Dynamics and flow regime of immersed granular collapse: role of fractal particle size distribution

Gravity-driven immersed granular flows are prevalent in both natural processes and industrial applications, exhibiting a significant sensitivity to particle size distribution of the constituent materials. This study employs a coupled computational fluid dynamics-discrete element method (CFD-DEM) approach to systematically investigate the collapse of polydisperse granular columns with fractal particle size distribution (FPSD) in fluids of varying viscosities. By maintaining a consistent initial volume fraction across different polydisperse systems, the findings indicate that the influence of FPSD on flow mobility is not primarily driven by changes in local pore pressure evolution but is instead governed by modifications in the particle–fluid interactions. An analysis of the relationship between the drag force at the flow front and spreading velocity reveals that the motion of flow front can serve as a reliable predictor for predicting its flow mobility, particularly in low-viscosity cases. Furthermore, two modified dimensionless parameters, incorporating an equivalent particle size, are introduced to reconstruct the flow regime diagram, which clarifies the role of FPSD under different fluid conditions. Finally, a theoretical model is developed to describe the motion of flow front during immersed polydisperse granular collapses. This model successfully predicts the correlation between normalized runout distance and fractal dimension of polydisperse particles, while also accounting for the effects of the initial column aspect ratio and particle density. These findings provide valuable insights into flow mobility of immersed polydisperse granular materials in more complex scenarios.