CFD modelling of offshore floating wind turbine rotors

Abstract

Significant research in the field of Offshore Floating Wind Turbine (OFWT) rotor aerodynamics has been documented in literature, including validated aerodynamic models based on Blade Element Momentum (BEM) and vortex methods, amongst others. However, the effects of platform induced motion on the rotor performance or any research related to such areas is rather limited. The project’s approach is based on an actuator disc (AD) technique implemented in a computational fluid dynamics (CFD) solver. The AD approach couples the Blade Element Theory (BET) for estimating rotating blade loads with a Navier Stokes (NS) solver to simulate the wake created by the turbine. Initially, results from a CFD-based AD numerical model for a fixed (non-surging) rotor are compared with those obtained from a BEM theory, existing experimental work and some computational methods. Furthermore, the project also focuses on the effect of tip speed ratio (TSR) on the rotor thrust and power coefficients. This is followed by the analysis of floating wind turbines specifically in relation to surge motion, through an AD technique implemented in CFD software, ANSYS Fluent®. The approach was slightly altered such that the BET-CFD coupling was implemented in a transient manner i.e. following the sinusoidal variation of the rotor surge position with time to account for the influence of regular waves on OFWT motion. The floating platform data extracted from the AD approach was compared to the non-surging turbine data obtained, to display platform motion effects clearly. An important step in the presented work is the adaptation of the AD model to model a full-scale offshore wind turbine. Floating (surging) analysis then followed with discussion of similarities and differences between data of both CFD models. Lastly, educated conclusions are presented from the observations of computed data, including wave motion effects on performance and therefore design challenges. The project explored the dependency of the rotor performance on the surge frequency and amplitude. It is observed that larger values resulted in larger cyclic loads on the rotor. Similarly, the effects of the sinusoidal surge motion on rotor aerodynamics was clearly captured by the CFD-based AD approach. Lastly, it is found that under specific surge conditions, the efficiency of a surging rotor may exceed that of a fixed rotor. This phenomenon has been observed in both rotors investigated in this study.