Settling of side-by-side particle pairs in density-stratified fluid at moderate Galileo numbers

We investigate how density stratification influences hydrodynamic interactions between a pair of settling particles. While numerical studies suggest that particle interactions in stratified fluids differ fundamentally from those in homogeneous fluids, experimental evidence and mechanistic understanding remain limited. We systematically examine the settling dynamics of isolated particles and side-by-side particle pairs in homogeneous and stratified environments, complemented by numerical simulations resolving the three-dimensional wake structures. The Galileo number is fixed at Ga≈178, and the Froude number is varied from Fr=6.4 to 19.4. In a homogeneous fluid, particle-pair separation is governed by two mechanisms: a short-range inter-particle interaction that deterministically sets the inclination of the particle wakes, and a long-range wake-induced particle–fluid interaction that sustains continued separation. Although the direct interaction decays rapidly beyond s>2D (where D is the particle diameter), lateral wake-induced motion persists. Density stratification suppresses this process by maintaining near-axisymmetric wakes, thereby inhibiting horizontal particle motion and significantly reducing the total separation distance.