高低刃脚异形钢围堰水力作用数值研究

建造桥梁深水桥墩多采用围堰施工，作用于围堰的水荷载是控制围堰结构设计及其下沉施工安全性和稳定性的关键因素，当前设计规范对围堰水力作用的考虑十分粗略。为探明某桥墩高低刃脚异形钢围堰施工过程中的水荷载特征，采用计算流体动力学方法分析围堰绕流流场，基于河道横断面地形资料足尺构建三维流体域，运用重叠网格技术高效建模模拟钢围堰下沉过程。研究结果表明，围堰两侧存在局部流动加速且呈不对称分布，围堰深水侧水流速度相对更大，围堰下方下游侧流速随河流深度增加呈降低趋势；围堰外表面压强除迎流侧为正压外，其余大部分均为负压，各区域负压随着下沉深度增加而逐步增大；下沉初期绕流场受围堰及护筒双重影响，随着围堰逐渐下沉至河床底，护筒影响逐步消失；流场各区域湍流强度随着下沉深度的增加整体呈现出先增大后减小的变化规律，最终最大湍流强度区域稳定于堰尾。围堰阻力系数及侧向力系数几乎不受来流流速影响，进一步验证了其无量纲性；阻力及侧向力随来流流速增大而增大；阻力随入水深度增大而增大，侧向力也整体表现出类似规律，但在入水深度8 m处受围堰异形区段三维流动效应影响，出现侧向力局部极小值的反常现象。

A Numerical Study on Hydrodynamic Forces of Abnormal Shape Steel Cofferdams with High-low Blade

Cofferdams are often used in the construction of deep water piers of bridges. The hydrodynamic load acting on cofferdams is a key factor to control the structural design of cofferdams and to insure the safety and stability of their subsidence process. The existing design specifications consider the hydrodynamic forces on cofferdams roughly to some degree. In order to understand the characteristics of hydrodynamic load during the construction of a special shaped steel cofferdam with high-low blade for bridge pier, the flow field around cofferdam is simulated by computational fluid dynamics (CFD) method. A threedimensional fluid domain is constructed based on the cross-sectional terrain data of the river channel. The subsidence process of steel cofferdam is simulated by overlapping grid technique efficiently. Results show that there is a partial acceleration of flow on both sides of the cofferdam, and the distribution of this acceleration effect is asymmetrical. The flow velocity of the deep water side of the cofferdam is relatively greater, and below the cofferdam the flow velocity of the downstream side decreases gradually with the increasing river depth. The external surface pressure on cofferdams are mostly negative, compared to the positive pressure at the upstream face. With the increase in subsidence depth, the negative pressure in each area increases gradually. In the initial stage, the water flow is influenced by the cofferdam and the protection tube. As the cofferdam gradually sinks to the riverbed, the influence of the protection tube gradually disappeared. Along with the increasing subsidence depth, the turbulence intensity in each region of the flow field shows an overall trend of increasing at first and then decreasing. Finally the maximum turbulence intensity area stabilizes at the wake of the cofferdam. The coefficients of drag and lateral force do not change with the incoming flow speed, which depends on its dimensionless property. The drag and lateral force increase with the increasing of incoming flow velocity, as well as the increasing underwater penetration. However, under the influence of the three-dimensional flow effect, the lateral force shows a local minimum when the sunk depth is about 8 m.