The Study of the Unsteady Aerodynamics and Stability of a Wind Turbine Under Flaoting Platform Motions

There is great interest in developing wind energy beyond the coast, known as offshore wind energy. In general, the quality of wind resources is better offshore than onshore. The wind speed is more stable, consistent and stronger with less turbulence intensity and smaller shear at sea than on land. Generally, there are two types of offshore wind turbines due to the design of the foundation: the fixed-base offshore wind turbine and the floating offshore wind turbine (FOWT). Floating platform wind turbines are more economically feasible in deep water than fixed base offshore wind turbines.

The aerodynamic performance of FOWTs is much more complex than fixed based wind turbines because of the flexibility of the floating platform. Due to the extra six degree-of-freedom of the floating platform, the inflow of the wind turbine rotors is highly influenced by the motions of the floating platform. These platform motions may result in unsteady inflow environment and lead to unsteady aerodynamic fatigue loads and power fluctuation.

The present study aims to study the high unsteady aerodynamic performance of the floating wind turbine under platform several single degree of freedom (DOF) motions and multiple coupled DOF motions. A lifting surface method with free wake model is used to predict the unsteady aerodynamic performance of the wind turbine. An instantaneous aerodynamic thrust damping of a wind turbine rotor based on Hilbert transform method is developed to analysis the stability of the wind turbine under various coupled platform motions.

A full scale U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) 5 MW floating wind turbine is chosen as the subject of the present study. The unsteady aerodynamic loads, the transient of wind turbine states and the instability of the wind turbine are discussed in details.

The unsteady aerodynamics (evolution of the tip wakes, angle of attack and induced velocity along the wind turbine blades and the lag effects on the blade and rotor aerodynamic performance) under platform single DOF or multiple DOFs motions have been calculated and analyzed in details. The results show that the shed vortices released from the trailing edge of the blades and tip vortices rolled from the near wakes have different level of scale of induced velocity on the wind turbine blades. Under single DOF platform motions, the difference of the air loads at each blade may leads to other DOF motions of the platform. Under multiple coupled DOF motions of the platform, at some phase angles between the DOF motions may increase the platform motion and lead the wind turbine-floating platform system into instable condition.