**3. Simulation assumptions**

In the analysis of sound pressure from wind turbine blade, generalized blade element momentum (BEM) method was used to compute the relative velocity field along the blade span. The outputs of BEM solver are relative velocity on the blade section, angle of attack, normal and tangential force coefficients on every section of blade which can be used to compute rotor loading forces and moments. The outputs from BEM solver are coupled to BPM noise prediction module for which, sound pressure level computations are done at a given wind speed, blade pitch angle and rotational speed of the machine.

In the BEM approach the total length of blade is discretized into several aerofoils at least 20 segments. Aerofoil can be assumed as half-infinite flat plate with finite thickness and aspect ratio. The flow over flat plate was assumed to be 2D incompressible and quasi uniform along the blade length which means that flow behavior does vary from one span station to another along the blade span. The overall shape of blade is approximated using selected aerofoils, viz. NACA 0012, NACA 6320 and *Trailing Edge Bluntness Noise Characterization for Horizontal Axis Wind Turbines [HAWT]… DOI: http://dx.doi.org/10.5772/intechopen.99880*

NACA 63215 while the turbulent boundary layer properties on suction and pressure side of aerofoils is computed from XFOIL module. The boundary layer data for aerofoil serve as input to the noise prediction module.

In the prediction of sound pressure levels, each blade segment is treated as a point source in near field and rotating blade as line source. In the far field sound prediction however, the rotor of turbine acts as point source when operating in a wind farm. Sound pressure level is thus calculated by logarithmic addition of individual sources relative to observer position. For the present simulation work, the receiver height was fixed at 2 m above the ground level and the source height was fixed at 80 m. The distance of the receiver location was set at 110 m, which is approximately the total turbine height (*HH + D*/2). This is in accordance with IEC 61400–11 regulations for measurements of acoustic emissions from wind turbines. HH is the hub height of turbine, and *D* is the rotor diameter in m.

A downwind scenario is considered as the worst case since sound waves bend in downward direction with respect to free stream wind and this results in amplification. Therefore, downwind receiver location is considered. The boundary conditions for the blade are Reynolds number, the angle of attack along the blade span. It is implemented to verify that blade element momentum (BEM) computed values do not exceed predefined threshold values as given in [6]. The blade pitch angle is set to 3.5° for sound pressure calculations and rotation speed for machine as 17 RPM.
