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

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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.