Numerical modeling of stall delay and dynamic stall phenomena on floating wind turbine rotors

Abstract

Floating offshore wind turbines have enormous potential to harvest wind energy. Notwithstanding this, floating offshore wind turbines are subject to more complex dynamic behaviour than fixed foundation wind turbines. Unsteady flow effects cause the rotor blade sections to operate at time varying angles of attack that subject the blades to combined Stall Delay and Dynamic Stall phenomena. Floating wind turbines are naturally more prone to experience such Stall Delay and Dynamic Stall conditions. The first objective of this project was to improve an existing open source tool by integrating dynamic stall and stall delay models in order to predict better the unsteady aerodynamics of floating offshore wind turbines. Additionally, in this thesis a detailed numerical investigation of the performance and aerodynamic characteristics of a full scale offshore floating wind turbine subjected to controlled conditions was carried out. The second and primary objective was to investigate how different stall delay and dynamic stall models influence the computer predictions for the aerodynamics loads and power of floating wind turbines. Through several simulations it was shown that the code repeatedly produced expected results for a given set of initial conditions. Verification tests showed that results converged by using a finer discretisation of elements. The results obtained by the code were also validated against experimental data acquired from the TuDelft rotor and the NREL Phase VI wind turbine, tested in both axial and yawed cases. This validation confirmed general agreement between simulations and experiments. Simulation results improved by the introduction of the stall delay and dynamic stall models. Considerable fluctuations were predicted by the simulations on the NREL 5MW floating wind turbine. However, through numerous simulations it was shown that the average power coefficient of a floating wind turbine is very similar to that produced by a fixed turbine. Also, it was seen that the wind turbine efficiency may be significantly affected by the platform's dynamics.