Collaborative Research: Internal waves impinging on near-critical slopes: multiscale numerical quantification of localized mixing and exchange with the interior

合作研究：撞击近临界斜坡的内波：局部混合和与内部交换的多尺度数值量化

• 批准号: 0825997
• 负责人: Alberto Scotti
• 金额: $ 20.77万
• 依托单位: University of North Carolina at Chapel Hill
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-09-01 至 2013-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0825997&HistoricalAwards=false
• 关键词: Collaborative; Research; Internal; waves; impinging

The project will pursue a numerical investigation of boundary processes driven by internal waves and tides incident on a sloping bottom. In particular, the project will give a quantitative assessment of the hypothesis that nonlinear breakdown of internal waves at critical and near-critical slopes results in mixed fluid that then disperses along isopycnals to the interior, a problem of fundamental importance to diagnose the global ocean circulation. There is indirect support of this hypothesis through observations of intermediate nepheloid layers in the vicinity of near-critical slopes as well as direct evidence through laboratory experiments, albeit at low Reynolds number. Accurate estimates of local dissipation rates and turbulent transport coefficients at these ?hot spots? will also be provided by the simulations. There is mounting evidence through field observations that bottom boundary processes are important to the exchange of fluid between slopes and the interior, to energy pathways in the ocean, the suspension and settling of sediment, and to local biological productivity. It is thus necessary to improve our understanding and our ability to numerically predict the internal wave/boundary interaction problem and a multi-scale approach, with potentially transformative outcomes, will bridge the large internal wave scales to the bottom boundary layer (BBL). Realistic simulations of bottom mixing processes have not been possible so far because of the scale disparity between the incident wave field and the turbulence, the high Reynolds number of pre-existing boundary layers as well as those that form in the case of critical slope, and the high numerical dissipation used for stability in current ocean models. The scale-separation problem is handled here through a novel hierarchical use of analytical or numerical solutions for the large-scale problem coupled to a large eddy simulation (LES) of bottom mixing. To provide the large-scale field when analytical theories are not available, we will employ a new baroclinic model based on Adaptive Mesh Refinement (AMR). A realistic description of small-scale processes will be obtained here by employing LES with a sophisticated sub-grid model. This sub-grid model has a scale-similarity component to account for deviations from isotropic, inertial-range behavior, it is not unduly dissipative, it does not need tunable coefficients, and it has a near-wall component to account for the turbulence at the boundary layer roughness scale. The AMR and LES will be two-way coupled. The AMR (when needed) will drive the LES, while the latter will provide approximated boundary conditions for the former allowing communication of boundary mixing into the interior. The simulations will be analyzed for BBL properties, phasing of local mixing rates, lateral dispersal of BL mixed fluid, and characteristics of the nonlinearly scattered wave. Functional relationships to the properties of the incoming wave and to the slope angle will be extracted. Comparisons with laboratory and field data will be performed. Numerical large-scale models at the regional or global level are asked to provide increasingly precise evaluations of oceanic impact on climate change, depend on parameterizations for boundary fluxes. The AMR and LES simulations proposed here will allow a careful assessment of bottom turbulence dynamics which could then be used to develop an accurate representation of the bottom mixing component of large-scale models. Mixing along slopes affects several characteristics of the shelf-edge/slope area, which in turn control the exchange of nutrients and pollutants between the boundary and the open ocean. On a broader horizon, mixing at hot spots on shelf slopes is a critical ingredient in the energy balance of the internal wave field. Two PhD students will be trained across the fields of oceanography, engineering and computational science. Undergraduate students will be given the opportunity to participate in the research.

该项目将对内波和潮汐入射到斜底的边界过程进行数值研究。特别是，该项目将对以下假设进行定量评估：在临界和接近临界的斜坡上，内波的非线性破裂导致混合流体，然后沿着等轴线向内陆扩散，这是诊断全球海洋环流的一个基本重要问题。通过观察近临界斜坡附近的中间云状层，以及实验室实验的直接证据，尽管雷诺数较低，这一假说得到了间接支持。这些热点的局部耗散率和湍流输送系数的准确估计？也将由模拟提供。通过实地观察，有越来越多的证据表明，底边界过程对于斜坡和内陆之间的流体交换、海洋中的能量途径、沉积物的悬浮和沉降以及当地的生物生产力都是重要的。因此，有必要提高我们对内波/边界相互作用问题的理解和数值预报的能力，而一种具有潜在变革性结果的多尺度方法将把大的内波尺度连接到底部边界层(BBL)。到目前为止，由于入射波场和湍流之间的尺度差异、预先存在的边界层以及在临界坡度情况下形成的边界层的雷诺数较高，以及当前海洋模式中用于稳定的高数值耗散，对海底混合过程的真实模拟是不可能的。在这里，尺度分离问题是通过一种新的层次化的方法来处理的，即大规模问题的解析解或数值解与底部混合的大涡模拟(LES)相耦合。为了在分析理论不可用的情况下提供大尺度场，我们将采用一种新的基于自适应网格加密(AMR)的斜压模式。在这里，通过采用具有复杂的子网格模型的大涡模拟，将获得对小规模过程的真实描述。该子网格模式有一个尺度相似分量来解释各向同性惯性范围行为的偏差，它不是过度耗散的，它不需要可调系数，它有一个近壁分量来考虑边界层粗糙度尺度上的湍流。AMR和LES将是双向耦合的。AMR(在需要时)将驱动LES，而后者将为前者提供近似的边界条件，从而允许将边界混合传递到内部。模拟将分析边界层性质、局部混合率的相位、边界层混合流体的横向弥散以及非线性散射波的特征。将提取入射波特性和斜率角之间的函数关系。将与实验室和现场数据进行比较。区域或全球一级的大型数值模式被要求对海洋对气候变化的影响提供越来越精确的评估，这取决于边界通量的参数设置。这里提出的AMR和LES模拟将允许对底部湍流动力学进行仔细的评估，然后可以用来开发大尺度模式的底部混合分量的准确表示。沿斜坡的混合影响陆架边缘/斜坡区域的几个特征，进而控制边界和开阔海洋之间的营养物质和污染物的交换。在更广阔的地平线上，陆架斜坡热点处的混合是内波场能量平衡的关键因素。两名博士生将接受海洋学、工程学和计算科学领域的培训。本科生将有机会参与这项研究。