328 research outputs found
High intensity tapping regime in a frustrated lattice gas model of granular compaction
In the frame of a well established lattice gas model for granular compaction,
we investigate the high intensity tapping regime where a pile expands
significantly during external excitation. We find that this model shows the
same general trends as more sophisticated models based on molecular dynamic
type simulations. In particular, a minimum in packing fraction as a function of
tapping strength is observed in the reversible branch of an annealed tapping
protocol.Comment: 5 pages, 4 figure
New criteria for cluster identification in continuum systems
Two new criteria, that involve the microscopic dynamics of the system, are
proposed for the identification of clusters in continuum systems. The first one
considers a residence time in the definition of the bond between pairs of
particles, whereas the second one uses a life time in the definition of an
aggregate. Because of the qualitative features of the clusters yielded by the
criteria we call them chemical and physical clusters, respectively. Molecular
dynamics results for a Lennard-Jones system and general connectivity theories
are presented.Comment: 31 pages, 11 figures, The following article has been accepted by The
Journal of Chemical Physics. After it is published, it will be found at
http://ojps.aip.org/jcpo
Multi-particle structures in non-sequentially reorganized hard sphere deposits
We have examined extended structures, bridges and arches, in computer
generated, non-sequentially stabilized, hard sphere deposits. The bridges and
arches have well defined distributions of sizes and shapes. The distribution
functions reflect the contraints associated with hard particle packing and the
details of the restructuring process. A subpopulation of string-like bridges
has been identified. Bridges are fundamental microstructural elements in real
granular systems and their sizes and shapes dominate considerations of
structural properties and flow instabilities such as jamming.Comment: 9 pages, 7 figure
Continuum percolation of simple fluids: Energetic connectivity criteria
During the last few years, a number of works in computer simulation have
focused on the clustering and percolation properties of simple fluids based in
an energetic connectivity criterion proposed long ago by T.L. Hill [J. Chem.
Phys. 23, 617 (1955)]. This connectivity criterion appears to be the most
appropriate in the study of gas-liquid phase transition. So far, integral
equation theories have relayed on a velocity-averaged version of this
criterion. We show, by using molecular dynamics simulations, that this average
strongly overestimates percolation densities in the Lennard-Jones fluid making
unreliable any prediction based on it. Additionally, we use a recently
developed integral equation theory [Phys. Rev. E 61, R6067 (2000)] to show how
this velocity-average can be overcome.Comment: 14 pages, 2 figure
Flow rate of polygonal grains through a bottleneck: Interplay between shape and size
We report two-dimensional simulations of circular and polygonal grains
passing through an aperture at the bottom of a silo. The mass flow rate for
regular polygons is lower than for disks as observed by other authors. We show
that both the exit velocity of the grains and the packing fraction are lower
for polygons, which leads to the reduced flow rate. We point out the importance
of the criteria used to define when two objects of different shape are
considered to be of the same size. Depending on this criteria, the mass flow
rate may vary significantly for some polygons. Moreover, the particle flow rate
is non-trivially related to a combination of mass flow rate, particle shape and
particle size. For some polygons, the particle flow rate may be lower or higher
than that of the corresponding disks depending on the size comparison criteria.Comment: 9 pages, 8 figure
Exact predictions from Edwards ensemble vs. realistic simulations of tapped narrow two-dimensional granular columns
We simulate via a Discrete Element Method the tapping of a narrow column of
disk under gravity. For frictionless disks, this system has a simple analytic
expression for the density of states in the Edwards volume ensemble. We compare
the predictions of the ensemble at constant compactivity against the results
for the steady states obtained in the simulations. We show that the steady
states cannot be properly described since the microstates sampled are not in
correspondence with the predicted distributions, suggesting that the postulates
of flat measure and ergodicity are, either or both, invalid for this simple
realization of a static granular system. However, we show that certain
qualitative features of the volume fluctuations difficult to predict from
simple arguments are captured by the theory.Comment: 11 pages, 6 figure
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