465 research outputs found

### Competition of mixing and segregation in rotating cylinders

Using discrete element methods, we study numerically the dynamics of the size
segregation process of binary particle mixtures in three-dimensional rotating
drums, operated in the continuous flow regime. Particle rotations are included
and we focus on different volume filling fractions of the drum to study the
interplay between the competing phenomena of mixing and segregation. It is
found that segregation is best for a more than half-filled drum due to the
non-zero width of the fluidized layer. For different particle size ratios, it
is found that radial segregation occurs for any arbitrary small particle size
difference and the final amount of segregation shows a linear dependence on the
size ratio of the two particle species. To quantify the interplay between
segregation and mixing, we investigate the dynamics of the center of mass
positions for each particle component. Starting with initially separated
particle groups we find that no mixing of the component is necessary in order
to obtain a radially segregated core.Comment: 9 pages, 12 figures (EPIC/EEPIC & EPS, macros included), submitted to
Physics of Fluid

### An exactly solvable model for driven dissipative systems

We introduce a solvable stochastic model inspired by granular gases for
driven dissipative systems. We characterize far from equilibrium steady states
of such systems through the non-Boltzmann energy distribution and compare
different measures of effective temperatures. As an example we demonstrate that
fluctuation-dissipation relations hold, however with an effective temperature
differing from the effective temperature defined from the average energy.Comment: Some further clarifications. No changes in results or conclusion

### Phase separation of a driven granular gas in annular geometry

This work investigates phase separation of a monodisperse gas of
inelastically colliding hard disks confined in a two-dimensional annulus, the
inner circle of which represents a "thermal wall". When described by granular
hydrodynamic equations, the basic steady state of this system is an azimuthally
symmetric state of increased particle density at the exterior circle of the
annulus. When the inelastic energy loss is sufficiently large, hydrodynamics
predicts spontaneous symmetry breaking of the annular state, analogous to the
van der Waals-like phase separation phenomenon previously found in a driven
granular gas in rectangular geometry. At a fixed aspect ratio of the annulus,
the phase separation involves a "spinodal interval" of particle area fractions,
where the gas has negative compressibility in the azimuthal direction. The heat
conduction in the azimuthal direction tends to suppress the instability, as
corroborated by a marginal stability analysis of the basic steady state with
respect to small perturbations. To test and complement our theoretical
predictions we performed event-driven molecular dynamics (MD) simulations of
this system. We clearly identify the transition to phase separated states in
the MD simulations, despite large fluctuations present, by measuring the
probability distribution of the amplitude of the fundamental Fourier mode of
the azimuthal spectrum of the particle density. We find that the instability
region, predicted from hydrodynamics, is always located within the phase
separation region observed in the MD simulations. This implies the presence of
a binodal (coexistence) region, where the annular state is metastable. The
phase separation persists when the driving and elastic walls are interchanged,
and also when the elastic wall is replaced by weakly inelastic one.Comment: 9 pages, 10 figures, to be published in PR

### On the Shape of the Tail of a Two Dimensional Sand Pile

We study the shape of the tail of a heap of granular material. A simple
theoretical argument shows that the tail adds a logarithmic correction to the
slope given by the angle of repose. This expression is in good agreement with
experiments. We present a cellular automaton that contains gravity, dissipation
and surface roughness and its simulation also gives the predicted shape.Comment: LaTeX file 4 pages, 4 PS figures, also available at
http://pmmh.espci.fr

### Force Dynamics in Weakly Vibrated Granular Packings

The oscillatory force F_b^ac on the bottom of a rigid, vertically vibrated,
grain filled column, reveals rich granular dynamics, even when the peak
acceleration of the vibrations is signicantly less than the gravitational
acceleration at the earth's surface. For loose packings or high frequencies,
F_b^ac 's dynamics are dominated by grain motion. For moderate driving
conditions in more compact samples, grain motion is virtually absent, but
F_b^ac nevertheless exhibits strongly nonlinear and hysteretic behavior,
evidencing a granular regime dominated by nontrivial force-network dynamics.Comment: 4 pages, 5 figure

### Avalanche Mixing of Granular Solids

Mixing of two fractions of a granular material in a slowly rotating
two-dimensional drum is considered. The rotation is around the axis of the
upright drum. The drum is filled partially, and mixing occurs only at a free
surface of the material. We propose a simple theory of the mixing process which
describes a real experiment surprisingly well. A geometrical approach without
appealing to ideas of self-organized criticality is used. The dependence of the
mixing time on the drum filling is calculated. The mixing time is infinite in
the case of the half-filled drum. We describe singular behaviour of the mixing
near this critical point.Comment: 9 pages (LaTeX) and 2 Postscript figures, to be published in
Europhys. Let

### Mixing and condensation in a wet granular medium

We have studied the effect of small amounts of added liquid on the dynamic
behavior of a granular system consisting of a mixture of glass beads of two
different sizes. Segregation of the large beads to the top of the sample is
found to depend in a nontrivial way on the liquid content. A transition to
viscoplastic behavior occurs at a critical liquid content, which depends upon
the bead size. We show that this transition can be interpreted as a
condensation due to the hysteretic liquid bridge forces connecting the beads,
and provide the corresponding phase diagram.Comment: submitted to PR

### Piling and avalanches of magnetized particles

We performed computer simulations based on a two-dimensional Distinct Element
Method to study granular systems of magnetized spherical particles. We measured
the angle of repose and the surface roughness of particle piles, and we studied
the effect of magnetization on avalanching. We report linear dependence of both
angle of repose and surface roughness on the ratio $f$ of the magnetic dipole
interaction and the gravitational force (\emph{interparticle force ratio}).
There is a difference in avalanche formation at small and at large
interparticle force ratios. The transition is at $f_c \approx 7$. For $f < f_c$
the particles forming the avalanches leave the system in a quasi-continuous
granular flow (\emph{granular regime}), while for $f > f_c$ the avalanches are
formed by long particle clusters (\emph{correlated regime}). The transition is
not sharp. We give plausible estimates for $f_c$ based on stability criteria.Comment: 9 pages, 7 figure

### Diffusion of a granular pulse in a rotating drum

The diffusion of a pulse of small grains in an horizontal rotating drum is
studied through discrete elements methods simulations. We present a theoretical
analysis of the diffusion process in a one-dimensional confined space in order
to elucidate the effect of the confining end-plate of the drum. We then show
that the diffusion is neither subdiffusive nor superdiffusive but normal. This
is demonstrated by rescaling the concentration profiles obtained at various
stages and by studying the time evolution of the mean squared deviation.
Finally we study the self-diffusion of both large and small grains and we show
that it is normal and that the diffusion coefficient is independent of the
grain size

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