Collaborative Research: Combined Waves and Currents over Multi-Scale Topography: From Boundary Layer Dynamics to Parameterization

多尺度海底地形是海岸体系中普遍存在的现象。表面波和流与这种地形的相互作用导致压力、速度和湍流产生的空间模式，从而导致底部阻力、驱动混合和耗散波能。在大多数波浪和环流研究中，这些过程没有得到解决，而是以总体摩擦参数来表示，这些参数通常是根据经验得出的。波浪流在多尺度地形上的动力学还不是很清楚，目前还没有根据多尺度地形特性先验计算波浪和水流的摩擦参数的方法。该项目将研究表面波和流与多尺度海底地形的相互作用，研究控制动力学，并利用理想化和自然珊瑚礁地形上的流动模拟和理论发展，开发适当的海底摩擦参数化方案。将讨论对阻力、波浪消散和混合的影响，从而改善对珊瑚礁和类似系统的理解和建模。该项目将通过PIs参与Moorea珊瑚礁LTER，为跨学科的珊瑚礁工作提供信息，并支持一名职业早期PI、一名博士后和一名博士后，并为六名本科生提供研究经验。项目组还将制定和实施新的K-12外联和教育活动。在以前的工作中，PI研究了地形上的边界层动力学，其特征是在典型的珊瑚礁参数范围内具有单一的长度尺度。这些结果以及对珊瑚礁地形的分析表明，一系列地形长度尺度(厘米-米)可能对海底摩擦有很大贡献。这个项目将研究不同的地形长度尺度如何在多尺度地形中共同作用，以确定组合的水流和振荡流的动力学、对水流的总阻力和波浪能量的耗散。这项工作将以单尺度地形的动力机制为指导，通过进行一系列计算流体力学模拟来研究跨越不同机制(惯性、阻力、应力主导)的多尺度地形上的组合波和流的物理特性，其中关键参数(波浪特性、水流、地形长度尺度尺度特性)是系统变化的。模拟将包括由离散长度尺度叠加组成的地形、具有连续长度尺度范围的表面以及珊瑚礁地形。模拟(使用带有LES闭合的OpenFOAM)将把流型分解到粗糙度元素尺度。模拟结果将采用空间和波系综平均的Navier-Stokes框架，以分析粗糙元尺度过程对振荡和定常流动动力学的影响，并将从波浪和水流中损失能量的机制进行量化。这些分析将为多尺度地形上的波浪消散和水流阻力的新参数化奠定基础，这些参数代表较小尺度的动力学，并可被纳入波浪和环流模型。这一裁决反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Multiscale bottom topography is ubiquitous in coastal systems. Interactions of surface waves and currents with this topography cause spatial patterns in pressure, velocities, and turbulence production that result in bottom drag, drive mixing, and dissipate wave energy. In most wave and circulation studies these processes are not resolved and are represented in bulk friction parameters, typically derived empirically. Dynamics of wavy flows over multiscale topography are not well understood and there is currently no method for computing friction parameters for waves and currents a priori from multiscale topography properties. This project will examine interactions of surface waves and currents with multiscale bottom topography, to investigate the controlling dynamics and develop appropriate bottom friction parameterization schemes, using modeling of flow over idealized and natural coral reef topography together with theoretical development. Implications for drag, wave dissipation and mixing will be addressed yielding improved understanding and modeling of reefs and similar systems. The project will inform interdisciplinary coral reef work through the PIs’ involvement with the Moorea Coral Reef LTER, and support an early career PI, a post-doc and a PhD student, and provide six undergraduates with research experiences. The project team will also develop and implement new K-12 outreach and education activities. In previous work, the PIs investigated boundary layer dynamics over topography characterized by a single length scale across a parameter range typical of reefs. Those results, along with analyses of reef topography, show that a range of topography length scales (cm - m) are likely to contribute substantially to bottom friction. This project will investigate how different topography length scales act together in multiscale topography to determine dynamics of the combined current and oscillatory flow, total drag on currents, and dissipation of wave energy. Using dynamical regimes for single-scale topography as a guide, the work will investigate the physics of combined waves and currents over multiscale topographies spanning different regimes (inertia-, drag-, stress-dominated) by conducting a series of computational fluid dynamics simulations in which key parameters (wave properties, current, topography length scales scaling properties) are systematically varied. Simulations will include topographies composed of superposed discrete length scales, surfaces with a continuous range of length scales, and reef topographies. Simulations (using OpenFOAM with LES closure) will resolve flow patterns down to roughness element scales. A spatially- and wave- ensemble-averaged Navier-Stokes framework will be applied to simulation results to analyze effects of roughness-element-scale processes on oscillatory and steady flow dynamics, and mechanisms by which energy is lost from waves and current will be quantified. These analyses will form the basis for new parameterizations for wave dissipation and drag on currents over multiscale topography that represent smaller-scale dynamics and can be incorporated into wave and circulation models.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.