Numerical Simulation of Admiralty Inlet, WA, with Tidal Hydrokinetic Turbine Siting Application

Thesis (Ph.D.)--University of Washington, 2012Tidal hydrokinetic energy has been recognized as a potential source of sustainable, renewable energy. In order to properly site turbines for commercial-scale development, the complex flow conditions in a potential deployment region must be understood. Viable locations for turbines are limited by many factors, including underwater space that is above the bottom boundary layer, below shipping traffic, within areas of strong currents, and yet avoids additional fatiguing stresses. The primary area of interest in the Puget Sound for commercial tidal energy development is Admiralty Inlet, which includes potentially disruptive flow features such as vortices and strong turbulence. This dissertation seeks to increase the body of knowledge of these features both from an oceanographic perspective and as they pertain to turbine site characterization. The primary means of studying Admiralty Inlet in this document is through numerical simulation of the region using the Regional Ocean Modeling System (ROMS). The model output is found to compare well with field data, capturing eddy fields, turbulence properties, relative tidal phases, and illuminating many flow features. Horizontal velocities in the simulation are, on average, approximately 75\% the size of those found in the data. This speed deficiency is inherited from the forcing model in which the Admiralty Inlet simulation is nested. The model output also shows that the flow field of this fjord-like estuary is largely affected by a headland on the northeast side of the Inlet. Vortices generated by this headland, Admiralty Head, are found to vary considerably depending on the tidal cycle. The eddies can persist beyond the half-cycle of generation to significantly affect the horizontal speed and other flow field properties in the subsequent half-cycle. Detailed analysis of the vertical vorticity governing equation shows that advection, tilting, stretching, and boundary generation are the most significant processes dictating the behavior of the vorticity. Turbulence modeling in the simulation is carried out via a k-&epsilon turbulence closure scheme. Comparisons of model output with high resolution field data show the model to perform reasonably well: predicted Reynolds stress and turbulent dissipation rate values are usually within a factor of two of the field data. The turbulent kinetic energy from the simulation compares well with field data that is restricted to the frequency range of classical turbulence. The energy density spectrum of the data is found to follow Kolmogorov's theory beyond the inertial subrange. Using this fact and Taylor's frozen field approximation, an inferred calculation for the turbulent kinetic energy is derived that spans the full frequency range of the data set. The output from the inferred calculation compares well with the full turbulent kinetic energy from the field data. Maps of metrics for tidal turbine siting are generated that address many considerations for turbine placement, and can be adjusted for the model's speed deficiency with a simple multiplication factor. Among the possible best locations for turbine deployment are north of Point Wilson on the west side of Admiralty Inlet and near the center of the channel between Point Wilson and Admiralty Head. These locations have a strong tidal resource available along with highly bi-directional tidal currents and low turbulence levels