Experimental Investigation Of Airfoil Trailing Edge Noise Reduction By Using TE Serrations

More and more wind turbine generators systems have been built in southern China in recent years. The aeroacoustic noises caused by blade rotating at high wind speed is prominent and is the main noise sources. As the generators are becoming nearer to residential areas, complaints from local people are also becoming more universal, which brings great economic loss to generators operators. More severely, the noise produced by generators has a negative influence on local residents’life. Therefore, it becomes increasingly important and urgent to resolve the aeroacoustic noise problem of wind turbine blade. For this purpose, this paper mainly focuses on experimental measurement and investigation of wind turbine blade noise reduction, which includes improvement of noise detection methods and experiment of noise reduction for different airfoils by inserting trailing edge serrations. This work is intended to provide some preliminary study for the design and optimization of wind turbine blade with low noise, and feasible noise measurement and noise assessment for wind turbine generators. The progresses are as follows:1. In this paper, instead of using single channel as reference and then computing the phase differences of other channels, multi-channels are simultaneously treated as references, and then the phase differences are calculated based on convolution method between one and another. This method greatly reduces the error of cross power spectrum matrix and improves the locating capacity of microphone array. Meanwhile, the time domain signal is got fully utilized by applying STFT and energy average method, which aims to remove the effect of sudden change of signal as much as possible;2. Based on above improvement, noise sources of a 1.5 MW wind turbine generators are detected by using DAMAS codes written by ourself. The distribution of noise sources on wind turbine rotor plane has two features. First, the noise strength of blade moving upward is at least 10 dB less than that of blade moving downward. Second, blade noise and hub noise are the main sources below 400 Hz. Above 400 Hz, the main sources is that caused by the interaction of blade rotating and incoming air. Meanwhile, sources on blades move toward the tip for increasing frequency. Compared with beamforming, the superiority of DAMAS mainly lies in three areas. Firstly, the size of main lobe is smaller, which can be relied to locate sources more accurately; Secondly, less and smaller false sources exist, which reduces the misinterpretation of noise sources. Finally, "edge effect" can be restrained effectively, which removes noise effect outside the scanning plane;3. Experimental investigation of three different models (two airfoils and one plate) noise reduction by the use of trailing edge serrations is done in an anechoic wind tunnel. The sound power spectrum reduces by 3-6 dB over a broad frequency range at mid to high frequencies. For increasing Re, the frequency range of noise reduction moves toward high frequencies, and widens according to 0.5< St< 1. The effects of angle of attack on noise reduction for different airfoils have features in common, but still show differences. One similar feature is that the reduction effect is better at moderate angle of attack than that at 0 degree. Not only are tonal noises with high magnitude removed, but also broad frequency noises at low to mid frequencies are effectively reduced. As for the influence of airfoil geometrical parameters on noise reduction, noise is not so much sensitive to the aspect ratio λ/h as to the serration height 2h. Moreover, the serration height 2h only takes effect over a frequency range at low frequencies under Re=1.2×105. Noise reduction effect is better when serration height is larger;4. There are various degree of noise reduction in streamwise fluctuant velocity spectrum at some points near the trailing edge area for different airfoils, with more than 4 dB at most. Sometimes this can even be associated with the development of sound power spectrum. Moreover, serrations can change the turbulence scale of the flow field. Therefore, the reduction of streamwise fluctuant velocity near trailing edge serrations may be an important reason for noise reduction. However, to understand the mechanism between the change of flow field and noise reduction, more exact measurement and analysis of flow field is a must.