A time-marching aerodynamic model for dual-rotor wind turbines (DRWT) is presented with the lifting surface method and free wake model. The performance of a reference DRWT by two axial connected NREL 5MW wind turbines is calculated to verify the algorithm. The output converges with remarkable oscillation. Free wakes swell under the coupling effect. The accumulated power of DRWT outranges single-rotor wind turbine (SRWT) by a limited extent under certain operating conditions. The front rotor (FR) produces the major portion of the power, especially for low inflow velocity. The unsteadiness of performance and spanwise load is further investigated, and the result shows that the power output of the rear rotor (RR) undergoes greater fluctuation than FR. The rotor-rotor induction strengthens both the rotors’ performance before they align and weakens it when they rotate away. The additional disturbance caused by FR’s tip vortices acts negatively on RR’s torque by upwashing the blade tip.

Preliminary numerical simulations are performed for validation of using the frequency response method on the study of aero-damping in a linear oscillating compressor cascade. The traveling wave method and influence coefficient method, based on the idea of the energy method, are also adopted for reference and comparison. The linearity of the unsteady pressure and the finite propagation of small flow disturbance is verified. The results of the aero-damping ratio obtained from different methods are in good agreement. Qualitative analysis is made on how the unsteady aerodynamic pressure distribution determines the overall aero-damping. When the frequency sweeps are conducted to measure aero-damping, the influence of aerodynamic stiffness cannot be ignored and the natural frequency offset caused by it should be taken into account. The existence of the two sidewalls in cascade has a certain impact on the measurement results, but it is still in an acceptable range. The application of the frequency response method on the study of aero-damping in the experiment is proved to be feasible and reliable in principle by numerical simulation.

The flexible deformation of the slender blade has non-negligible effects on the performance of modern megawatt wind turbines, especially under marine environments where additional six degrees of motion is introduced by floating platform. In this research, an aeroelastic coupling model is established based on the lifting surface free vortex wake (LS-FVW) method and geometrically exact beam theory (GEBT) method. By adopting the model developed, the effects of the surge motion on the unsteady aeroelastic performance of floating offshore wind turbines (FOWTs) are investigated with the NREL 5 MW wind turbine. The results show that, the surge motion will increase the average power and thrust of FOWT, while the blade deformation has the opposite effect. The increase of the amplitude and frequency of the surge motion will increase the fluctuation amplitude and frequency of the power, thrust and structural deformation of the wind turbine. Besides, the smaller the amplitude or frequency of the surge, the stronger the ability of the wind turbine-floating platform system to restore stability.