Wind-induced 4P harmonic excitation and blade vibrations in wind turbines: Sensitivity to tower flexibility and yaw-system behaviour

The interaction between operational harmonics and structural modes can affect vibration levels, fatigue loading, and structural integrity in modern wind turbines. This study investigates the sensitivity of blade vibration to tower flexibility and yaw-system behaviour in horizontal-axis wind turbines. The analysis focuses on the resonance between the first blade edgewise mode and the 4P operational harmonic, a phenomenon whose sensitivity to tower flexibility and yaw-system behaviour has received limited attention. A reduced-order dynamic model is used to perform a parametric sensitivity analysis considering wind shear, tower shadow, and wake-induced velocity deficits. The results show that blade vibration is highly sensitive to wake-deficit position, with maximum response near rotor azimuths of 0° and 180°. Furthermore, the flexible yaw configuration with backlash produces the highest vibration levels, while the steel tower configuration exhibits larger responses than the corresponding concrete tower configuration. These findings provide guidance for mitigating vibration and structural loads.