Modelling density segregation in immersed binary granular column collapse through the coupled LBM-DEM method

Transient granular flows immersed in a viscous fluid are prevalent in geological hazards (e.g., submarine landslides) and civil engineering applications (e.g., fresh concrete delivery), where particle segregation plays a critical role in shaping flow dynamics and mobility. The present study investigates density-driven segregation in immersed granular column collapses, a crucial, yet challenging, phenomenon due to complex fluid-particle interactions at the particle scale. Using the coupled LBM-DEM simulation tool that was validated by laboratory experiments, this study investigated the effects of different particle density ratios and fluid viscosities on density segregation through macro-/micro-analyses. Results show that higher density ratios lead to more pronounced segregation, with light particles migrating to the surface and heavy particles concentrating near the base, enhancing the overall mobility of the flow. Microscopically, light particles experience larger normalized average contact and hydrodynamic forces, but lower coordination numbers than heavy particles, indicating stronger driving pressures promoting their relatively upward transport. Furthermore, density segregation is primarily driven by an imbalance between gravity and buoyancy-like forces, with drag forces playing a minimal role in vertical segregation. These insights advance our understanding of density segregation in immersed granular flows and have significant implications for improving predictive models of submarine landslides and other fluid-laden granular hazards.