Passive load alleviation by morphing blades for tidal turbines
This thesis presents a novel passive load control to reduce the unsteady hydrodynamic loads on the blades of tidal turbines, enabling lighter, more resilient and less expensive turbines. Existing load control strategies are inadequate to mitigate the high frequency loads experienced by tidal turbines. The ideal solution should pro vide a fast, local control action on every section of the blade. Research on wind turbines suggests that the most promising option are trailing edge flaps, and passive, flexible materials are recommended over rigid control surfaces to maximise the device reliability. Beyond some pioneering examples of morphing blades, the fundamental principles underlying their efficacy have yet to be fully understood, withholding further development and the adoption by the industry. I present a numerical investigation of morphing blades to show the principles underlying unsteady load alleviation by morphing blades, and I demonstrate their capabilities via proof-of-concept experiments. I develop a low-order model where the blade flexibility is represented by a torsional spring that controls the blade pitch motion, and I optimise the system for a specific turbine design operating in different flow conditions. The fluctuations of the root bending moment can be reduced up to 99% when the turbine operates in shear flow, and by 87% when operating in large wave conditions. The system is governed by the blade flexibility, but the blade inertia, material damping, and unsteady flow phenomena can affect the load-alleviating performance greatly. To verify the system capabilities, I conduct a series of experiments in FloWave, a 25 m wide, 2 m deep, circular tank testing facility, using a 1:15 scale turbine and custom designed passively-pitching blades. The system consistently reduces the fluctuations of the root bending moment, thrust and torque over a range of different tip speed ratios. While the experiments featured a passively-pitching blade, the results are a good indication of the potential of morphing blades, and the analytical low-order code is equally representative of a rigid blade with a flexible trailing edge. This work aims to underpin the future development of morphing blades by providing a simple, yet reliable numerical model, and by proving experimentally the capabilities of this technology.