164 research outputs found
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
Anisotropic permeabilities evolution of reservoir rocks under pressure: new experimental and numerical approaches
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Why Effective Medium Theory Fails in Granular Materials
Experimentally it is known that the bulk modulus, K, and shear modulus, \mu,
of a granular assembly of elastic spheres increase with pressure, p, faster
than the p^1/3 law predicted by effective medium theory (EMT) based on
Hertz-Mindlin contact forces. To understand the origin of these discrepancies,
we perform numerical simulations of granular aggregates under compression. We
show that EMT can describe the moduli pressure dependence if one includes the
increasing number of grain-grain contacts with p. Most important, the affine
assumption (which underlies EMT), is found to be valid for K(p) but breakdown
seriously for \mu(p). This explains why the experimental and numerical values
of \mu(p) are much smaller than the EMT predictions.Comment: 4 pages, 5 figures, http://polymer.bu.edu/~hmaks
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
Stress response inside perturbed particle assemblies
The effect of structural disorder on the stress response inside three
dimensional particle assemblies is studied using computer simulations of
frictionless sphere packings. Upon applying a localised, perturbative force
within the packings, the resulting {\it Green's} function response is mapped
inside the different assemblies, thus providing an explicit view as to how the
imposed perturbation is transmitted through the packing. In weakly disordered
arrays, the resulting transmission of forces is of the double-peak variety, but
with peak widths scaling linearly with distance from the source of the
perturbation. This behaviour is consistent with an anisotropic elasticity
response profile. Increasing the disorder distorts the response function until
a single-peak response is obtained for fully disordered packings consistent
with an isotropic description.Comment: 8 pages, 7 figure captions To appear in Granular Matte
Hydromechanical behavior of heterogeneous carbonate rock under proportional triaxial loadings
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