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

采用图像分析方法，对床面高程突变的明渠水流湍流特性进行了实验研究。在本研究中，有或没有有前向台阶的明渠水流被处理。在具有前向台阶的情况下，改变台阶的标高，以便在通道的中间形成非对称的凹形部分，我们称之为沟槽部分。在单个后向台阶的情况下，对比了有无沙粒和粗糙度的湍流流动；在沟槽情况下，发现在狭小的沟槽纵横比范围内，沟槽断面内产生了一个振荡的水跃。一开始，这种现象被认为是上游台阶分离涡与水面相互作用的结果。然而，这种周期振荡是由于在一定的水流条件下，在前向台阶下游产生的平均速度和波速之间的不平衡造成的。利用图像分析技术，即粒子跟踪测速技术(PTV)和粒子图像测速技术(PIV)，对沟槽内的湍流特性进行了研究。为此，使用了高速摄像机和高分辨率摄像机。结果表明，消能效果的提高与沟槽段的湍动程度密切相关。对于后向台阶的情况，研究发现，当床层被沙粒粗糙时，重新附着的长度缩短了约20%-这是一个在其他地方从未报道过的新发现。

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.