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

基于 PIV 测量获得的大型数据库的海岸底部边界层湍流特征

• 批准号: 0648490
• 负责人: Joseph Katz
• 金额: $ 55.14万
• 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: 2007
• 资助国家: 美国
• 起止时间: 2007-03-15 至 2010-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0648490&HistoricalAwards=false
• 关键词: Characterization; Turbulence; Coastal; Bottom; Boundary

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.

OCE-0648490Intlectual Furect：海洋动力学，沉积物传输，污染物分散和生物学过程的预测需要在底部边界层中正确建模湍流。由于苛刻且巨大的环境条件，获得对建模至关重要的特征性数据，这对于建模至关重要。该项目扩展了我们努力测量沿海海洋底部边界层的流动结构和湍流，并研究了它们对循环，波浪，底部地形，高程和分层的依赖。使用潜水粒子图像速度（PIV）系统进行10 m的分析范围进行湍流测量，该系统可以独立于任何方向对齐两个样本区域，例如平均电流，波浪或彼此倾向于测量3-D流量特征。 PIV数据包括两个时间序列的瞬时2-D速度分布。随着重大变化，数据解析长度尺度在1.2 mm至1 m之间，从而直接计算耗散速率或雷诺从结构函数中压力。先前部署的结果显示：a）雷诺应力的变化和波相的平均电流。 b）与实验室数据一致的边界层外部的雷诺应力下降随着边界外部的升高。发现应力比例平均电流。 c）随着海拔高度的增加，湍流产生耗散率的减少，从30 cm的海拔略低至1.5 m的非常低的值。缺失的能量最有可能起源于底部很近的高产量，并通过平均流量，波浪和湍流来运输。 d）在中等的雷诺数中，但典型的潮汐流，导致雷诺的压力和生产的事件间歇性地发生在“阵风”期间，而耗散的时间很少随着时间而变化。关于间歇事件的有意义的统计数据需要一个大数据库。 e）湍流能谱看起来与雷诺数增加的通用光谱更相似，但仍表明各种尺度（包括耗散范围）各向异性。 f）以波相的重复变化（SGS）应力和能量通量可重复变化，提供了直接的证据，表明波诱导的过滤会改变所有尺度上湍流的能量级联过程。基于这些先前的观察结果，本研究的目标是：a）识别，测量和随后建模特定的机制，这些机制主要是由底栖 - 沿底部界面附近的湍流产生，这是由于电流和波的相互作用而导致的，底部的相互作用具有粗糙的底部。 b）确定雷诺应力变化与波相的变化的原因，包括由于波诱导的高度时波诱导的应压，波浪与底部波纹的相互作用，相对于平均电流的波动方向的相互作用以及湍流通过波的垂直运输的方向。分析将检查对平均流量和雷诺压力的剖面的影响。 c）测量波诱导的过力对SGS能量通量的影响，并在不同尺度上对能量级联过程和湍流能量谱产生影响。实现这些目标所需的大型数据库将在LEO-15附近的两个部署中记录。 PIV数据将与测量的平均电流，方向和幅度同时获得，底部粗糙度，温度光谱，密度谱和浮力通量的影响：社会经济问题：海洋循环的正确建模对于预测气候，天气和人类对沿海海洋的影响至关重要。此外，污染物，养分和沉积物的运输会影响沿海的经济，健康，旅游，渔业和粮食生产。数据和分析将有助于改善海洋运输，循环和混合的预测。对未来科学家的教育：与巴尔的摩城市学校的教育外展工作将继续涉及巴尔的摩理工学院的高中生，作为他们必要的研究实践的一部分。本科生将继续参与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培训的研究生。他们的教育包括研究，并在海洋学，流体力学，仪器，生物学和数学方面进行了专门的课程。 JHU提供的联合学位课程促进了与具有不同背景的教师的互动。