135 research outputs found
Internal Avalanches in a Granular Medium
Avalanches of grain displacements can be generated by creating local voids
within the interior of a granular material at rest in a bin. Modeling such a
two-dimensional granular system by a collection of mono-disperse discs, the
system on repeated perturbations, shows all signatures of Self-Organized
Criticality. During the propagation of avalanches the competition among grains
creates arches and in the critical state a distribution of arches of different
sizes is obtained. Using a cellular automata model we demonstrate that the
existence of arches determines the universal behaviour of the model system.Comment: 4 pages (Revtex), Four ps figures (included
Solid-fluid transition in a granular shear flow
The rheology of a granular shear flow is studied in a quasi-2d rotating
cylinder. Measurements are carried out near the midpoint along the length of
the surface flowing layer where the flow is steady and non-accelerating.
Streakline photography and image analysis are used to obtain particle
velocities and positions. Different particle sizes and rotational speeds are
considered. We find a sharp transition in the apparent viscosity ()
variation with rms velocity (). In the fluid-like region above the depth
corresponding to the transition point (higher rms velocities) there is a rapid
increase in viscosity with decreasing rms velocity. Below the transition depth
we find for all the different cases studied and the
material approaches an amorphous solid-like state deep in the layer. The
velocity distribution is Maxwellian above the transition point and a Poisson
velocity distribution is obtained deep in the layer. The observed transition
appears to be analogous to a glass transition.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
The S shape of a granular pile in a rotating drum
The shape of a granular pile in a rotating drum is investigated. Using
Discrete Elements Method (DEM) simulations we show that the "S shape" obtained
for high rotation speed can be accounted for by the friction on the end plates.
A theoretical model which accounts for the effect of the end plates is
presented and the equation of the shape of the free surface is derived. The
model reveals a dimensionless number which quantifies the influence of the end
plates on the shape of the pile. Finally, the scaling laws of the system are
discussed and numerical results support our conclusions
Avalanche statistics and time-resolved grain dynamics for a driven heap
We probe the dynamics of intermittent avalanches caused by steady addition of
grains to a quasi-two dimensional heap. To characterize the time-dependent
average avalanche flow speed v(t), we image the top free surface. To
characterize the grain fluctuation speed dv(t), we use Speckle-Visibility
Spectroscopy. During an avalanche, we find that the fluctuation speed is
approximately one-tenth the average flow speed, and that these speeds are
largest near the beginning of an event. We also find that the distribution of
event durations is peaked, and that event sizes are correlated with the time
interval since the end of the previous event. At high rates of grain addition,
where successive avalanches merge into smooth continuous flow, the relationship
between average and fluctuation speeds changes to dv Sqrt[v]
Mixing and segregation of granular materials in chute flows
Mixing of granular solids is invariably accompanied by segregation, however, the fundamentals of the process are not well understood. We analyze density and size segregation in a chute flow of cohesionless spherical particles by means of computations and theory based on the transport equations for a mixture of nearly elastic particles. Computations for elastic particles (Monte Carlo simulations), nearly elastic particles, and inelastic, frictional particles (particle dynamics simulations) are carried out. General expressions for the segregation fluxes due to pressure gradients and temperature gradients are derived. Simplified equations are obtained for the limiting cases of low volume fractions (ideal gas limit) and equal sized particles. Theoretical predictions of equilibrium number density profiles are in good agreement with computations for mixtures of equal sized particles with different density for all solids volume fractions, and for mixtures of different sized particles at low volume fractions (v<0.2), when the particles are elastic or nearly elastic. In the case of inelastic, frictional particles the theory gives reasonable predictions if an appropriate effective granular temperature is assumed. The relative importance of pressure diffusion and temperature diffusion for the cases considered is discussed
Dynamics of granular avalanches caused by local perturbations
Surface flow of granular material is investigated within a continuum approach
in two dimensions. The dynamics is described by a non-linear coupling between
the two `states' of the granular material: a mobile layer and a static bed.
Following previous studies, we use mass and momentum conservation to derive
St-Venant like equations for the evolution of the thickness R of the mobile
layer and the profile Z of the static bed. This approach allows the rheology in
the flowing layer to be specified independently, and we consider in details the
two following models: a constant plug flow and a linear velocity profile. We
study and compare these models for non-stationary avalanches triggered by a
localized amount of mobile grains on a static bed of constant slope. We solve
analytically the non-linear dynamical equations by the method of
characteristics. This enables us to investigate the temporal evolution of the
avalanche size, amplitude and shape as a function of model parameters and
initial conditions. In particular, we can compute their large time behavior as
well as the condition for the formation of shocks.Comment: 25 pages, 12 figure
Continuous Avalanche Segregation of Granular Mixtures in Thin Rotating Drums
We study segregation of granular mixtures in the continuous avalanche regime
(for frequencies above ~ 1 rpm) in thin rotating drums using a continuum theory
for surface flows of grains. The theory predicts profiles in agreement with
experiments only when we consider a flux dependent velocity of flowing grains.
We find the segregation of species of different size and surface properties,
with the smallest and roughest grains being found preferentially at the center
of the drum. For a wide difference between the species we find a complete
segregation in agreement with experiments. In addition, we predict a transition
to a smooth segregation regime - with an power-law decay of the concentrations
as a function of radial coordinate - as the size ratio between the grains is
decreased towards one.Comment: 4 pages, 4 figures, http://polymer.bu.edu/~hmaks
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