Collaborative Res: Physics of lutoclines and laminarization extracted from turbulence-resolved numerical investigations on sediment transport in wave-current bottom boundary layer

最近的数值研究表明，存在四种不同的波致细泥沙输运机制，从充分混合输运到卢倾斜的形成，最后在一定的泥沙可利用性和沉降速度范围内形成完全的流层化。该数值模型基于简化了细泥沙（小颗粒响应时间）的欧拉-欧拉两相公式，同时解决了湍流-泥沙相互作用的所有尺度。该项目将通过数值模拟进一步研究与这些制度相关的四个关键科学问题。首先，通过一系列精心设计的模拟，构建以波雷诺数、体积理查德森数（沉积物有效性）和无因次沉降速度为函数的流态的完整相图。结果将突出潮汐和波浪边界层对沉积物的响应之间的主要差异。其次，随着对层压化开始的更好理解，该模型将得到加强，以支持非牛顿流变学，以便研究流变应力和湍流调制之间的相互作用，从而确定流动形式的转变和水动力耗散。第三，将平均电流加入到仿真中。最近在新西兰怀亚普河陆架的实地研究发现，波流相互作用可能会增加泥层厚度和运移。然而，如果水流太强，沉积物会被重新夹带并充分混合，从而阻止重力流的形成。最后，该模型将扩展到更粗颗粒的运输。首先将研究由于波雷诺数减小而引起的粘土和粉土的同步输运。下一步将是用直接正交矩法模拟一个完整的多分散系统。多分散模拟工作使我们能够深入了解在以泥质为主的环境中导致观察到的微地层的过程。先前对大陆架的几次实地观测揭示了波浪流驱动的沉积物输运导致的各种海底状态。这些海底状态的出现有几个关键的含义。例如，卢托斜的形成表明在床附近捕获了细沉积物，由此产生的大密度异常可能通过波浪支撑的重力流在陆架上产生显著的近海沉积物运输。当表面波在泥泞的海床上传播时，在风暴减弱阶段，由于流体泥浆层变得分层，通常观察到高波耗散率。最近对泥沉积的微地层学研究表明，一个由三部分组成的沉积微结构可能与波浪支撑重力流事件期间发生的过程有关。模拟波浪引起的流体-泥浆输运的主要挑战是泥沙与湍流之间的耦合、湍流的过渡性质、流变性和输运的多分散性。本研究解决了这些挑战，其结果将有助于进一步解释在以淤泥为主的沿海环境中观察到的关键过程。该项目将提高我们对大陆边缘细粒沉积物的再悬浮和输送的认识，这是沉积物源汇研究的关键因素。该研究还将提高海底表层性质的预测能力，这对水下勘探和海浪预测至关重要。利用已有的波浪槽，本科生将进行一项动手实验，以可视化波浪边界层的存在和床附近混合过程的间歇性。这个新设计的实验将用于每年在每个机构的夏季会议期间进行的工程推广活动。在第三年，这个实验将被添加到特拉华大学本科流体力学实验室的课程中。

Recent numerical investigations reveal the existence of four distinct regimes of wave-induced fine sediment transport ranging from well-mixed transport, to the formation of a lutocline, and eventually a complete flow laminarization over a range of sediment availabilities and settling velocities. The numerical model is based on an Eulerian-Eulerian two-phase formulation simplified for fine sediment (small particle response time) while resolving all the scales of turbulence-sediment interactions. This project will further investigate four critical science issues related to these regimes via numerical simulations. Firstly, a complete phase map will be constructed of flow regimes as a function of wave Reynolds number, bulk Richardson number (sediment availability) and nondimensional settling velocity with a series of carefully designed simulations. The results will highlight the major differences between the tidal and wave boundary layers in response to sediments. Secondly, with a better understanding of the onset of laminarization, the model will be enhanced to support non-Newtonian rheology in order to study the interplay between rheological stress and turbulence modulation in determining the transition of flow regimes and hydrodynamic dissipation. Thirdly, mean current will be added to the simulations. Wave-current interaction may enhance the mud layer thickness and transport, as observed in a recent field study at the shelf of Waiapu River (New Zealand). However, if the current is too strong, sediments can be re-entrained and become well-mixed and hence the formation of gravity flow is prevented. Finally, the model will be expanded for transport of coarser grains. Concurrent transport of clay and silt due to decreasing wave Reynolds number will be first studied. Next step will be to simulate a complete polydispersed system using a direct quadrature method of moments approach. Polydispersed simulation efforts allow insights into the processes causing the observed microstratigraphy in mud-dominant environments.Several prior field observations on continental shelves reveal a variety of seabed states due to wave-current driven sediment transport. The occurrences of these seabed states have several critical implications. For example, the formation of a lutocline indicates trapping of fine sediments near the bed and the resulting large density anomaly may yield significant offshore sediment transport on the shelf through wave-supported gravity flows. When surface waves propagate over a muddy seabed, high wave dissipation rate is often observed during the waning stage of a storm as the fluid mud layer becomes laminarized. A recent microstratigraphy study of mud deposits suggests a three-part sedimentary microfabric that can be associated by processes occur during wave-supported gravity flow events. The main challenges of modeling wave-induced fluid mud transport are the coupling between sediment and turbulence, the transitional nature of turbulent flow, rheology and the polydispersed nature of transport. This research addresses these challenges and the results will be valuable in further interpreting critical processes observed in the mud-dominant coastal environment.The project will improve our understanding of the resuspension and delivery of fine sediment across the continental margin, which is a critical element of the sediment source to sink study. This study will also improve the ability to predict the surface layer properties of the seabed which is critical to underwater exploration and wave prediction. Using wave tanks already available, a hands-on laboratory experiment to visualize the existence of wave boundary layer and the intermittent nature of the mixing process near the bed will be developed by undergraduate students. This newly-designed experiment will be used in Engineering outreach activities taking place annually during the summer session of each institution. In Year 3, this experiment will be added to the curriculum in the undergraduate fluid mechanics laboratory at U. Delaware.