An experimental study on the aeromechanics and wake characteristics of a novel twin-rotor wind turbine in a turbulent boundary layer flow

The aeromechanic performance and wake characteristics of a novel twin-rotor wind turbine (TRWT) design, which has an extra set of smaller, auxiliary rotor blades appended in front of the main rotor, was evaluated experimentally, in comparison with those of a conventional single-rotor wind turbine (SRWT) design. The comparative study was performed in a large-scale wind tunnel with scaled TRWT and SRWT models mounted in the same incoming turbulent boundary layer flow. In addition to quantifying power outputs and the dynamic wind loadings acting on the model turbines, the wake characteristics behind the model turbines were also measured by using a particle image velocimetry system and a Cobra anemometry probe. The measurement results reveal that, while the TRWT design is capable of harnessing more wind energy from the same incoming airflow by reducing the roots losses incurred in the region near the roots of the main rotor blades, it also cause much greater dynamic wind loadings acting on the TRWT model and higher velocity deficits in the near wake behind the TRWT model, in comparison with those of the SRWT case. Due to the existence of the auxiliary rotor, more complex vortex structures were found to be generated in the wake behind the TRWT model, which greatly enhanced the turbulent mixing in the turbine wake, and caused a much faster recovery of the velocity deficits in the turbine far wake. As a result, the TRWT design was also found to enable the same downstream turbine to generate more power when sited in the wake behind the TRWT model than that in the SRWT wake, i.e., by mitigating wake losses in typical wind farm settings.