Abstract
Granular flows are complex and evolving systems where grains interact with each other and, if immersed, interact with an ambient fluid. These flows occur at different velocities and state variables, and could behave like solids, liquids or even gases. Granular flows are involved in many circumstances and scales, from geophysical mass flows such as landslides, debris flows, pyroclastic flows, and snow avalanches, to industrial processes like pharmaceuticals, food production, and construction. For simplicity, granular flows are commonly studied with a monodisperse distribution of grains (e.i., grains with nearly the same size); however, among these flows, the grains involved in these processes have different sizes, a property termed as polydispersity.This thesis focuses on the study of granular flows and, specifically, on the influence that polydispersity has on granular flows. We explore the effect that polydispersity has on steady flows with low inertia, where granular materials can be considered as solids, and high inertia, where granular materials can be considered as fluids. Additionally, we study dry and immersed granular flows in the granular column collapse configuration, that is a benchmark geometry for studying granular flows with phases of acceleration and deceleration.We study granular flows by means of experimental and numerical methods. The numerical simulations of granular flows are done with discrete element methods (DEM) and, for immersed cases, we use a coupled finite element method (FEM) with DEM. We also conduct a controlled experimental campaign in the triaxial test apparatus where we systematically vary the polydispersity level, aiming to study the strength of polydisperse granular materials in quasi-static conditions. Furthermore, we do the physical modelling of immersed and dry gravity-driven flows in the granular column collapse configuration. Our goal is to explore the influence of polydispersity on granular flows and to identify the influence of the basal fluid pressure on the mobility of granular flows. For the experiments, we use spherical beads, exclusively focusing on the effect that size polydispersity has on granular flows.Our results allow us to conclude that the shear strength of granular materials is independent of the size polydispersity from a quasistatic condition to a condition of high inertia. For very large inertial conditions, the shear strength of polydisperse materials is smaller compared to that of monodisperse materials. We found that this difference arises from distinct variations in geometric and force parameters belonging to the contact and force network. Additionally, we provide evidence that immersed granular flows are strongly influenced by an increase in polydispersity levels. We show that the difference between monodisperse and polydisperse materials essentially arises from different evolutions of the basal fluid pressure. The initiation of polydisperse flows is delayed compared to monodisperse flows, due to a sustained negative fluid pressure change with large amplitude. Then, as the flow deposits, polydisperse systems reach longer runout distances due to the generation of exceeding pore pressure that lasts longer than the exceeding pore pressure provoked by monodisperse systems. Finally, we propose a model that links flow kinetic energy with the mobility of granular flows, which applies to different polydispersity levels, and has been successfully validated through simulations and experiments. The results of this thesis provide new insights into the role of polydispersity in both dry and immersed granular flows.