Monodisperse Dry Granular Flows on Inclined Planes : Role of Roughness

Recent studies have pointed out the importance of the basal friction on the dynamics of granular flows. We present experimental results on the influence of the roughness of the inclined plane on the dynamics of a monodisperse dry granular flow. We found experimentally that it exists a maximum of the friction for a given relative roughness. This maximum is shown to be independent of the angle of the slope. This behavior is observed for four planes with different bump sizes (given by the size of the beads glued on the plane) from 200 microns to 2 mm. The relative roughness corresponding to the maximum of the friction can be predicted with a geometrical model of stability of one single bead on the plane. The main parameters are the size of the bumps and the size of the flowing beads. In order to obtain a higher precision, the model also takes into account of the spacing between the bumps of the rough plane. Experimental results and model are in good agreement for all the planes we studied. Other parameters, like the sphericity of the beads, or irregularities in the thickness of the layer of glued particles, are shown to be of influence on the friction.Comment: 17 pages, 15 figures, submitted to EPJ

Bidisperse granular avalanches on inclined planes: A rich variety of behaviors

Experiments were performed to provide insight into the flow behavior and structure of bimodal mixtures of grains in gravity-driven, free-surface flows. Unsteady unconfined flows were produced by releasing instantaneously a dry granular mass, composed of two particle sizes, over a rough inclined plane. As a result of size segregation, the small particles are found at the bottom of the flow and final deposit, the large particles are found at the free surface, but also on the lateral borders and at the front of the flow. The lateral and vertical inhomogeneous repartitions of particles lead to two main effects that are completely absent in monodispersed flows. The outline effect results from the accumulation of large beads on the periphery of the flow depending on the value of the relative friction of each particle species on the plane. This effect in turn causes a narrowing of the flow and/or an increase of length of the final deposit. The interface effect results of the interaction between layers of different size particles and causes the modification of the thickness of the deposit. These effects occur simultaneously and their combination leads to a great variety of behaviors. In this investigation, evidence of the diversity of behaviors is presented as the size ratio, relative friction and concentration of each particle species are varied

Discharge flow of a bidisperse granular media from a silo

International audienceThe discharge flow in a cylindrical and a rectangular silo using both monodisperse and bidisperse mixtures of spherical glass beads is studied experimentally. The flow rate is measured using a precision balance for a large range of particle diameters, size ratios, and outlet diameters. A simple physical model is proposed to describe the flow of bidisperse mixtures. It gives an expression for the flow rate and predicts that the bulk velocity follows a simple mixture law. This model implies that a mixture diameter cannot be simply defined. Moreover it is shown that bidisperse granular media allow for the transport of coarse particles below their jamming conditions

DENSIFICATION SUPERFICIELLE DE MATÉRIAUX POREUX PAR CHOC LASER

Les chocs laser permettent d'obtenir une densification superficielle sur un matériau poreux. La profondeur affectée est de l'ordre de quelques centaines de microns. Cette étude présente les résultats numériques obtenus avec un code hydrodynamique monodimensionnel lagrangien décrivant l'évolution d'une cible soumise à un choc laser. Le comportement des matériaux poreux est modélisé en utilisant le principe du modèle P-α de Herrmann. Le code nous permet d'obtenir le profil de densification dans la cible en fonction des conditions d'irradiation, de la nature du matériau et de sa porosité initiale. Des expériences ont été réalisées sur des échantillons de poudre d'aluminium et d'acier fritté (Distaloy AE). La configuration de plasma confiné a été utilisée : la cible est recouverte par un milieu transparent au rayonnement incident, ce procédé augmente la pression et sa durée de maintien par rapport à une irradiation directe. Des échantillons ont été observés en microscopie optique. Le profil de porosité résiduelle est estimé par analyse d'image. Les résultats obtenus avec le code sont en bon accord avec les résultats expérimentaux.Laser-driven shock-waves are used as a surface treatement for compacting porous materials. The compaction depth is typically a few hundreds microns. The behavior of the porous medium is described through a compaction model based on the P-α theory of Herrmann. This model has been introduced into a one-dimensional finite difference hydrodynamic code describing the behavior of a target under the action of a laser-generated shock-wave. The code enables us to compute the compaction depth as a function of irradiation conditions, nature and initial porosity of the material. Experiments are performed on aluminum powder and sintered porous steel (Distaloy AE). The samples are covered with a transparent window acting as a confinement for the plasma generated by absorption of the incident radiation on the target surface. This process increases both the pressure and the duration of the applied pressure in comparison with a bare target irradiation. Samples are observed by optical microscopy. The residual porosity is estimated by image analysis. Experimental results and computed compaction profiles correlate well

Assessing the homogeneity of powder mixtures by on-line electrical capacitance

4th International Conference on Conveying and Handling of Particulate Solids, Budapest, HUNGARY, MAY 27-30, 2003International audienceIn this paper, we try to improve our comprehension of one of the most important challenge in the particulate solids industry: the assessment of the homogeneity of powder mixtures. A two-electrode electrical capacitance method is used to measure the permittivity of mixtures while flowing in a 1-D set up. An emphasis is placed on the possibility of following homogeneity evolution through permittivity measurement, without the necessity to calculate the volumetric proportions of each component from an effective medium formulation. The methodology is applied to examine segregation in a funnel, as well as the efficiency of a laboratory drum mixer

LASER SHOCK COMPACTION OF POROUS MATERIALS

Laser-driven shock-waves are used as a surface treatement for compacting porous materials. The compaction depth is typically a few hundreds of microns. The behavior of the porous medium is described through a compaction model based on the α theory of Herrmann. This model has been introduced into a one-dimensional finite difference hydrodynamic code describing the behavior of a target under the action of a laser-generated shock-wave. The code enables us to compute the compaction depth as a function of irradiation conditions, nature and initial porosity of the material. Experiments are performed on aluminum powder. Samples are observed by optical microscopy. The residual porosity is estimated by image analysis. Experimental results and computed compaction profiles correlate well

Laser shock flier impact simulation of particle-substrate interactions in cold spray

International audienceCoating-substrate adhesion in cold spray is a paramount property, the mechanisms of which are not yet well elucidated. To go into these mechanisms, due to the intrinsic characteristics of the cold spray process (particle low-temperature and high velocity) direct observation and control of inflight particles and related phenomena cannot be done easily. For this reason, an experimental simulation of the particle-substrate reactions at the particle impingement was developed. This simulation is based on original flier impact experiments from laser shock acceleration. Relevant interaction phenomena were featured and studied as a function of shearing, plastic deformation, phase transformation primarily. These phenomena were shown to be similar to those involved in cold spray. This was ascertained by the study of the Cu-Al metallurgically reactive system using SEM, TEM, EPMA, and energy balance and diffusion calculations. This simulation could also be used to feed finite element modeling of cold spray and laser shock flier impact