Control of Coherent Vortex Shedding from Separated Open Channel Flow.

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The turbulent structure of the backward-facing step flow, including the reverse flow region, in open-channel flows was investigated by making use of two sets of two-component Laser Doppler anemometers (LDA). The turbulence characteristics and reattachment properties were revealed. The time-averaged reattachment length was dependent on both the Reynolds number and Froude number in open-channel flows. In particular, it should be noted that the reattachment length became smaller in supercritical flows than in subcritical flows. The instantaneous reattachment point moved over a distance of the time averaged reattachment length. This suggested strongly that a coherent structure of the kolk-boil vortex might be generated due to the unsteady and low-frequency motions of the reattachment point.The structures of dynamic pressure and shear stress were analyzed on the basis of the momentum equation. These calculated values coincided well with the experimental values measured by the present LDA sy … More stem. The separated step flow and its recirculation in open-channel flows were similar to that in boundary-layer and duct step flows. But, the former was more complicated than the latter, because the pressure was more relaxed by the existence of the free surface.Next, the spectral analyses of pressure and velocity fluctuations revealed that a coherent low-frequency motion existed near the reattachment of the step. In order to reveal the coherent structures in the separated shear layer of the step flows and also in the downstream of the reattachment point, the space-time correlation measurements of the velocity-pressure and velocity-velocity combinations were conducted by using two sets of the Laser Doppler anemometers and the pressure transducer. Quasi-periodic trains of vortical structure were generated due to the Kelvin-Helmholtz instability in the same manner as in the mixing layer, and they were convected toward the reattachment point. The possibility was discussed that these mixing-layer type coherent vortices might trigger a shedding of the large-scale horseshoe vortex from the reattachment point in open-channel flows. Then, this horseshoe vortex could have developed up to the free surface and become boil vortex, which was observed in actual rivers and estuaries. Less