## 1 Introduction  
As a clean and renewable energy, wind power has the advantages of low cost, low environmental pollution and wide availability. These unique advantages of wind power make it an important part of the sustainable energy mix in many countries. However, wind energy also has some drawbacks that hinder its global use. The noise caused by wind turbines has become one of the primary sources polluting the urban environment today. Wind turbine noise caused by the movement of the blades through the air is often seen as an essential aspect causing great annoyance as compared with other noise sources [1]. In Danish regulation [2], a modern wind turbine should be set up at least four times its tip height away from residential areas. Even segregated by such a long distance, a turbine still produces a sound pressure level (SPL) of more than 40 dB (A), which almost equals some common home appliances [3]. A wind turbine operating under wind shear and in yaw produces periodic changes of blade loading, which intensifies the amplitude modulation (AM) of the generated noise, and thus can give more annoyance to the people living nearby. If the aerodynamic noise of a wind turbine can be reduced without limiting its rotational speed, wind energy can be utilized with a high efficiency. Therefore, in this paper, aerodynamic characteristics and noise features of a wind turbine in wind shear and yaw is studied. Noise from a wind turbine or wind farm often has a feature of amplitude modulation with a varying sound pressure amplitude during its operation. There are two types of AM noise: the one is created by changing the blade noise directivity to receiver [4] and the other is caused by non-uniform inflow and operation, for example the operation in wind shear and yaw. There are three methods (Japanese F-S method [5], UK method [6] and min-max method [7]) to characterize the modulation depth. Recently, [PERSON] et al. [8] studied the amplitude modulation caused by a wind turbine wake using an advanced Computational Fluid Dynamics (CFD) (EllipSys3D/AL) and a sound propagation model based on solving parabolic wave equation and found that the modulation depth can reach 4-5 dB even with the same strength of noise source. The prediction formulae of trailing edge noise were summarized first using data from an experiment by [PERSON] and [PERSON] in 1981 [9]. In 1985, [PERSON] [10] used the helicopter noise formulae synthesized by [PERSON] to calculate the emission noise of several two-blade, low-power downwind horizontal axis wind turbines, and the results were in good agreement with the measured data. In 1986, [PERSON] continued the research based on a similar model [11]. Back to 1981, [PERSON] [12] utilized an approach for the low-frequency noise of a wind turbine. A rather complex airfoil self-noise prediction model (known as the Brooks, Pope and Marcolini: BPM model) using wind tunnel acoustic measurements of National Advisory Committee for Aeronautics (NACA) 0012 airfoils was published by [PERSON] et al. [13] in 1989. In 1996, [PERSON] and [PERSON] [14] used the BPM model and the inflow noise model by [PERSON] [15] modified from [PERSON]'s model [16] to calculate the noise emission of a Vestas V27 wind turbine. In 2003, the National Renewable Energy Laboratory (NREL) [17] developed its NREL AirFoil Noise (NAFNoise) based on the BPM model. The BPM was improved by including the real blade geometry for predicting wind turbine noise generation by [PERSON] et al. from Technical University of Denmark (DTU) in 2005 [18]. In 2009, [PERSON] [19] studied wind shear effects on noise at different locations and at different time instants of the day and of the year using field data collected at wind farm sites. [PERSON] [20] investigated the influence of wind shear on the amplitude modulation of wind turbine noise in 2015. Wind shear and atmospheric turbulence effects on wind turbine noise using the Monin-Obukhov similarity theory was studied in 2016 by [PERSON] and [PERSON] [21].