Characterization of Turbulence in the Coastal Bottom Boundary Layer Based on a Large Database Obtained From PIV Measurements

OCE-0648490智能优点：预测海洋动力学、沉积物传输、污染物扩散和沿海海洋的生物过程需要对底部边界层中的湍流进行适当的建模。由于要求苛刻且变化极大的环境条件，获得对建模至关重要的具有良好特征的数据是一项挑战。该项目扩展了我们测量沿海海洋底部边界层的流动结构和湍流的努力，并研究它们对环流、波浪、海底地形、海拔高度和层结的依赖关系。湍流测量是使用可潜式粒子图像测速(PIV)系统进行的，该系统具有10 m的剖面范围，可以在任何方向上独立地对齐两个样品区域，例如，与平均水流、波浪或相互倾斜的样品区域，以测量三维流动特征。PIV数据由两个瞬时二维速度分布的时间序列组成。在不同的放大倍数下，数据可以分辨1.2 mm到1 m的长度范围，从而可以直接计算结构函数的耗散率或雷诺应力。以往部署的结果表明：a)雷诺应力和平均流随波相的变化。B)雷诺应力随边界层外部高度的升高而减小，这与实验室数据一致。结果表明，应力随平均电流的变化而变化。C)湍流产生耗散比随海拔高度的升高而减小，从海拔30 cm处略低于30 cm处的值下降到1.5m处的很低值，损失的能量很可能来源于靠近底部的高生产量，并由平均流、波浪和湍流向上输送。D)在中等雷诺数，但典型的潮汐流动中，导致雷诺应力和产量的事件在“阵风”期间间歇性地发生，而耗散随时间变化不大。有关间歇性事件的有意义的统计数据需要一个大型数据库。E)随着雷诺数的增加，湍流能谱与普适能谱更接近，但仍表现出各向异性，包括耗散范围。F)亚网格尺度(SGS)应力和能量通量随波相的可重复变化提供了直接证据，表明波浪应变改变了所有尺度上的湍流能量级联过程。基于这些先前的观察，本研究的目标是：a)查明、测量并随后模拟在海底-上层界面附近由水流和波浪与粗糙海底相互作用而产生的主导湍流的具体机制。B)确定雷诺应力随波相变化的原因和贡献因素，包括高海拔海浪引起的应变引起的生产循环变化、波与底部波纹的相互作用、波相对于平均流的方向以及波对湍流的垂直输送。分析将检查结果对平均流动和雷诺应力的剖面和尺度的影响。C)测量波浪应变对SGS能量通量的影响，以及由此对不同尺度上的能量级联过程和湍流能谱的影响。在LEO-15附近的两次部署期间，将记录实现这些目标所需的大型数据库。PIV数据将与平均水流、波浪方向和幅度、海底粗糙度、温度谱、密度分布和浮力通量的测量同时获得。广泛影响：社会经济问题：正确的海洋环流模拟对于预测气候、天气和人类对沿海海洋的影响至关重要。此外，污染物、营养物和沉积物的运输影响到沿海的经济、健康、旅游业、渔业和粮食生产。这些数据和分析将有助于改进对海洋运输、环流和混合的预测。未来科学家的教育：与巴尔的摩城市学校的教育外联工作将继续让巴尔的摩理工学院的高中生参加为期一年的研究体验，作为他们要求的研究实践的一部分。将继续让本科生参加实地考察和数据分析，以此作为激励他们参与海洋学的一种手段。该项目将支持两名将接受海洋学家培训的研究生。他们的教育包括研究和专门针对海洋学、流体力学、仪器仪表、生物和数学的课程。JHU提供的联合学位课程促进了与具有不同背景的教职员工的互动。

OCE-0648490Intellectual Merit: Predictions of ocean dynamics, sediment transport, pollutant dispersal and biological processes in the coastal ocean require proper modeling of turbulence in the bottom boundary layer. Obtaining well-characterized data, which is essential for modeling, is a challenge due to the demanding and enormously variable environmental conditions. This project extends our effort to measure the flow structure and turbulence in the bottom boundary layer of the coastal ocean, and study their dependence on circulation, waves, bottom topography, elevation and stratification. Turbulence measurements are performed using a submersible Particle Image Velocimetry (PIV) system with a 10 m profiling range that can align two sample areas independently in any direction, e.g. with mean current, with waves or inclined to each other to measure 3-D flow features. The PIV data consists of two time series of instantaneous, 2-D velocity distributions. With varying magnifications, the data resolve length scales ranging between 1.2 mm to 1 m, enabling direct calculation of dissipation rate or Reynolds stresses from structure functions. Results from previous deployments show: a) Variations of Reynolds stresses and mean current with wave phase. b) Decrease of Reynolds stresses with elevation in outer portions of the boundary layer, consistent with laboratory data. It is found that stresses scale with mean current. c) Decrease of turbulence production dissipation ratio with increasing elevation, from slightly below one at 30 cm elevation to very low values at 1.5 m. The missing energy most likely originates from high production very near bottom, and is transported up by mean flow, waves and turbulence. d) At moderate Reynolds numbers, but typical to tidal flows, events contributing to Reynolds stresses and production occur intermittently during periods of "gusts", while dissipation changes little with time. Meaningful statistics on intermittent events requires a large database. e) Turbulent energy spectra appear more similar to the universal spectrum with increasing Reynolds number, but still indicate anisotropy at all scales, including dissipation range. f) Repeatable variations of sub-grid scale (SGS) stresses and energy flux with wave phase provide direct evidence that wave-induced straining modifies the energy cascading process of turbulence at all scales. Based on these previous observations, objectives of the present study are: a) To identify, measure and subsequently model specific mechanism dominating turbulence production near the benthic-pelagic interface resulting from interactions of currents and waves with a rough bottom. b) To determine causes and contributors to variations of Reynolds stresses with wave phase, including cyclic changes in production due to wave-induced straining at high elevations, interactions of waves with bottom ripples, orientation of waves relative to mean currents and vertical transport of turbulence by waves. Analysis will examine resulting effects on profiles and scaling of mean flow and Reynolds stresses. c) To measure effect of wave-induced straining on SGS energy fluxes and resulting impact on the energy cascading process and turbulent energy spectra at different scales. A large database required for achieving these objectives will be recorded during two deployments near LEO-15. The PIV data will be acquired concurrently with measurements of mean current, direction and amplitude of waves, bottom roughness, temperature spectra, density profiles and buoyancy flux.Broader impact:Socio-Economic Issues: Proper modeling of oceanic circulation is essential for predictions of climate, weather and human impact on the coastal ocean. Furthermore, transport of pollutants, nutrients and sediment affect the economy, health, tourism, fisheries and food production along the coast. The data and analysis will contribute to improved predictions of oceanic transport, circulation and mixing. Education of future Scientists: Educational outreach effort with the Baltimore City Schools will continue involving senior high-school students from the Baltimore Polytechnic Institute in a yearlong research experience, as part of their required Research Practicum. On-going participation of undergraduates in field trips and data analysis as a means of motivating them to get involved in oceanography will be continued. The project will support two graduate students that will be trained as oceanographers. Their education includes research, and specially geared courses in oceanography, fluid mechanics, instrumentation, biology and mathematics. A joint degree program available at JHU facilitates interaction with faculty having diverse backgrounds.