To study the output power and wake flow characteristics of a wind turbine with swept blades, taking the blade tip offset and the location of the sweep start as two variables, the straight blade of the DTU-LN221 baseline airfoil was optimally designed with sweep. Then the designed wind turbine was numerically simulated, and the swept blade with the best optimal output power characteristics was selected for the wind tunnel test. The results indicate that for both forward and backward swept blades, increasing the blade tip offset and the sweep start location could decrease the power and thrust coefficients. Compared with the backward swept design, the forward swept design significantly improved the blades’ power characteristics. By adopting swept blades instead of straight blades, wind turbines could generate more power at high tip speed ratios, especially in yaw conditions. The streamwise velocity recovery of the wind turbine with swept blades was slower than that with straight blades as the lateral velocity near the wake region was higher than that with straight blades. Besides, the wind turbine with swept blades had a greater turbulence intensity of the wake near the wake center than that with straight blades with or without yaw condition.

As wind-power installations increase rapidly worldwide in both number and capacity, the noise problems created by wind turbines, which increasingly are being installed near residential areas, are becoming more pressing. For noise-reduction purposes, a wind turbine can be viewed as being a mechanism for generating aerodynamic noise, and key to that noise reduction is a precisely localized sound source. In this paper, the acoustic signal of an S-wing horizontal-axis wind turbine with a diameter of 1.4 m is acquired using a 60-channel microphone array. Using beamforming, the sound-source distribution and frequency characteristics are analyzed in detail, showing different frequency bands at different wind speeds and tip speed ratios. The results show that the sound pressure level of the sound source (i) decreases gradually with increasing frequency band and (ii) increases gradually with increasing wind speed and tip speed ratio. The radial location of the sound source approaches the blade tip gradually, but the variance is not large. The relevant experimental data and conclusions provide a research basis for exploring the mechanism for the aerodynamic noise and ways to reduce it.

Wind tunnel experiments at a model scale have been carried out to investigate the flow characteristics in the near wake of a wind turbine. Time resolved particle image velocimetry (TRPIV) measurements are applied to visualize the wind turbine wake flow. The instantaneous vorticity, average velocities, turbulence kinetic energy, and Reynolds stresses in the near wake have been measured when the wind turbine is operated at tip speed ratios (TSRs) in the range of 3–5. It was found that wind turbine near the wake flow field can be divided into a velocity increased region, a velocity unchanged region, and a velocity deficit region in the radial direction, and the axial average velocities at different TSRs in the wake reach inflow velocity almost at the same radial location. The rotor wake turbulent kinetic energy appears in two peaks at approximately 0.3R and 0.9R regions in the radial direction. The Reynolds shear stress is less than the Reynolds normal stresses, the axial Reynolds normal stress is larger than the Reynolds shear stress and radial Reynolds normal stress in the blade root region, while the radial Reynolds normal stress is larger than the Reynolds shear stress and axial Reynolds normal stress in the blade tip region. The experimental data may also serve as a benchmark for validation of relevant computational fluid dynamics (CFD) models.

The dominant type of vibration in wind turbine blades is flapwise vibration, which causes fatigue loading owing to the out-of-plane alternating load, with blade breakage occurring when the load is below the strength limit. Consequently, the mechanism and distribution of alternating-load generation must be understood to reduce the fatigue load. The strain data in the flapwise direction at the characteristic position of the operating wind turbine were collected to learn the characteristics of the spanwise distribution of the strain power spectral density (PSD) amplitude, to explore the fatigue damage-prone area and the generation mechanism, and to reveal the influence of the rotational speed on the spanwise distribution of the strain. It was evident that linked torsional and edgewise vibrations were predominant and that the strain PSD amplitude of the 1P loading component was nonlinear because of the Brazier effect in the transition region. The fatigue failure susceptible region of the rotating blade is more accurately identified. Except for the 0.10 R position, the blade 1P component 0.70–0.75 R spanwise region is more prone to failure. The rotation speed increases the dynamic fluctuation of spanwise strain.

Although the aerodynamic loading of wind turbine blades under various conditions has been widely studied, the radial distribution of load along the blade under various yaw conditions and with blade flapping phenomena is poorly understood. This study aims to investigate the effects of second-order flapwise vibration on the mean and fluctuation characteristics of the torque and axial thrust of wind turbines under yaw conditions using computational fluid dynamics (CFD). In the CFD model, the blades are segmented radially to comprehensively analyze the distribution patterns of torque, axial load, and tangential load. The following results are obtained. (i) After applying flapwise vibration, the torque and axial thrust of wind turbines decrease in relation to those of the rigid model, with significantly increased fluctuations. (ii) Flapwise vibration causes the blades to reciprocate along the axial direction, altering the local angle of attack and velocity of the blades relative to the incoming wind flow. This results in the contraction of the torque region from a circular shape to a complex “gear” shape, which is accompanied by evident oscillations. (iii) Compared to the tangential load, the axial load on the blades is more sensitive to flapwise vibration although both exhibit significantly enhanced fluctuations. This study not only reveals the impact of flapwise vibration on wind turbine blade performance, including the reduction of torque and axial thrust and increased operational fluctuations, but also clarifies the radial distribution patterns of blade aerodynamic characteristics, which is of great significance for optimizing wind turbine blade design and reducing fatigue risks.

To accurately calculate the unsteady dynamic load and output performance of the rotor in the stand-alone large-scale wind turbine, a novel added mass force expression $m_A=128/75\rho R^3$ is derived and corresponding dynamic wake model is established using the acceleration potential flow theory and the blade element momentum in the present study. Moreover, induced velocity delay, wind shear, and yaw are considered in the proposed model. Referring to IEC61400-1 standard of the wind turbine, numerical models of turbulence, gust and wind direction change are established to investigate the output performance of the rotor under the combined effect of aerodynamic force, gravity, and centrifugal force. The obtained results show that the axial load and flapwise bending moment of the blade root are mainly determined by the axial aerodynamic component, while the tangential load and edgewise bending moment are mainly determined by the gravity component of the blade. It is found that the wind shear has a significant impact on the axial load and flapwise bending moment. Meanwhile, the axial thrust and rotor power are mainly affected by the inflow wind speed. It is observed that when the yaw angle exceeds 9.25°, the axial thrust and power of the rotor decrease obviously. The research results are compared with the experimental data from the NREL, it is found that the proposed model provides a new choose for the calculation of added mass force, and it has more calculation accuracy compared with classical BEM method in the prediction of unsteady dynamic load and output performance of the rotor.