24,190 research outputs found
Force Density Function Relationships in 2-D Granular Media
An integral transform relationship is developed to convert between two important probability density functions (distributions) used in the study of contact forces in granular physics. Developing this transform has now made it possible to compare and relate various theoretical approaches with one another and with the experimental data despite the fact that one may predict the Cartesian probability density and another the force magnitude probability density. Also, the transforms identify which functional forms are relevant to describe the probability density observed in nature, and so the modified Bessel function of the second kind has been identified as the relevant form for the Cartesian probability density corresponding to exponential forms in the force magnitude distribution. Furthermore, it is shown that this transform pair supplies a sufficient mathematical framework to describe the evolution of the force magnitude distribution under shearing. Apart from the choice of several coefficients, whose evolution of values must be explained in the physics, this framework successfully reproduces the features of the distribution that are taken to be an indicator of jamming and unjamming in a granular packing. Key words. Granular Physics, Probability Density Functions, Fourier Transform
A New Approach based on Langevin type Equation for Driven Granular Gas under Gravity
We propose a novel approach based on a Langevin equation for fluctuating
motion of the center of mass of granular media fluidized by energy injection
from a bottom plate. In this framework, the analytical solution of the Langevin
equation is used to derive analytic expressions for several macroscopic
quantities and the power spectrum for the center of mass. In order to test our
theory, we performed event-driven molecular dynamics simulations for one- and
two-dimensional systems. Energy is injected from a vibrating bottom plate in
the one-dimensional case and from a thermal wall at the bottom in the
two-dimensional case. We found that the theoretical predictions are in good
agreement with the results of those simulations under the assumption that the
fluctuation-dissipation relation holds in the case of nearly elastic collisions
between particles. However, as the inelasticity of the interparticle collisions
increases, the power spectrum for the center of mass obtained by the
simulations gradually deviates from the prediction of theoretical curve.
Connection between this deviation and violation of the fluctuation-dissipation
relation is discussed.Comment: 13 pages, 11 figures; corrected typos, to be published in the AIP
Conference Proceedings of the IUTAM-ISIMM Symposium "Mathematical and
Physical Instances of Granular Flows" held in Reggio Calabria, September
14-18, 200
Power law scaling of early-stage forces during granular impact
We experimentally and computationally study the early-stage forces during
intruder impacts with granular beds in the regime where the impact velocity
approaches the granular force propagation speed. Experiments use 2D assemblies
of photoelastic disks of varying stiffness, and complimentary discrete-element
simulations are performed in 2D and 3D. The peak force during the initial
stages of impact and the time at which it occurs depend only on the impact
speed, the intruder diameter, the mass density of the grains, and the elastic
modulus of the grains according to power-law scaling forms that are not
consistent with Poncelet models, granular shock theory, or added-mass models.
The insensitivity of our results to many system details suggest that they may
also apply to impacts into similar materials like foams and emulsions.Comment: 5 pages, 4 figure
Contact tribology also affects the slow flow behavior of granular emulsions
Recent work on suspension flows has shown that contact mechanics plays a role
in suspension flow dynamics. The contact mechanics between particulate matter
in dispersions should depend sensitively on the composition of the dispersed
phase: evidently emulsion droplets interact differently with each other than
angular sand particles. We therefore ask: what is the role of contact mechanics
in dispersed media flow? We focus on slow flows, where contacts are
long-lasting and hence contact mechanics effects should be most visible. To
answer our question, we synthesize soft hydrogel particles with different
friction coefficients. By making the particles soft, we can drive them at
finite confining pressure at all driving rates. For particles with a low
friction coefficient, we obtain a rheology similar to that of an emulsion, yet
with an effective friction much larger than expected from their microscopic
contact mechanics. Increasing the friction coefficient of the particles, we
find a flow instability in the suspension. Particle level flow and fluctuations
are also greatly affected by the microscopic friction coefficient of the
suspended particles. The specific rheology of our "granular emulsions" provides
further evidence that a better understanding of microscopic particle
