Particle-size segregation and rheology coupling in dense granular flows

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.