DEVELOPMENT OF AN ACTUATOR LINE MODEL FOR SIMULATION OF FLOATING OFFSHORE WIND TURBINES

Floating offshore wind turbines (FOWTs) are a solution to harvest wind energy in deep water wind farms thanks to higher wind energy potential than onshore configurations. Fast and effective numerical models explaining the impact of platform motion on wind fields are fundamental to harvesting the potential energy in large offshore wind farms. This project developed a CFD-based Computationally-Efficient approach based on an actuator line model (ALM) to study FOWTs. A dedicated C++ library was implemented in the OpenFOAM toolbox to complement reasonable accuracy and affordable computational effort while investigating the effects of the platform motions on the wake evolution. Two well-known turbine cases were studied, including NREL Phase VI with adequate available experimental data and NREL 5-MW being representative of a large-size wind turbine. It was well documented that, at the same displacement, pitch and surge motions have the most considerable impact on turbine performance due to their inherent effect on 3D local wind inclination in the relative frame. The ALM implementation decreased computational cost as just about 400k and 600k grids are necessary for performance assessment of the NREL Phase VI case and NREL 5-MW case with reasonable accuracy. Visualizing the followed flow fields proved the ability of the ALM code in capturing vortices trajectory, potential blade-vortex interactions, vortex pairing, and vortex ring state phenomenon in FOWTs. It is shown that a high amplitude or frequency of motion can result in the dynamic stall or vortex ring state. Even though a motion affects the turbine performance marginally, the wake can still be dominated due to complex flow conditions like vortex interactions or pairing.