This thesis examines methods for designing and analyzing kinetic turbines based on blade element momentum (BEM) theory and computational fluid dynamics (CFD). The underlying goal of the work was to assess the potential augmentation of power production associated with enclosing the turbine in an expanding duct. Thus, a comparison of the potential performance of ducted and non-ducted turbines was carried out. This required de ning optimal turbine performance for both concepts. BEM is the typical tool used for turbine optimization and is very well established in the context of wind turbine design. BEM was suitable for conventional turbines, but could not account for the influence of ducts, and no established methodology for designing ducted turbines could be found in the literature. Thus, methods were established to design and analyze ducted turbines based on an extended version of BEM (with CFD-derived coe cients), and based on CFD simulation. Additional complications arise in designing tidal turbines because traditional techniques for kinetic turbine design have been established for wind turbines, which are similar in their principle of operation but are driven by flows with inherently different boundary conditions than tidal currents. The major difference is that tidal flows are bounded by the ocean floor, the water surface and channel walls. Thus, analytical and CFD-based methods were established to account for the effects of these boundaries (called blockage effects) on the optimal design and performance of turbines. Additionally, tidal flows are driven by changes in the water surface height in the ocean and their velocity is limited by viscous effects. Turbines introduced into a tidal flow increase the total drag in the system and reduce the total flow in a region (e.g. a tidal channel). An analytical method to account for this was taken from the eld of tidal resource assessment, and along with the methods to account for ducts and blockage effects, was incorporated into a rotor optimization framework. It was found that the non-ducted turbine can produce more power per installed device frontal area and can be operated to induce a lesser reduction to the flow through a given tidal channel for a given level of power production. It was also found that by optimizing turbines for array con gurations that occupy a large portion of the cross sectional area of a given tidal channel (i.e. tidal fences), the per-device power can be improved signi cantly compared to a sparse-array scenario. For turbines occupying 50% of a channel cross section, the predicted power improves is by a factor of three. Thus, it has been recommended that future work focus on analyzing such a strategy in more detail. / Graduate

This study presents a numerical investigation of ducted tidal turbines, employing three-dimensional Reynolds-averaged Navier-Stokes simulations. Bidirectional ducted turbines are modelled with and without aperture, referred to as ducted and open- centre turbines respectively. The work consists of two investigations. In the first, the turbine rotors are represented by actuator discs, a simplification which captures changes in linear momentum and thus the primary interaction of the turbine with the flow through and around the duct, while greatly reducing computational complexity. In the second investigation, the turbine rotors are represented through a CFD-integrated blade element momentum model, employing realistic rotor data, capturing swirl and blade drag in addition to the extraction of linear momentum. Both modelling techniques were employed to investigate the performances of bare, ducted, and open-centre turbines, relating these to the flow fields exhibited. For axial flow, substantial decreases in power generated by the ducted and open-centre turbines were found, relative to a bare turbine of equal total device diameter. For open-centre turbines, an increase in aperture size leads to a further reduction in power generated. Increased blockage was shown to positively affect the performance of all devices. Two further measures of performance were employed: power density, normalising the power by the rotor area, and basin efficiency, relating the power generated to the overall power removed from the flow. Moderate increases in power density can be achieved for the ducted and open-centre devices, while their basin efficiencies are of similar value to that of the bare turbine. For yawed inflow, the performance of the bare turbine decreases, whilst that of the ducted and open-centre turbines increases. This is due to an increased flow velocity following flow acceleration around the inlet lip of the duct and also an increase in effective blockage as ducts present greater projected frontal area when approached non-axially.