Experimental investigation on inter-particle settling dynamics of multiple spherical particles released side by side at intermediate Reynolds numbers

The settling of solid particles in fluid constitutes a fundamental and crucial aspect with applications spanning various natural phenomena and engineering processes, including sediment transport and wastewater treatments. This paper delves into an experimental investigation aimed at comprehending the settling dynamics and self-organization of multiple spherical particles settling side by side at intermediate Reynolds numbers. The study employs an electromagnetic release device, previously developed for controlled settling of particles under gravity, ensuring simultaneous release with zero initial rotation and velocity. This research captures settling trajectories and provides insight into the flow fields surrounding particles by utilizing particle tracking and particle image velocimetry. The experiments systematically investigate the influence of the settling patterns, the flow fields, the velocities of particles, and their dependence on Reynolds number Re (Re = 52–258), the number of particles n (n = 3–8), as well as the initial spacing between particles l0* (l0* = 0–2). The results consistently reveal a left–right symmetry about the centerline in settling patterns, flow fields, and particle rotations across all values of n, l0*, and Re. The final settling pattern exhibits distinct shapes dependent on l0*: a “V” or “M” shape for l0* 0.2, a “concave-downward” shape for 0.2 l0* 2, and a “straight-line” shape for l0* ≥ 2. The lateral spread of particles increases with time, particularly pronounced with smaller l0* and larger Re, attributed to strong repulsive forces between neighboring particles. Correspondingly, the maximum of horizontal velocities reduces from outside to inside and increases with decreasing l0* and increasing Re. The inner vortices are smaller than the outer vortices, which causes the lateral spread. The vertical spread increases with n but remains insensitive to Re. The average terminal settling velocities for all particles in the array are consistently smaller than those for single particles, as a portion of kinetic energy contributes to horizontal motions.