基于三维同步测量的墩柱周围水沙作用机理研究

The local scour around piers is due to the sediment transport under the action of three-dimensional and unsteady flow structures. It is a major cause for the failure of underwater structures such as bridges. Since the complex flow and sediment motions can hardly be measured synchronously with conventional techniques, our current understanding of the mechanism of flow-sediment interactions is not deep enough to propose well-performed scour prediction formula and mathematical models. In this study, a stereoscopic particle image velocimetry (SPIV) system is developed to synchronously measure the flow and sediment motions around a cylinder pier. Measurements are conducted separately for the clear-water flow and the sediment-laden flow in an open channel with either a flat bed or a scour hole. The experimental results for the two flows are used to analyze the correlations between flow structures and the bed shear stress, and between flow structures and sediment motions, respectively. The former correlation identifies the dominant flow structure in the scour process and characteristics indicating its scour capacity. The latter correlation establishes the dynamical importance of flow structures in generating bed shear stress. Finally, comparison between the two correlations reveals the representation degree of the bed shear stress for the sediment transport capacity of local flow. The achievements are of great importance for improving the performance of both scour prediction formula and numerical simulation models.

墩柱周围的局部冲刷是三维、非稳态水流结构与泥沙相互作用的结果，是引起桥梁等涉水建筑坍塌的重要原因。由于常规的测量方法难以同时对水流和泥沙开展三维瞬态测量，目前关于墩柱周围水流对泥沙输运机理的理解还不够深入，不利于构建预测性能良好的局部冲刷计算公式和数学模型。本项目通过研发体视粒子图像测速（SPIV）技术，提出水沙同步测量方法，实现对水流和泥沙运动的三维、同步观测。在此基础上，开展相同水流和地形条件下的清水及输沙圆柱绕流实验，分别测得墩柱周围的三维流场和水沙同步运动场。通过建立水流结构与泥沙运动以及床面剪切力之间的相关关系，识别出在墩柱周围局部冲刷过程中起主导作用的水流结构，得到表征该水流结构冲刷能力的的特征参数；建立床面剪切力与水流结构之间的动力关系，分析床面剪切力对墩柱周围水流输沙能力的表征程度。研究成果可为完善局部冲刷计算公式和数学模型提供理论和试验依据，具有重要科学意义和实用价值。

桥梁局部冲刷是水流、床沙、结构物三者相互作用的结果，是引起桥梁损毁事故的主要原因，准确预测桥梁在洪水作用下的冲刷特征，依赖于对墩柱周围复杂水沙作用机理的深入认识。本项目以圆柱绕流及其导致的河床局部冲刷现象为研究对象，依次完成了高频体视粒子图像测速（SPIV）系统研发、水沙三维同步测量方法研究，开展了平整床面及局部冲刷条件下的圆柱绕流模型实验和圆柱上游水流结构与泥沙运动作用机理分析。项目取得的主要进展和成果包括：对SPIV技术进行了较为全面的总结和评价；通过硬件设计、制造、集成和软件开发，研发成功了一套具有自主知识产权的高频SPIV系统；深入分析了粒子图像时间间隔对SPIV技术测量精度的影响，提出了开展SPIV测量时应当遵循的基本准则；基于粒子图像分割、双目视觉三维重构和三维粒子示踪测速技术，实现了水流流场与泥沙颗粒运动的三维同步测量；发现圆柱上游马蹄涡系结构在动态变化过程中主要表现为零流、回流和过渡三种模态特征；进一步完善了对马蹄涡系统在来流大尺度相干结构作用下沿流向大幅度振荡的机理认识，从统计上揭示了马蹄涡的垂向位置、尺寸及强度等主要特征随着流向振荡而发生趋势性演化的规律；获得了圆柱上游马蹄涡结构和下降流与床面剪切应力之间的相关函数，发现两种水流结构对起始冲刷阶段的墩前床沙具有同等重要的动力作用。项目的技术研发成果在水动力学实验量测领域具有推广应用价值，理论研究成果有助于深入理解圆柱绕流这一普遍而又复杂的流动现象，可为构建更加合理的墩柱周围水沙动力作用模型提供理论支撑。