrodynamic
problem is completely described by two scaled parameters and the aspect ratio
of the box. Marginal stability analysis predicts a spontaneous symmetry
breaking instability of the strip state, similar to that predicted recently for
a different set of constitutive relations. If the system is big enough, the
marginal stability curve becomes independent of the details of the boundary
condition at the driving wall. In this regime, the density perturbation is
exponentially localized at the elastic wall opposite to the thermal wall. The
short- and long-wavelength asymptotics of the marginal stability curves are
obtained analytically in the dilute limit. The physics of the symmetry-breaking
instability is discussed.Comment: 11 pages, 14 figure

### Velocity correlations in dense granular gases

We report the statistical properties of spherical steel particles rolling on
an inclined surface being driven by an oscillating wall. Strong dissipation
occurs due to collisions between the particles and rolling and can be tuned by
changing the number density. The velocities of the particles are observed to be
correlated over large distances comparable to the system size. The distribution
of velocities deviates strongly from a Gaussian. The degree of the deviation,
as measured by the kurtosis of the distribution, is observed to be as much as
four times the value corresponding to a Gaussian, signaling a significant
breakdown of the assumption of negligible velocity correlations in a granular
system.Comment: 4 pages, 4 Figure

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