Effects of gate-release configurations and initial saturation conditions on granular column collapse

This study investigates two–phase granular collapse under varying saturation levels and gate openings and provides a detailed analysis of the flow and deposition phenomena. A quasi-two-dimensional horizontal flow channel was initially filled with a granular column composed of aluminium oxide beads with an interstitial fluid, either water or a glycerol–water solution. The rapid gate retraction triggered partial or complete collapse, which was recorded by high–speed imaging and analyzed via image processing to extract velocity fields, runout distances, descent heights, and collapse durations. Deposit morphology and flowing layer areas were quantified from free–surface and velocity profiles at characteristic times. The results show that nonlinear coupling between saturation levels and gate openings governs collapse dynamics. Increasing saturation enhances runout distance and descent height by reducing interparticle cohesion, whereas over–saturation under partial openings produces outlet cavity formation and jamming. Collapse behavior is jointly controlled by gate opening and fluid viscosity. Larger openings increase runout but yield nonmonotonic trends in descent height and collapse duration. Higher viscosity reduces runout distances and slows collapse, yet under over–saturation the final deposition length remains largely unchanged. These findings underscore the importance of accounting for the combined effects of saturation, viscosity, and geometry when predicting and mitigating granular–fluid collapse hazards.