Invited Talks
Name = Don Candela
email = candela@physics.umass.edu
Institution = University of Massachusetts Amherst
Title = Using NMR to Study Granular Media
Nuclear magnetic resonance (NMR) can serve as a powerful,
noninvasive probe of dense three-dimensional granular media. NMR has
been used to study a variety of granular states including static
packings, quasistatic tapping flow, continuous rotating drum flow, and
rapid granular flows such as the vibrofluidized bed. In our lab at
UMass we have used NMR to measure density and granular temperature
profiles in vibrofluidized beds, providing a quantitative test of
granular hydrodynamic theory. Using this work as an example, I will
provide a brief tutorial for nonspecialists on the principles and
capabilities of NMR as an experimental probe of granular systems.
Name = David L. Johnson
email = johnson10@slb.com
Institution = Schlumberger-Doll Research
Title = Linear and nonlinearelasticity of granular media
We consider the linear and non-linear elasticity ofunconsolidated
granular aggregates, considered as a function ofapplied stress, which
may be anisotropic. A major considerationis the path-dependence of
the stress-strain relation. Forexample, if the system is compressed
and sheared, the order inwhich this is done is important. For special
cases in which theinter-granular forces are path independent, we
derive expressionsfor the so-called "3rd order" nonlinear elastic
constants. Wecompare our results for the speeds of sound as a
function ofstress against experimental data on both
hydrostaticallycompressed and on uniaxially strained systems of loose
glassbeads. We present results on molecular dynamic simulations of
thedependence of the bulk and shear modulii as a function
ofhydrostatic stress. Here we see that the bulk modulus is
wellunderstood but the shear modulus depends upon grain rotations
andtranslations in a manner not captured by the simple theory.Initial
attempts to understand this in terms of correlated oruncorrelated
particle relaxation are only partially successful
Name = Mark Shattuck
email = shattuck@ccny.cuny.edu
Institution = Levich Institute, City College of New York
Title = Granular Kinetic Theory
Granular materials are ubiquitous in nature and of great importance in
industries ranging from food preparation to pharmaceuticals to coal
processing. They also represent a new forum in which to study
collective behavior and pattern formation in a system with weak scale
separation, but one in which the microscopic scale is very large
ranging from tens of microns to centimeters. The large-scale
microstructure allows unprecedented experimental access to the
underlying particle motions and provides an interesting test bed for
kinetic theories of hard spheres that incorporate inelastic particle
interactions.
In our laboratory, we combine experiment, simulation, and theory to
examine the validity of kinetic theories of dense inelastic hard
spheres, which have been developed over the last 15 years using Grad's
13-moment method, Enskog theory, or revised Enskog theory. All of
these theories lead to equations similar to the Navier-Stokes
equations for dense gases but with the addition of a temperature loss
rate term in the energy equation. We find that transport coefficients
predicted by the Grad 13-moment approach agree (typically within 20
percent) with event-driven molecular dynamics simulation of a
homogeneously heated gas of inelastic hard spheres. These simulations
have been previously validated and quantitatively reproduce the
complex patterns found in experiments in thin oscillated granular
layers. The same simulations agree to within 1 percent with experiments in
dilute gravity-driven flow past a wedge producing granular shockwaves.
Further, numerical integrations of the Navier-Stokes equations for
dense gases also agree with the simulations to within 1 percent. We are
currently investigating dense rotating flows and find near
Maxwell-Boltzmann velocity distributions in agreement with the
assumptions of kinetic theory
In this presentation an introduction to the kinetic theory of dense
granular gases will be presented, with an emphasis on basic concepts.
Name = Greg Voth
email = gvoth@wesleyan.edu
Institution = Wesleyan University
Title = Fluidization and collapse of driven granular materials
While theories have been developed that accurately describe many properties
of granular solids and gasses, a comprehensive understanding of how
materials transition between these states has remained elusive. This is
particularly problematic because many systems exhibit both flow types in
different spatial domains, and so an adequate understanding of the
transitional regime is crucial to understanding even the steady state of
these flows. I will discuss what we know and identify some open questions
about granular fluidization by examining experimental and computational
studies of two systems: a vertically shaken system and a Couette shear
flow. Important issues we will consider include: Can a fluidization
transition be defined? What are the relevant parameters that control
whether a specific system will collapse or not? And are there useful
general (flow-independent) criteria for predicting collapse?
Contributed Talks
(in order of receipt)
Name = Paul Nakroshis
email = pauln@maine.edu
Institution = University of Southern Maine
talk = yes-talk
Title = Force Fluctuations and Angular Rotations in a driven granular
array.
A quasi-two dimensional array of cylinders was pushed along a horizontal
aluminum track at constant velocity and the total driving force was sampled
at 10 KHz. Operating in a continuous motion regime, we found driving force
fluctuations that follow a power law distribution with a sharp upper cutoff,
a behavior which persisted even while varying the number of cylinders, the
geometry and the driving velocity. We also recorded the angular rotations of
the individual cylinders and report on how the total force correlates with
the total angular velocity within the array, as well as the vital role
dilatency plays in the collapse of the array.
