Settling of localized particle plumes in a quiescent water tank

The fall or rise of inertial particles plumes in an initially quiescent fluid is ubiquitous in natural and industrial processes. Disentangling the influence of the numerous parameters of these systems on the plume velocity is thus a major issue. Moreover, understanding the energy transfer between the two phases and the structure of induced flow is another very relevant question. A specific experiment has been designed to tackle these problems. A continuous plume of inertial particles released above the center of a still water tank generates a large-scale recirculation flow. The present experimental study investigates the stationary settling dynamics and induced flow properties using various particle populations with particle-to-fluid density ratios from 3.8to14.3, Archimedes numbers 4to580, and mass fractions 10-6to10-3, revealing an unexplored region of the parameter space. The fluid and particle velocities are measured simultaneously by two cameras, utilizing optical filtering and image postprocessing. It is found that the settling in the laboratory frame generally follows the terminal velocity predicted using the Schiller-Naumann drag. The particle velocity relative to the surrounding fluid is unaffected by the particle mass fraction and always hindered due to the kinetic energy transferred to the fluid. This transfer of energy is most effective for low Archimedes numbers. All modifications of the settling behavior due to the particle mass fraction are caused by the back-reaction of the induced flow on the particles; no direct particle-particle interactions are observed.