Interactions of a Vortical Flow with Dispersed Particles Below a Rotor

摘要

Measurements were made of the vortically dominated flow produced by a rotor hovering in ground effect over a mobile sediment bed. Time-resolved particle image velocimetry and particle tracking velocimetry were used to measure the two-phase flow as the rotor wake interacted with the bed. Mobilized sediment particles were tracked, with the objective to relate the particle motion to individual vortical structures and turbulent motions of the fluid. Although the tip vortices trailed from the rotor blades were the dominant flow features, secondary flow structures and organized turbulent motions were identified to affect the process of particle uplift. A vortex-induced secondary vortical structure convected downstream in between two consecutive blade tip vortices, and in a dynamic interplay with the tip vortices was shown to contribute to the uplift process of particles from the bed. Most tip vortices trailed a region of negative streamwise velocity fluctuations, which decelerated the particles that could subsequently be picked up by a succeeding tip vortex. A quadrant analysis showed the preferential distribution of turbulence events that could be correlated to sediment mobilization and uplift. Positive wall-normal velocity fluctuations (ejection and outward interaction events) were induced primarily by the tip vortices and correlated with the uplift of particles from the bed and their subsequent suspension.