Granular collapse in fluids: Dynamics and flow regime identification

The collapse of granular material in fluids is a prevalent phenomenon in both natural and industrial processes, displaying a notable sensitivity to initial configuration of the system. This study is specifically oriented towards falling process of collapsing material under various fluid conditions, employing the computational fluid dynamics-discrete element method (CFD-DEM) to primarily investigate the dynamics and scaling laws of deposit morphology of collapsed material. Through a comprehensive analysis of particle sedimentation in fluids, we introduce a refined inertial characteristic time for granular collapse within the inertial regime. Subsequently, we propose modifications to conventional fluid-particle density ratio and Reynolds number, aiming to enhance the accuracy of depicting collapse dynamics and identifying flow regimes across diverse column heights and fluid conditions. Finally, we construct a phase diagram of flow regimes using modified dimensionless numbers, emphasizing the role of column height in transition between viscous and inertial regimes. These parameters demonstrate enhanced relevance in governing the collapse of immersed granular columns, thereby contributing to a more nuanced understanding of fluid-particle interations in dense granular flows under different regimes.