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 experiencing less friction compared to the front. By studying the basal slip we are able to explain the complex relation between the mixture ratio of the two species and the flow speed. Overall, this thesis highlights the important role of the distinct dynamics of large and small particles in size-segregating granular flows

Segregation of large particles in dense granular flows: A granular Saffman effect?

We report on the scaling between the lift force and the velocity lag experienced by a single particle of different size in a monodisperse dense granular chute flow. The similarity of this scaling to the Saffman lift force in (micro) fluids, suggests an inertial origin for the lift force responsible for segregation of (isolated, large) intruders in dense granular flows. We also observe an anisotropic pressure/stress field surrounding the particle, which potentially lies at the origin of the velocity lag. These findings are relevant for modelling and theoretical predictions of particle-size segregation. At the same time, the suggested interplay between polydispersity and inertial effects in dense granular flows with stress- and strain-gradients, implies striking new parallels between fluids, suspensions and granular flows with wide application perspectives

Three-Dimensional Time-Resolved Trajectories from Laboratory Insect Swarms

Aggregations of animals display complex and dynamic behaviour, both at the individual level and on the level of the group as a whole. Often, this behaviour is collective, so that the group exhibits properties that are distinct from those of the individuals. In insect swarms, the motion of individuals is typically convoluted, and swarms display neither net polarization nor correlation. The swarms themselves, however, remain nearly stationary and maintain their cohesion even in noisy natural environments. This behaviour stands in contrast with other forms of collective animal behaviour, such as flocking, schooling, or herding, where the motion of individuals is more coordinated, and thus swarms provide a powerful way to study the underpinnings of collective behaviour as distinct from global order. Here, we provide a data set of three-dimensional, time-resolved trajectories, including positions, velocities, and accelerations, of individual insects in laboratory insect swarms. The data can be used to study the collective as a whole as well as the dynamics and behaviour of individuals within the swarm

Breaking Size-Segregation Waves in Granular Avalanches

We experimentally prove the existence of the theoretically predicted breaking size-segregation wave within a binary granular avalanche. This complex structure involves the recirculation of particles through a pattern of shocks and rarefaction waves, and causes large particles to accumulate at the avalanche front and small particles in the tail. Using the non-intrusive imaging technique of refractive-index matching we study particle-size segregation inside the flow---far from the sidewall---on an inclined moving-bed channel. In this configuration the bottom layers of the flow are dragged upslope while upper layers are avalanching downslope due to gravity; effectively, the flow remains stationary in the reference frame of the observer. This allows us to time-average discrete particle positions in the steady-state flow and arrive at a continuous particle concentration. The measured particle concentration and particle trajectories match qualitatively with the theoretical predictions

Mercury DPM: fast, flexible particle simulations in complex geometries part II: applications

MercuryDPM is a particle-simulation software developed open-source by a global network of researchers. It was designed ​ab initio to simulate realistic geometries and materials, thus it contains several unique features not found in any other particle simulation software. These features have been discussed in a companion paper published in the DEM7 conference proceedings; here we present several challenging setups implemented in MercuryDPM ​ . Via these setups, we demonstrate the unique capability of the code to simulate and analyse highly complex geotechnical and industrial applications.These tups implemented include complex geometries such as (i) a screw conveyor, (ii) steady-state inflow conditions for chute flows, (iii) a confined conveyor belt to simulate a steady-state breaking wave, and(iii)aquasi-2D cylindrical slice to efficiently study shear flows.​MercuryDPM is also parallel, which we showcase via a multi-million particle simulations of a rotating drum. We further demonstrate how to simulate complex particle interactions, including: (i)deformable, charged clay particles; and (ii) liquid bridges and liquid migration in wet particulates, (iii) non-spherical particles implemented via superquadrics. Finally, we show how to analyse and complex systems using the unique micro-macro mapping (coarse-graining) tool MercuryCG

Comparative return of imports into the state of Negri Sembilan during the years, 1916 and 1915.

Supplement to the F.M.S Government Gazette, May 11th, 1917. It also contains 'Comparative return of exports into the state of Negri Sembilan during the years, 1916 and 1915'

Asymmetric Particle Behavior during Size Segregation

We have performed oscillatory shear experiments on a bidisperse granular mixture in combination with refractive index matched scanning in order to resolve particle motion in three-dimensions during size-segregation. We find that small particles segregate faster in regions where the large particle concentration is high and large particles segregate slower in regions where the small particle concentration high. This dependency of particle segregation speed on local relative volume fraction results from a fundamental asymmetry in the underlying dynamics of the two species. We observe that this asymmetry affects the segregation behavior of the mixture at both the bulk and particle scale. For example, mixtures containing relatively more small particles take longer to segregate; and during segregation small particles are quicker to reach the bottom of the flow compared to larges particles reaching the top. The predictions of a theoretical model incorporating this asymmetric dependence on volume fraction show a significant improvement compared to the standard model with a symmetric dependence. Besides having repercussions for the understanding of size segregation on a fundamental level, this discovery expands the connections to processes such as traffic flow, sedimentation and particle diffusion, which exhibit similar asymmetric behavior