Aerodynamics modelling of floating offshore wind turbines

An unsteady Blade Element Momentum (uBEM) method is presented for the analysis of Floating Offshore Wind Turbines (FOWTs). FOWTs allow wind turbines to be placed in deeper waters with stronger and more consistent winds. However, the floating platform motions result in motions of the wind turbine not found in fixed-bottom wind turbines, thus the widely-used blade element momentum (BEM) method, a static analysis method, cannot be used to calculate wind turbine blade forces. Different unsteady aerodynamic phenomena associated with FOWTs are analysed, with corrective models developed for an accurate analysis of FOWT aerodynamics. The NREL 5MW virtual wind turbine was studied in axial flow and sinusoidal surge motions at different frequencies and amplitudes, usingfull-3D computational fluid dynamics simulations. The effects studied include the unsteady airfoil effect, stall delay effect, tip-loss, and transient wake states. The unsteady airfoil effect is studied based on experimental data from the Ohio State University, and a modified Beddoes-Leishman model is presented based on airfoil shape parameters and an optimisation-based methodology. The stall-delay effect was accounted for using the Du & Selig model, and a novel methodology for real-time stall delay incorporating the unsteady airfoil effect is presented. A novel correction and modelling methodology is presented for tip-losses, through a correction to Glauert's implementation of the Prandtl tip-loss factor, which incorporates changes in tip speed ratio as well as changes in wake state. Buhl's version of the Glauert correction for the turbulent wake state was used as a basis to model the propeller and propeller brake states of the wind turbine, which are rare for fixed-bottom wind turbines but can be expected to occur periodically for FOWTs. The dynamic wake model of Øye is optimised to take into account the level of unsteadiness found in FOWTs and work in conjunction with other unsteady models. Finally, these models are incorporated into the uBEM method and a FOWT aerodynamics analysis tool is created with low computational time, and more realistic force predictions. Future studies can extend these methodologies to more motion degrees of freedom.Doctor of Philosophy (MAE