A study on the aerodynamics of floating wind turbine rotors

Abstract

Understanding the impact of wave-induced dynamic effects :on the aerodynamic power performance of Offshore Floating Wind Turbines (OFWTs) is crucial for developing floating wind turbines cost-effectively in order to harness wind energy in deep water sites. The complexity of the wake of an OFWT has not yet been fully understood and measurements and numerical simulations are essential in this regard. An experiment to investigate the aerodynamics of a mode OFWT was undertaken at the University of Malta. Established experimental techniques used to analyse fixed HAWTs were applied and modified for the floating wine turbine condition. The effects of wave induced motions on the rotor aero dynamic variables were analysed in detail. An open source free-wake vortex code was also used to examine whether certain phenomena observed in the experiments could be reproduced numerically by the lifting line method. Results from hot-wire measurements and free-wake vortex simulations have shown that for OFWTs surge-induced torque fluctuations are evident. At high tip speed ratios a discrepancy in the mean power coefficient between the fixed and floating conditions was found from both measurements and numerical simulations. Free wake simulations on a large scale OFWT were also carried out to investigate the wake geometry and aerodynamic performance under realistic and extreme environmental conditions. Complex wake phenomenon were observed under the impact of extreme wave conditions. It was found that the difference between the mean power coefficient under surge conditions and the steady power coefficient depends on platform surge frequency, surge amplitude and the rotor operating conditions. Using the results from the free-wake vortex simulations, an analysis of a number of wind turbine wake characteristics was carried out in order to identify possible reasons behind the increase in the aerodynamic torque and thrust variations with tip speed ratio.