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

组合波流沿岸底部边界层内部的湍流

• 批准号: 1031040
• 负责人: Joseph Katz
• 金额: $ 51.75万
• 依托单位: Johns Hopkins University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-09-01 至 2013-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1031040&HistoricalAwards=false
• 关键词: Turbulence; Inner; Part; Combined; Wave

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.

该项目扩展了同一团队的工作，测量沿海海洋底部边界层（BBL）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。使用粒子图像测速 (PIV) 进行了原位测量，它以前所未有的 3.5 柯尔莫哥洛夫尺度分辨率提供两个独立样本区域中二维速度分布的时间序列。来自其他传感器（包括 ADV）的数据也可用。对 BBL 内部获得的大型数据集的分析提供了平均速度、雷诺剪切应力、剪切产生和耗散速率、能谱和涡流丰度的剖面。分析表明，平均速度剖面中出现了一个拐点，这表明流动不稳定，该拐点位于平均电流测井层下方，但远高于底部波纹。包括波感生速度畸变在内的一些论点表明，这种变形发生在水流与其下方较薄的波边界层 (WBL) 之间的界面处。使用剪切速度和粗糙度比例缩放平均速度仅在拐点以上才有效。相关的不稳定性表现为比稳定的粗糙壁边界层高得多的剪切产生峰值，以及小规模涡流数量的快速增加。后者增加了耗散率并改变了能谱。水流和波浪边界层之间的过渡还涉及宽雷诺应力峰值和超过耗散率的剪切产生。该项目的重点是阐明拐点附近的流动和湍流，以及 WBL 内流动和波浪与粗糙度相互作用所产生的湍流。最初依靠现有数据，随后依靠将获得的广泛但有选择性的额外现场数据，该研究将解决以下问题：（1）当前 WBL 界面的拐点、相关不稳定性和高湍流持续特征是否存在？ (2) 平均水流、波浪引起的运动（相对于水流的振幅、偏移和方向）和底部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中，湍流产生的峰值非常靠近粗糙度元素的界面。问题是，除了拐点峰值之外，WBL 内是否也存在这样的界面湍流产生峰值。 (4) 与粗糙度相互作用产生的涡流的特征强度、规模和丰度是多少？它们与当前波底规模有何关系？它们会影响拐点区域和 BBL 其余部分的涡流规模吗？它们如何影响湍流统计？这些涡流与稳定粗糙壁边界层中的结构不同吗？这些差异的含义是什么？为了提供关于沿海 BBL 中的海流-波浪-湍流相互作用的有意义的图片，包括上述问题，有必要获取和分析不同雷诺数、平均海流与波幅之比、波浪相对于平均海流的方向以及两者相对于底部的方向的大量数据库。 涟漪。因此，在大西洋大陆架沿线几个地点的两次现场部署期间，可用数据库将得到扩展。这些实验的特点是一个 PIV 平面与平均水流对齐，另一个 PIV 平面与波浪或粗糙度对齐，以及局部底部粗糙度的详细声学映射。更广泛的影响：BBL 中湍流的正确建模对于预测海洋环流、气候和天气，以及污染物、营养物和沉积物的运输以及沿海水域的相关生物过程至关重要。波浪、水流和粗糙度的结合使得 BBL 建模特别具有挑战性。对最先进仪器获得的数据进行分析是开发建模工具的重要一步。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、后续分析和出版物，作为激励他们从事海洋学职业的行之有效的手段。该团队还将延续长期的惯例，让邻近学校（巴尔的摩理工学院）的高中生参与为期一年的研究实习项目，这是他们必修课程的一部分。