Mechanisms of Vortex-Induced Particle Transport from a Mobile Bed below a Hovering Rotor

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A study has been conducted to examine rotor-generated blade tip vortices that pass near to a ground plane covered with mobile sediment particles and to explore whether they induce a pressure field that may affect the problem of rotor-induced dust fields. It was hypothesized that fluctuating pressures lower than ambient at the ground could potentially affect the processes of sediment particle mobilization and uplift into the flow. To investigate the relationship between the vortex wake characteristics and the motion of the mobilized sediment particles, single-phase and dual-phase (particle) flow experiments were conducted using a small laboratory-scale rotor hovering overing a ground plane. Time-resolved particle image velocimetry was used to quantify the flow velocities in the rotor wake and near the ground plane, and particle tracking velocimetry was used to quantify the particle velocities. Measurements were also made of the unsteady pressure over the ground plane using pressure transducers that were sensitive enough to resolve the small induced pressures. Time-histories of the measured responses showed significant pressure fluctuations occurred before, during, and after the rotor wake impinged upon the ground. While it was not possible to separate out the effects of pressure forces from other forces acting on the particles, the present work has shown good evidence of vortex-induced pressure effects on the particles in that particle trajectories significantly deviated from the directions of the surrounding flow in the immediate presence of the vortices. The characteristics of the pressure responses produced at the ground by vortices passing nearby was also predicted using a model based on unsteady potential flow theory, and was used to help interpret the measurements. The vortex strength (circulation), height of the vortex above the ground, and the vortex convection velocity, were all shown to affect the pressures at the ground and were likely to affect particle motion.