An Unsteady Hydrodynamic Model For Tidal Current Turbines

Due to concerns about the impacts of carbon emissions on the environment, the security of supply of electricity and the likelihood of achieving "peak-oil" in the near future, governments have legislated to reduce reliance on fossil fuels. An attractive alternative is power obtained from tidal currents, and the coast of the British Isles is especially hydraulically active. Tidal energy converters typically resemble wind turbines however, unlike wind turbines, they are expected to operate in an environment which is singularly hostile, and will also be expected to generate power in non-ideal operating conditions. This thesis is concerned with the ability to model individual and groups of tidal devices including their mutual interactions. The ability to capture unsteady inflow conditions at realistic array spacing requires preservation of turbine wakes over a sufficiently large range at spatial resolutions and over time durations which are not feasible using standard computational fluid dynamics software. This thesis has combined methodologies developed for helicopter wake modelling with techniques used in naval architecture for modelling thick maritime propellers into a computational tool. The particular formulation of the Navier-Stokes equations employed allows the determination of the unsteady pressure and force distributions on a turbine rotor due to the effects of a neighbouring device, even if it is operating some significant distance upstream. The constituents of the method of this thesis are developed and applied to "proof-of-principle" studies. These include flow past static and oscillating 2-D aerofoils and past a 3-D wing, wind turbine and tidal turbine configuration. The results from these studies demonstrate that the model is convergent and capable of capturing the time dependant forces on these devices, and by comparison with analytical or experimental results, or via inter-model comparison begins the process of calibration and validation of the model. The method is then applied to flow past groups of turbines in various array configurations, and a coaxial, contra-rotating device. The outcome of this work is a decision making tool which can be used to improve success and reduce risk in tidal power array planning, optimise device configurations and is translatable back into rotorcraft or naval architecture usage.

This research addresses the need for an improved characterisation of the onset flow turbulence and the unsteady hydrodynamic blade loads on tidal turbines for the purposes of predicting fatigue life. A new, extensive set of parameters which characterise the magnitudes of the turbulent fluctuations, the anisotropy and the scales of the turbulence at a tidal energy site have been presented. A novel application of rapid distortion theory estimated the velocity fluctuations to be amplified by 15% due to the presence of the turbine. The turbulence was also predicted to be well correlated over the outer span of a turbine blade at the frequencies of interest. Together, these results enabled a set of non-dimensional parameters describing the turbulence induced forcing on a turbine blade to be established. A model-scale horizontal-axis turbine was used to investigate the unsteady blade load response in a still-water towing tank. A set of wind tunnel tests of the S814 foil were also conducted and used to demonstrate that the lift on the blades could have been degraded by 10% at the relatively low Reynolds numbers at which the turbine was tested, relative to full-scale. This was owing to dominant laminar separation bubbles. Single frequency planar oscillations of the turbine were used to quantify the contribution of hydrodynamic unsteadiness to the blade-root bending moment. For attached flow, the unsteady bending moment was found to amplify the steady loads by up to 15 %. The total hydrodynamic added mass was up to 2.7 times larger than from non-circulatory forcing and decreased with frequency. Dynamic inflow theory and a returning wake model were able to provide qualitative predictions of these results at low frequencies. At low tip-speed ratios, phenomena consistent with delayed separation and dynamic stall were characterised and the unsteady loading was up to 25% larger than the steady load. Linear superposition of the single frequency responses was also demonstrated to offer a reliable technique to model the response to a multi-frequency forcing and to a large eddy.

