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

本文对具有非对称空腔的明渠水流进行了研究。流动也可以被认为是带有不同展弦比的空腔的后退流动。在所研究的流动情况下，流动在空穴的入口处显示出一个临界深度，并且在空穴的下游产生了一个水力跃迁。根据初步的目测结果，随着空腔高宽比的增大，水跃向上游向空腔移动。当水跃的位置变得非常接近空腔时，水跃突然开始振荡。这一现象可以认为是分离涡与水面相互作用的结果，为了了解这些流动特征的机理，引入了图像分析技术来测量空洞区周围纵向垂直截面上的二维速度场。利用高速摄像机和高分辨率摄像机对示踪剂颗粒在激光光源下的连续流动图像进行了采集。利用PIV或PTV等粒子图像测速技术可以获得二维速度矢量。在本研究中，PTV用于恒定流，PIV用于非恒定振荡水跃。结果表明，当空泡长度大于入口气流的再附着长度时，能量耗散变大。此外，在振荡水跃的情况下，速度演化图清楚地显示了水流的时变特性。

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.