Three-Dimensional Coherent Flow Structures during Incipient Particle Motion

Abstract

In this study, 3D coherent flow structures during incipient particle entrainment under unidirectional flow conditions were experimentally investigated. Time-resolved tomographic particle tracking velocimetry (TOMO-PTV) and the shake the box (STB) algorithm were applied to measure 3D-velocities in a hydraulic laboratory flume. Quadrant and octant analyses were performed, their probabilities of occurrence were determined, and the Q-criterion was used to visualize the dominant 3D-flow structures during particle dislodgement. The results showed that octants related to sweeps and ejections were dominant with a probability of 70%. Fifty ms prior to particle entrainment, only one single contributing octant was observed. At particle entrainment, a sweep, three retrograde vortices, and a prograde vortex paired with a retrograde vortex were identified in the flow field. Considering the whole measurement volume, a group of three hairpin-like structures passed the particle during entrainment. In conclusion, the TOMO-PTV method is well suited and enables novel insights on the interaction between coherent structures and incipient sediment motion.