分层强剪切内波环境中柱体失稳机理研究

The pile foundations in harbors and bridge piers in estuaries are affected by the unsteady, stratified and strong shear environment. The instability mechanism of the foundations is extremely complicated as the instability is closely related with the hydrodynamic characteristics of environmental flows as well as the foundation scouring and silting. In the present research, the stability problem of piles in off-shore regions and estuaries will be generalized as a problem of pile stress state, foundation scouring and silting in strong shear stratified flows under the internal wave in density stratified flows. A mathematical internal wave model in near shore regions and estuaries will be established based on the laboratory experiments, numerical simulations, in-situ monitoring and theoretical analysis. The generation, propagation and degeneration of internal waves, the corresponding impacts on cylinders, as well as the foundation scouring and silting will be systematically studied to reveal the instability mechanisms of cylinders in strong shear stratified flows. In the further study, the southern branch of the Yangtze estuary with seasonal stratification characteristics will be set to be a typical study region. A coupling model of internal wave, hydrodynamics, sediment and piles will be developed and verified with the in-situ salinity, sediment, hydrodynamic and geographic data. The stable conditions and instability mechanisms for cylinders under the multi-factor coupling situations will be explored and the relationship between the instability of cylinders and the environmental factors, such as the shear intensity, will be concluded. The research results have great significance on the improvement of the safety and stability for piles in coastal and estuarine constructions, and could provide theoretical support to the amendments of relevant design specifications and industrial standards.

海港码头桩基及河口桥梁墩柱受非恒定、层化、强剪切环境流体作用，其稳定性与环境流体水动力特性及基础冲淤密切相关，失稳机理极为复杂。本项目拟运用海洋密度分层流研究中的内波理论，将近岸及河口桩柱稳定问题概化为分层强剪切内波环境中桩柱受力及基础冲淤问题，采用物理模型试验、数值模拟计算、现场监测和理论分析相结合的方法，对近岸及河口内波数学模型的建立，内波产生、传输、消亡过程，内波对柱体的作用规律及对柱基河床的冲淤效应进行系统研究，揭示层化强剪切环境流体中柱体失稳机理。在此基础上，以具有季节性分层特性的长江口南支为典型区域，建立内波-水动力-泥沙-结构桩柱耦合模拟模型，借助盐度、含沙量、水动力及地形监测资料，率定验证数值模型，探索多因素耦合作用下柱体稳定条件及失稳机理，提出剪切强度等环境要素与柱体失稳的响应关系。成果对提高近岸及河口建筑物桩柱安全有重要意义，可为相关设计规范与行业标准的修订提供支持。

项目背景及研究内容：海港码头桩基及河口桥梁墩柱受非恒定、层化、强剪切环境流体作用，其稳定性与环境流体水动力特性及基础冲刷密切相关。运用海洋密度分层流研究中的内波理论，将近岸及河口桩柱稳定问题概化为分层剪切内波环境中桩柱受力及基础冲淤问题，采用物理模型试验、数值模拟计算、理论分析相结合的方法，对内波数学模型的建立，内波的产生、传输、消亡过程，内波对柱体的作用规律及对柱基河床的冲刷效应进行系统研究，揭示层化强剪切环境流体中柱体失稳机理。在此基础上，依托典型分层区域建立内波水动力模拟模型，探索实尺度分层环境下内波的剪切效应，成果为提高近岸及河口建筑物桩柱安全提供支持。.重要结果、关键数据：特定分层环境中，内孤立波与柱体间的相互作用力随柱型、内波波幅、上下层水深比等要素的不同而不同。静水环境中内波引起方柱和圆柱的无量纲水平作用力系数分别达到0.45、0.22，方柱受力受波幅影响更为敏感。基于总水深的无量纲波幅在0.025-0.075时，方柱与圆柱的无量纲水平作用力系数之比约为1.4-1.75。柱体受力系数与内波波幅呈正相关，但与上下层水深比呈负相关。上凸与下凹型内波与单柱及双柱相互作用时，引起的上下层水体的紊动特性及强度均不同，研究得到了不同波幅条件下扰动强弱转化临界无量纲间距的分布规律、影响因素、拟合公式。研发了沉积物颗粒输运模型，实现了流体与颗粒物相互作用的“负载颗粒流动的四向耦合”算法，据此研究并揭示了内波产生的近底流场对底质运移的影响。本项研究获得了内波造床及与柱体相互作用的特性与规律。.科学意义：（1）定量描述分层环境中内波与不同底地形相互作用过程中能量损耗规律，以及能量分布与底床泥沙输移的关系，揭示了内波的造床作用。（2）不同特性内波对柱体剪切力的空间分布及随时间的周期性演化规律，揭示了柱体破坏特性与失稳机制。