Study on the effect of the turbulent bursting phenomenon on sediment entrainment using 3D particle image velocimetry and particle tracking velocimetry

Analyzing the cause of sediment entrainment is the most fundamental knowledge to understand the various morphodynamic phenomenon in a natural river. Earlier researchers proposed that the sediment starts to move when the magnitude of the momentum of flow acting on the sediment particle exceeds a certain threshold value. However, recent studies revealed that the sediment is transported by the instantaneous turbulent motions within the boundary layer, making the threshold disappeared. Such motions are generally created by the turbulent bursting phenomenon where a sequence of cyclic events repeatedly occurs and are in charge of creating large momentum near the bed. In order to examine the effect of the turbulent bursting phenomenon on sediment entrainment, this study performed laboratory experiments based on optical measurement techniques. 3D particle image velocimetry and particle tracking velocimetry were applied to measure the velocity field and to identify the sediment movement, respectively. Octant analysis and proper orthogonal decomposition were applied for the measured velocity field to find the dominant turbulent motion inducing sediment entrainment. Finally, the variation of the pressure field was analyzed when the sediment starts to move.