Interaction between open-channel turbulence and separation bubble in a cavity under a flow with a large water surface deformation

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Open-channel flow with a non-symmetric cavity was investigated in this research. The flow can also be considered as a back-step flow with a cavity of various aspect rations. In the flow situation investigated, the flow shows a critical depth at the inlet of the cavity and a hydraulic jump is created downstream of the cavity. According to the preliminary visual investigation, the hydraulic jump shifts upstream towards the cavity as the aspect ratio of the cavity increases. When the location of the hydraulic jump becomes very close to the cavity, the hydraulic jump suddenly begins to oscillate. This phenomenon can be considered as a result of the interaction between separation vortices and water surface.In order to understand the mechanism of these flow features, image analysis techniques were introduced for measurements of two-dimensional velocity fields in a longitudinal vertical cross-section around a cavity area. A high-speed video camera and a high-resolution video camera were used to capture consecutive flow images of tracer particles visualized by a laser light sheet. Two-dimensional velocity vectors can be obtained by using particle image velocimetry techniques such as PIV or PTV. In this research, PTV was used for a steady flow and PIV for unsteady oscillating hydraulic jump. It was made clear that the energy dissipation becomes large when the length of the cavity is greater than the reattachment length of the inlet flow. In addition, the time-dependent flow characteristics were made clear by a velocity evolution diagram in the case of an oscillating hydraulic jump.