Interaction of inertial particles falling in a quiescent, density-stratified, two-layer medium

Motivated by observations and modelds of the May 18, 1980, eruption of Mount St. Helens, WA, we studied the interaction, pair dispersion, and Lagrangian features of spherical particles falling in a two-liquid, quiescent medium composed of combinations of oil, ethanol, and water-glycerin mixtures. Spherical glass particles of diameters 1, 2, 4 mm were released continuosly at constant rate using a customized system that kept the inital particle separation consistant. Particle trajectories and velocities were quantified using a particle tracking velocimetry system composed of two high-speed cameras mounted perpendicularly. On the order of a thousand particles were tracked for each scenario. We explored the role of Galileo number, particle/fluid density ratio, the buoyancy jump between two fluid layers, and the initial separation of the falling particles upon their settling behavior. The results show the significant modulation generated by fluid stratification and the density interface on the dispersion and Lagransian features of the falling particles.