Experimental investigation into airfoil self-noise, blade-tower interaction noise and wind farm noise character

Abstract

An experimental investigation into airfoil self-noise (trailing edge noise, tip vortex formation noise and bluntness trailing edge noise) and blade-tower interaction (BTI) noise from rotating airfoils, is presented. Investigation into these noise sources enables characterisation of wind turbine noise and the results can be compared with measurements taken in the vicinity of a wind farm. Investigation of airfoil self-noise and BTI noise was conducted on NACA 0012 airfoils mounted on a rotor at 0, 5, and 10 degrees angle of attack. Flow measurements were carried out in an anechoic wind tunnel (AWT) using hot-wire anemometry and acoustic measurements were conducted in an anechoic room and in the AWT with point microphones and microphone arrays. With hot-wire anemometry, the state of the boundary layer in the wake of trailing edge was determined and microphone arrays were used for beamforming. For characterisation of wind farm noise, acoustic and meteorological measurements were carried out at several locations in close vicinity to a wind farm. Wind farm noise was simultaneously measured indoors and outdoors for when the wind farm was operational and non-operational together with measurements of wind speed and direction. Good agreement is found between the peak trailing edge noise for stationary and rotating airfoils. Additionally, the comparison between measured and predicted airfoil self-noise using predictions based on BPM models is good at all angles of attack. BTI noise is shown to manifest at distinct acoustic pressure pulses, which occur at the blade pass frequency. This noise source is shown to be the likely origin of wind farm infrasound. Furthermore, it is shown that the combination of BTI noise and steady thickness noise contains a large number of higher harmonics, which are the most likely source of wind farm tonal low-frequency noise. It is concluded, that since wind farm infrasound and lowfrequency noise can exceed the normal hearing threshold, BTI appears to be an important noise source on a wind turbine. Characterisation of wind farm noise shows that tonal wind farm infrasound and low-frequency noise (ILFN) are measured frequently under stable and very stable atmospheric conditions which are dependent on the time of year. This noise was compared with several hearing thresholds and it is shown that ILFN can be perceived by the human auditory system up to 4 km away from a wind farm.