Turbulence in the Inner Part of a Combined Wave-Current Coastal Bottom Boundary Layer

该项目扩展了同一团队的工作，测量了沿海海洋底部边界层的流动结构和湍流，并研究了它们对环流、波场和海底地形的依赖关系。使用粒子图像测速仪(PIV)进行了现场测量，它提供了两个独立样本区的二维速度分布的时间序列，分辨率达到前所未有的3.5科尔莫戈罗夫尺度。来自其他传感器的数据，包括ADV，也可以获得。对边界层内部获得的大数据集的分析提供了平均速度、雷诺切应力、剪切产生率和耗散率、能谱和涡旋丰度的轮廓。分析表明，在平均流速剖面上出现拐点，这表明水流不稳定，拐点在平均水流对数层以下，但远高于底部波纹。包括波浪诱导速度的扭曲在内的几个论点表明，这种拐点是在海流与其下方较薄的波边界层(WBL)的交界处发展的。用剪切速度和粗糙度标度平均速度仅在拐点以上有效。相关的不稳定性表现为在比稳定的粗糙壁边界层高得多的海拔处出现切变生产峰值，以及小尺度涡旋数量的迅速增加。后者提高了耗散率，改变了能谱。水流边界层和波浪边界层之间的过渡还涉及到宽广的雷诺应力峰值和超过耗散率的剪切产生。本项目的重点是阐明拐点附近的流动和湍流，以及由流动和波浪与粗糙度相互作用产生的湍流。首先依赖现有的数据，然后根据将获得的大量但有选择性的现场数据，该研究将解决以下问题：(1)海流-WBL界面的拐点、相关的不稳定性和高湍流是否具有持久的特征？(2)平均流、波浪诱导的运动(相对于水流的幅度、漂移和方向)和海底地形的组合如何影响拐点及其相关的雷诺应力、生产和耗散率？(3)在实验室稳定的边界层和冠层流动中，湍流的生产峰值非常接近于带有粗糙元素的界面。问题是，除了拐点峰值外，这种界面湍流产生峰是否也存在于WBL内。(4)与粗糙度相互作用产生的涡旋的特征强度、尺度和丰度是什么？它们与当前波底尺度有何关系？它们有没有影响大气层的拐点区域和其他区域的涡旋规模？它们对湍流统计数据有何影响？这些涡旋与填充在稳定粗糙壁面边界层中的结构是否不同？这些不同的含义是什么？为了提供关于沿岸边界层中流-波-湍流相互作用的有意义的图像，包括上述问题，有必要获得并分析在不同雷诺数、平均流与波幅比、波相对于平均流的方向以及两者相对于底部波纹的方向下的大量数据库。因此，在大西洋大陆架沿线的几个地点进行两次实地部署期间，现有数据库将得到扩展。这些实验将包括一个PIV平面与平均流、另一个平面与波浪或粗糙度的对准以及对局部底部粗糙度的详细声学测绘。广泛影响：对边界层中的湍流进行适当的模拟对于预测海洋环流、气候和天气以及沿海水域中污染物、营养物质和沉积物的传输以及相关的生物过程至关重要。海浪、海流和粗糙度的结合使得对边界边界层的建模特别具有挑战性。分析由最先进的仪器获得的数据是开发建模工具的关键一步。未来科学家的教育：该项目将支持两名研究生(博士)，他们将接受海洋学、流体力学、光学和仪器方面的广泛培训。本科生将继续广泛参与实地考察、随后的分析和出版，作为激励他们追求海洋学职业的一种行之有效的手段。该团队还将继续保持邀请邻近学校(巴尔的摩理工学院)的高中生参加为期一年的研究实践项目的长期习惯，这是他们必修课程的一部分。

This project extends efforts by the same team to measure the flow structure and turbulence in the bottom boundary layer (BBL) of the coastal ocean, and study their dependence on circulation, wave field and bottom topography. In-situ measurements have been performed using Particle Image Velocimetry (PIV), which provides time series of 2-D velocity distributions in two independent sample areas, at an unprecedented resolution of 3.5 Kolmogorov scales. Data from other sensors, including an ADV, are also available. Analysis of large datasets obtained in the inner part of the BBL provides profiles of mean velocity, Reynolds shear stress, shear production and dissipation rates, energy spectra, and abundance of eddies. Analysis shows that an inflection point develops in the mean velocity profile, which indicates flow instability, below the mean current log layer, but well above the bottom ripples. Several arguments, including distortion of wave induced velocity, suggest that this inflection develops at the interface between current and thinner wave boundary layer (WBL) below it. Scaling of mean velocity with shear velocity and roughness scales is effective only above the inflection point. Associated instabilities are manifested by a shear production peak at much higher elevations than those in steady rough-wall boundary layers, as well as a rapid increase in the number of small-scale eddies. The latter increases the dissipation rate and modifies the energy spectra. Transition between current and wave boundary layers also involves broad Reynolds stress peaks and shear production exceeding the dissipation rate.This project focuses on elucidating the flow and turbulence near the inflection point, as well as on turbulence generated by interactions of flow and waves with roughness within the WBL. Relying initially on available data, and subsequently on extensive, but selective additional in situ data that will be obtained, the study will address the following questions: (1) Are the inflection, associated instabilities and high turbulence persistent characteristic features of current-WBL interfaces? (2) How are the inflection and associated Reynolds stresses, production and dissipation rates affected by combinations of mean current, wave-induced motion (amplitude, excursion and direction relative to current) and bottom topography? (3) In laboratory steady boundary layers and in canopy flows, turbulence production peaks very near the interface with roughness elements. The question is whether such an interfacial turbulence production peak exists also within the WBL, in addition to the inflection point peak. (4) What are the characteristic strength, scale and abundance of eddies generated by interactions with roughness? How are they related to the current-wave-bottom scales? Do they affect the scale of eddies populating the inflection area and rest of BBL? How do they affect the turbulence statistics? Do these eddies differ from structures populating steady rough wall boundary layers, and what is the implication of these differences?In order to provide a meaningful picture on current-waves-turbulence interactions in the coastal BBL, including the above questions, it is essential to obtain and analyze a substantial database at varying Reynolds numbers, ratios of mean current to wave amplitude, orientation of waves relative to mean current, and orientation of both relative to bottom ripples. Consequently, the available database will be extended during two field deployments in several sites along the Atlantic Continental Shelf. These experiments will feature alignment of one PIV plane with mean current and the other one with waves or roughness as well detailed acoustic mapping of the local bottom roughness.Broader impact: Proper modeling of turbulence in the BBL is essential for predictions of oceanic circulation, climate and weather, as well as transport of pollutants, nutrients and sediment and associated biological processes in coastal waters. The combination of waves, currents and roughness makes modeling of the BBL particularly challenging. Analysis of data obtained by state-of-the-art instruments is an essential step in development of modeling tools.Education of future Scientists: This project will support two graduate (PhD) students, who will be broadly trained in oceanography, fluid mechanics, optics and instrumentation. Undergraduate students will continue to be involved extensively in field trips, subsequent analysis and publications, as a proven means of motivating them to pursue careers in oceanography. The team will also continue the long-term custom of engaging senior high-school students from a neighboring school (Baltimore Polytechnic) in a yearlong, research practicum project, which is part of their required curriculum.