Coherent structures and their interactions in smooth open channel flows

The study presents experimental results of coherent structures and their interactions in a smooth open channel flow based on measurement of instantaneous two-dimensional velocity vectors with particle image velocimetry. The sampled data were analyzed through techniques of ensemble average, vortex extraction, and proper orthogonal decomposition (POD). Redistribution of turbulent kinetic energy is observed in the near-surface region. The spanwise vortices, which are closely related to hairpin vortices, exhibit a clear dependence on Reynolds number of the flow. Hairpin vortex packets and long streamwise vortices are identified as typical large-scale and super-scale coherent structures, respectively, and their interaction is revealed by examining the relationship between the population density of spanwise vortices and the coefficient functions of the first POD mode. Interactions between large-scale and super-scale structures have been recognized to support the hypothesis of closed-loop feedback cycle.