海上风力机支撑结构局部冲刷的动力机制及基于流动控制的防冲刷技术研究

The offshore wind farm is vibrant developing in the new century as one emergent industry to provide electric energy, which is one kind of clear and renewable energy sources. But one problem must be severely faced when building the turbine's base, i.e. the oceanic environment identified by tides, waves and winds, especially during Typhoons. The local scouring induced by combined waves and currents decreases the buried depth of the turbine's base with piers, which really threatens the safety of the structures. The present proposal is aimed at seeking the mechanisms of the marine hydrodynamics and the relative sediment dynamics, and further at the potential development of the practical and efficient technology to protect the base from local scouring. The vertical single cylinder is to be taken as the physical model, and the combination of higher Stokes waves and uniform currents are to be adopted as the hydrodynamic forces. The Detached-Eddy Simulation (DES) will be established and used to investigate the turbulence. In respect to turbulent coherent structure research, the DES is much more superior to RANS model. With the DES model, the proposal will be focused on the research of the turbulence mechanism, referring to the horseshoe vortex induced by flow separation in front of the obstacle, and the vortex street in the lee wake. Both of the large eddy structures are the critical key forces for the local scouring around the single cylinder. With the furtherly understanding of the turbulent flows, which controls the transportation of the local sediment, some key dynamic parameters, for example the amplification of the local bed stress around the cylinder, are desired to be discovered and achieved by means of mathematic description. The researches of local scouring are mostly paid on the summary of the scouring depth by means of the physical experiments. Seldom efforts are focused on the dynamics of sediment particles. Compared to the macro description of the sediment motions, the behaviors of single particles are basic and benefit to seek the mechanisms of sediment dynamics. Besides of physical model, the Discrete-Element Model (DEM) will be established to investigate the particle dynamics, which is one technological innovation in the research of local scouring. Both the DES model for the turbulent flows, and the DEM for the single particles motion, are focused on the micro dynamics, and desired to investigate deeply the key hydrodynamic forces and the relative sediment responses. With the insight into the key dynamic force parameters for local scouring, some protect structures are desired to be put forward by means of controlling the turbulence induced by waves and currents.

近海风力机支撑结构在波流海洋环境下遭受局部泥沙淘蚀，减小了桩基入土有效深度，改变了结构振动响应频率，严重威胁结构安全，且海上施工难度大、维护成本较高。本项研究提炼物理模型为直立式单圆柱，采用高阶Stokes波和定常流作为动力条件，发展针对波流耦合场的分离涡（DES）模型，着重探讨波流条件下局部马蹄涡、尾涡等特征涡的演化特征及动力要素，量化局部床面应力放大因子与流动条件的内在关联。对于局部泥沙运动，将发展流固耦合的泥沙颗粒流模型(DEM)，从泥沙运动的细观角度开展研究，探讨水沙耦合作用的内在机制，进而分析总结局部冲刷的宏观描述方法。通过水沙细观运动的深入研究，从控制关键波流水动力要素这一点上探求削弱典型流动的技术手段，为主动防冲、减冲措施的设计提供理论支撑。该项研究不仅具有基础研究的学术价值，且对于海上风电场建设所面临的基础安全问题的解决具有一定的应用价值。

国家自然科学基金《海上风力机支撑结构局部冲刷的动力机制及基于流动控制的防冲刷技术研究》项目执行期自2016年1月至2019年12月，直接研究经费66万元。该项目以海上风力机支撑结构为研究背景，研究波流作用下桩柱局部冲刷及相应的抑制措施。项目的创新点聚焦于自主计算模型的建立及计算软件的研发，进而借助数值模型及物理模型实验，研究桩柱局部泥沙运动的细观物理过程，并探讨促淤减冲的工程措施。. 该项目在带自由表面水流的高精度数值模拟方法及计算软件的研发方面投入了大量的人力、物力。在项目申请人长期以来数值模型研发的基础上，发展了RANS和LES混合的分离涡模型（DES）。研究工作涉及网格生成、模型开发、代码编写、计算程序验证及实际应用。针对混合模型DES存在的“灰区”问题，开发了湍流合成模块，提升了模型性能。在项目执行期间，开展了多项实际应用研究。DES模型应用于浅水环境下沙丘地形条件下的高精度数值模拟，成功模拟了局部地形条件下流动分离、再附着等湍流运动特征，较好的模拟了相关统计量，包括雷诺应力、湍动能能谱等。DES模型应用于水利工程中常见的丁坝绕流数值模拟。针对沿岸斜坡地形条件下系列布置丁坝的局部涡流场进行了研究工作，描述了局部涡系组成及演化特征；描述了坝田区与主流区流动交换；描述了涡流场的空间演化特征。DES模型应用于柱群绕流的模拟，研究了柱群阻力特征、尾流场特征、波浪绕射透射等物理问题。DES模型的成功应用为进一步关于柱体局部流场的高精度数值模拟打下了坚实的基础。.　　颗粒流离散单元模型（DEM）的开发经历了计算软件的框架设计、代码编写、验证的系列工作，基本成型。采用单颗粒自由沉降物理模型实验、颗粒群集聚实验、孤立波与碎石体相互作用的过程观察对模型进行了严格的验证，模拟结果良好。但DEM模型还存在很多有待提高之处，包括非均匀颗粒的流固耦合模式化、极细颗粒的粗化模式化、非圆形大颗粒模拟、流固作用力的完善等方面的研究工作。.项目执行期发表研究论文３０余篇，培养硕士研究生８名（在读２人），博士研究生2名（在读2人）。获得软件著作权２项，申请中３项。.项目经费严格按照预算执行，未出现违规问题。目前尚有部分结余，将用于波流水槽建设及测量设备的购置。