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

协作研究：从波流底部边界层沉积物输运的湍流解析数值研究中提取的卢斜层和层化物理

• 批准号: 1130217
• 负责人: Tian-Jian (Tom) Hsu
• 金额: $ 24.31万
• 依托单位: University of Delaware
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-09-15 至 2014-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1130217&HistoricalAwards=false
• 关键词: Collaborative; Res; Physics; lutoclines; laminarization

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.

最近的数值研究表明，存在四种不同的波浪引起的细小沉积物输运模式，从充分混合的输运到卢斜线的形成，以及最终在一系列沉积物可用性和沉降速度上的完全流动层流。该数值模型基于针对细沉积物（小颗粒响应时间）进行简化的欧拉-欧拉两相公式，同时解决了湍流-沉积物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，将根据波雷诺数、体积理查森数（沉积物可用性）和无量纲沉降速度构建完整的流态相图。结果将突出潮汐边界层和波浪边界层对沉积物响应的主要差异。其次，随着对层流开始的更好理解，该模型将得到增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，以确定流动状态和流体动力耗散的转变。第三，平均电流将被添加到模拟中。正如最近在怀阿普河（新西兰）陆架的一项实地研究中所观察到的那样，波流相互作用可能会增加泥浆层的厚度和运输。然而，如果水流太强，沉积物可能会被重新夹带并充分混合，从而阻止重力流的形成。最后，该模型将扩展到粗粮的运输。首先将研究由于波雷诺数减少而导致的粘土和淤泥的同时输送。下一步将是使用矩量直接求积法来模拟完整的多分散系统。多分散模拟工作可以深入了解在泥浆为主的环境中引起所观察到的微地层学的过程。先前对大陆架的几次现场观测揭示了由于波流驱动的沉积物输送而导致的各种海底状态。这些海底状态的出现具有几个重要的意义。例如，卢斜线的形成表明床附近的细沉积物被捕获，由此产生的大密度异常可能会通过波浪支撑的重力流在陆架上产生显着的近海沉积物运输。当表面波在泥质海底传播时，在风暴减弱阶段，由于流体泥浆层层化，通常会观察到较高的波浪耗散率。最近对泥浆沉积物的微观地层学研究表明，由三部分组成的沉积微结构可以与波浪支撑重力流事件期间发生的过程相关联。模拟波浪引起的流体泥浆输送的主要挑战是沉积物与湍流之间的耦合、湍流的过渡性质、流变性和输送的多分散性质。这项研究解决了这些挑战，其结果对于进一步解释在以泥浆为主的沿海环境中观察到的关键过程将很有价值。该项目将提高我们对大陆边缘细小沉积物的再悬浮和输送的理解，这是沉积物从源到汇的研究。这项研究还将提高预测海底表层特性的能力，这对于水下勘探和波浪预测至关重要。本科生将使用现有的波槽，开发一个动手实验室实验，以可视化波边界层的存在以及床附近混合过程的间歇性。这个新设计的实验将用于每个机构每年夏季会议期间举行的工程外展活动。在第三年，该实验将被添加到特拉华大学本科生流体力学实验室的课程中。