Simulation of size segregation in granular flow with material point method

Abstract

Abstract. Segregation is common in granular flows consisting of mixtures of particles differing in size or density. In gravity-driven flows, both gradients in total pressure (induced by gravity) and gradients in velocity fluctuation fields (often associated with shear rate gradients) work together to govern the evolution of segregation. Since the local shear rate and velocity fluctuations are dependent on the local concentration of the components, understanding the co-evolution of segregation and flow is critical for understanding and predicting flows where there can be a variety of particle sizes and densities, such as in nature and industry. Kinetic theory has proven to be a robust framework for predicting this simultaneous evolution but has a limit in its applicability to dense systems where collisions are highly correlated. In this paper, we introduce a model that captures the coevolution of these evolving dynamics for high density gravity driven granular mixtures. For the segregation dynamics we use a recently developed mixture theory (Fan & Hill 2011, New J. Phys; Hill & Tan 2014, J. Fluid Mech.) which captures the combined effects of gravity and fluctuation fields on segregation evolution in high density granular flows. For the mixture flow dynamics, we use a recently proposed viscous-elastic-plastic constitutive model, which can describe the multi-state behaviors of granular materials, i.e. the granular solid, granular liquid and granular gas mechanical states (Fei et al. 2016, Powder Technol.). The platform we use for implementing this model is a modified Material Point Method (MPM), and we use discrete element method simulations of gravity-driven flow in an inclined channel to demonstrate that this new MPM model can predict the final segregation distribution as well as flow velocity profile well. We then discuss ongoing work where we are using this platform to test the effectiveness of particular segregation models under different boundary conditions.

A modified calculation of particle buoyant forces in vibro-fluidized beds

“…Granular materials attempt to exhibit spatially segregation of different components after vibration, such as the Brazil nut effect, which is self-organization displayed by systems that are out of equilibrium. To explain this segregation behavior, several mechanisms have been proposed, such as void-filling [1], convection [2], condensation [3], and some new mechanisms, such as buoyancy [4], asymmetric flux model [5], material point method [6], etc. Void-filling mechanism and convection mechanism are based on experimental observation, which can explain Brazil nut effect intuitively, but isn't suitable for reverse Brazil nut effect, and is also limited to the boundary conditions [7].…”