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努力，以测量沿海海洋底部边界层（BBL）中的流动结构和湍流，并研究它们对环流，波场和底部地形的依赖性。原位测量已进行粒子图像测速（PIV），它提供了时间序列的2-D速度分布在两个独立的样品区，在前所未有的分辨率为3.5柯尔莫哥洛夫尺度。来自其他传感器的数据，包括ADV，也是可用的。在BBL的内部获得的大型数据集的分析提供的平均速度，雷诺剪切应力，剪切生产和耗散率，能量谱，和丰富的涡流的配置文件。分析表明，在平均流速剖面中出现了一个拐点，表明流动不稳定，该拐点位于平均流速测井层以下，但远高于底部波纹。几个参数，包括扭曲的波诱导的速度，表明这种拐点的发展在当前和薄波边界层（WBL）之间的接口低于it. Scaling平均速度与剪切速度和粗糙度尺度是有效的，只有在拐点以上。相关联的不稳定性表现为剪切生产高峰在更高的海拔比那些在稳定的粗糙壁边界层，以及在小尺度涡的数量迅速增加。后者增加了耗散率并修改了能谱。水流和波浪边界层之间的过渡也涉及到宽的雷诺应力峰值和剪切产生超过耗散率。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WBL内粗糙度的相互作用产生的湍流。最初依赖于现有的数据，并随后在广泛的，但选择性的额外的原位数据，将获得，研究将解决以下问题：（1）拐点，相关的不稳定性和高湍流的持续特征的电流WBL接口？(2)平均海流、波浪诱导运动（振幅、偏移和相对于海流的方向）和海底地形的组合如何影响拐点和相关的雷诺应力、产生率和耗散率？(3)在实验室稳定边界层和冠层流动中，湍流产生的峰值非常接近粗糙元素的界面。问题是，除了拐点峰之外，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流的特征强度、尺度和丰度是什么？它们与海流波底尺度有何关系？它们是否影响了BBL的拐点区域和其余部分的涡旋规模？它们如何影响湍流统计数据？这些涡流与稳定粗糙壁边界层中的结构不同吗？这些不同的含义是什么？为了提供一个有意义的图片流波湍流的相互作用，在沿海BBL，包括上述问题，它是必不可少的，以获得和分析大量的数据库在不同的雷诺数，平均电流波振幅的比率，波相对于平均电流的方向，以及两者相对于底部波纹的方向。因此，在沿着大西洋大陆架的几个地点进行两次实地部署期间，将扩大现有的数据库。这些实验的特点是将一个PIV平面与平均海流对准，另一个平面与波浪或粗糙度对准，以及对当地海底粗糙度进行详细的声学测绘。更广泛的影响：对BBL中的湍流进行适当的建模对于预测海洋环流、气候和天气，以及污染物、营养物和沉积物的输送以及沿海沃茨中相关的生物过程至关重要。波浪、海流和粗糙度的组合使得BBL的建模特别具有挑战性。未来科学家的教育：本项目将资助两名研究生（博士），他们将接受海洋学、流体力学、光学和仪器仪表方面的广泛培训。本科生将继续广泛参与实地考察、后续分析和出版物，这是激励他们从事海洋学职业的行之有效的方法。该团队还将继续从邻近学校（巴尔的摩理工学院）聘请高中生参加为期一年的研究实习项目的长期习俗，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.