Flow characterization at a turbulent tidal energy site in Minas Passage, Bay of Fundy
LE3 .A278 2022
Master of Science
Mathematics & Statistics
Characterization of site-specific hydrodynamic variables in an important factor in the successful design of tidal in-stream power systems. The Minas Passage, one of the Bay of Fundy’s tidal channels, presents significant potential for tidal energy development because of its highly energetic flows. Flow in Minas Passage is highly turbulent (𝑅𝑒 ≈ 1.4 × 10!). Although great strides have been made towards the complete characterization of the water column in Minas Passage and at similar sites in Nova Scotia, some aspects of the flow are yet to be explored in depth. This thesis uses observational data from five-beam Acoustic Doppler Current Profilers (ADCPs) to examine some of these aspects, namely (1) the vertical structure of the flow, (2) the turbulence characteristics at the site, and (3) the impacts of wind forcing on flow structure. Vertical profiles of along-stream velocity are fit using two methods: logarithmic law of the wall, and an adapted logarithmic law of the wall with a wake function. Fitting of the logarithmic law of the wall, a well-established method for estimating the mean velocity in a rough-walled turbulent boundary layer, results here in well-fitted estimations of the velocity profile near the seabed (𝑧 ≤ 5𝑚). Observational profiles are found to deviate from the fitted curves in the upper portion of the water column, measuring significantly faster flows than predicted by the logarithmic law. The adapted wake law model, which includes a cubic wake term to describe flow behaviour outside of the boundary layer, is found to result in much better fits throughout the entirety of the water column, with remarkably low error values across all flow speeds (𝑁𝑅𝑀𝑆𝐸 ≤ 1%). Turbulence metrics including vertical beam amplitude (acoustic backscatter), variance in the vertical beam, and turbulent kinetic energy are examined and found to comply well with homogeneous isotropic turbulence theory and with observations at similar tidal sites. Vertical velocity spectra are also computed at varying depths below the free surface, and the presence of an inertial subrange complying with Kolmogorov theory is consistently noted in the absence of wind-generated surface gravity waves. Analysis of vertical structure and turbulence metrics is then repeated for periods of high wind. The parameters extracted from the logarithmic law and adapted model procedures during windy times do not differ significantly from those calculated for the mean flow conditions. Elevated vertical beam variance and TKE are, however, noted near the free-surface in the presence of wind. Spectral analysis shows consistent peaks in the mid-frequency range which coincide with short (5-8 s) waves when wind speeds surpass a threshold of approximately 10 𝑚/𝑠. This is of particular interest at the Minas Passage site because it presents a mechanism for validation of a network of X-Band marine radars at the site.
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