198 research outputs found
Velocity statistics in excited granular media
We present an experimental study of velocity statistics for a partial layer
of inelastic colliding beads driven by a vertically oscillating boundary. Over
a wide range of parameters (accelerations 3-8 times the gravitational
acceleration), the probability distribution P(v) deviates measurably from a
Gaussian for the two horizontal velocity components. It can be described by
P(v) ~ exp(-|v/v_c|^1.5), in agreement with a recent theory. The characteristic
velocity v_c is proportional to the peak velocity of the boundary. The granular
temperature, defined as the mean square particle velocity, varies with particle
density and exhibits a maximum at intermediate densities. On the other hand,
for free cooling in the absence of excitation, we find an exponential velocity
distribution. Finally, we examine the sharing of energy between particles of
different mass. The more massive particles are found to have greater kinetic
energy.Comment: 27 pages, 13 figures, to appear in Chaos, September 99, revised 3
figures and tex
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
Bubble kinematics in a sheared foam
We characterize the kinematics of bubbles in a sheared two-dimensional foam
using statistical measures. We consider the distributions of both bubble
velocities and displacements. The results are discussed in the context of the
expected behavior for a thermal system and simulations of the bubble model.
There is general agreement between the experiments and the simulation, but
notable differences in the velocity distributions point to interesting elements
of the sheared foam not captured by prevalent models
The Behavior of Granular Materials under Cyclic Shear
The design and development of a parallel plate shear cell for the study of
large scale shear flows in granular materials is presented. The parallel plate
geometry allows for shear studies without the effects of curvature found in the
more common Couette experiments. A system of independently movable slats
creates a well with side walls that deform in response to the motions of grains
within the pack. This allows for true parallel plate shear with minimal
interference from the containing geometry. The motions of the side walls also
allow for a direct measurement of the velocity profile across the granular
pack. Results are presented for applying this system to the study of transients
in granular shear and for shear-induced crystallization. Initial shear profiles
are found to vary from packing to packing, ranging from a linear profile across
the entire system to an exponential decay with a width of approximately 6 bead
diameters. As the system is sheared, the velocity profile becomes much sharper,
resembling an exponential decay with a width of roughly 3 bead diameters.
Further shearing produces velocity profiles which can no longer be fit to an
exponential decay, but are better represented as a Gaussian decay or error
function profile. Cyclic shear is found to produce large scale ordering of the
granular pack, which has a profound impact on the shear profile. There exist
periods of time in which there is slipping between layers as well as periods of
time in which the layered particles lock together resulting in very little
relative motion.Comment: 10 pages including 12 figure
Understanding the dynamics of segregation bands of simulated granular material in a rotating drum
Axial segregation of a binary mixture of grains in a rotating drum is studied
using Molecular Dynamics (MD) simulations. A force scheme leading to a constant
restitution coefficient is used and shows that axial segregation is possible
between two species of grains made of identical material differing by size.
Oscillatory motion of bands is investigated and the influence of the frictional
properties elucidated. The mechanism of bands merging is explained using direct
imaging of individual grains
Creep motion in a granular pile exhibiting steady surface flow
We investigate experimentally granular piles exhibiting steady surface flow.
Below the surface flow, it has been believed exisitence of a `frozen' bulk
region, but our results show absence of such a frozen bulk. We report here that
even the particles in deep layers in the bulk exhibit very slow flow and that
such motion can be detected at an arbitrary depth. The mean velocity of the
creep motion decays exponentially with depth, and the characteristic decay
length is approximately equal to the particle-size and independent of the flow
rate. It is expected that the creep motion we have seeen is observable in all
sheared granular systems.Comment: 3 pages, 4 figure
Breakdown of Energy Equipartition in a 2D Binary Vibrated Granular Gas
We report experiments on the equipartition of kinetic energy between grains
made of two different materials in a mixture of grains vibrated in 2
dimensions. In general, the two types of grains do not attain the same granular
temperature, Tg = 1/2m v^2. However, the ratio of the two temperatures is
constant in the bulk of the system and independent of the vibration velocity.
The ratio depends strongly on the ratio of mass densities of the grains, but is
not sensitive to the inelasticity of grains. Also, this ratio is insensitive to
compositional variables of the mixture such as the number fraction of each
component and the total number density. We conclude that a single granular
temperature, as traditionally defined, does not characterize a multi-component
mixture.Comment: 4 pages, 5 figures, submitted to Physical Review Letters, updated
reference
Deformation and flow of a two-dimensional foam under continuous shear
We investigate the flow properties of a two-dimensional aqueous foam
submitted to a quasistatic shear in a Couette geometry. A strong localization
of the flow (shear banding) at the edge of the moving wall is evidenced,
characterized by an exponential decay of the average tangential velocity.
Moreover, the analysis of the rapid velocity fluctuations reveals self-similar
dynamical structures consisting of clusters of bubbles rolling as rigid bodies.
To relate the instantaneous (elastic) and time-averaged (plastic) components of
the strain, we develop a stochastic model where irreversible rearrangements are
activated by local stress fluctuations originating from the rubbing of the
wall. This model gives a complete description of our observations and is also
consistent with data obtained on granular shear bands by other groups.Comment: 5 pages, 2 figure
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