Name = Jaehyuk Choi
email = jaehyuk@mit.edu
Institution = Dept of Math, MIT
talk = yes-talk
Title = Diffusion in gravity driven dense granular flows
Abstract: We measure the diffusion of particles in a silo drainage using direct
particle tracking. The mean squared deviations are observed to grow in
proportion to time indicating cage rearrangement. The diffusion rates, however,
are very small compared with mean flow. We also measure the rearrangement
time scales from the distributions of velocity fluctuations and topological
correlation function.
Name = Lou Kondic
email = kondic@oak.njit.edu
Institution = New Jersey Institute of Technology
talk = yes-talk
Title = Extended granular temperature
Kondic and Behringer
We consider the role of elastic energy in the context of granular materials
undergoing shear flow. Depending on the ratio of pressure to Young's modulus
of the material from which grains are made and the typical velocity of
shearing, there is a transition from a regime in which kinetic energy is the
dominant form of energy storage to one in which elastic energy is dominant. We
then consider a generalization of the granular temperature that includes both
types of terms and changes smoothly from one regime to the other. We conclude
by relating this generalized temperature to the concepts based on equilibrium
statistical mechanics.
Name = Allison Ferguson
email = shamrock@brandeis.edu
Institution = Brandeis University
talk = yes-talk
Title = Signatures of Large-Scale Spatial Structures in Dense Granular
Flows
Measurements of impulse distributions P(I) on a simulated 2D gravity
driven hopper system show flow-velocity-invariant behavior of the
distribution for impulses larger than the average impulse. For
small impulses, however, P(I) decreases significantly with flow
velocity, a phenomenon which can be attributed exclusively to
collisions between grains undergoing frequent
collisions. Visualizations of the system also show that these
frequently colliding particles tend to form increasingly large linear
clusters as the flow velocity decreases. A model is proposed for the
form of P(I), given distributions of cluster size and velocity, which
accurately predicts the observed form of the distribution. Thus the
impulse distribution provides some insight into the formation and
properties of these "dynamic" force chains.
Name = John R Williams
email = jrw@mit.edu
Institution = MIT
talk = yes-talk
Title = A two dimensional Discrete Element Model is used to study the
deformation of granular materials under slow quasi-static deformations.
The results of uni-axial compression tests under steady confinement
show that the particles move by forming distinct circulation
cells. The size of these cells grows as the deformation proceeds until
a shear band is formed. This behavior is exhibited by a range of
particle shapes and for a wide range of inter-particle friction and
stiffness. The formation of these structures whose characteristic
length is significantly larger than the particle size has implications
for continuum analysis of granular materials. At failure the
characteristic length may be as large as the sample size. Therefore
continuum assumptions involving the definition of an elementary volume
of the material may be invalid because of the formation of the
circulation structures within the material. A brief review of the
latest advances in the Discrete Element Method will also be given.
Name = Martin Z. Bazant
email = bazant@mit.edu
Institution = Department of Mathematics, MIT
talk = yes-talk
Title = Towards a model of cooperative diffusion in dense granular flows
There is currently no mathematical model to describe particle dynamics
in a slow, dense granular flow, such as gravity-driven drainage from a
silo. Although classical continuum models, based on the concept of
diffusing voids, are able to capture the mean velocity profile,
particle mixing (relative to the mean flow) is grossly over-estimated
if the presumed microscopic picture is taken seriously. We demonstrate
these successes and failures by an exact analysis of the void model
compared with drainage experiments. The basic problem is that
particles move independently in the Void Model, whereas in a real
dense flow, they clearly move with strong local correlations related
to dynamical random packings.
As an alternative, we propose a new stochastic model in which
particles undego strongly correlated random motion in response to
diffusing `spots' of influence, which correspond to extended regions
of slightly reduced density. Although it is eventually unstable to
density fluctuations, the Spot Model is in good agreement with
experimental measurements, at least for early times. The model also
predicts spatial correlations in local velocities, which we later
observed in experiments. This could be interpreted as direct
experimental evidence for the existence of spots.
Collaborators: Jaehyuk Choi, Ruben Rosales, Chris Rycroft, Camilo
Guaqueta (MIT), Arshad Kudrolli (Clark).
Name = GLAND
email = gland@geologie.ens.fr
Institution = ECOLE NORMALE SUPERIEURE, PARIS
talk = yes-talk
Title = Effects of Grain Size Heterogeneities on Packing Order and Stress
Distribution in Compact Granular Assemblies.
We use Molecular Dynamics numerical simulations in order to study the
connection between microscopic parameters and macroscopic properties
of compacted granular assemblies.
