Detection of coherent structures associated with bursting events in an open-channel flow by a two-point measuring technique using two laser-Doppler velocimeters

Large-scale structures in the wall region of an open-channel flow were investigated experimentally using the two-point measuring technique of Komori and Ueda [J. Fluid Mech. 152, 337 (1985)] which used two laser-Doppler velocimeters and a conditional (pattern-) averaging technique. Instantaneous streamwise and vertical velocities were measured simultaneously at two positions—a reference and a movable position—and the spatial and temporal distributions of the fluctuating velocity vectors, Reynolds stress, and turbulent kinetic energy associated with the bursting events were computed using a pattern-averaging technique based on the variable-interval time-averaging method. The results clearly show the time evolution of the coherent structure associated with the bursting events. They also supported the flow visualization results published hitherto, and gave a physical and qualitative basis for them.