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.
The objective of this project is to study the characteristics of flow and turbulence in the combined wave-current, bottom boundary layer of the coatal ocean. At small scales, turbulence in the Benthic Boundary layer (BBL) affects biological activities, and plays a primary role in sediment, nutrient and pollutant transport, which impact human health, tourism, fisheries and food production in coastal communities. At large scales, turbulence depletes energy and momentum from currents and waves, and subsequently dissipates it. Coastal circulation models, which do not resolve the turbulence must relate the production, transport and dissipation of small-scale turbulence to large-scale flows, which are driven by wind, tidal pressure gradients and waves. Studies of the impact of turbulence models on circulation predictions have indicated that results are sensitive to the model performance, and that improvements in bottom stress modeling are essential. However, technical obstacles associated with the challenging environment in the BBL have left this need largely unfulfilled. Estimates of vertical Reynolds shear stress profiles are particularly hampered by difficulties in separating wave unsteadiness from turbulent fluctuations. Consequently, field measurements in the inner part of the BBL are scarce and have been mostly performed nearshore. Consequently, in most efforts turbulence properties have been estimated indirectly. To obtain detailed data on the flow structure and turbulence in the BBL, our group has introduced the oceanic application of two dimensional Particle Image Velocimetry (PIV), which provides instantaneous distribution of two velocity components over a sample area. We have also used in situ digital holography for simultaneously profiling the distributions of particles, shear strain and dissipation rates in the water column. In laboratories, PIV has become a primary experimental tool for characterizing complex flows, including studies relevant to oceanography. However, technical challenges have limited the in-situ application of PIV. Our group has played a leading role in applications of PIV and holography to study oceanic turbulence and particles distributions at resolutions that match laboratory data. Large databases of time- and spatially-resolved velocity distributions obtained during our field tests have enabled us to examine the flow in the inner part of the BBL at unprecedented resolution, which competes with that achievable in laboratories. Profiles of mean velocity, Reynolds and wave-induced shear stresses, wave-turbulence correlations, turbulence production and dissipation rates, as well as frequency and wavenumber energy spectra have been obtained under varying conditions of currents, waves, and measured bottom topography. Analysis of these results begins to elucidate how the interplay between currents, waves and wall roughness affect near-bed turbulence. Also, surprisingly, the results also demonstrate the substantial effects of mixing induced by thin, near-bed schools of small fish (10 mm) on the boundary layer structure, which has never been shown before. In terms of education, this project supported two PhD students. The PI teaches graduate courses in fluid mechanics and optical instrumentation and undergraduate courses in thermodynamics and jet propulsion. We also involved undergraduate students and senior high-school students in this effort. They built instruments, developed software, analyzed data and performed research. Four undergraduate students participated in the field trips to acquire data off the New Jersey coast, as well as in preparation and calibration of instruments, such as bottom scanning sonar, cameras, data acquisition systems and acoustic Doppler profilers.
