A novel integrated approach to efficiently model viscous effects on wave-structure interaction in extreme sea

许多用于开发海洋油气资源、利用海洋可再生波浪能、潮流能和海上风能的海洋结构物已经或将要设计和运行。在这些结构的设计过程中，必须考虑它们在可能遇到的最坏情况（极端海况）下的响应。在这种情况下，破碎波的冲击和粘性效应被广泛认为是重要的。这些因素使设计中常用的频域线性或二阶波浪绕射分析失效。然而，具有对波浪冲击进行建模的能力的计算流体动力学（CFD）工具可能需要几天或几周的时间才能为所需的大海域中的结构的响应产生可靠的结果，所述海域的尺寸在3D中为数十或数百波长的水平并且对于许多波浪周期。基于完全非线性势理论（FNPT）的替代工具具有相对较高的计算效率，例如准任意拉格朗日欧拉有限元方法（QALE-FEM）可以在一夜之间完成模拟。然而，它们不能处理破碎波的冲击，并考虑粘性/效应。因此，如何有效地模拟粘性/湍流以及与波浪-结构物相互作用相关的破碎波冲击仍然是海洋工程和海洋工程的关键挑战。本项目将通过开发一种新的方法来有效地模拟大范围三维极端波浪与海洋结构物之间的相互作用，并考虑粘性/湍流效应和破碎波冲击，从而开展研究以应对挑战。新方法通过将CFD工具和基于FNPT的方法集成在一种方法中，从而发挥了两者的优势。预计其计算效率与基于FNPT的QALE-FEM相似，即在现代PC上在一夜之间模拟所需的大型3D海域中具有粘性和波浪破碎的波浪-结构相互作用。这一新的发展使得在时域内模拟大型浮式结构物在极端波浪作用下的运动成为可能，从而得到更符合实际的结果。结果表明，所提出的方法的可行性和有前途的功能。

Many offshore structures for exploiting oil/gas in ocean and for harnessing marine renewable wave energy, tidal current energy and offshore wind energy have been and will be designed and operated. During the design of these structures, it is essential to consider their responses in the worst situation possibly met(extreme sea). In such situation, the breaking wave impact and the viscous effects are widely recognized to be important. These factors disqualified the well-established linear or second-order wave diffraction analysis based in the frequency domain which has been usually used during the design. However, the Computational Fluid Dynamics (CFD) tools with ability to model the wave impact and viscously may take several days or weeks to produce reliable results for the response of structures in a required large sea area with dimensions at the level of tens or hundreds of wavelengths in 3-D and for many wave periods. Alternative tools based on the fully nonlinear potential theory (FNPT) have relatively higher computational efficiency, e.g. the Quasi Arbitrary Lagrangian Eulerian Finite Element method (QALE-FEM) may complete the simulation within an overnight. However, they cannot deal with the breaking wave impact and take the viscous/ effects into account. Therefore, how to efficiently model viscosity/turbulence and the breaking wave impact associated with wave-structure interaction remains to be a key challenge in offshore and marine engineering. This project will carry out the research to tackle the challenge by developing a novel approach to efficiently model the interaction between large-domain 3D extreme waves and the offshore structures with consideration of viscous/turbulent effects and breaking wave impact. The new method takes the advantage of the CFD tools and the FNPT based methods by integrating them in a single approach. It is expected to have the computational efficiency at a similar level to the FNPT based QALE-FEM , i.e. simulating wave-structure interaction with viscosity and wave breaking in a required large 3D sea area on modern PCs within an overnight. The new development may make it possible to simulate large floating structures subjected to extreme waves in time domain and so give more realistic results.A preliminary test has been carried out. The results demonstrate the feasibility and the promising features of the proposed approach.