Coupled aero-servo-elastic method for floating offshore wind turbine wake analysis

A new coupled aero-servo-elastic method is developed to model unsteady loads and wakes of Floating Offshore Wind Turbines (FOWTs) with elastic blades, in which rotor speed control and blade pitch control strategies are coupled with an aero-elastic model based on Actuator Curve Embedding (ACE) method and nonlinear finite element rotating beam model in a lab code-HEU-FOWT. The method is capable for efficient aero-servo-elastic simulation of FOWT(s) including wakes on relatively coarse Cartesian grids without requiring empirical tip loss corrections. Tests of an isolated blade and a rotating uniform cantilever beam indicate the static deformations, natural frequencies, modal shapes, and centrifugal stiffening effects are well predicted. Validations of a bottomfixed NREL 5 MW wind turbine show good accuracy for various aero-servo-elastic results in a wide range of wind speeds, and the magnitude of blade tip torsion is found significantly overpredicted by about 26 times (comparing - 2.71 degrees to -0.1 degrees) at rated conditions if the aerodynamic center offset effects are neglected, leading to the rotor thrust and blade tip out-of-plane deformation being underpredicted by 10.5% and 15.0%, respectively. Coupled aero-servo-elastic wake analyses of a NREL 5 MW wind turbine under specified surge motion show the two control strategies (constant power and constant torque mode) significantly reduce the overall far wake deficit by 38.1% and 35.6%, while blade elasticity only reduces the same quantity by 2.7%.