Turbulence Modifications and Phase Couplings in Ground Effect under Simulated Brownout Conditions

A detailed investigation was made into the particle-laden turbulent flow produced by a rotor hovering in ground effect over a mobile sediment bed. Measurements of the dual-phase flow were made using time-resolved particle image velocimetry and particle tracking velocimetry as the rotor wake and its embedded vorticity approached the bed. Mobilized sediment particles were individually identified and tracked in the flow, with the objective of relating any changes in the vortical flow and turbulence characteristics of the carrier flow to the action of the dispersed particle phase. It was observed that, in general, a two-way coupling between the phases was produced and in some cases the coupling was very significant. Specifically, it was shown that the uplifted particles altered the carrier flow near the sediment bed, leading to an earlier distortion of the blade tip vortices and to the accelerated diffusion of concentrated vorticity. The uplifted particles were also seen to modify the global turbulence field, and when sufficient particle concentrations built up the particles began to attenuate the overall levels of turbulence. Even in regions with lower particle concentrations, the turbulence was attenuated by the indirect action of the particles through their distortions to the tip vortices, which were otherwise a significant source of turbulence production. After the tip vortices had sufficiently diffused further downstream, the uplifted particles were also found to increase the anisotropy of turbulence that was present in the flow.