Laboratory experiments are conducted to study the rheological behaviour of highconcentration granular-liquid mixtures. Steady uniform free-surface flows are obtained using a recirculating flume. Cases in which a loose deposit forms underneath the flow are contrasted with runs for which basal shear occurs along the flume bottom. The granular motions are observed through the channel sidewall, and analysed with recently developed Voronoï imaging methods. Depth profiles of mean velocity, solid concentration, and granular temperature are obtained, and complemented by stress estimates based on force balance considerations. These measurements are used to probe variations in rheological behaviour over depth, and to clarify the role of the granular temperature. The flows are found to evolve a stratified structure. Distinct sublayers are characterized by either frictional or collisional behaviour, and transitions between one and the other occur at values of the Stokes number which suggest that viscous effects intervene. The observed frictional behaviour is consistent with shear cell tests conducted at very low shear rates. On the other hand, the collisional data corroborate both the Bagnold description and the more recent kinetic theories of granular flows, provided that one accounts for the inertia of the interstitial liquid.

We study dry flows of two types of spheres down an inclined, rigid, bumpy bed in the absence of sidewalls. The flow is assumed to be steady and uniform in all but the direction normal to the free surface, collisions between particles are dissipative, and the sizes and masses of the particles are not too different. We restrict our analysis to dense flows and use an extension of kinetic theory to predict the concentration of the mixture and the profile of mixture velocity. A kinetic theory for a binary mixture of nearly elastic spheres that do not differ by much in their size or mass is employed to predict profiles of the concentration fraction of one type of sphere. We also determine the ratio of the radii and of the masses of the two species for which there is no segregation. We compare the predictions of the theory to the results of numerical simulations.

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A set of stereoscopic imaging techniques is proposed for the measurement of rapidly flowing dispersions of opaque particles observed near a transparent wall. The methods exploit projective geometry and the Voronoı diagram. They rely on purely geometrical principles to reconstruct 3D particle positions, concentrations, and velocities. The methods are able to handle position and motion ambiguities, as well as particle-occlusion effects, difficulties that are common in the case of dense dispersions of many identical particles. Fluidization cell experiments allow validation of the concentration estimates. A mature debris-flow experimental run is then chosen to test the particle-tracking algorithm. The Voronoı ¨stereo methods are found to perform well in both cases, and to present significant advantages over monocular imaging measurements.

We consider the evolution of particle segregation in collisional flows of two types of spheres down rigid bumpy inclines in the absence of sidewalls. We restrict our analysis to dense flows and use an extension of kinetic theory to predict the concentration of the mixture and the profiles of mixture velocity and granular temperature. A kinetic theory for a binary mixture of nearly elastic spheres that do not differ by much in their size or mass is employed to predict the evolution of the concentration fractions of the two types of spheres. We treat situations in which the flow of the mixture is steady and uniform, but the segregation evolves, either in space or in time. Comparisons of the predictions with the results of discrete numerical simulation and with physical experiments are, in general, good.