非线性波浪流场有粘和无粘分区数值模拟双向耦合方法研究

Offshore floating platforms are complex engineering systems, during their lifetime of operations these platforms are exposed to various environmental loads such as current, wind and waves. Ensuring platform and occupant safety against those environments is of paramount concern to the designer from the perspective of safety, reliability and longevity. For given design inputs of payloads and environment, typical design spiral of an offshore floater starts with hull sizing to minimize weight and cost under the design constraints dictated by fabrication, transportation, installation and operating conditions and options. The evaluation of the design constraints involves global performance analysis in various environmental conditions by using semi-empirical potential-flow-based motion. The model tests provide design parameters that cannot be derived from the potential-flow-based analytic tools such as wave run-up, air gap, green water, slamming, springing/ringing and vortex induced motion. There exist the discrepancy between prediction and model test results being beyond the adjustment of empirical formula and result into major modification of the hull design, and some cases even change of the design concepts, which lead to considerable delay and increase of project schedule and cost. This worst scenario occurs when unexpected physical phenomena that could not be modeled by the analytic model are observed in the model test. These unexpected physical phenomena are mostly related to nonlinear fluid force and viscous effects...So the motivations to develop real size numerical ocean for floating platform is the hot research topics in ocean engineering field. It is well known that scaled model tests only properly model the gravitational and inertial forces (Froude scale) and not the viscous forces (Reynolds scale). It is the same conclusion for scaled numerical simulation. The available numerical models for strongly nonlinear interactions between water waves and floating platform considering viscous effects are mainly based on solving the Navier–Stokes (NS) equations. So the time step and grid size most be small enough to properly model the real viscous effects and avoiding unwanted numerical damping for nonlinear wave propagation. Even using now days supercomputer, a huge of computational time is still needed. Since NS are computationally complex, it is not feasible to use the viscous flow model in large flow domains where the nonlinear and irregular waves propagate. So efficient numerical method which can reflect the nonlinear wave propagation as well as properly model the viscous flow is very useful. ..In this proposal, it is noticed that far field incoming waves and radiated waves from the floating platforms are mainly controlled by gravitational and inertial forces, the non-viscous flow wave model, such as Green-Naghdi (GN)wave model has achieved low complexity which can simulate nonlinear waves in large domain with high efficiency. The inertial forces and the viscous forces are only important in the near flow field. So the idea to couple the viscous (NS) model with non-viscous(GN) takes advantage of the relatively low computational cost of the GN compared to that of NS, and of the ability to approximate physical flow over wide ranges in the far-field (away from floating platforms) where the effects of viscosity and vortices can be neglected. The key challenge of the coupling between viscous (NS) model non-viscous GN model is properly realize mutual couple of velocity field and pressure filed near the coupling surface.

采用实际尺度海洋平台和实际的风浪流海况进行超越水池实验的深海装备数值模拟实验仍然面临诸多技术挑战。当前，精细考虑粘性效应模拟波浪对于海洋结构作用普遍在全部计算流域采用粘性流动模型。人们为数值造波和消波所开辟的计算流域远大于实际关注的海洋结构物所占据的流域，同时，全流域必须采用极密的网格和极小的时间步长，以保障结构物附近粘流模拟的合理性和远场波浪传播模拟的正确性。如果不采用超级计算机，数值水池的计算时间仍然远远大于实际物理水池的试验时间，提高波浪模拟的计算效率是实尺度海洋数值模拟面临诸多技术挑战中必须克服的困难之一。实际上，远离物体的入射波浪和外传辐射波浪粘性影响甚微，本项目因此提出结构物附近流域采用粘流模型，远离物体的波浪流域采用无粘模型，有粘和无粘分区耦合以实现数值模拟的高效率，具体创新是针对无粘流层析波浪模拟方法和粘流笛卡尔网格模拟方法提出实现双向耦合的高效计算方法并完成验证。

采用实际尺度海洋平台和实际的风浪流海况进行超越水池实验的深海装备数值模拟实验仍然面临诸多技术挑战。当前，精细考虑粘性效应模拟波浪对于海洋结构作用普遍在全部计算流域采用粘性流动模型。人们为数值造波和消波所开辟的计算流域远大于实际关注的海洋结构物所占据的流域，同时，全流域必须采用极密的网格和极小的时间步长，以保障结构物附近粘流模拟的合理性和远场波浪传播模拟的正确性。如果不采用超级计算机，数值水池的计算时间仍然远远大于实际物理水池的试验时间，提高波浪模拟的计算效率是实尺度海洋数值模拟面临诸多技术挑战中必须克服的困难之一。实际上，远离物体的入射波浪和外传辐射波浪粘性影响甚微，本项目因此提出了结构物附近流域采用粘流模型，远离物体的波浪流域采用无粘模型，有粘和无粘分区耦合以实现数值模拟的高效率，具体创新是针对无粘流层析波浪模拟方法和粘流笛卡尔网格模拟方法提出并实现了双向耦合的高效计算方法并完成验证，针对长峰不规则波、短峰不规则波、三维绕射问题、考虑辐射影响的三维波物相互作用问题都开展了验证，结果表明本项目提出的耦合方法大幅提高了计算效率，保证了计算精度，对船舶、海洋平台等结构物水动力预报具有重要意义。