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.

Experimental study of impulse waves generated by viscoplastic fluid

Zhenzhu Meng

Landslide-generated waves, also called impulse waves, occur as a result of the intrusion of landslides (such as rock falls, debris flows, and avalanches) into bodies of water (such as lakes, reservoirs, and seas). The objective of this thesis was to study the momentum transfer from the slide material to the body of water, in order to develop a better understanding of how the slide material's properties affect the wave generation and to discover alternative modeling approaches to existing empirical equations. Previous experimental studies have usually used blocks and granular materials to mimic natural landslides. However, many landslides in real worlds have been idealized as viscoplastic fluids in theoretical and numerical studies. No studies have used viscoplastic material in experimental studies of landslide-generated waves. The originality of this thesis lies in the use of a viscoplastic material called Carbopol Ultrez 10, an artificial aqueous micro-gel whose rheological behavior can be described using the Herschel-Bulkley model. Carbopol's cohesive and deformable properties are different to both block and granular slides. Further, Carbopol is transparent and can easily be seeded with micro-seeding particles, so its velocity field can be measured using particle image velocimetry (PIV). As a comparison of Carbopol, I also used a granular material named polymer-water balls whose density is close to that of Carbopol. The investigations of this thesis are as follows: I conducted two series of experiments. First, I observed waves generated by Carbopol, water balls, and mixtures of them using high-speed cameras, to investigate the role of slide material's properties in wave prediction. Second, I conducted PIV experiments with Carbopol to investigate the internal dynamic of slide-water interaction. (1) I developed a theoretical model that combined the momentum conservation of twophase flow in a control volume (Zitti et al., 2016) and viscoplastic theory (Ancey et al., 2012). With the experimental results obtained from PIV measurements, I analyzed the drag force and hydrostatic force that act on stopping the sliding mass, and validated the theoretical model. (2) I developed empirical equations using the dimensionless groups that emerged from the governing equations to quantify the wave characteristics (for example, maximum wave amplitude and height) as functions of the slide parameters. Using empirical equations, I compared the characteristics of waves generated by cohesive Carbopol and cohesionless water balls, and discussed the effect of slide cohesion on wave generation. (3) Taking advantage of a purely data-driven approach that strictly relies on the dataset and does not need any physical constraints in advance, I applied an artificial neural network method to predict wave characteristics under complex configurations, such as dealing with an experimental dataset with several different slide materials (Carbopol, water balls, and mixtures of them). (4) Using a panel data model called random coefficient model, I predicted the time series data of wave characteristics from the time series data of slide parameters on impact. Given the slide parameters on impact by the viscoplastic theory, the temporal wave characteristics were quantified from the parameters of the slide material at the initial stage (at rest on the slope and then starting to move).

EPFL2020

Experimental study of impulse waves generated by viscoplastic fluid

Zhenzhu Meng

EPFL2020