Open-Channel Turbulence Structure of Flow with Separation and Large Free-Surface Deformation

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Turbulence properties of an open-channel flow with a sudden change of bed elevation were experimentally investigated Using an image analysis method. In the present study, open-channel flows having a backward-facing step with or without a forward-facing step were treated. In the case with a forward-facing step, the elevation of the step was varied so that a non-symmetric concavity section was, formed in the middle of the channel, which we called a trench section. In the case of a single backward-facing step, turbulent flows with or without sand, roughness were investigated for comparison.In the trench case, it was found that an oscillatory hydraulic jump is generated within the trench section for a narrow range of the trench aspect ratio. In $he beginning, this phenomenon was considered as a result of the interaction between separation vortices from the upstream step and water surface. However, it was made clear that the periodic oscillation was caused by the imbalance between the mean velocity and the speed of wave created just downstream of the forward-facing step for a certain flow condition. The turbulence properties within the trench section were clarified using the image analysis ,techniques referred to as the Particle Tracking Velocimetry (PTV) and the Particle Image Velocimetry (PIV). A high-speed video camera and a high resolution video camera were utilized for these purposes. It was made clear that the increase of energy dissipation is closely related to the level of turbulence in the trench section. For the case of a backward-facing step, it was found that the reattachment length becomes about twenty percent shorter when the bed was roughened by sand elements, -which is a new finding never reported, elsewhere.