International audienceUsing molecular dynamic simulations, we investigate the characteristics of dense flows of model cohesive grains. We describe their rheological behavior and its origin at the scale of the grains and of their organization. Homogeneous plane shear flows give access to the constitutive law of cohesive grains which can be expressed by a simple friction law similar to the case of cohesionless grains, but intergranular cohesive forces strongly enhance the resistance to the shear. Then we show the consequence on flows down a slope: a plugged region develops at the free surface where the cohesion intensity is the strongest. Moreover, we measure various indicators of the microstructure within flows which evidence the aggregation of grains due to cohesion and we analyze the properties of the contact network (force distributions and anisotropy). This provides new insights into the interplay between the local contact law, the microstructure and the macroscopic behavior of cohesive grains

Granular materials segregate by size when sheared, which increases the destructive power in avalanches and causes demixing in industrial flows. Here we present a concise theory to describe this phenomenon for systems that for the first time include particles of arbitrary size. The evolution of the grainsize distribution during flow is described based on mass and momentum conservation. The theory is derived in a five-dimensional space, which besides position and time, includes a grainsize coordinate. By coupling the theory with a simple constitutive law we predict the kinematics of the flow, which depends on the grainsize dynamics. We show that the underpinning mechanism controlling segregation is a stress variation with grainsize. The theory, solved by a finite difference scheme, is found to predict the dynamics of segregation consistent with results obtained from discrete element simulations of polydisperse granular flow down inclined planes. Moreover, when applied to bimixtures, the general polydisperse theory reveals the role of grainsize contrast.

No abstract

Using discrete numerical simulations, we have studied the flow down a rough inclined plane of a bidisperse assembly of frictional cohesionless disks. Our study focuses on steady uniform flows, once a stable segregation has developed inside the flowing layer. The material is segregated in three layers: a basal layer (small grains), a superficial layer (large grains), and a mixed layer in the center, so that the average diameter of the grains increases from the bottom to the top. From the measurement of the profiles of velocity, solid fraction, and stress components, we show that the rheological law of such a polydisperse material may be described by a local friction law, which extends the result obtained for quasimonodisperse granular flows. This law states that the effective friction coefficient depends approximately linearly on a generalized inertial number, taking into account the average diameter of the grains.

Mass size distributions of the major elements of the mineral aerosol of northem Sahara were obtained from measurements carried out with an inertial cascade impactor. A fitting procedure by data inversion was applied to these data. This technique considers the major mechanisms of nonideal behavior such as particle bounce-off, wall losses and cross sensitivity which affect the accuracy of impactor measurements of aerosol size distribution. Accurate size distributions of Saharan aerosol (range 0.1-20 gm) were so obtained for different conditions of soil mobilization. These measurements, coupled with wind speed information, show the shape of the size distribution for desert aerosols in the considered range to be characterized by a similar pattern with a common mode of particles having diameter between 1 and 20 [am whatever the weather conditions. A second mass peak of submicron particles appears when the wind speed increases above the threshold value for erosion. Scanning electron microscope photographs indicate that the whole granulometric spectrum of the sampled aerosol between 0.1 and 20 [am mainly consists of clay easily disaggregated by sandblasting. The common origin of submicron particles with those between 1 and 20 [am is established by showing the similar composition of dusts in the both modes. The results suggest that the presence of submicron mineral particles during dusty and sandstorm conditions is consistent with a sandblasting process.13,927 the confr&erie of E1 Abiod Sidi Cheikh (Algeria) and to Y. Callot of University of Oran (Algeria) for their kind assistance. We also would like to mention the friendly cooperation of P. Blanc for SEM and EDXA examinations and B. Chatenet for her technical support, especially for illustration. We are particularly grateful to hypothesis that this population of very fine particles W.F. Fitzgerald and F. Dulac for reviewing our manuscript. This originates from windblown dust, and is derived from the work was supported by the Centre National de la Recherche soils by a sandblasting process. Scientifique (France).