Acoustics Australia Vol. 40, No. 1, April 2012 - 7 WIND TURBINE NOISE MECHANISMS AND SOME CONCEPTS FOR ITS CONTROL Con J. Doolan, Danielle J. Moreau and Laura A. Brooks School of Mechanical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia INTRODUCTION Climate change policies have forced governments around the world to mandate large increases in wind power. Consequently, wind power is now one the fastest growing energy sources, with worldwide generation predicted to increase from 150 TWh in 2008 to 1068 TWh (per annum) by 2030 [1]. In Australia, wind energy production is set to increase from 4 TWh in 2007-8 to over 40 TWh by 2030. Wind energy increases will mean that many more wind turbines will be installed, inevitably closer to more people and their residences. Noise from wind turbines is a serious and controversial issue and it can be expected to become more of a concern as wind power production is increased. Surveys [2] show that noise from wind turbines is annoying to people and that it is perceived to be more annoying than other forms of industrial noise at the same level. To accommodate the expected increase in the number of installed wind farms and to reduce public disquiet, there needs to be more research and development into how wind turbine noise is generated and how it can be controlled. The purpose of this paper is to review the aeroacoustic source mechanisms that are on a wind turbine blade and possible methods for reducing their strengths. An engineering analysis is performed that gives an indication of the frequencies that contain most of the energy for each type of source. Some recently published results on wind farm noise will be discussed that suggest that the noise from multiple wind turbines can interact, creating intermittent regions of increased noise amplitude. Daytime noise measurements taken several hundred meters from a South Australian wind farm are also presented. These measurements show noticeable amplitude modulation that is similar to that of European data. An explanation for the noise phenomena is suggested in this paper along with some conceptual ideas for its control. WIND TURBINE AERODYNAMIC NOISE GENERATION MECHANISMS The major noise sources on a wind turbine are located at the gearbox and the fast moving outer blade tip region [3]. Gearboxes on modern turbines are now very quiet [4] and therefore the dominant noise sources are located on the blade. These noise sources are aeroacoustic in origin and in order to understand them, a review of blade aerodynamics is first necessary. Figure 1 shows an idealised picture of a wind turbine outer blade tip moving through air. The major aerodynamic phenomena that influence noise are shown. Ahead of the blade is atmospheric (or other) turbulence. When the blade interacts with these turbulent eddies, unsteady lift is generated by the blade. The unsteady lift creates a dipole-like sound source located at the blade leading edge [5]. This is called inflow or leading-edge interaction noise and has a dipole-like directivity pattern. The flow of air over the blade surface creates a boundary layer, due to the viscous shear present between the blade and the air. The flow conditions on large wind turbine means this boundary layer will usually transition to a turbulent state by the time the air reaches the trailing edge. Turbulence by itself is a very inefficient radiator of sound [6], but when turbulent eddies pass a sharp edge (such as the trailing edge of a wind turbine blade), the acoustic waves created by turbulence are reinforced via an edge diffraction mechanism [7], making them much more efficient. This is known as trailing edge noise [8] and is the major noise source on a wind turbine [4, 9, 10]. An important quality of trailing edge noise is its directivity pattern, which is different from a monopole or dipole. Figure 2 illustrates the directivity pattern of trailing edge noise, assuming that the frequency of sound emitted from the trailing edge is high enough so that the airfoil can be considered a semi-infinite half- plane. Most of the sound is radiated forward of the blade (in what is known as a cardioid directivity pattern), in the direction of rotation, while little is radiated behind. This explains the “swish” character of wind turbine noise whereby an observer on the ground will periodically receive fluctuations in acoustic energy as the blade rotates. Here, “swish” is defined as the amplitude modulation of broadband aerodynamic noise created by the blades at the blade passing frequency, which is usually about 1 Hz [11]. The received acoustic signal has both a high The aerodynamic noise production mechanisms of modern horizontal axis wind turbines are reviewed. An engineering analysis of the time and frequency scales from three noise sources, leading edge turbulence interaction noise, trailing edge noise and blade-tower interaction noise is presented. The analysis shows that noise sources are present from low-frequencies (1-4 Hz) to over 500 Hz for a representative wind turbine. The results of the analysis are used to explain amplitude modulation observed during noise measurements at a European wind farm. Daytime noise measurements close to a South Australian wind farm are also presented that show amplitude modulation. The paper concludes with a description of conceptual ideas for the control of wind turbine noise.