Sediment-Bed-Turbulence Coupling in Oscillatory Flows: Fully Resolved Numerical Experiments and Modeling

1133363河流和沿海地区的河流泥沙输送影响了沙丘形成、海滩侵蚀和地貌演变的大规模地貌过程，可能使人类住区流离失所，并破坏植被和农业基础设施，产生强烈的社会经济影响。大型多沙水流的大规模预报计算通常采用简化的模型(I)起动运动和由此产生的推移质输运，以及(Ii)悬浮泥沙输运。这些模型基于基于平均床面剪应力的准稳定模型和基于平流-扩散输运的连续介质模型，忽略了颗粒惯性，在准确预测泥沙侵蚀、夹带和输移方面能力有限。缺乏准确的泥沙起动判据和泥沙起沙函数仍然是发展更好的泥沙运动预报模型的两个最大障碍。主要的研究目标是量化这些相互作用对泥沙开始侵蚀、夹带、悬浮和沉积的影响。与颗粒-颗粒和颗粒-流体相互作用相关的广泛的时空尺度使得在实际尺度上对泥沙运动进行建模极具挑战性。这项研究建立在以下主要假设的基础上：(I)近壁湍流结构的动力学以及由此对泥沙颗粒产生的破坏稳定的阻力和提升力的大小和时间尺度的变化是制定侵蚀开始预测标准的关键；(Ii)泥沙的流态化和沉降引起的颗粒惯性和悬浮泥沙浓度的变化在很大程度上影响湍流边界层外部和内部区域壁面事件的时空演变。建议使用高保真数值模拟和建模来验证这些假设。智力上的优点：这项研究的创新之处在于开发和使用了一种基于第一性原理的全分辨模拟(FRS)方法，不需要阻力和升力模型，用于研究泥沙的起动运动。这项工作将首次提供关于沉积物颗粒上的阻力和升力大小的时间变化、与这些变化相关的时间尺度以及它们与湍流边界层中的扫掠暴发事件的关系的数据。我们将揭示由扫掠爆发湍流事件引起的阻力和升力的时间变化对初始运动的影响，确定床面剪切力(Shield准则)、局部加速度(Sleath参数)和冲量(Diplas‘s概念)对侵蚀开始的作用。FRS的研究改变了颗粒大小、流动雷诺数和体积负荷，也将产生流动参数，以便基于连续介质方法开发更好的侵蚀、冲刷、输送和沉积的分析和工程预测。这些由全分辨数值试验获得的流动参数有可能改变目前在实际尺度上模拟泥沙运动的实践。广泛影响：这些研究方法和模型适用于其他几个流体-颗粒系统，如生物应用中的流体-结构相互作用和微型空气飞行器中的扑翼、多孔介质中的惯性流动、氧煤燃烧等。这项研究将产生一个数值水/风隧道(NWT)，这是一个流体-颗粒系统的虚拟教育工具。西北大学的几个方面将被纳入研究生课程和大学荣誉学院(UHC)的课程。在正在进行的K-12外联活动期间，将为当地学校教师举办培训讲习班，并向学生演示以模拟为基础的工程和科学在河流和沿海沉积物预测中的作用。

1133363ApteSediment transport, in rivers and coastal regions, affects large-scale geomorphic processes of dune formation, beach erosion and landform evolution that can displace human settlements, as well as destroy vegetation and agricultural infrastructure with strong socio-economic impact. Large-scale predictive computations of large-scale sediment-laden flows typically employ simplistic models for (i) incipient motion and resultant the bedload transport, and (ii) suspended sediment transport. These models are based on quasi-steady models based on averaged bed-shear stress for erosion onset and advective-diffusive transport based continuum models, neglecting particle inertia and are limited in their ability to accurately predict sediment erosion, entrainment and transport. Lack of accurate criteria for onset of incipient motion and sediment pickup function remain two of the biggest hurdles in developing better predictive models for sediment transport.A numerical investigation of sediment-bed-turbulence interactions is proposed in oscillatory turbulent boundary layers representative of coastal environments. The primary research objective is to quantify the effect of these interactions on the onset of erosion, entrainment, suspension, and deposition of sediments. The broad range of spatio-temporal scales associated with particle-particle and particle-fluid interactions makes modeling of sediment transport at practical scales extremely challenging. The proposed research builds upon the following main hypotheses: (i) the dynamics of near-wall turbulence structure and resultant variations in the magnitudes and time-scales of the destabilizing drag and lift forces on sediment grains are critical in formulating predictive criteria for onset of erosion, and (ii) particle inertia and variations in suspended sediment concentration due to fluidization and settling of sediments substantially affects the spatio-temporal evolution of the wall-events in the outer as well as inner regions of a turbulent boundary layer. Use of high-fidelity numerical simulations and modeling are proposed to test these hypotheses.Intellectual Merit: The novelty of this research is in the development and use of a fully resolved simulation (FRS) approach based on first principles, without requiring models for drag and lift forces, for the study of sediment incipient motion. This work will, for the first time, provide data on the temporal variations in the magnitude of drag and lift forces on sediment grains, the time-scales associated with these variations, and their correlation to the sweep-burst events in turbulent boundary layers. Effect of temporal variations in drag and lift forces, due to the sweep-burst turbulence events, on incipient motion will be revealed, identifying the roles of bed shear stress (Shield's criterion) and local accelerations (Sleath's parameter), and impulse (Diplas' concept) on onset of erosion. The FRS studies varying the grain size, flow Reynolds number, and volume loading, will also yield flow parameterizations for development of better analytical and engineering predictions of erosion, scour, transport, and deposits based on continuum approaches. These flow parameterizations obtained from fully resolved numerical experiments have the potential to transform current practices in modeling sediment transport on practical scales.Broader Impact: The research methodologies and models are applicable to several other fluid-particle systems, such as fluid-structure interactions in biological applications and flapping wings in micro-air vehicles, inertial flow through porous media, oxy-coal combustion, among others. This research will yield a Numerical Water/Wind Tunnel (NWT), a virtual educational tool for fluid-particle systems. Several aspects of the NWT will be integrated into graduate courses and University Honors College (UHC) curriculum. Training workshops for local school teachers and demonstrations to students will be held on the role of simulation-based engineering and science in predictions of river and coastal sediments during the ongoing K-12 outreach activities.