Comportement d'un assemblage de billes non frottantes dans la limite géométrique

8 PAGESNational audienceNous étudions numériquement le comportement d'un matériau granulaire modèle constitué de billes sphériques élastiques identiques non frottantes dans la limite géométrique macroscopique (à savoir la triple limite où les sollicitations extérieures sont très lentes et les grains extrêmement rigides et très nombreux). Nous montrons que les coefficients de frottement macroscopique statique et dynamique coïncident, que l'assemblage ne présente aucune dilatance, que le matériau satisfait un critère de rupture de Lade-Duncan et que ces résultats ne sont pas affectés par la nature du contrôle (en contrainte ou en volume) imposé. La résistance au cisaillement s'explique par l'anisotropie géométrique et mécanique que développe le matériau sous sollicitation

Shear flow of sphere packings in the geometric limit

3 pagesInternational audienceWe investigate the behavior of a model granular material made of frictionless, nearly rigid equal-sized beads, in the quasistatic limit, by numerical simulation. In the macroscopic geometric limit (that is the macroscopic, rigid and quasistatic limits), with either volume or normal stress controlled simulations, static and dynamic macroscopic friction coefficients coincide, dilatancy vanishes and the material satisfies a Lade-Duncan failure criterion. The macroscopic shear strength stems from both contact network and force anisotropy

Strategic tradeoffs in competitor dynamics on adaptive networks

Recent empirical work highlights the heterogeneity of social competitions such as political campaigns: proponents of some ideologies seek debate and conversation, others create echo chambers. While symmetric and static network structure is typically used as a substrate to study such competitor dynamics, network structure can instead be interpreted as a signature of the competitor strategies, yielding competition dynamics on adaptive networks. Here we demonstrate that tradeoffs between aggressiveness and defensiveness (i.e., targeting adversaries vs. targeting like-minded individuals) creates paradoxical behaviour such as non-transitive dynamics. And while there is an optimal strategy in a two competitor system, three competitor systems have no such solution; the introduction of extreme strategies can easily affect the outcome of a competition, even if the extreme strategies have no chance of winning. Not only are these results reminiscent of classic paradoxical results from evolutionary game theory, but the structure of social networks created by our model can be mapped to particular forms of payoff matrices. Consequently, social structure can act as a measurable metric for social games which in turn allows us to provide a game theoretical perspective on online political debates.Comment: 20 pages (11 pages for the main text and 9 pages of supplementary material

Transport en solution et en suspension par le fleuve Congo (Zaïre) et ses principaux affluents de la rive droite

Ce travail est une synthèse des connaissances actuelles du bilan des transports en solution et en suspension sur un grand bassin forestier équatorial. Une large place est faite aux mesures effectuées dans le cadre de l'opération grands bassins fluviaux du programme PIRAT. Les résultats obtenus sur le fleuve Congo à Brazzaville (95% de la superficie totale du bassin soit 3.5 X 10 puissance 6 km2) chiffrent les exportations moyennes de la façon suivante : matières dissoutes : 61.1 X 10 puissance 6 t an -1; et matières en suspension : 30.6 X 10 puissance 6 t an -1. La caractérisation hydrochique de la matière dissoute révèle une faible minéralisation des eaux du fleuve. (Résumé d'auteur

Analysis of inhomogeneous materials at large strains using fast Fourier transform

invited lecture at the IUTAM Symposium - Stuttgart 2001International audienceThis paper focuses on a numerical method which has been recently developed to analyze the response of highly inhomogeneous materials, often with complex microstructure. This numerical method is based on Fast Fourier Transforms and allows to make direct use of digital images of the ”real” microstructure in the numerical simulation. The case of elastic nonhomogeneous phases is reduced to an integral equation (Lippman- Schwinger equation) which is solved iteratively. A nice feature of the method is that it involves a multiplication in Fourier space, a multiplication in real space, a FFT and an inverse FFT. The two first operations can be easily parallelized. It has been extended to various nonlinear behaviours (elastoplasticity, phase transformations, 3d analysis of texture evolution in polycrystals). Our work has been mainly concentrated on providing reference results to assess the accuracy of theoretical estimates for nonlinear compos- ites with simple behaviours (ideally plastic or power-law materials)

A Parametric Propagator for Pairs of Sum Constraints with a Discrete Convexity Property

International audienceWe introduce a propagator for pairs of Sum constraints, where the expressions in the sums respect a form of convexity. This propagator is parametric and can be instantiated for various concrete pairs, including Deviation, Spread, and the conjunction of Linear ≤ and Among. We show that despite its generality , our propagator is competitive in theory and practice with state-of-the-art propagators