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

OCE-0648490智力价值：预测沿海海洋的海洋动力学、沉积物迁移、污染物扩散和生物过程需要对底部边界层的湍流进行适当的建模。由于环境条件要求苛刻且变化巨大，获取对建模至关重要的充分表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、底部地形、海拔和分层的依赖性。湍流测量是使用潜水式粒子图像测速 (PIV) 系统进行的，其分析范围为 10 m，可以在任何方向上独立对齐两个样本区域，例如在方向上。使用平均电流、波浪或相互倾斜来测量 3-D 流量特征。 PIV 数据由两个瞬时二维速度分布时间序列组成。通过不同的放大倍率，数据可解析 1.2 mm 至 1 m 之间的长度尺度，从而能够根据结构函数直接计算耗散率或雷诺应力。先前部署的结果显示： a) 雷诺应力和平均电流随波相的变化。 b) 雷诺应力随着边界层外部的升高而减小，与实验室数据一致。研究发现应力随平均电流变化。 c) 湍流产生耗散比随着海拔的增加而降低，从 30 cm 海拔处略低于 1 到 1.5 m 处的非常低的值。缺失的能量很可能源自底部附近的高产量，并通过平均流、波浪和湍流向上输送。 d) 在中等雷诺数下，但对于潮汐流来说是典型的，导致雷诺应力和生产的事件在“阵风”期间间歇性地发生，而耗散随时间变化很小。对间歇性事件进行有意义的统计需要一个大型数据库。 e) 随着雷诺数的增加，湍流能谱看起来与通用能谱更加相似，但仍然表明所有尺度上的各向异性，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，表明波引起的应变改变了所有尺度湍流的能量级联过程。基于这些先前的观察，本研究的目标是： a) 识别、测量并随后模拟在底栖-远洋界面附近由水流和波浪与粗糙底部相互作用产生的湍流产生的主导机制。 b) 确定雷诺应力随波相变化的原因和影响因素，包括由于高海拔处波浪引起的应变而产生的产量循环变化、波浪与底部波纹的相互作用、波浪相对于平均水流的方向以及波浪湍流的垂直传输。分析将检查对平均流量和雷诺应力的分布和缩放的影响。 c) 测量波致应变对 SGS 能量通量的影响以及对不同尺度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 数据将与平均海流、波浪方向和幅度、底部粗糙度、温度谱、密度剖面和浮力通量的测量同时获取。更广泛的影响：社会经济问题：正确的海洋环流建模对于预测气候、天气和人类对沿海海洋的影响至关重要。此外，污染物、营养物和沉积物的输送影响沿海地区的经济、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：巴尔的摩城市学校的教育外展工作将继续让巴尔的摩理工学院的高中生参与为期一年的研究体验，作为他们所需研究实习的一部分。将继续让本科生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。该项目将支持两名研究生接受海洋学家培训。他们的教育包括海洋学、流体力学、仪器仪表、生物学和数学方面的研究和专门课程。约翰霍普金斯大学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。

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.