Horizontal Axis Tidal Turbines (HATTs) can experience amplified, time varying hydrodynamic loads during operation due to dynamic stall. Elevated hydrodynamic loads impose high structural loads on turbine blades, thus appreciably shortening machine service life. An improved characterization of the unsteady hydrodynamic loads on tidal turbine blades is therefore necessary to enable more reliable predictions of their fatigue life and to avoid premature failures. This thesis reports on a Computational Fluid Dynamics (CFD) analysis of the unsteady blade loading of a scale-model HATT taking dynamic stall into account. Numerical simulations are performed both in two-dimensional (2-D) and three-dimensional (3-D) using the commercial CFD solver ANSYS Fluent.After a brief description of the theories and methods involved, the behaviour of flow at low Reynolds number around a NACA-0012 aerofoil pitching in a sinusoidal pattern that induces dynamic stall is studied firstly to validate the numerical method and the choice of turbulence models. Then full 3-D computations of a rotating scale-model HATT rotor are presented for steady and periodic unsteady inflow situations, respectively. The reliability of the 3-D numerical method is evaluated by comparing the blade loads, especially the out-of-plane blade-root bending moment (defined as being about an axis normal to the rotor axis), with measurement data obtained from experimental tests conducted at the University of Strathclyde's Kelvin Hydrodynamics Laboratory towing tank. Analyses in the steady velocity study are documented for a broad range of rotor speeds and flow velocities. Furthermore, investigations of 3-D flow separation and scale effects on blade loads are also performed.The periodic unsteady velocity study aims to examine the out-of-plane blade-root bending moment response to harmonic axial motion, deemed representative of the free-stream velocity perturbations induced by the unsteady flow. Parametric tests on oscillatory frequencies and amplitudes are carried out in order to analyse the HATT blade hydrodynamic behaviour under different flow patterns. Detailed flow field data is analysed to understand 3-D dynamic stall from a modelling perspective.It is concluded that the results by the present study provide significant insights into the flow physics occurring around the HATT rotor blades under various flow conditions. The CFD method can be used for designing more advanced HATT rotors, it also can be used to fine tune the computationally faster lower order Blade Element Momentum (BEM) methods for parametric design studies where experimental data is not available, particularly at the challenging rotor operating conditions involving flow separation and dynamically varying hydrodynamic behaviours.

Tidal turbines harness hydrokinetic energy resulting from ocean tidal flows to generate power. This type of power generation is a potential source of clean, reliable renewable energy. However, the technology is still under development. The effects of unsteady flow conditions, specifically surface gravity waves, on tidal turbines have not been completely analyzed. The effects of waves on performance characteristics were assessed for a model horizontal axis tidal turbine selected by the Department of Energy and designed by the National Renewable Energy Laboratory (NREL). The performance characteristics of the 1/25th scale model turbine were tested under unsteady flow conditions. Parameters including wave height, wave length and tow speed for the experiment were scaled to properly model flow conditions that a horizontal axis tidal turbine was expected to experience at a full scale. First, turbine rotational speed, torque and thrust were measured for steady flow conditions and unsteady flow conditions characterized by a range of incoming waves. Turbine performance characteristics, including thrust and power coefficients, were obtained as functions of rotor tip speed ratio for the unsteady flow conditions tested. The second experiment involved a detailed fluid flow survey in the near wake of the turbine with and without one of the waves utilized in the first experiment, as measured by Acoustic Doppler Velocimeters. The results provided a characterization of velocity fields in the near wake of the turbine, necessary information for the placement of multiple turbines in a larger array.

Covers theoretical concepts in offshore mechanics with consideration to new applications, including offshore wind farms, ocean energy devices, aquaculture, floating bridges, and submerged tunnels This comprehensive book covers important aspects of the required analysis and design of offshore structures and systems and the fundamental background material for offshore engineering. Whereas most of the books currently available in the field use traditional oil, gas, and ship industry examples in order to explain the fundamentals in offshore mechanics, this book uses more recent applications, including recent fixed-bottom and floating offshore platforms, ocean energy structures and systems such as wind turbines, wave energy converters, tidal turbines and hybrid marine platforms. Offshore Mechanics covers traditional and more recent methodologies used in offshore structure modelling (including SPH and hydroelasticity models). It also examines numerical techniques, including computational fluid dynamics and finite element method. Additionally, the book features easy-to-understand exercises and examples. Provides a comprehensive treatment for the case of recent applications in offshore mechanics for researchers and engineers Presents the subject of computational fluid dynamics (CFD) and finite element methods (FEM) along with the high fidelity numerical analysis of recent applications in offshore mechanics Offers insight into the philosophy and power of numerical simulations and an understanding of the mathematical nature of the fluid and structural dynamics with focus on offshore mechanic applications Offshore Mechanics: Structural and Fluid Dynamics for Recent Applications is an important book for graduate and senior undergraduate students in offshore engineering and for offshore engineers and researchers in the offshore industry.