桥墩局部冲刷三维地形及水流结构特性试验研究

本文开展不同水流条件下单、双墩绕流冲刷试验，基于SFM方法实现冲刷地形三维重构，并使用粒子图像测速技术获取桥墩绕流二维流场，定量分析水流结构与冲刷地形之间耦合关系。结果表明：（1）通过运动摄像重构技术可实现冲刷地形三维重构，冲刷坑为不规则倒置圆锥体，模型前方及两侧冲刷深度最大，后方呈凸起形态。（2）随水流强度增大，坑深与坑径均线性增大，坑后沙堆长度与高度均增大，沙丘“尾鳍”夹角减小。冲刷坑面积与体积均随水流强度增大而增大，且随坑深增大呈开口向上抛物线趋势增大。（3）剪切应力下切作用将形成较深冲刷坑，位于桥墩两侧大尺度流向涡向下游延伸，促进冲刷坑后方倾斜凹槽形成。尾涡分离弱化，挟沙能力降低，桥墩下游出现泥沙堆积。

Experimental study on three-dimensional topography and flow structure around bridge piers under local scour

In this study, a laboratory experiment was conducted to examine the behaviors of flow scour around one bridge pier and two bridge piers under different flow conditions. The SFM method was used to achieve three-dimensional reconstruction of the scour pit topography; A PIV system was adopted to measure the two-dimensional flow fields. We also present a quantitative analysis of the coupling relationship between the flow structure and scour topography. The results show (1) Three-dimensional reconstruction of scour topography can be well completed by the SFM method. The scour pit is an irregular inverted cone shape, with the largest depth in front and both sides of the model and a hump shaped sand dune behind it. (2) As flow intensity increases, the depth and diameter of the scour pit are increased linearly toward a parabolic shape, and its area and volume are also increased. The sand dune behind the pit grows in length and height, and its tail fin angle decreases. (3) The deep scour pit can be related to the downcutting by large flow shear. Large-scale streamwise vortices from both sides of the bridge pier extend to the downstream, promoting the formation of inclined grooves behind the pit. The shed vortices are weakened along the flow, and the sediment carrying capacity is reduced, resulting in sediment accumulation downstream of the pier.