Discrete element method simulation analysis of the generation mechanism of cooperative behavior of disks falling in a low-density particle bed

Pacheco-Vázquez and Ruiz-Suárez reported an interesting cooperative behavior for disks falling in a particle bed. This behavior involved the formation of upward and downward convex configurations during the falling of five steel disks into a bed of polystyrene particles. We used discrete element method simulations to investigate the generation mechanism for this cooperative behavior. Particles with a diameter of 5.0 mm and a density of 14.0 kg/m³ were placed randomly in a container with a width of 900 mm or 2700 mm and a height of 2700 mm. Model spheres with the same mass and diameter as the steel disks with a diameter of 25.4 mm and a thickness of 5.0 mm were then dropped into the particle bed, and we investigated the cooperative behavior of the model spheres. Similar cooperative behaviors were observed for the containers with widths of 900 mm and 2700 mm, indicating that the container side walls do not affect the occurrence of this behavior when the width is larger than 900 mm. The falling velocity of each disk was strongly dependent on the packing fractions over the disk and the flow velocity of the bed particles around the disks. Based on these results, the generation mechanism of the upward and downward convex configurations is discussed.