Particle Size Segregation in Dense Granular Flows

Flows of granular materials widely occur in many industrial and natural environments. Particle-size segregation---the separation of grains of different sizes during flow---plays a crucial role in the behaviour of such flows. In spite of considerable efforts over the last years to investigate size-segregation, the phenomenon is still only partially understood. In particular for dense gravity-driven flows, where small particles percolate to the bottom of the flow and large particles rise to the top, against gravity, the particle-scale behavior and mechanisms are poorly understood. This thesis presents an experimental and particle simulation investigation of size-segregation in dense gravity-driven bi-disperse granular flows in various three-dimensional configurations. Namely, an experimental shear-box, a numerical chute flow, an experimental moving-bed channel, and a numerical moving-bed channel. Using the non-intrusive imaging technique Refractive Index Matched Scanning (RIMS) we investigate the particle motion inside the bulk of the experimental configurations. The moving-bed channel was designed from the ground up with the use of the RIMS technique in mind. The results obtained through both experiments and simulations demonstrate the existence of a property we call size-segregation asymmetry'. This property is fundamental to the process of size-segregation and explains a number of specific features that arise on the bulk-scale of the flow. It can be simply and intuitively explained as follows: When a large particle in a granular flow is surrounded by many small particles it rises to the free-surface at a low velocity compared to the sinking velocity of a small particle in a flow surrounded by many large particles. We quantify this behaviour by measuring the segregation velocities as a function of local particle concentration. Comparison of the bulk concentration field with predictions of an asymmetric size-segregation model allow us to link the particle-scale behaviour to a number of bulk-scale features. Segregation at the front of granular avalanches produces a recirculation zone, known as a breaking size-segregation wave', in which large particles are initially segregated upwards, sheared towards the front of the flow, and overrun before being resegregated again. This recirculation zone separates a coarse particle front from a small-particle tail. By making use of a moving-bed channel, which permits the creation of a continuous gravity-driven flow, we are able to visualise, using RIMS, the complex internal structure of breaking size-segregation waves for the first time. We find that size-segregation asymmetry plays a role in the observed structure. In particular, it is seen that a few large particles are swept a long way upstream inside the small-particle tail and take a very long time to recirculate. Additionally, a basal slip is observed that is linked to the local presence of the different sized particles, with the tail of the flow experi