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With ongoing global warming, snow avalanche dynamics may change as snow cohesion and friction strongly depend on temperature. In the field, a diversity of avalanche flow regimes has been reported including fast, sheared flows and slow plugs. While the significant role of cohesion and friction has been recognized, it is unclear how these mechanical properties affect avalanche flow regimes. Here, we model granular avalanches on a periodic inclined plane, using the distinct element method to better understand and quantify how inter-particle cohesion and ground friction influences avalanche velocity profiles. The cohesion between particles is modeled through bonds that can subsequently break and form, thus representing fragmentation and aggregation potentials, respectively. The implemented model shows a good ability to reproduce the various flow regimes and transitions as observed in nature: for low cohesion, highly sheared and fast flows are obtained while slow plugs form above a critical cohesion value and for lower ground frictions. Simulated velocity profiles are successfully compared to experimental measurements from the real-scale test site of Vallée de la Sionne in Switzerland. Even though the model represents a strong simplification of the reality, it offers a solid basis for further investigation of relevant processes happening in snow avalanches, such as segregation, erosion and entrainment, with strong impacts on avalanche dynamics research, especially in a climate change context., PLoS ONE, 17 (2), ISSN:1932-6203

This repository provides the data and routines to plot figures 5 to 15 of the article "An experimental scaling law for particle-size segregation in dense granular flows" published in J. Fluid Mech. Tomás Trewhela tomas.trewhela@epfl.ch (from October 2021: tomas.trewhela@uai.cl) Lausanne, March 2021, {"references":["van der Vaart , K., Gajjar , P., Epely-Chauvin , G., Andreini , N., Gray , J.M.N.T. & Ancey , C. (2015) Underlying asymmetry within particle size segregation. Phys. Rev. Lett. 114, 238001.","Thornton , A.R., Weinhart , T., Luding , S. & Bokhove , O. (2012) Modeling of particle size segregation: calibration using the discrete particle method. Intl J. Mod. Phys. C 23, 1240014."]}

It is essential to consider the fluidity of a debris-flow front when calculating its impact. Here, we flume-tested monogranular and bigranular debris flows, and compared the results to those of numerical simulations. We used sand particles with diameters of 0.29 and 0.14 cm at two mixing ratios, of 50% and 50% (5:5), and 30% and 70% (3:7), respectively. Particle segregation was recorded using a high-speed video camera. We evaluated the fronts of debris flows at 0.5-s intervals. We then numerically simulated one- dimensional debris flows under the same conditions, and we used the mean particle diameter when simulating mixed-diameter flows. For monogranular debris flows, the experimental and simulated results were in good agreement in terms of flow depth, front velocity, and flux, but the bigranular debris flows were not well-simulated; the simulated flow depth was less than that found experimentally, and the front velocity and flux were greater. The differences may be attributable to the fact that the dominant shear stress was caused by the concentration of smaller sediment particles in the lower flow layers; such inverse gradations were detected in the debris flow bodies. In this situation, most shear stress is supported by smaller particles in the lower layers; the debris-flow characteristics become similar to those of monogranular flows. Consequently, the calculated front velocities were underestimated; particle segregation at the front of bigranular debris flows did not affect fluidity either initially or over time.

We consider the segregation of spheres of equal size and different density flowing over an inclined plane, theoretically and computationally by means of distinct element method (DEM) simulations. In the first part of the work, we study the settling of a single higher-density particle in the flow of otherwise identical particles. We show that the motion of the high-density tracer particle can be understood in terms of the buoyancy and drag forces acting on it. The buoyancy force is given by Archimedes principle, with an effective volume associated with the particle, which depends upon the local packing fraction, $\phi $. The buoyancy arises primarily from normal forces acting on the particle, and tangential forces have a negligible contribution. The drag force on a sphere of diameter $d$ sinking with a velocity $v$ in a granular medium of apparent viscosity $\eta $ is given by a modified Stokes law, ${F}_{d} = c\pi \eta dv$. The coefficient ($c$) is found to decrease with packing fraction. In the second part of the work, we consider the case of binary granular mixtures of particles of the same size but differing in density. A continuum model for segregation is presented, based on the single-particle results. The number fraction profile for the heavy particles at equilibrium is obtained in terms of the effective temperature, defined by a fluctuation–dissipation relation. The model predicts the equilibrium number fraction profiles at different inclination angles and for different mass ratios of the particles, which match the DEM results very well. Finally, a complete model for the theoretical prediction of the flow and number fraction profiles for a mixture of particles of different density is presented, which combines the segregation model with a model for the rheology of mixtures. The model predictions agree quite well with the simulation results.

Granular segregation, spontaneous demixing of components in. owing mixtures, is a phenomenon of practical importance and occurs in a number of natural systems. We present a model to describe granular segregation due to density differences in a dense granular flow. Such flows are a prototype of "jammed" systems for which a new thermodynamics has been proposed arising from analysis of glassy systems and packed powders. In the context of this new framework, we show that granular segregation is determined by the effective temperature defined by the fluctuation-dissipation relation. The description has no fitted parameters and is validated by experiments. The work shows the utility of the effective temperature for the analysis of transport in granular flows and indicates the promise of the new thermodynamics for the description of practical systems. Copyright (c) EPLA, 2008.

