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

Johns Hopkins University, Baltimore MD

Investigators

Abstract

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.