CFD study of induction and finite blade effect on a floating horizontal-axis wind turbine

Abstract

As the need for abundant and reliable renewable energy increases, there is a growing interest in floating wind turbines, which would allow to harness the wind resource in areas where bottom-founded wind turbines cannot be used. However, the movements of the platform are expected to cause unsteady aerodynamics effects, including different wake dynamics, a variation of the induction field at the rotor and blade-vortex interactions. Consequently, there is no general consensus on whether Blade-Element Momentum (BEM) codes could be employed to model the aerodynamics of floating wind turbines. This poses a serious issue as BEM models are widely used in industry practice.
This project proposes to analyze the impact of surge motion on the induction field of a horizontal-axis wind turbine. A suitable actuator disc model is developed, followed by an actuator line model that allows to study the effect of the finite number of blades. Both models are implemented in OpenFOAM, an open-source CFD software. The simulations are run for a range of case studies with imposed baseline thrust, amplitude of the thrust variations, surge frequency and surge amplitude and the resulting induction factors are compared to those obtained with a dynamic inflow model, to assess whether a momentum method could lead to accurate predictions. Particular attention is given to the identification of the wake states of the streamtube, since both turbulent wake state and vortex ring state imply a breakdown of momentum theory.
The results of the actuator disc simulations show that in all cases there is a good agreement with the induction factors obtained with the examined dynamic inflow model. Turbulent wake state is only entered when a high thrust coefficient is reached at low frequencies, while propeller state is only entered when a negative thrust coefficient is reached at low frequencies. Furthermore, no signs of vortex ring state were detected.
Some of the cases considered during the first part of the project were also run with the actuator line model, to examine the finite blade effect. A rotor with three blades was modelled. The resulting disc average induction factors are in excellent agreement with those obtained with the actuator disc model, while the induction at the disc center is lower. The contour plots show that the conclusions on the wake states entered by the streamtube remain valid. It is advised to test this model at different tip speed ratios and for rotors with different numbers of blades.
Overall, this project contributes to a better understanding of the aerodynamics of floating wind turbines and gives confidence in the possibility of using momentum methods during their design phase. Furthermore, the CFD models developed are a flexible tool that may be used for future research on related topics.