About Computational Modeling of Structures Submerged in Flows in Sub-Grid Scale

ABSTRACT In the computational modeling of water environments there are cases where slender vertical structures, like bridge piers, are part of the modeling domain. In this work, four strategies for modeling the presence of bridge piers were studied: Mesh Refinement (MR), Nodal Island (NI), Additional Stress Term (AST) and Increasing Bottom Roughness (IBR). For each method, advantages and disadvantages were discussed. AST was the most favorable of the techniques studied due to the fact that it drastically decreases the time of computation, as well as providing the possibility to choose the drag coefficient according to the pier geometry and hydrodynamic condition. Comparing the results of longitudinal centerline velocity profiles, it has been observed that the AST method provides similar results to the RM method from a distance of 100 m downstream of the pier. For this reason, the research concludes that the AST method is appropriate to evaluate the flow distant from the structures, outside the near wake. This work presents a sensibility study of the pier drag coefficient to make the reader aware of the influence of this parameter.

摘要：在水环境数值模拟中，常会将细长竖向结构物（如桥墩）纳入模拟域范畴。本研究针对桥墩建模的四类方法展开系统研究：网格细化法（Mesh Refinement, MR）、节点岛法（Nodal Island, NI）、附加应力项法（Additional Stress Term, AST）以及床面糙率增大法（Increasing Bottom Roughness, IBR），并逐一剖析了各方法的优势与局限。在所探讨的四类技术中，附加应力项法表现最优，因其可大幅缩减计算时长，同时还能根据桥墩几何形态与水动力条件灵活选取拖曳系数（drag coefficient）。通过对比纵向中心线流速剖面（longitudinal centerline velocity profiles）的模拟结果，发现在桥墩下游100 m处，附加应力项法的计算结果与网格细化法高度相近。基于此，本研究得出结论：附加应力项法适用于评估结构物远场（近尾流区（near wake）之外）的水流运动。此外，本文针对桥墩拖曳系数开展了敏感性分析，以帮助读者明晰该参数对模拟结果的影响。