Aerodynamic simulations of offshore wind turbines by using a potential flow approach

Abstract

This thesis explores the aerodynamic performance of floating offshore wind turbines (FOWTs) using a potential flow approach. The primary objective is to understand how the output power of an FOWT is affected by imposing different motions on the substructure. To this end, I employ an aerodynamic model based on the well-known unsteady vortex-lattice method (UVLM) to simulate the aerodynamic behaviour of the turbines in combination with a UVLM-oriented mesh generator (UVLMeshGen, developed at the University of Bergen) to build the aerodynamic grids. As part of this thesis, I have also integrated into UVLMeshGen the capacity of: meshing spar-like substructures, generating boundary surfaces intended for representing sea waves, and the kinematics associated with both the substructure’s motion and sea waves. Initially, the aerodynamic model of fixed and floating offshore wind turbines were validated against well-established benchmarks, including the NREL 5 MW, DTU 10 MW RWT, and Sandia 13.2 MW turbines. Following this, various motion scenarios were simulated to investigate the effects of individual and combined heave, surge, and wind turbine pitch motions on the power output. The results showed that the surge motion has the most significant impact on the power, while the heave motion has the least. However, the heave motion greatly affects the shape of the wake. In addition, I analyze the effect of including sea waves, as a boundary surface with imposed kinematics, on the output power of stand-still FOWTs. The findings indicate that higher wave amplitudes produces a slight increase in power output, the so-called blocking effect in aeronautics. Such findings undoubtedly require further investigations in this direction to fully understand and characterize such a phenomenon. Finally, simulations of multiple FOWTs demonstrated the importance of considering wake interactions when designing wind farm layouts. FOWTs placed in close proximity showed reduced power output due to wake interference, highlighting the need for accounting wake interactions when optimizing wind farm layouts and operating wind farms. This research contributes to the field of renewable energy by enhancing the predictive capabilities of aerodynamic simulations for FOWTs. It provides a framework for future studies to incorporate more complex wave-structure interactions and optimise the design and placement of floating wind turbines to maximise the power production.