## 1 Introduction  
The generation and propagation of wind turbine aerodynamic noise exhibit several special characteristics as compared to some other industrial noise problems. Considering the generation of wind turbine aerodynamic noise, the rotor aerodynamic noise level depends on airfoil profiles at cross sections, blade shape (the spanwise distribution of chord, twist, and airfoils), rotor size, rotational speed, yaw angle, tilt angle, pitch setting, the angular position of blades, wind speed profile, inflow turbulence level, density, and viscosity of air. From the perspective of aerodynamic noise propagation (ANPropagation), aerodynamic noise source (ANSource) of large wind turbines is often located at high altitude which leads to more significant problem for ANPropagation over long range. Intrinsically, the study of wind turbine noise is a multi-disciplinary subject which should consider wind farm aerodynamic noise generation (ANGeneration), ANPropagation, and energy production. However, ANGeneration from a single airfoil has a similar basis in the nature of aerodynamics and aeroacoustics as that from a large wind farm. Sophisticated numerical methods are needed to understand the physical mechanisms of flow induced noise. Computational aero-acoustics (CAA) methods were widely applied favored by the high-performance computing technology. However,advanced large eddy simulation (LES)/CAA method still requires heavy computational effort. Consequently, conventional CAA methods are neither efficient to model wind farm noise source, nor to predict long range ANPropagation. In the following paragraphs, feasible aerodynamic and aero-acoustic modeling techniques are briefly reviewed and the numerical methods employed in the current study are introduced. As a prerequisite for wind turbine noise source and propagation modeling, a proper aerodynamic tool should be considered. There are many methods developed for wind turbine aerodynamic simulations. The classic blade element momentum (BEM) method [1; 2] is a commonly used semi-engineering technique that simulates the aerodynamics of a single wind turbine. With a BEM model, one can apply engineering approaches to compute the noise source level of a wind turbine. When the prediction accuracy increases, the model complexity as well as the computational cost also increases. It is known that the CFD based methods predict the rotor aerodynamic forces and the wake flow behind the rotor more accurately. Considering long-distance ANPropagation of wind turbines, CFD methods become time consuming because the wake flow should be simulated in the large range whose size is the same as that of the ANPropagation. In the present study, the velocity field is the input data for the ANPropagation solver. Instead of conventional CFD methods, other sophisticated methods are developed for wind turbine applications, such as the AD/AL/AS (actuator disc /actuator line /actuator surface) methods, these methods have good trade-off between model accuracy and computational efficiency. The actuator disc method [3; 4] is applied in this study which has relatively high computational efficiency. The rotor is assumed as a permeable disc. The aerodynamic loadings of the rotor are treated as external volume force terms of the momentum equations of Navier-Stokes (NS) equations. The rotor disc of a horizontal axis wind turbine (HAWT) has an inherent axisymmetric characteristic. In this study, to reduce computational time the 3D-AD method is simplified to 2D-AD. The boundary condition of rotor center is axisymmetric, such that the obtained flowfield with 2D-NS equations will be exactly identical to that of the 3D case. Since the axisymmetric assumption is made, it has to be mentioned that the solution only holds correct for axisymmetric uniform inflow conditions and rotor under no yaw and tilt manipulations. Dealing with the modeling of ANSource, the CAA methods, based on high fidelity techniques [5; 6; 7; 8; 9; 10], can model the physical details of flow induced noise mechanisms.