Dense granular flows -debris or pyroclastic flows, avalanches, granular flows in heaps, silos or rotating drums-are ubiquitous to many natural and industrial processes. Their grain composition is often polydisperse, with particles differing in shape, size or density. In a phenomenon called particle-size segregation, grains of different sizes are rearranged by the very same mechanical forcing that generates the granular flow in the first place. If the flow's rheology is determined by grain characteristics and grain-grain interactions, a grain rearrangement should change the flow's rheology. Although recent constitutive relations for dense granular flows and size segregation models are able to describe well their respective processes, the interplay between rheology and particle-size segregation remains poorly understood. In this dissertation, I present an experimental study that aims to describe the coupling between rheology and particle-size segregation in dense granular flows. To address this objective, experiments in three experimental setups were carried out: (i) a two-dimensional oscillating shear cell, (ii) a three-dimensional oscillating shear box, and (iii) a three-dimensional conveyor belt flume. In these three configurations, I focused on the experimental determination of bulk species composition, segregation rates, velocity fields and the strain-rate tensor invariants. For that, the Refractive Index Matching (RIM) and Particle Tracking Velocimetry (PTV) techniques were intensively applied. In the two-dimensional shear cell, the rate and mechanism by which a large particle (intruder) segregated were found to be determined by the particles' size ratio, its rotation, and the strain rate around it. A scaling law for the segregation flux function was determined from the three-dimensional shear box single-intruder experiments. Four key observations were made: the segregation rate scaled; (i) linearly to the shear rate, for small and large intruders; (ii) linearly to the size ratio for a large intruder; and (iii) quadratically to size ratio for a small intruder. Finally, (iv) all the intruders' trajectories were well-fitted by a quadratic law, a sign of a pressure-dependent segregation rate. For the mono- and bidisperse stationary granular avalanches in the conveyor belt, I found two flow regions: a bulged-convective front and a well-arranged layered tail. Independently of concentration, the time-averaged velocity profiles were found to be Bagnold-like and segregation rate was found to be highest at the front, where dilation and shear rate values were also high. All these experimental results described a strong oupling between rheology and particle-size segregation, and ultimately provided a segregation rate dependence on normal stresses, shear rate and size ratio.

Flows of granular materials widely occur in many industrial and natural environments. Particle-size segregation---the separation of grains of different sizes during flow---plays a crucial role in the behaviour of such flows. In spite of considerable efforts over the last years to investigate size-segregation, the phenomenon is still only partially understood. In particular for dense gravity-driven flows, where small particles percolate to the bottom of the flow and large particles rise to the top, against gravity, the particle-scale behavior and mechanisms are poorly understood. This thesis presents an experimental and particle simulation investigation of size-segregation in dense gravity-driven bi-disperse granular flows in various three-dimensional configurations. Namely, an experimental shear-box, a numerical chute flow, an experimental moving-bed channel, and a numerical moving-bed channel. Using the non-intrusive imaging technique Refractive Index Matched Scanning (RIMS) we investigate the particle motion inside the bulk of the experimental configurations. The moving-bed channel was designed from the ground up with the use of the RIMS technique in mind. The results obtained through both experiments and simulations demonstrate the existence of a property we call `size-segregation asymmetry'. This property is fundamental to the process of size-segregation and explains a number of specific features that arise on the bulk-scale of the flow. It can be simply and intuitively explained as follows: When a large particle in a granular flow is surrounded by many small particles it rises to the free-surface at a low velocity compared to the sinking velocity of a small particle in a flow surrounded by many large particles. We quantify this behaviour by measuring the segregation velocities as a function of local particle concentration. Comparison of the bulk concentration field with predictions of an asymmetric size-segregation model allow us to link the particle-scale behaviour to a number of bulk-scale features. Segregation at the front of granular avalanches produces a recirculation zone, known as a `breaking size-segregation wave', in which large particles are initially segregated upwards, sheared towards the front of the flow, and overrun before being resegregated again. This recirculation zone separates a coarse particle front from a small-particle tail. By making use of a moving-bed channel, which permits the creation of a continuous gravity-driven flow, we are able to visualise, using RIMS, the complex internal structure of breaking size-segregation waves for the first time. We find that size-segregation asymmetry plays a role in the observed structure. In particular, it is seen that a few large particles are swept a long way upstream inside the small-particle tail and take a very long time to recirculate. Additionally, a basal slip is observed that is linked to the local presence of the different sized particles, with the tail of the flow experiencing less friction compared to the front. By studying the basal slip we are able to explain the complex relation between the mixture ratio of the two species and the flow speed. Overall, this thesis highlights the important role of the distinct dynamics of large and small particles in size-segregating granular flows.