Settling dynamics of vertically aligned particles in single and two-layer fluid media

Settling dynamics of vertically aligned particles was examined in single and two-layer fluid media. Water, ethanol, water-glycerin mixture, and oil were considered for the single fluid media, while the two-layer fluids were composed of ethanol and oil, with the upper layer thickness varying from 40 mm to 160 mm. Experiments were performed in a 600 mm height tank using approximately 800 spherical glass particles of 4 mm diameter and 2200 kg/m³ density, released at frequencies f_p = 4, 6, and 8 Hz. The single fluid media particles exhibited two general trajectories. At high Galileo number (Ga) in water and ethanol, particles showed preferential in-plane motions, while at low Ga in water-glycerin mixture and oil, they showed nearly vertical paths. However, particles in the lower part of the two-layer fluids near the interface showed lateral dispersions depending on the upper layer thickness, even at low Ga. For all single fluid media, particles showed higher settling velocities than a single particle while experiencing reduced drag. The drag reduction was enhanced when the particle separation was close. However, particles settled slower in the lower part of the two-layer fluids due to the additional buoyancy caused by the entrained lighter fluid from the upper layer. Drag reduction due to the leading particles became less significant as the particles no longer maintained their straight path seen in the single oil medium. An analysis of the particle pair dispersion showed its dependence on Ga and initial separation.