Michele Larcher

Rheological stratification in experimental free-surface flows of granular-liquid mixtures

Laboratory experiments are conducted to study the rheological behaviour of high-concentration 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 Voronoi 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.

Set of measurement data from flume experiments on steady uniform debris flows

The paper documents a detailed set of measurements from laboratory experiments involving open–channel flows of granular–liquid mixtures. Polyvinyl Chloride pellets mixed with clear water are used, and represent simplified analogues of the mixtures of rocks and muddy water encountered in stony inertial debris flows. The flows are examined in steady uniform conditions, achieved by setting up a closed loop with an inclined channel and a high–speed conveyor belt. Such idealized conditions make it possible to estimate stresses within the flowing mixture, and to accumulate statistics of local granular configurations and motions. These are extracted from video sequences imaged through the sidewall, using algorithms based on the Voronoï diagram. Estimates of granular concentration and granular temperature are derived from local grain patterns and from a detailed analysis of Lagrangian and Eulerian velocity correlations. Depth profiles of kinematic and dynamic quantities are then obtained for various ratios of solid to liquid discharges. These data were earlier used to probe depth variations of rheological behaviour in granular–liquid flows. To make them available for other purposes, they are assembled here into a comprehensive dataset, and provided in digital form in the electronic supplement to this special issue.

DYNAMIC IMPACT OF A DEBRIS FLOW FRONT AGAINST A VERTICAL WALL

Recent experimental results obtained at the University of Trento show that a liquid-granular wave can impact against a vertical obstacle producing two different mechanisms of reflection, depending on the Froude number: if the front is sufficiently fast, the flow is completely deviated in the vertical direction, producing a vertical jet-like bulge, while if it is relatively slow it can be totally reflected in direction nor mal to the obstacle. The standard theoretical approaches for the analysis of the dynamic impact of a granular front against an obstacle take into account only the second mechanism described above and are obtained from the mass and momentum balances applied to the reflected bore under the hypothesis of homogeneous fluid. We extend this approach to the case of a two-phase granular-liquid mixture, taking into account the presence of a deposit of granular material near the wall, as observed in the experiments. Furthermore we propose a theoretical analysis of the formation of the vertical bulge, that is usually observed for Froude numbers larger than one, and propose an original analytical expression to estimate the dynamic impact forces also in this situation.The theoretical approaches we propose are suitable to describe the experimental results with a reasonable agreement.

Segregation and mixture profiles in dense, inclined flows of two types of spheres

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.

Debris Flows: Recent Advances in Experiments and Modeling

Abstract We report on recent advances in experiments and modeling of particle–fluid flows that are relevant to an understanding of debris flows. We first describe laboratory experiments on steady, inclined flows of mixtures of water and a single idealized granular phase that focus on the differences in the depths and the velocity of the two phases and that provide evidence for the importance of collisional exchange of momentum and energy between the particles. We then indicate how a relatively simple rate-dependent rheological model for the particles that incorporates yield may be used in the context of a two-phase mixture theory that distinguishes between the depths of the fluid and particle phases to reproduce what is seen in the experiments on both uniform and non-uniform flows. Finally, because a phenomenological extension of kinetic theory for dense, inclined flows of identical particles has recently been developed, we outline a kinetic theory for dense, inclined flows of two types of particles and water as a possible alternative to existing phenomenological theories.

Size Segregation in Dry Granular Flows of Binary Mixtures

We phrase and solve a problem of particle segregation in a dry flow of two sizes of spheres down an inclined bed in a wide channel. The flow is assumed to be steady and fully‐developed, 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 profiles of the volume fractions of the two species. For a particular ratio of the radii and of the masses of the two species, segregation is not predicted.

The Influence of Size Segregation in Particle‐Fluid Flows

We phrase and solve a problem of particle segregation in a flow of two sizes of spheres in water that is driven above by a clear fluid down an inclined erodible bed in a wide channel. The flow is assumed to be steady and fully‐developed, collisions between particles are assumed to dissipate little energy, and the sizes and masses of the particles are not too different. For particles of the same material with diameters that differ by ten percent, the total particle flux is twice that for a single species with a diameter equal to the average of the two diameters, at least for the angle of inclination that we employ.

Segregation in Dense, Dry, Inclined Flows of Binary Mixtures of Grains

We phrase and solve a problem of particle segregation in a dry flow of a binary mixture of spheres down an inclined bed in a wide channel. The flow is assumed to be steady and fully-developed, 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 profiles of one concentration fraction in a mixture of spheres with different sizes made of the same material. We compare the predictions to the results of numerical simulations.