Abstract
Noise prediction from streamlined bodies such as wind turbine blades can be predicted accurately using CFD computations that use spatio-temporal turbulence models at the expense of high computational power. In this work, empirical methods proposed from BPM, Grosveld and Lowson are used to compute numerically to analyse the influence of rotor solidity factor on broadband trailing edge noise from a 2 MW horizontal axis wind turbine with a blade length of 37 m. Inputs to acoustic solver are the velocity vector field and boundary layer data which are obtained using blade element momentum and X-Foil. The outputs from acoustic solver are directivity and far field sound pressure on a receiver located at distance of 120 m from tower base. The results have shown that for a wind speed of 10 m/s measured at 10 m above ground, sound power level was found to increase between mid-bands to high frequencies for all three methods. Rotor solidity effect was illustrated at constant rotational speed of 17 RPM and receiver height of 0.5 m located in downwind position. A minimum difference of 1.5dBA was found at f ~ 100 Hz for Lowson method and maximum of ~ 2.8dBA at 1 kHz between two and three blade rotor. For BPM and Grosveld methods however, the sound levels were ~ 5dBA lower for two blade rotor than three blade rotor between f ~ 100 Hz and f ~ 1 kHz. The study also demonstrated that as number of blades increase by integral multiples, the effect on noise radiation from trailing edge of blades increase by 2-5dBA due to amplitude modulation.
Highlights
In the past decade, size of wind turbines has grown considerably faster from a few kilo-watt to multi megawatt type in order to produce power for various needs of the society [1]
It can be inferred that the maximum difference in sound levels between three blade and two blade rotor is in order of 5 decibel A- weighted (dBA) between 63 Hz and 8 kHz for all three methods
Since the human perception is sensitive to certain band of frequencies, the A-weighted filter correction was used for sound power levels (SPL) values of trailing edge noise dominant between 1 kHz and 5 kHz
Summary
Size of wind turbines has grown considerably faster from a few kilo-watt to multi megawatt type in order to produce power for various needs of the society [1]. Many empirical and experimental studies on aerofoil self-noise mechanisms have been conducted, which provided useful results that are able to predict the noise from trailing edge surfaces in turbulent wind field [6]. It has been found that for low Mach number flows efficient sound generation is produced when the turbulent boundary layer undergoes edge scattering at the trailing edge surface [2,3,4, 7, 8, 11,12,13]. This paper attempts to analyse such effect and aims to predict the 1/3rd octave band sound power level from a 2 MW horizontal axis wind turbine with a blade length of 37 m using three different trailing edge noise models, viz. BPM (Brookes, Pope and Marcolini), Lowson and Grosveld. According to IEC 61400–11 standard, the worst case microphone position is usually downwind and recommended distance equivalent to hub height plus the half rotor diameter has been implemented for assessing the intensity of sound level [4, 7,8,9, 11, 16]
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