Identifying spatial transitions in heterogenous granular flow

It is well known that heterogeneous granular flows exhibit collisional, dense and creep regimes that can coexist in space. How to correctly predict and control such complex phenomena has many applications in both mitigation of natural hazards and optimization of industrial processes.However, it still remains a challenge to establish a predictive granular rheology model due to the lack of understanding of the internal structure variation across different regimes and its interaction with the boundary. In this work, we use DEM simulations to investigate the internal structure of heterogeneous granular flow developed at the center of rotating drum systems. By systematically varying the side wall conditions, we are able to generate various heterogeneous flow fields under different levels of boundary effects. Our extensive simulation results reveal a highly relevant micro-structural quantity δθ= | θ_c- θ_f| , where θ_c and θ_f are the preferred direction of inter-particle contacts and the preferred direction of inter-particle force transmissions, respectively. We show that δθ can characterize the internal structure of granular flow in collisional, dense and creep regimes, and its variation can identify the transition between them. In particular, in dense and collisional regimes, the classical rheological relation between bulk friction μ and inertia number I holds, while in the creep regime, such relation breaks down and μ instead depends on δθ. Our findings hold for all investigated flow fields regardless of the level of boundary effect imposed, and regardless of the amount of shear experienced. δθ thus provides a unified micro-structural characterization for heterogeneous granular flow in different regimes, and lays the foundation of establishing microstructure-informed granular rheology models.