Phase Transitions in Granular Packings

We describe the contact network of granular packings by a frustrated lattice gas that contains steric frustration as essential ingredient. Two transitions are identified, a spin glass transition at the onset of Reynolds dilatancy and at lower densities a percolation transition. We describe the correlation functions that give rise to the singularities and propose some dynamical experiments

Effect of friction in a toy model of granular compaction

We proposed a toy model of granular compaction which includes some resistance due to granular arches. In this model, the solid/solid friction of contacting grains is a key parameter and a slipping threshold Wc is defined. Realistic compaction behaviors have been obtained. Two regimes separated by a critical point Wc* of the slipping threshold have been emphasized : (i) a slow compaction with lots of paralyzed regions, and (ii) an inverse logarithmic dynamics with a power law scaling of grain mobility. Below the critical point Wc*, the physical properties of this frozen system become independent of Wc. Above the critical point Wc*, i.e. for low friction values, the packing properties behave as described by the classical Janssen theory for silos

A Simple Geometrical Model for Solid Friction

We present a simple model for the friction of two solid bodies moving against each other. In a self consistent way we can obtain the dependence of the macroscopic friction force as a function of the driving velocity, the normal force and the ruggedness of the surfaces in contact. Our results are discussed in the context of friction laws used in earthquake models.Comment: 9 pages, plain TeX, preprint HLRZ 24/9

Contact forces in regular 3D granular pile

We present exact results for the contact forces in a three dimensional static piling of identical, stiff and frictionless spheres. The pile studied is a pyramid of equilateral triangular base (``stack of cannonballs'') with a FCC (face centered cubic) structure. We show in particular that, as for the two dimensional case, the pressure on the base of such a pile is uniform.Comment: 10 pages, 5 figure

Calculation of the separation streamlines of barchans and transverse dunes

We use FLUENT to calculate the wind profile over barchans and transverse dunes. The form of the streamlines of flow separation at the lee side of the dunes is determined for a symmetric barchan dune in three dimensions, and for the height profile of a measured transverse dune field in the Len\c{c}\'ois Maranhenses.Comment: 6 pages including 5 figures. Proceedings of PSIS 200

Lattice-Boltzmann Simulations of Transport Phenomena and Structuring in Suspensions

We simulate particles suspended in a fluid by means of the lattice-Boltzmann method and its extension to particle suspensions as introduced by Ladd et al. in order to study transport phenomena and structuring effects of particles in the vicinity of sheared rigid walls. We find that a particle free region arises near walls, which has a width depending on the shear rate and the particle concentration. The wall causes the formation of parallel particle layers at low concentrations, where the number of particles per layer decreases with increasing distance to the wall. These findings are in good agreement with a phenomenological theory of Muckenfuss and Buggisch. We also study the velocity distributions of suspended particles which turn out to be non-Gaussian, but exponential

Cluster size distribution of infection in a system of mobile agents

Clusters of infected individuals are defined on data from health laboratories, but this quantity has not been defined and characterized by epidemy models on statistical physics. For a system of mobile agents we simulate a model of infection without immunization and show that all the moments of the cluster size distribution at the critical rate of infection are characterized by only one exponent, which is the same exponent that determines the behavior of the total number of infected agents. No giant cluster survives independent on the magnitude of the rate of infection.Comment: preprint for Physica A, proceedings of Medyfinol in La Seren

Lattice Boltzmann Models for Complex Fluids

We present various Lattice Boltzmann Models which reproduce the effects of rough walls, shear thinning and granular flow. We examine the boundary layers generated by the roughness of the walls. Shear thinning produces plug flow with a sharp density contrast at the boundaries. Density waves are spontaneously generated when the viscosity has a nonlinear dependence on density which characterizes granular flow.Comment: 11 pages, plain TeX, preprint HLRZ 23/9

Scaling of the propagation of epidemics in a system of mobile agents

For a two-dimensional system of agents modeled by molecular dynamics, we simulate epidemics spreading, which was recently studied on complex networks. Our resulting network model is time-evolving. We study the transitions to spreading as function of density, temperature and infection time. In addition, we analyze the epidemic threshold associated to a power-law distribution of infection times.Comment: 10 pages, 7 Postscript figure