MEASUREMENTS OF THE TWO-PHASE VORTICAL FLOW AND TURBULENCE CHARACTERISTICS BELOW A ROTOR
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Time-resolved particle image and particle tracking velocimetry measurements were made in the particle-laden turbulent flow environment below a rotor hovering over a mobile sediment bed. The results were also compared to the near-wall flow produced by a nominally equivalent two-dimensional wall jet. The objective of the work was to understand the fluid dynamic mechanisms of how the mean flow, stochastic turbulence, and concentrated vorticity produced by the rotor affected the mobilization and pickup of particles from the sediment bed. Another objective was to better understand the assumptions that would be required for the development of models that are more applicable to rotor-induced particle mobilization. It was shown that the mean flow in the boundary layer at the ground below the rotor was similar to that of a wall jet. However, the instantaneous flow field and turbulence characteristics between these two flows were significantly different. Mobilized particles of 45--63 micron diameter (with a particle Reynolds number of less than 30 and a Stokes number of about 60) were individually identified and tracked, with the objective of relating any changes in the temporal evolution of the vortical flow and turbulence characteristics of the carrier flow phase to its coupling to the dispersed particle phase. The processes of particle mobilization and pickup from the bed were found to correlate to the Reynolds stresses and discrete turbulence events, respectively. The mean flow and turbulence characteristics were modified by the presence of particles in the near-wall region, showing clear evidence of two-way coupling between the phases of the resulting two-phase flow. Specifically, it was shown that the uplifted particles altered the carrier flow near the sediment bed, leading to an earlier distortion of the flow induced by the blade tip vortices and to the accelerated diffusion of the vorticity that they contained. The uplifted particles were also seen to modify the overall turbulence field, and when sufficient particle concentrations built up, the particles began to attenuate the turbulence levels. Even in regions with lower particle concentrations, the turbulence was found to be attenuated by the indirect action of the particles because of the distortions to the tip vortices, which were otherwise a significant source of turbulence production. After the tip vortices had diffused further downstream from the rotor, the uplifted particles were also found to increase the anisotropy of the resulting turbulence in the flow.