Effect of turbulent coherent motion on the onset of particle transport using synchronized 3D PIV and PTV

A series of laboratory experiments was conducted to understand the role of unsteady coherent motions of turbulent flow in the onset of particle motion. Turbulent eddies having coherent structures constantly interact with sediment particles within the bottom boundary layer. Therefore, a 3D turbulent flow field and grain trajectories were obtained by the simultaneous operation of three-dimensional (3D) particle image velocimetry and particle tracking velocimetry. Characteristics of the grain motions were investigated with the various particles in the various flow conditions. Based on the Stokes number of sediments, the spectral density of flow field revealed that turbulent eddies in the production subrange initiate grain motion. The proper orthogonal decomposition extracted the flow fields in the production subrange and the quadrant analysis was used to define dominant turbulent event initiating particle motions in the different flow conditions. The events of outward interaction and sweep initiated particle motion under low-velocity conditions, whereas those of sweep and ejection frequently occurred at the onset of particle motion under high-velocity conditions. Based on the results of the quadrant analysis, we proposed a transition curve that determines the dominant turbulent event depending on the flow condition on the conventional Shields diagram