Influence of grain properties on jumps during free-surface granular flows

Overview & Synopsis

Problems involving granular flows comprise anything from thousands to trillions of particles, if not more, and may feature liquid, solid or gaseous states. While the solid and gaseous states have been associated with various models, the liquid state presents further challenges as the material dissipates energy through collisions and frictional contacts. Particle collisions in granular flows occur in different frequencies depending on the shear rate and density of the material, making it difficult to develop appropriate behavioural models. In many real-world situations, variations in the shear rate and density of the flow, which may happen for example through impact with an obstacle or with a changing slope, significantly change the free surface and subsequently modify the internal properties of the flow. These changes are not fully understood, especially in terms of their effect on problems at the engineering scale. To partially address this gap, this thesis focuses on studying free surface deformations in open channel granular flows, at slow and fast regimes, referred to as subcritical and supercritical, respectively.Changes in the surface of supercritical flows are examined in the context of granular jumps, which commonly occur when a fast incoming flow experiences an abrupt slowdown and an increase in density. These changes are generally accompanied by the rise of the free surface as a means to maintain mass and momentum conservations. Granular jumps typically occur during the retainment of avalanches and landslides by structures. Estimating the height of the granular mass near those structures becomes relevant to prevent an overflow, risking infrastructures and populations downstream. To address this, this work studies a wide range of granular jumps using discrete element method (DEM) simulations in two and three-dimensional configurations, followed by the development of corresponding scaling laws that facilitate the estimation of the final height of the flow.Moving on to the subcritical flow regime, the changes in the free surface are analysed in the context of bulldozing grains, akin to a snowplough dragging snow. The interaction between the bulldozed and the flowing layers is studied experimentally using a conveyor belt setup, displacing a granular mass towards an obstacle, which causes the surface to undulate. The surface undulations are detected through the development of a novel technique involving x-ray radiography from two perpendicular directions. These radiographs are then used to unveil the flow field underneath the free surface by using x-ray rheography, complemented with three-dimensional DEM simulations.The results from these setups address a significant gap in understanding the changes in the free surface of granular flows. First, the simulations of granular jumps reveal a strong influence of the frictional properties of the basal slip of the incoming flow, which alters the transitional and final properties of granular jumps. To quantify the overall jump, scaling laws for both the jump length and the effective friction are proposed to contribute to the estimation of the final height of the flow after the jump, relying on incoming flow parameters. Predicting the jump height accurately is crucial for many practical reasons, including for improving the design of retaining structures against mass movements such as avalanches. Second, it is shown that all the experiments involving the conveyor belt setup systematically feature a dip in the centre of the flume at the start of the undulation. The observed dip unveils the presence of strong secondary flows under the free surface, which are quantified using the recorded radiographs. Further analysis of the radiographs, alongside the complementary simulations, demonstrates that the walls and the bottom friction are essential in the development of secondary flows.