Particle-scale studies elucidating visco-collisional rheology in granular-fluid flows

Abstract: We study the particle-scale dynamics that give rise to bulk flow behaviours of highly concentrated particle-fluid mixtures using discrete element method (DEM) simulations. We utilize boundary conditions of a stress-controlled shear cell and vary material properties and applied stresses systematically. We find the bulk rheology transitions smoothly among what might be called viscous, collisional, and visco-collisional behaviours based on average shear rate dependence as well as dominance of local interaction. Using specific measures of particle-scale dynamics, we find that the transitions in system rheologies coincide with statistically significant changes in the fabric'' (i.e., strong force network and coordination numbers), thegranular temperature'' (i.e., fluctuation energy) and relative importance of fluid and interparticle contacts. Armed with these measures and previous theoretical work, we provide a foundation for understanding the particle-scale physics that gives rise to meso-scale rheologies and transitions from one to the next. We determine physics-based formulations to better represent dynamic behaviors for a wide range of material properties and loading conditions. Our study broadens the applicability of rheological models to natural and engineered systems by providing a foundation for a more general constitutive model.