Particle-scale studies elucidating visco-collisional rheology in granular-fluid flows
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), the ``granular 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.