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

多尺度海底地形在海岸系统中普遍存在。表面波和海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，而是以通常凭经验得出的体摩擦参数表示。多尺度地形上波浪流的动力学还没有得到很好的理解，目前还没有方法从多尺度地形特性中先验地计算波浪和水流的摩擦参数。该项目将研究表面波和海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水流模型以及理论发展，制定适当的海底摩擦参数化方案。阻力，波耗散和混合的影响将得到解决，提高理解和珊瑚礁和类似系统的建模。该项目将通过PI参与Moorea Coral Reef LTER为跨学科珊瑚礁工作提供信息，并支持早期职业PI，博士后和博士生，并为六名本科生提供研究经验。项目团队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调查边界层动力学地形特征的一个单一的长度尺度在一个典型的珊瑚礁的参数范围。这些结果，沿着对礁地形的分析，表明一系列地形长度尺度（厘米-米）可能对底摩擦产生重大影响。本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，以确定组合电流和振荡流的动力学，电流的总阻力和波能的耗散。使用单尺度地形的动力学机制作为指导，这项工作将通过进行一系列计算流体动力学模拟，系统地改变关键参数（波浪特性，电流，地形长度尺度缩放特性），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浪和电流的物理特性。模拟将包括由叠加的离散长度尺度组成的地形、具有连续范围的长度尺度的表面和礁石地形。模拟（使用OpenFOAM与LES封闭）将解决流态下到粗糙度元素尺度。空间和波系综平均的Navier-Stokes框架将应用于模拟结果，以分析粗糙度元素尺度过程对振荡和定常流动动力学的影响，并量化波浪和水流能量损失的机制。这些分析将成为多尺度地形上波浪耗散和水流阻力的新参数化的基础，这些地形代表较小尺度的动力学，并可纳入波浪和环流模型。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估来支持。

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.