Three-Dimensional Coherent Flow Structures during Incipient Particle Motion
Johannes Schobesberger, Petr Lichtneger, Christoph Hauer, Helmut Habersack, Christine Sindelar
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.