跨海桥梁大尺度基础波浪荷载计算方法对比研究∗

A Comparative Study on Wave Load Calculation Methods Regarding to Large-scale Foundation for Sea-crossing Bridge

摘要:

波浪荷载是跨海桥梁基础的主要环境荷载之一。为了探究大尺度桥梁基础上各类波浪荷载计算方法的差异，分别采用了当前研究中较常用的 Morison 方程、规范、基于边界元法的三维绕射理论和基于 Navier?Stokes 方程的 CFD 方法计算了某跨海桥梁大尺度圆端型沉井基础在规则波作用下的波浪荷载，研究了不同结构相对尺度对各方法计算结果的影响，并重点比较分析了后两种数值方法的三维流场分布，三维压力场分布，以及非线性二阶力效应的影响。结果表明，在结构相对尺度为 0.2 左右时，上述方法的计算结果接近，随着结构相对尺度的增加，边界元与 CFD 方法依然吻合较好，规范方法给出了较为保守的波浪荷载，Morison 方程可能高估了波浪拖曳力的贡献。边界元与 CFD 方法计算得到的速度场与压力场结果整体趋势一致，但在沉井前端自由液面与物面，沉井侧面的波谷等局部区域会产生明显差异，这可能是由于二者考虑绕射效应与黏性效应的理论不同。在边界元方法中考虑非线性二阶力效应会使其波浪荷载结果小幅度增大，并在频谱中增加差频二阶力及和频二阶力成分，考虑非线性二阶力效应后，边界元与 CFD 方法的频谱曲线更加吻合，边界元方法能更精确地计算波浪荷载。边界元与 CFD 方法均能合理地给出大尺度结构物波浪荷载结果，研究结论可为跨海桥梁工程波浪荷载的准确计算提供了依据。

Abstract:

Wave load is one of the main environmental loads on the foundation of sea-crossing bridges. In order to investigate the difference between various wave load calculation methods on large-scale bridge foundations， the Morison equation， the code， the three-dimensional diffraction theory based on the boundary element method and the CFD method based on the Navier Stokes equation are used to calculate the wave load on a large-scale round ended caisson foundation of a sea-crossing bridge， respectively. The influence of the relative scale of the structure on the results of each method is studied. The distribution of three-dimensional flow field and pressure field of the latter two numerical methods is compared in detail and the influence of the nonlinear second-order force effect is focused on. It is found that when the relative scale of the structure is about 0.2， the results of the above method are similar. With the increase of the relative scale， the boundary element method is still in good agreement with the CFD method， while the code gives a more conservative wave load， and the Morison equation may overestimate the contribution of the wave drag force. The results of flow the field and pressure field obtained by the boundary element method and CFD method are consistent in trend， but there are obvious differences in local areas such as free surface and object surface at the front of the open caisson and wave trough at the side of the open caisson， which may be due to the difference between the two theories that consider the diffraction effect and the viscosity effect， respectively. The second-order force effect will increase the wave load slightly and add the difference frequency and sum frequency components into the frequency spectrum. After considering the nonlinear second-order force effect， the frequency spectrum curve of the boundary element method is more consistent with that of the CFD method， and the boundary element method can calculate the wave load more accurately. Both the boundary element method and CFD method can reasonably give the wave load results of large-scale structures. The results provide a basis for the accurate calculation of wave load on sea-crossing bridge engineering.