Investigation of Sediment Entrainment in Brownout Using High-Speed Particle Image Velocimetry

To advance the understanding of the brownout phenomenon, the dual-phase flow environment induced by a rotor hovering above a sediment bed was studied using high-speed flow visualization and particle image velocimetry (PIV). The high frame rate of the camera, combined with advanced particle recognition software, permitted an understanding of the temporal evolution of the rotor wake in ground effect, simultaneously with the processes of sediment uplift. High-resolution PIV measurements in the surface boundary layer showed large excursions in the ground shear produced by the convecting wake vortices, these excursions being correlated with localized, intermittent increases in sediment entrainment rates. Once entrained, significant quantities of sediment were trapped and vertically transported by the vortexinduced upwash field. Large sediment particles were often spun out of the vortical flow, and proceeded in a modified saltation trajectory. In particular, the surface and upwash velocities were shown to strengthen significantly during the viscous merging of adjacent wake vortices. This mechanism proved fundamental in defining the concentration of entrained sediment, and also the maximum height to which sediment could be transported. Particles reaching sufficient heights were observed to recirculate into the rotor wake, and convect back towards the ground, causing sediment ejection through the process of reingestion bombardment. While providing new insight into the time- and length-scales associated with sediment entrainment by a rotor wake, the observations made here also bring into question the validity of equilibrium particle flux models currently being used for brownout simulations.