interactions is of broad relevance for dispersed media flows
Comparison of different capillary bridge models for application in the discrete element method
Weakly wetted granular material is the subject of many studies. Several
formulations were proposed to calculate the capillary forces between wet
particles. In this paper some of such models have been implemented in a
DEM-framework, and simulation results were compared to experimental
measurements. Also, the influence of capillary model type on macro parameters
like local shear viscosity and cohesive parameters of sheared material have
been investigated through the simulation of spherical beads using a DEM-model
of a split-bottom shear-cell. It was concluded that the water content,
simulated with the help of capillary bridge models, changes the
macro-properties of the simulated granular material. Different capillary bridge
models do not influence the macroscopic results visibly
Discrete modelling of capillary mechanisms in multi-phase granular media
A numerical study of multi-phase granular materials based upon
micro-mechanical modelling is proposed. Discrete element simulations are used
to investigate capillary induced effects on the friction properties of a
granular assembly in the pendular regime. Capillary forces are described at the
local scale through the Young-Laplace equation and are superimposed to the
standard dry particle interaction usually well simulated through an
elastic-plastic relationship. Both effects of the pressure difference between
liquid and gas phases and of the surface tension at the interface are
integrated into the interaction model. Hydraulic hysteresis is accounted for
based on the possible mechanism of formation and breakage of capillary menisci
at contacts. In order to upscale the interparticular model, triaxial loading
paths are simulated on a granular assembly and the results interpreted through
the Mohr-Coulomb criterion. The micro-mechanical approach is validated with a
capillary cohesion induced at the macroscopic scale. It is shown that
interparticular menisci contribute to the soil resistance by increasing normal
forces at contacts. In addition, more than the capillary pressure level or the
degree of saturation, our findings highlight the importance of the density
number of liquid bonds on the overall behaviour of the material
The effects of packing structure on the effective thermal conductivity of granular media: A grain scale investigation
Structural characteristics are considered to be the dominant factors in
determining the effective properties of granular media, particularly in the
scope of transport phenomena. Towards improved heat management, thermal
transport in granular media requires an improved fundamental understanding. In
this study, the effects of packing structure on heat transfer in granular media
are evaluated at macro- and grain-scales. At the grain-scale, a gas-solid
coupling heat transfer model is adapted into a discrete-element-method to
simulate this transport phenomenon. The numerical framework is validated by
experimental data obtained using a plane source technique, and the
Smoluschowski effect of the gas phase is found to be captured by this
extension. By considering packings of spherical SiO2 grains with an
interstitial helium phase, vibration induced ordering in granular media is
studied, using the simulation methods developed here, to investigate how
disorder-to-order transitions of packing structure enhance effective thermal
conductivity. Grain-scale thermal transport is shown to be influenced by the
local neighbourhood configuration of individual grains. The formation of an
ordered packing structure enhances both global and local thermal transport.
This study provides a structure approach to explain transport phenomena, which
can be applied in properties modification for granular media.Comment: 11 figures, 29 page
From liquid to solid bonding in cohesive granular media
We study the transition of a granular packing from liquid to solid bonding in
the course of drying. The particles are initially wetted by a liquid brine and
the cohesion of the packing is ensured by capillary forces, but the
crystallization of the solute transforms the liquid bonds into partially
cemented bonds. This transition is evidenced experimentally by measuring the
compressive strength of the samples at regular intervals of times. Our
experimental data reveal three regimes: 1) Up to a critical degree of
saturation, no solid bonds are formed and the cohesion remains practically
constant; 2) The onset of cementation occurs at the surface and a front spreads
towards the center of the sample with a nonlinear increase of the cohesion; 3)
All bonds are partially cemented when the cementation front reaches the center
of the sample, but the cohesion increases rapidly due to the consolidation of
cemented bonds. We introduce a model based on a parametric cohesion law at the
bonds and a bond crystallization parameter. This model predicts correctly the
phase transition and the relation between microscopic and macroscopic cohesion.Comment: 20
- …