基于SFM方法的不同倾角桥墩绕流局部冲刷特性试验

桥墩绕流冲刷是水流、泥沙和桥墩三者相互作用的结果，桥墩周围复杂的水沙运动关系是造成桥梁水毁的重要原因。定量分析桥墩冲刷地形特征规律及其与水流之间的相互作用关系，是深入研究桥墩绕流水沙作用机理及实际工程应用的重要突破口。本文开展两种坡降条件下不同倾角均匀沙动床桥墩绕流冲刷试验，并沿下游方向设置四个模型倾角0°、5°、10°、15°。使用粒子图像测速系统（Particle Image Velocimetry，PIV）测量桥墩绕流二维流场，并基于运动摄像恢复结构技术（Structure From Motion，SFM）实现冲刷地形三维重构，在此基础上分析床面冲刷三维地形结构和绕流流场特征，以构建紊流结构与冲刷地形相互耦合作用关系。结果表明：（1）SFM方法可实现冲刷地形三维结构重构，冲刷试验平衡时，模型前方和两侧冲刷坑较深，后方冲刷坑出现凸起，沿水流方向倾斜顺延上升至床面。（2）冲刷坑尺寸、面积和体积均随水流强度增大而增大，随倾角增大而减小。不同截面处冲刷坑面积、体积随坑深呈开口向上抛物线趋势增大。（3）桥墩模型对后方流向流速扰动范围随倾角增大而减小，对展向流速影响范围随倾角增大而增大。（4）随模型倾角增加，旋转强度与剪切应力影响范围均减小。剪切应力下切较易形成桥墩周围较深冲刷坑，而位于桥墩两侧大尺度流向涡向下游延伸，将促使桥墩后侧方浅长凹槽形成。

Experiment of Local Scour Characteristics of Flow Around Bridge Piers with Different Inclination Angles Based on SFM

The flow scour around bridge piers is the result of the interaction among the flow, sediment and the pier. The complex relationship between water and sediment movement around bridge piers is an important cause of bridge water damage. Quantitative analysis of scour topographic characteristics of bridge piers and its interaction with the flow is an important breakthrough in deeply understanding of the mechanism of the flow and sediment interaction around bridge piers and its practical engineering application. In this paper, the flow scour experiment around bridge piers of uniform sediment moving bed was carried out with different inclination angles under two kinds of slope conditions, and four model inclination angles of 0°, 5°, 10° and 15° were set along the downstream direction. The PIV system was employed to measure the two-dimensional flow field around bridge piers, and the SFM method was used to achieve the three-dimensional reconstruction of scour topography. Based on this, the characteristics of three-dimensional topography structure of bed surface scour and flow field were analyzed, together to construct the coupling relationship between the two. The results show that: (1) The SFM method can be applied to reconstruct the three-dimensional structure of scour topography. As the scour experiment is balanced, the scour depth in front and both sides of the model is deeper relatively, and the rear is convex, rising to the bed surface along the flow direction. (2) The size, area and volume of the scour hole increase with the increase of the flow intensity, and decrease with the increase of the inclination angle. As the depth of scour hole increases, the cross section area and three dimension morphology increase in parabola with opening upward. (3) As the inclination angle of the bridge pier model increases, the disturbance region of the rear streamwise velocity decreases and the influence region on the spanwise velocity increases. (4) As the model inclination angle increases, influence regions of swirling-strength and shear stress decrease. The deep scour hole around bridge piers can be easily formed by the flow shear stress, and the large-scale streamwise vorticity on both sides of the bridge pier extends downstream, which leads to the formation of shallow long grooves on the back two sides of the bridge pier.