Dynamic simulation of the formation of granular media and study of their properties

A numerical model is presented which describes the evolution of a system containing a large number of deformable spherical grains based on Newton's second law. Starting from an initial state with fixed positions, velocities and grain characteristics, the system evolution is simulated by successive steps. The acceleration of each grain results from the application of an external force and from interactions with other particles. These contact forces are evaluated as functions of the grain deformations during the collisions considered as elastic. The grain bed can be deposited between vertical walls as well as with periodical conditions in the lateral directions. The properties of these packings submitted to mechanical stresses are characterized by using numerical codes which operate on unstructured tetrahedral grids on the scale of the individual grains

Random packings of spiky particles : Geometry and transport properties

Spiky particles are constructed by superposing spheres and prolate ellipsoids. The resulting nonconvex star particles are randomly packed by a sequential deposition algorithm. The geometry, the conductivity, and the permeability of the resulting packings are systematically studied, in relation with the individual grain characteristics. Overall correlations are proposed to approximate these properties as functions of the grain equivalent size and sphericity index

Barometric pumping of a fractured porous medium

International audienceBarometric pumping plays a crucial role in the release of trace gases from fractured porous media to the atmosphere, and it requires a rigorous and complete modeling in order to go beyond the approximate schemes available in the literature. Therefore, a coupled set of convection and convection-diffusion equations for a slightly compressible fluid in unsteady conditions should be solved. The numerical methodology is presented, and it is applied to conditions close to the ones of the Roselend Natural Laboratory (France). The precision of the code is assessed and the mechanism of barometric pumping is explained. The usual schematization by simple vertical fractures is shown to be only qualitative. Finally, barometric pumping is shown to be efficient in a narrow range of parameter values; its efficiency is a decreasing function of the matrix porosity and of the fracture density

Propriétés acoustiques des milieux poreux secs et saturés

Les propriétés acoustiques de milieux poreux sont étudiées dans le cadre général de la théorie de l'homogénéisation, en supposant que l'échelle caractéristique des pores est petite devant la longueur d'onde, en appliquant les développements successifs du formalisme de Botin et Auriault (1993). Pour les milieux secs, on détermine la célérité d'une onde plane (de compression ou de cisaillement), et éventuellement la correction de polarisation, la dispersion de célérité ainsi que l'atténuation due à la diffraction de Rayleigh. Différents types de milieux modèles ainsi que des matériaux réels imagés par microtomographie X sont étudiés. Dans les milieux poreux saturés, le comportement acoustique est décrit par une équation de type élastique dans la phase solide et par les équations de Navier-Stokes dans le fluide interstitiel. La perméabilité dynamique et les célérités des ondes de compression et de cisaillement sont déterminées pour les mêmes milieux que précédemment

Acoustic properties of saturated porous media

This work addresses the propagation of acoustic waves in porous media. The theoretical developments or earlier works are generalized to establish dynamic poro-elastic equations for a compressible fluid in a pore space which may consist in multiple independent pore components, closed or percolating. These equations involve several effective coefficients, to be determined by solving closure problems on the microscale. Systematic applications are presented, for various kinds of model reconstructed media, and for real media imaged by microtomography

Bicontinuous minimal surface nanostructures for polymer blend solar cells

This paper presents the first examination of the potential for bicontinuous structures such as the gyroid structure to produce high efficiency solar cells based on conjugated polymers. The solar cell characteristics are predicted by a simulation model that shows how the morphology influences device performance through integration of all the processes occurring in organic photocells in a specified morphology. In bicontinuous phases, the surface de. ning the interface between the electron and hole transporting phases divides the volume into two disjoint subvolumes. Exciton loss is reduced because the interface at which charge separation occurs permeates the device so excitons have only a short distance to reach the interface. As each of the component phases is connected, charges will be able to reach the electrodes more easily. In simulations of the current-voltage characteristics of organic cells with gyroid, disordered blend and vertical rod (rods normal to the electrodes) morphologies, we find that gyroids have a lower than anticipated performance advantage over disordered blends, and that vertical rods are superior. These results are explored thoroughly, with geminate recombination, i.e. recombination of charges originating from the same exciton, identified as the primary source of loss. Thus, if an appropriate materials choice could reduce geminate recombination, gyroids show great promise for future research and applications

Transport properties of heterogeneous materials derived from Gaussian random fields: Bounds and Simulation

We investigate the effective conductivity ($\sigma_e$) of a class of amorphous media defined by the level-cut of a Gaussian random field. The three point solid-solid correlation function is derived and utilised in the evaluation of the Beran-Milton bounds. Simulations are used to calculate $\sigma_e$ for a variety of fields and volume fractions at several different conductivity contrasts. Relatively large differences in $\sigma_e$ are observed between the Gaussian media and the identical overlapping sphere model used previously as a `model' amorphous medium. In contrast $\sigma_e$ shows little variability between different Gaussian media.Comment: 15 pages, 14 figure

Structure-property correlations in model composite materials

We investigate the effective properties (conductivity, diffusivity and elastic moduli) of model random composite media derived from Gaussian random fields and overlapping hollow spheres. The morphologies generated in the models exhibit low percolation thresholds and give a realistic representation of the complex microstructure observed in many classes of composites. The statistical correlation functions of the models are derived and used to evaluate rigorous bounds on each property. Simulation of the effective conductivity is used to demonstrate the applicability of the bounds. The key morphological features which effect composite properties are discussed

Photométrie physique

La possibilité de mesures spectrophotométriques précises avec les cellules photoélectriques rend désirable la définition physique des qualités lumineuses. Le choix d'une fonction arbitraire de visibilité permet de définir la luminosité monochromatique et sa valeur relative, caractéristique de la couleur. Un outillage technique peut être rapidement mis au point pour effectuer les mesures objectives utilisant ces définitions