Understanding Brownout using Near-Wall Dual-Phase Flow Measurements

The two-phase flow environment produced by 1- and 2-bladed laboratory-scale rotors operating inground- effect over a mobile sediment bed was studied. Time-resolved flow visualization, particle image velocimetry, and particle tracking velocimetry were used. The high imaging rate of these systems allowed the time-history of the rotor wake interactions with the bed to be documented, providing a better understanding of the transient mechanisms that lead to the uplift of sediment and the formation of dust clouds. Three size ranges of characterized glass microspheres were used to represent the sediment particles. The near-wall measurements exposed at least six fundamental uplift and sediment transport mechanisms below the rotor: 1. Creep, 2. Modified saltation and saltation bombardment, 3. Unsteady pressure loading effects, 4. Vortex induced trapping, 5. Reingestion bombardment (local and global), and 6. Secondary suspension. The highest sediment entrainment levels occurred within the wake impingement zone, mainly from the action of the blade tip vortices on the bed. Once entrained, significant quantities of sediment were intermittently trapped in the vortex-induced upwash field. Secondary sediment suspension was more prevalent with the 2-bladed rotor because of the merging of adjacent blade tip vortices and higher upwash velocities. The reingestion of the suspended particles into the vortex flow was also observed, often causing bombardment ejection of sediment from the underlying bed.