^{1}

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In the present work, Large Eddy Simulation (LES) of turbulent flows over a NACA 0015 airfoil is performed. The purpose of such numerical study is to relate the aerodynamic surface pressure with the noise generation. The results from LES are validated against detailed surface pressure measurements where the time history pressure data are recorded by the surface pressure microphones. After the flow-field is stabilized, the generated noise from the airfoil Trailing Edge (TE) is predicted using the acoustic analogy solver, where the results from LES are the input. It is found that there is a strong relation between TE noise and the aerodynamic pressure. The results of power spectrum density show that the fluctuation of aerodynamic pressure is responsible for noise generation.

As one of the important renewable energy source, wind energy development is growing very fast in recent years. To achieve the ambition of 100% renewable energy in 2050, wind energy is considered as the most important renewable energy source in Denmark. Continuously improving wind energy efficiency has become the major task to keep wind power as an ever growing portion of the world renewable energy. However, among some other problems, public acceptance due to noise from wind turbines becomes a significant barrier for future wind energy development. The issue of wind turbine noise becomes a large factor of uncertainty during wind turbine design and wind farm planning. There is a need of developing wind turbine noise prediction tool that can be used to design low noise wind turbines and silent wind farms.

So far the methods that can be used for wind turbine noise prediction range from the rule of thumb models to advanced Computational Aero-Acoustic methods (CAA). From the simple models to the CAA methods, the computational time grows exponentially. Some of the most widely used methods are summarized below:

The rule of thumb models.

Semi-empirical engineering models: BPM [

More theoretically based models: Amiet [

CAA methods: Acoustic analogy [

The engineering tools are convenient to provide reasonable good predictions. The CAA methods are able to give physical understanding of the flow induced noise mechanisms. Unless DNS is applied, modelling of turbulence cannot be avoided where LES is so far a reliable model for turbulent flow and noise investigations. The aim of the present study is to focus on turbulent flows over an airfoil and its noise generation. By understanding more about the flow characteristics with respect to noise generation, it is possible to design low noise airfoils with special treatment at trailing edge.

The numerical approach solving the Navier-Stokes equations and the TE noise is presented in this section. Based on the results from LES, the acoustic analogy is performed to calculate noise level at receiver.

The Reynolds number of the current study is around 3 million. Although sound propagation is based on compressible air, the flow can still be considered as incompressible because the flow and acoustic solver can be split [

The turbulent stresses in Equation (1) is based on the mix model [_{x}Δ_{y}Δ_{z})^{1/3} is the reference filter size based on the grid volume. The model constants are C = 0.04 and α = 0.5.

In this work, the formulation proposed by Farassat [

The right hand sides of Equations (5) and (6) are the integrations of time history variables obtained from flow calculations. The variables include wall normal velocity v_{n}, the Mach number of the source in the radiation direction M_{r}, the pressure on the solid wall surface p, the distance between source and receiver r, the angle between the radiation direction and the local wall normal direction θ.

The wind tunnel used for the surface pressure measurements is optimized for aerodynamic performance. Since TE noise signal is difficult to obtain directly, the surface pressure measurements are performed to evaluate the noise source mechanisms [

A C-type mesh is created for numerical simulation. The mesh is block-structured with 48^{3} grid points in each block. The total number of blocks is 216 which contains a total number of grid points around 24 million. The first wall cell size is in the order of 10^{−5} chord and the ratio of ∆x/∆y is around 25 along the airfoil wall surface. An iso-vorticity plot is shown in

An additional computation is carried out for the same airfoil with a serrated TE. The flow and mesh configurations are the same as the previous calculation. As seen from _{1} and SE_{1} are located at x/c = 0.9. The other two curves are plotted for location x/c = 0.64. As it is seen, the signal closer to TE contains more energy than the one in the upper stream. Also it is observed that near the TE region, the serrated airfoil has less spectral density amplitude which might indicate less noise generation.

The acoustic computation is performed when the turbulent flow is numerically stabilized. The integration of sound source is through Equation (6) where only the loading noise plays the role. The time history data of acoustic pressure is recorded during the simulation. After that, the Sound Pressure Level (SPL) of the original airfoil and the serrated airfoil is compared at a same receiver point which is located at 3-chord distance normal to the suction side of TE. It is interesting to see that the difference of the SPL is quite similar to the one we see in

Numerical simulations of flow and noise generation of a NACA 0015 airfoil at Re = 3 × 10^{6} are carried out. The incompressible pressure signals at TE are compared with wind tunnel experiments where some general agreements are found at low frequencies. The geometry of the original airfoil is then modified with serrated TE. At a relatively low angle of attack of 4˚, the computations show that it is the small part of the TE that dominates the noise level.

The current numerical study is supported by the European Union’s Seventh Programme for research, technolo- gical developement and demonstration under the grant agreement No 322449. The auther wish to acknowledge the Energy-Technological Development and Demonstration Program under the project (J.nr. 64012-0146) and the Danish Council for Strategic Research under the project (Sags nr. 0603-00506B).

Weijun Zhu,Wenzhong Shen, (2015) Airfoil Trailing Edge Noise Generation and Its Surface Pressure Fluctuation. Journal of Power and Energy Engineering,03,14-19. doi: 10.4236/jpee.2015.37003