Confined packings of spherical grains of equal size with identical mechanical
and surface properties spontaneously exhibit structures with local Hexagonal
Close-Packed (HCP) order. We observed that introducing grain size
heterogeneities (for instance by using binary mixtures) inhibits the local
organisation of HCP crystals. We studied numerically structural transitions in
binary mixtures of grains with different size ratios and different compositions
(fractions of each specie). We use both local and global order parameters to
characterize the geometry and directional properties of these structures, which
result from collective rearrangement of grains. We also analyse mechanical
properties of the packings (elastic moduli), using macroscopic response
measurements. We discuss the problem of homogenisation of the structures of
packings (towards Random Close Packing) obtained by adding of small amounts of
grains of different size, and its implications on the mechanical responses.
Name = Scott Johnson
email = smjohns@mit.edu
Institution = MIT
talk = yes-talk
Title = THE DISCRETE ELEMENT MODELING APPROACH TO ADDRESSING GRANULAR
MATERIAL PROBLEMS
Discrete element modeling (DEM) has become a popular method to analyze a
variety of granular discontinua where continuum methods are inappropriate.
There are several current applications of DEM to engineering problems,
including modeling of the near well-bore region in drilling operations, hopper
flow, pharmaceutical powder mixing, and particle behavior in fluidized beds.
The usefulness of DEM in addressing these problems has led to several advances
over the past 30 years, a brief overview of which is provided.
Many of the problems currently of interest to researchers, however, require a
3-D formulation, and addressing this has been the focus of past and current
research efforts at the MIT Intelligent Engineering Systems Laboratory. A
recent project, which developed a new method of modeling ellipsoidal shapes
with C1 parametric continuity called the Equivalent Spheres Method, is
discussed. A brief discussion of its applicability and computational
requirements is also presented.
Posters
(in order of reciept)
Name = Benjamin Dupuy
email = bdupuy@mit.edu
Institution = MIT
talk = yes-poster
Title = Fingering instabilities in particle-laden flow down an inclined
plane
The instability of a two-dimensional moving contact line is studied for a thin
liquid film of a high-concentrated suspension flowing down an inclined plane,
leading to the formation of rivulets. We perform experiments in various ranges
of volume fraction, particle size and particle density and obtain measurements
of the characteristic wave length, tip velocity and position of the onset. A
laser imaging technique was used to obtain data for rivulet shapes and contact
angles. Comparisons are made with the existing data and theoretical
models.
Name = Fabricio Q. Potiguar
email = potiguar@levdec.engr.ccny.cuny.edu
Institution = City College of CUNY
talk = yes-poster
Title= Structural study of Granular assmblies
We measured some functions, which are standard in measuring the
structure of liquids, for sheared granular systems. We are mainly
interested in the incoherent scattering function which, as in the glass literature, can give us a good way to mearuse the granular system's temperature.
Name = Mason Klein
email = mklein@fas.harvard.edu
Institution = Harvard-Smithsonian CFA
talk = yes-poster
Title = The role of interstitial gas in the segregation of vertically
vibrated granular media
We report experimental studies of vibrofluidized mixtures of like-sized bronze
and glass spheres. We show how changes in the vibration conditions (amplitude
and frequency) and the interstitial gas affect the formation of spatial
patterns, including segregation of the two species and hysteretic
behavior.
Name = Ruopeng Wang
email = wangrp@mit.edu
Institution = Harvard Smithsonian Center for Astrophysics
talk = yes-poster
Title= Study of homogeneous gas fluidization with both phenomenal
observation and hyperpolarized 129Xe NMR
We present our recent work on the gas-fluidized glass beads with diameter in
the range 45 to 70 micrometers. This type of particles is classified as Geldart
A, and was observed to demonstrate that, in homogeneous fluidization of the
particles, hysteretic behavior in terms of bed height and pressure drop as gas
flow rate was cycled up and down. Our measurements of bed height as a function
of gas flow rate exhibited this phenomenon, and were also able to show that
obvious homogeneous fluidization only occurred when the flow rate was
decreasing in the backward cycle. When the flow rate increased in the forward
cycle, the height of the particle bed stayed still until the minimum bubbling
flow rate was reached, and then increased significantly and suddenly to
accommodate bubbles. We also measured 129Xe NMR spectra and gas velocity
distribution at different gas flow rate, which shows correlation to the results
available from phenomenal observation.
Name = Alexander Lobkovsky
email = leapfrog@mit.edu
Institution = MIT
talk = yes-poster
Description = Physics of erosion driven by subsurface flow
We attempt to understand the fundamental a spects of erosion in a
lab-scale experiment. In the experiment, water is injected into a
pile of glass beads. When it emerges on the surface of the pile, it
incises channels which grow fed by the seeping water. A laser
tomography setup allows us to extract time dependent topographic map
of the eroding slope. We will present our current understanding of
the initiation and growth of the channels.
Name = Emily Gardel/Nalini Easwar
email = egardel@email.smith.edu
Institution = Smith College
talk = yes-poster
Description = Force and Velocity Fluctuations in 2D Granular Flow
