Dynamic stall in floating wind turbine airfoils under pitch motion

The six-degree-of-freedom motions of floating offshore wind turbines (FOWTs) complicate airfoil flow and promote dynamic stall relative to fixed-bottom wind turbines. This study investigates the effect of pitch motion on the flow field around airfoils of FOWTs. This study employs computational fluid dynamics and investigates the Delft University airfoils DU35 and DU21, and the National Advisory Committee for Aeronautics airfoil NACA64 at 25.2%, 57.7%, and 89.2% blade span of the National Renewable Energy Laboratory 5 MW wind turbine. First, the three-dimensional pitch motion of the wind turbine is transformed to the two-dimensional complex motion of the airfoil. Then, the aerodynamic coefficients, separation vortex development, and surface load variations of these airfoils are studied under the complex motion and compared with those in the airfoil pitching case. The results show that under pitch motion, the dynamic stall phenomenon propagates toward the blade tip. During backward pitch, the lift coefficients (Cls) of the airfoils exhibit increases of 39.25%, 12.97%, and 6.32%, and the onset of stall is delayed by −0.7728°, 0.2333°, and 8.2537°, respectively. Stall severity is suppressed. During forward pitch, the Cls of the airfoils exhibit decreases of 22.57%, 11.45%, and 3.33%, and the onset of stall is advanced by 0.4145°, 8.8956°, and 4.1771°, respectively. Stall severity is increased. Pitching motion increases the aerodynamic loading on the blade-tip airfoil. The results provide a basis for controlling airfoil dynamic stall under complex motions and optimizing the operational stability of FOWTs.