Tomás Alfredo Trewhela Palacios

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A conveyor belt experimental setup to study the internal dynamics of granular avalanches

Christophe Ancey, Tomás Alfredo Trewhela Palacios

This paper shows how a conveyor belt setup can be used to study the dynamics of stationary granular flows. To visualise the flow within the granular bulk and, in particular, determine its composition and the velocity field, we used the refractive index matching (RIM) technique combined with particle tracking velocimetry and coarse-graining algorithms. Implementing RIM posed varied technical, design and construction difficulties. To test the experimental setup and go beyond a mere proof of concept, we carried out granular flow experiments involving monodisperse and bidisperse borosilicate glass beads. These flows resulted in stationary avalanches with distinct regions whose structures were classified as: (i) a convective-bulged front, (ii) a compact-layered tail and, between them, (iii) a breaking size-segregation wave structure. We found that the bulk strain rate, represented by its tensor invariants, varied significantly between the identified flow structures, and their values supported the observed avalanche characteristics. The flow velocity fields' interpolated profiles adjusted well to a Bagnold-like profile, although a considerable basal velocity slip was measured. We calculated a segregation flux using recent developments in particle-size segregation theory. Along with vertical velocity changes and high expansion rates, segregation fluxes were markedly higher at the avalanche's leading edge, suggesting a connection between flow rheology and grain segregation. The experimental conveyor belt's results showed the potential for further theoretical developments in rheology and segregation-coupled models.

SPRINGER2021

Large particle segregation in two-dimensional sheared granular flows

Christophe Ancey, Tomás Alfredo Trewhela Palacios

We studied the segregation of single large intruder particles in monodisperse granular materials. Experiments were carried out in a two-dimensional shear cell using different intruder and media diameters, whose quotient defined a size ratio R that ranged from 1.2 to 3.333. When sheared, the intruders segregated and rotated at different rates, which depended on their R values and depth. The vertical intruder trajectories as a function of time were curved due to nonconstant depth-dependent segregation rates. An analysis that considered the lithostatic pressure distribution and a size ratio dependence was done to capture the trajectories and the general segregation rate behavior. As a result of a strain rate analysis, we observed a greater expansion rate around the intruders when R values were larger, which in turn promoted faster segregation. Experiments with large R values showed that intruder rotation was weak and local shear rates were low. In contrast, experiments with R closer to unity resulted in strong intruder rotation, high local shear rates, and contraction below the intruder. Therefore, an intruder with a diameter close to that of the medium was likely to segregate due to a rotation mechanism. We propose that large particle segregation depends on size ratio, local expansion rate, and, to a lesser extent, the local shear rate. Based on our observations we redefine large particle segregation as two well-defined processes dependent on R and the local strain rate.

2021

Particle-size segregation and rheology coupling in dense granular flows.

Tomás Alfredo Trewhela Palacios

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.

EPFL2021