394 research outputs found
Transverse Instability of Avalanches in Granular Flows down Incline
Avalanche experiments on an erodible substrate are treated in the framework
of ``partial fluidization'' model of dense granular flows. The model identifies
a family of propagating soliton-like avalanches with shape and velocity
controlled by the inclination angle and the depth of substrate. At high
inclination angles the solitons display a transverse instability, followed by
coarsening and fingering similar to recent experimental observation. A primary
cause for the transverse instability is directly related to the dependence of
soliton velocity on the granular mass trapped in the avalanche.Comment: 3 figures, 4 pages, submitted to Phys Rev Let
A constitutive law for dense granular flows
A continuum description of granular flows would be of considerable help in
predicting natural geophysical hazards or in designing industrial processes.
However, the constitutive equations for dry granular flows, which govern how
the material moves under shear, are still a matter of debate. One difficulty is
that grains can behave like a solid (in a sand pile), a liquid (when poured
from a silo) or a gas (when strongly agitated). For the two extreme regimes,
constitutive equations have been proposed based on kinetic theory for
collisional rapid flows, and soil mechanics for slow plastic flows. However,
the intermediate dense regime, where the granular material flows like a liquid,
still lacks a unified view and has motivated many studies over the past decade.
The main characteristics of granular liquids are: a yield criterion (a critical
shear stress below which flow is not possible) and a complex dependence on
shear rate when flowing. In this sense, granular matter shares similarities
with classical visco-plastic fluids such as Bingham fluids. Here we propose a
new constitutive relation for dense granular flows, inspired by this analogy
and recent numerical and experimental work. We then test our three-dimensional
(3D) model through experiments on granular flows on a pile between rough
sidewalls, in which a complex 3D flow pattern develops. We show that, without
any fitting parameter, the model gives quantitative predictions for the flow
shape and velocity profiles. Our results support the idea that a simple
visco-plastic approach can quantitatively capture granular flow properties, and
could serve as a basic tool for modelling more complex flows in geophysical or
industrial applications.Comment: http://www.nature.com/nature/journal/v441/n7094/abs/nature04801.htm
The Feeling of Color: A Haptic Feedback Device for the Visually Disabled
Tapson J, Gurari N, Diaz J, et al. The Feeling of Color: A Haptic Feedback Device for the Visually Disabled. Presented at the Biomedical Circuits and Systems Conference (BIOCAS), Baltimore, MD.We describe a sensory augmentation system designed to provide the visually disabled with a sense of color. Our system consists of a glove with short-range optical color sensors mounted on its fingertips, and a torso-worn belt on which tactors (haptic feedback actuators) are mounted. Each fingertip sensor detects the observed objectpsilas color. This information is encoded to the tactor through vibrations in respective locations and varying modulations. Early results suggest that detection of primary colors is possible with near 100% accuracy and moderate latency, with a minimum amount of training
Effective boundary conditions for dense granular flows
We derive an effective boundary condition for granular flow taking into
account the effect of the heterogeneity of the force network on sliding
friction dynamics. This yields an intermediate boundary condition which lies in
the limit between no-slip and Coulomb friction; two simple functions relating
wall stress, velocity, and velocity variance are found from numerical
simulations. Moreover, we show that this effective boundary condition
corresponds to Navier slip condition when GDR MiDi's model is assumed to be
valid, and that the slip length depends on the length scale that characterises
the system, \emph{viz} the particle diameter.Comment: 4 pages, 5 figure
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
Experimental compaction of anisotropic granular media
We report on experiments to measure the temporal and spatial evolution of
packing arrangements of anisotropic and weakly confined granular material,
using high-resolution -ray adsorption. In these experiments, the
particle configurations start from an initially disordered,
low-packing-fraction state and under vertical solicitations evolve to a dense
state. We find that the packing fraction evolution is slowed by the grain
anisotropy but, as for spherically shaped grains, can be well fitted by a
stretched exponential. For a given type of grains, the characteristic times of
relaxation and of convection are found to be of the same order of magnitude. On
the contrary compaction mechanisms in the media strongly depend on the grain
anisotropy.Comment: to appear in the european physical journal E (EPJE
Shear bands in granular flow through a mixing length model
We discuss the advantages and results of using a mixing-length, compressible
model to account for shear banding behaviour in granular flow. We formulate a
general approach based on two function of the solid fraction to be determined.
Studying the vertical chute flow, we show that shear band thickness is always
independent from flowrate in the quasistatic limit, for Coulomb wall boundary
conditions. The effect of bin width is addressed using the functions developed
by Pouliquen and coworkers, predicting a linear dependence of shear band
thickness by channel width, while literature reports contrasting data. We also
discuss the influence of wall roughness on shear bands. Through a Coulomb wall
friction criterion we show that our model correctly predicts the effect of
increasing wall roughness on the thickness of shear bands. Then a simple
mixing-length approach to steady granular flows can be useful and
representative of a number of original features of granular flow.Comment: submitted to EP
Is Random Close Packing of Spheres Well Defined?
Despite its long history, there are many fundamental issues concerning random
packings of spheres that remain elusive, including a precise definition of
random close packing (RCP). We argue that the current picture of RCP cannot be
made mathematically precise and support this conclusion via a molecular
dynamics study of hard spheres using the Lubachevsky-Stillinger compression
algorithm. We suggest that this impasse can be broken by introducing the new
concept of a maximally random jammed state, which can be made precise.Comment: 6 pages total, 2 figure
Stationary shear flows of dense granular materials : a tentative continuum modelling
We propose a simple continuum model to interpret the shearing motion of
dense, dry and cohesion-less granular media. Compressibility, dilatancy and
Coulomb-like friction are the three basic ingredients. The granular stress is
split into a rate-dependent part representing the rebound-less impacts between
grains and a rate-independent part associated with long-lived contacts. Because
we consider stationary flows only, the grain compaction and the grain velocity
are the two main variables. The predicted velocity and compaction profiles are
in apparent agreement with the experimental or numerical results concerning
free-surface shear flows as well as confined shear flow
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