72 research outputs found
Particle displacements in the elastic deformation of amorphous materials: local fluctuations vs. non-affine field
We study the local disorder in the deformation of amorphous materials by
decomposing the particle displacements into a continuous, inhomogeneous field
and the corresponding fluctuations. We compare these fields to the commonly
used non-affine displacements in an elastically deformed 2D Lennard-Jones
glass. Unlike the non-affine field, the fluctuations are very localized, and
exhibit a much smaller (and system size independent) correlation length, on the
order of a particle diameter, supporting the applicability of the notion of
local "defects" to such materials. We propose a scalar "noise" field to
characterize the fluctuations, as an additional field for extended continuum
models, e.g., to describe the localized irreversible events observed during
plastic deformation.Comment: Minor corrections to match the published versio
Feasibility study on traffic decongestion strategies at Maseru bridge border post
The decongestion strategy of Maseru Bridge was informed by a research exercise investigating different border post decongestion measures and border precinct typologies. Generic design parameters for border post decongestion strategies were compiled. Each measure and typology was evaluated against the generic design parameters. A short list of three decongestion strategies was determined. The capacity requirements of each was determined based on data surveys at the existing border crossing which determined the travel needs for light vehicles, public transport vehicles, freight vehicles and pedestrians. Design years and directional and seasonal peaks were taken into consideration when the patronage forecast was estimated. A spreadsheet based queueing model was developed to determine the number of entry lanes, number of parking bays and queueing capacity required for each decongestion strategy. The model took consideration of a large variety of design related constraints or parameters. The generic design parameters in conjunction with the capacity requirements as provided by the queueing model were taken into account in the conceptual design of the different border precincts.Paper presented at the 34th Annual Southern African Transport Conference 6-9 July 2015 "Working Together to Deliver - Sakha Sonke", CSIR International Convention Centre, Pretoria, South Africa.The Minister of Transport, South AfricaTransportation Research Board of the US
Numerical Study of the Stress Response of Two-Dimensional Dense Granular Packings
We investigate the Green function of two-dimensional dense random packings of
grains in order to discriminate between the different theories of stress
transmission in granular materials. Our computer simulations allow for a
detailed quantitative investigation of the dynamics which is difficult to
obtain experimentally. We show that both hyperbolic and parabolic models of
stress transmission fail to predict the correct stress distribution in the
studied region of the parameters space. We demonstrate that the compressional
and shear components of the stress compare very well with the predictions of
isotropic elasticity for a wide range of pressures and porosities and for both
frictional and frictionless packings. However, the states used in this study do
not include the critical isostatic point for frictional particles, so that our
results do not preclude the fact that corrections to elasticity may appear at
the critical point of jamming, or for other sample preparation protocols, as
discussed in the main text. We show that the agreement holds in the bulk of the
packings as well as at the boundaries and we validate the linear dependence of
the stress profile width with depth.Comment: 7 pages, 5 figure
Segregation by thermal diffusion of an intruder in a moderately dense granular fluid
A solution of the inelastic Enskog equation that goes beyond the weak
dissipation limit and applies for moderate densities is used to determine the
thermal diffusion factor of an intruder immersed in a dense granular gas under
gravity. This factor provides a segregation criterion that shows the transition
between the Brazil-nut effect (BNE) and the reverse Brazil-nut effect (RBNE) by
varying the parameters of the system (masses, sizes, density and coefficients
of restitution). The form of the phase-diagrams for the BNE/RBNE transition
depends sensitively on the value of gravity relative to the thermal gradient,
so that it is possible to switch between both states for given values of the
parameters of the system. Two specific limits are considered with detail: (i)
absence of gravity, and (ii) homogeneous temperature. In the latter case, after
some approximations, our results are consistent with previous theoretical
results derived from the Enskog equation. Our results also indicate that the
influence of dissipation on thermal diffusion is more important in the absence
of gravity than in the opposite limit. The present analysis extends previous
theoretical results derived in the dilute limit case [V. Garz\'o, Europhys.
Lett. {\bf 75}, 521 (2006)] and is consistent with the findings of some recent
experimental results.Comment: 10 figure
Sensitivity of the stress response function to packing preparation
A granular assembly composed of a collection of identical grains may pack
under different microscopic configurations with microscopic features that are
sensitive to the preparation history. A given configuration may also change in
response to external actions such as compression, shearing etc. We show using a
mechanical response function method developed experimentally and numerically,
that the macroscopic stress profiles are strongly dependent on these
preparation procedures. These results were obtained for both two and three
dimensions. The method reveals that, under a given preparation history, the
macroscopic symmetries of the granular material is affected and in most cases
significant departures from isotropy should be observed. This suggests a new
path toward a non-intrusive test of granular material constitutive properties.Comment: 15 pages, 11 figures, some numerical data corrected, to appear in J.
Phys. Cond. Mat. special issue on Granular Materials (M. Nicodemi Editor
Navier-Stokes transport coefficients of -dimensional granular binary mixtures at low density
The Navier-Stokes transport coefficients for binary mixtures of smooth
inelastic hard disks or spheres under gravity are determined from the Boltzmann
kinetic theory by application of the Chapman-Enskog method for states near the
local homogeneous cooling state. It is shown that the Navier-Stokes transport
coefficients are not affected by the presence of gravity. As in the elastic
case, the transport coefficients of the mixture verify a set of coupled linear
integral equations that are approximately solved by using the leading terms in
a Sonine polynomial expansion. The results reported here extend previous
calculations [V. Garz\'o and J. W. Dufty, Phys. Fluids {\bf 14}, 1476 (2002)]
to an arbitrary number of dimensions. To check the accuracy of the
Chapman-Enskog results, the inelastic Boltzmann equation is also numerically
solved by means of the direct simulation Monte Carlo method to evaluate the
diffusion and shear viscosity coefficients for hard disks. The comparison shows
a good agreement over a wide range of values of the coefficients of restitution
and the parameters of the mixture (masses and sizes).Comment: 6 figures, to be published in J. Stat. Phy
Anisotropy in granular media: classical elasticity and directed force chain network
A general approach is presented for understanding the stress response
function in anisotropic granular layers in two dimensions. The formalism
accommodates both classical anisotropic elasticity theory and linear theories
of anisotropic directed force chain networks. Perhaps surprisingly, two-peak
response functions can occur even for classical, anisotropic elastic materials,
such as triangular networks of springs with different stiffnesses. In such
cases, the peak widths grow linearly with the height of the layer, contrary to
the diffusive spreading found in `stress-only' hyperbolic models. In principle,
directed force chain networks can exhibit the two-peak, diffusively spreading
response function of hyperbolic models, but all models in a particular class
studied here are found to be in the elliptic regime.Comment: 34 pages, 17 figures (eps), submitted to PRE, figures amended,
partially to compare better to recent exp. wor
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