Connecting Flow over Complex Terrain to Hydrodynamic Roughness on a Coral Reef
Connecting Flow over Complex Terrain to Hydrodynamic Roughness on a Coral Reef
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DOI：
10.1175/jpo-d-18-0013.1
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发表时间：
2018-07-01
影响因子：
3.5
通讯作者：
Monismith, Stephen G.
中科院分区：
文献类型：
--
作者：
Rogers, Justin S.;Maticka, Samantha A.;Monismith, Stephen G.
复杂地形上的水流会对底部产生压力，导致阻力、湍流和边界层的形成。但是，尽管水动力学粗糙度z（0）在预测流动和混合方面很重要，但人们对它与复杂地形的关系知之甚少。为了解决这一差距，我们进行了广泛的实地观测流量和水深测量的精细尺度使用流体透镜技术在奥福岛，美属萨摩亚的浅珊瑚礁。我们开发了一个有效的厘米尺度的非静力学模型的珊瑚礁，和阻力的结果比较好的观测。总阻力是由压力差产生的形状阻力引起的，并且仅是相对深度和空间平均流向斜率的函数，与k-型粗糙度的缩放一致，其中k是粗糙度高度，是边界层厚度。我们将复杂的礁表面近似为波浪形底形的叠加，并提出了一种简单的方法，用于从水深表面的深度和流向斜率的空间均方根和线性系数a（1）预测z（0），类似于波浪形底形的其他研究结果。虽然当地的速度分布变化很大，水平平均是一致的日志层近似。精确计算形状阻力所需的模型网格分辨率是主导水平水动力尺度的O（10-50）倍，主导水平水动力尺度由流向斜率谱中的峰值确定。在这项研究中采取的方法可能适用于其他复杂的地形，可以探索其他设置。
Flow over complex terrain causes stress on the bottom leading to drag, turbulence, and formation of a boundary layer. But despite the importance of the hydrodynamic roughness scale z(0) in predicting flows and mixing, little is known about its connection to complex terrain. To address this gap, we conducted extensive field observations of flows and finescale measurements of bathymetry using fluid-lensing techniques over a shallow coral reef on Ofu, American Samoa. We developed a validated centimeter-scale nonhydrostatic hydrodynamic model of the reef, and the results for drag compare well with the observations. The total drag is caused by pressure differences creating form drag and is only a function of relative depth and spatially averaged streamwise slope, consistent with scaling for k--type roughness, where k is the roughness height and is the boundary layer thickness. We approximate the complex reef surface as a superposition of wavy bedforms and present a simple method for predicting z(0) from the spatial root-mean-square of depth and streamwise slope of the bathymetric surface and a linear coefficient a(1), similar to results from other studies on wavy bedforms. While the local velocity profiles vary widely, the horizontal average is consistent with a log-layer approximation. The model grid resolution required to accurately compute the form drag is O(10-50) times the dominant horizontal hydrodynamic scale, which is determined by a peak in the spectra of the streamwise slope. The approach taken in this study is likely applicable to other complex terrains and could be explored for other settings.