Mechanisms of Sediment Entrainment and Transport in Rotorcraft Brownout

Abstract

To advance the understanding of the phenomenon of rotorcraft brownout, a dual-phase flow environment induced by a small-scale 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 and tracking software, permitted an understanding of the temporal evolution of the rotor wake in ground effect simultaneously with the processes of sediment entrainment and transport by the rotor flow. High-resolution near-wall PIV measurements showed that large excursions in the surface boundary layer were produced by the convecting rotor wake vortices. These excursions acted to suppress an equilibrium state in the boundary layer within the zone of vortex impingement on the ground. The highest sediment entrainment levels were observed to occur within this impingement zone, which can be attributed to the increase in groundwash and wall shear produced beneath the vortices. Once entrained, significant quantities of sediment were then trapped and locally suspended by the vortex-induced upwash field. This effect resulted in a noticeable level of intermittency in the initial vertical transport of sediment from the ground. The ground and upwash flow velocities were shown to strengthen significantly during the viscous merging of adjacent wake vortices. This mechanism proved fundamental in defining the concentration of suspended sediment, as well as the maximum height to which sediment could be transported. Sediment particles reaching sufficient heights were observed to recirculate into the rotor wake, and convect back towards the ground at a high speed. This process caused sediment ejection by means of bombardment or "splash." The classical process of saltation bombardment was also visualized for larger particles whose inertia prevented them from being suspended in the vortical flow. While providing new insight into the time- and length-scales associated with sediment transport 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.