Measurements of the Plume-like Three-Dimensionality of Rotor-Induced Dust Fields

To better understand the problem of rotor-induced particle motion and rotorcraft brownout, time-resolved, dual-phase particle image velocimetry and particle tracking velocimetry measurements were made in the flow produced by a small laboratory rotor that was hovering over a mobile sediment bed. The main goal was to further investigate the three-dimensionality of the rotor wake and how its interaction with the sediment bed led to the formation of distinct columnar plumes of uplifted sediment particles. Dual-phase flow measurements were made in both vertical and horizontal planes around the rotor and near the ground. The results showed that the rotor wake was extremely three-dimensional, with non-uniform velocities near the ground that resulted in the radially asymmetric uplift of particles throughout the flow field. The helicoidal vortex filament developed wave-like displacements along their lengths, which caused some parts of the filaments to convect closer to the ground. The parts of the filaments that were closer to the ground were observed to interact with the near-wall flow there, sometimes causing the boundary layer to briefly detach and introduce secondary vorticity into the flow. Adjacent turns of the interdigitated helicoidal vortex filaments often merged together, creating significantly larger upwash/downwash regions near the ground, which produced discrete bursts of sediment mobilization and uplift.