PhD Defence Marnix van Schrojenstein Lantman

a study on fundamental segregation mechanisms in dense granular flows 

Marnix van Schrojenstein Lantman is a PhD student in the Department of Thermal and Fluid Engineering (TFE). His supervisor is prof.dr. A.R. Thornton from the Faculty of Engineering Technology (ET). 

Segregation in dense granular flows occurs due to particles having different properties, with particle size and density playing a dominant role among others such as shape and surface roughness. A good understanding of segregation of realistic materials is required to avoid costly unnecessarily long or re-mixing operations in industrial plants and to predict the evolution of natural hazards like avalanches and pyroclastic flows.

Segregation in sheared granular flows is normally described in terms of kinetic sieving, where the larger particles act as a sieve for smaller particles, and squeeze expulsion, where larger particles are squeezed out of their layer in the opposite direction of the smaller particles. The aim of this research is to better understand the micro-mechanical origins of  segregation by numerical simulations and to develop models that can qualitatively predict segregation. The considered system is a monodisperse flow with a single large intruder, effectively removing kinetic sieving, but keeping squeeze expulsion.

First, the analogy to a single particle in a standard Newtonian fluid is taken by considering a model of buoyancy, drag and lift forces. Two remarkable discoveries are: (i) an upstream velocity is measured which is correlated to the lift force and (ii) the granular buoyancy force being different to Archimedes' law. Further investigations into the buoyancy force show that the difference stems from a lack of scale separation between the bulk particles and the intruder. For increasing intruder size, the number of contacts per intruder surface area reduces, effectively reducing the buoyancy force. This contact mechanism is captured accurately by a Voronoi volume correction to Archimedes' law.

The second approach is to visualise the mechanisms of segregation by analysing how the intruder size, density and friction affects the granular flow. This is done by converting the discrete particle simulation data into smooth conservative continuum (density, velocity, stress) fields with a technique called coarse graining. These fields show that a large intruder does not fit inside a layer of bulk particles leading to an anisotropic stress field. This observation has inspired new scalings for the lift force on an intruder, proportional to the shear rate and viscosity gradient of the bulk flow. Simulations for many different flows have been performed to confirm this hypothesis.

The segregation strength of an intruder depends on the granular flow. Hence, simulations of granular flows with continuum methods are performed. A generalised µ(I)-rheology in a split-bottom shear cell setup has been simulated, with a new correction for low inertial values. Results show improvement compared to the classical µ(I)-rheology, however further corrections are recommended.

The fundamental mechanisms discovered in this thesis have improved the understanding of individual particles in granular flows, which can be used to develop more accurate continuum models for segregation. The developed micro-based force model can be used as starting point to develop more sophisticated models that could aid in the engineering of granular materials by balancing size with density and other realistic particle properties with the goal of reducing segregation.