Flow Environment and Organized Turbulence Structures at the Ground Below a Rotor

Time-resolved particle image velocimetry measurements of the flow environment and turbulence structures were made at a ground plane below laboratory-scale hovering rotors operating in ground effect. The results were also compared to the flows produced by a two-dimensional wall jet that was generated by the flow from a nozzle. The measurements were conducted with and without mobile sediment beds. The objective of the work was to understand the fluid dynamic mechanisms of how the mean flow, stochastic turbulence, and concentrated vorticity contained in coherent vortex structures, affected the mobilization and pickup of sediment particles from the bed. Another objective was to better understand the boundary conditions that would be required for the development of entrainment models that are applicable to the modeling of rotor-induced particle flows that lead to brownout conditions. Below the rotor the mean flow in the boundary layer over the ground plane was found to be similar to that measured for the wall jet. The instantaneous flow field and the turbulence characteristics, however, were found to be significantly different between the rotor and wall-jet type flows. Measurements made of the heterogeneous dual-phase flow environment below the rotor better exposed the complex interplay between the carrier and dispersed phases of the flow. The turbulence characteristics were found to be modified by the presence of the particles in the near-wall region. Using particle flux measurements, the processes of sediment mobilization and pickup from the bed were found to be correlated to the Reynolds stresses and discrete turbulence events, respectively.