Floating offshore wind turbines enable renewable energy expansion into deep-water regions with stronger, more consistent winds. However, they are subject to continuous platform motion from wind and waves, which complicates blade loading and wake aerodynamics. While surge and pitc
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Floating offshore wind turbines enable renewable energy expansion into deep-water regions with stronger, more consistent winds. However, they are subject to continuous platform motion from wind and waves, which complicates blade loading and wake aerodynamics. While surge and pitch effects have been extensively studied, the aerodynamic influence of roll motion remains underexplored. Roll motion uniquely induces a non-uniform tangential velocity field across the rotor, which would have implications on the blade loading, structure fatigue and the wake aerodynamics.
This thesis presents a baseline analysis of blade loading and near-wake aerodynamics for a FOWT under prescribed roll motions. A coupled high-fidelity GPU-based Large-Eddy Simulation code GRASP resolved turbulent wake dynamics, while OpenFAST calculated blade loads. Coupling was achieved via the Filtered Actuator Line Method and the AspFAST application programming interface. The IEA 15MW reference turbine was modeled under steady, uniform inflow, with variations in roll amplitude (5° and 10°), roll frequency (0.03Hz and 0.05Hz), and tip-speed ratio (TSR 7 and TSR 9) in a full factorial test set.
Results show that roll motion leads to asymmetric fluctuations across the rotor plane. Normal forces display strong vertical asymmetry, with greater variations in the upper side of the rotor due to its larger distance from the roll center. A kinematic analysis deriving the variation of tangential velocity both azimuthally and over time supported this finding. Tangential forces exhibit lateral variation, with a left-right asymmetry across the rotor plane, though the magnitude of variation is similar on both sides. Both roll amplitude and frequency increase the magnitude of loading fluctuations, while TSR influences their blade spanwise distribution.
Power Spectral Density analysis reveals that roll introduces spectral content at the roll frequency and at 1P sideband frequencies, the latter arising from modulation of the 1P frequency by roll. Higher roll amplitudes, frequencies, and TSRs amplify these load fluctuations. While the prominence of the 1P frequency is negligible at the blade, it is strong in the near wake along with the roll frequency, 3P frequency, and 3P sidebands.
The unsteady blade loads directly shape the near wake. The stable helical vortex system of a bottom-fixed turbine is replaced by an oscillatory corkscrew structure that breaks down earlier and more chaotically. Increased roll amplitude causes stronger lateral oscillations and fragmented vortices, while higher roll frequency produces shorter wavelength perturbations, accelerating vortex pairing. Higher TSR strengthens the initial vortices and accelerates breakdown. Standard deviation of velocity fields in the wake confirms larger fluctuations at the top of the rotor due to its greater distance from the roll axis.
This study establishes a direct link between roll-induced blade loads and near-wake dynamics, providing a foundation for improved FOWT design, control, and wake modeling. Future work should assess far-wake impacts, the persistence of lateral velocities induced by rotational motion, and inertial effects under highly unsteady roll conditions.
