Unsteady aerodynamics of offshore floating wind turbines in platform pitching motion using vortex lattice method

• Unsteady aerodynamics of floating wind turbines has been investigated.
• A numerical code which is using vortex lattice method was developed.
• A turbulent wake state arises when a floating wind turbine is operated at a low-speed inflow condition.
• The rotor experiences a turbulent wake state when the floating platform undergoes upward pitching motion.

As the flow states of an offshore floating wind turbine (OFWT) differ from those of an onshore fixed wind turbine, it is questionable as to whether the aerodynamic load prediction of a turbine using conventional blade element momentum theory (BEMT) is accurate. The aim of this paper is to show the characteristics of aerodynamic load predictions using the vortex lattice method (VLM). Washizu's experimental data, which was measured under a similar flow state of a floating wind turbine, is used for validation. The prediction shows good results compared to those of an experiment. To determine the unsteady aerodynamics of a floating wind turbine, the NREL 5 MW wind turbine model is used for the simulation of a floating wind turbine. These results show that a turbulent wake state (TWS), which is undesirable condition and cannot predicted in BEMT simulation, arises when a floating wind turbine is operated at a low-speed inflow condition. In addition, the rotor experiences a TWS when the floating platform undergoes upward pitching motion.