Acoustic Noise Measurement Downstream of an Oscillating Wind Turbine Blade Section

Acoustic measurements were performed using microphone downstream of a 2-D wind turbine blade section in wind tunnel. The experiments have been carried out in both static and oscillatory pitching cases. The latter is usually experienced by the blades in actual circumstances. The microphone was 1.5 chords downstream of the airfoil and the measurements were conducted at three transverse positions, i.e. behind the trailing edge, midway between the trailing edge and the ground and very close to the ground. A CFD simulation of the flowfield has also been conducted using Fluent to correlate the acoustic behavior to the phenomena observed in the flowfield around the blade. The results show that the acoustic noise heard by a listener located on the ground is higher and stronger than that positioned downstream of the trailing edge, showing the ground effect on acoustic noise reverberation. The aerodynamic noise heard by the listener, changes from a treble to bass sound as the angle of attack increases. Beyond stall, the flow is dominated by the vortices shed into wake and the acoustic noises would be at very low frequencies which would result in a bass sound accompanied by structural vibration. In high angle of attack range, such noises can hardly be heard by a normal person but have a very destructive role on blade structure.