Low-Frequency Noise Propagation from a Small Wind Turbine Tower
A major barrier to the acceptance of small wind turbines is that they are perceived to be noisy particularly when mounted on monopole towers rather than traditional guy-wired ones. Noise emission from a 2.4 kW downwind turbine due to its 10.2 m monopole t
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Abstract A major barrier to the acceptance of small wind turbines is that they are perceived to be noisy particularly when mounted on monopole towers rather than traditional guy-wired ones. Noise emission from a 2.4 kW downwind turbine due to its 10.2 m monopole tower was investigated. Tower vibration was measured using 24 accelerometers. A finite-element tower model combined with simple assumptions for the turbine and wind loads allowed the noise to be obtained from solution of the wave equation. The measured vibration levels were matched to the tower model amplitudes. Sound pressure level produced by the fluid–structure interaction reached 30 dB at about 11 m from the tower and decreased to 5 dB 1 km away. Propagation switched from cylindrical to hemispherical when the distance was about 200 times larger than the tower height.
Keywords Small wind turbine tower Low frequency noise propagation Fluid– structure interaction Acoustic-structure interaction
1 Introduction Vibration generation and consequent noise propagation have been one of the issues in the development of sustainable wind energy. Less noisy turbines have become available in recent decades due to improved design of airfoils and other components such as bearings and generators. Still, there are some missed areas which need more investigation. One of those is how the tower contributes to total noise generation. Monopole towers are thought to amplify the generated noise like a speaker, at low frequency and particularly in the infrasound range. Furthermore, E. Mollasalehi Q. Sun D. H. Wood (&) Department of Mechanical and Manufacturing Engineering, University of Calgary, Calgary, Canada e-mail: [email protected]
Y. Zhou et al. (eds.), Fluid-Structure-Sound Interactions and Control, Lecture Notes in Mechanical Engineering, DOI: 10.1007/978-3-642-40371-2_40, Ó Springer-Verlag Berlin Heidelberg 2014
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a tower will behave more like a line source than the nacelle and blades, and thus may cause significant noise even if its sound power level is low. Vibration from nacelle components and blades can be easily transferred to the tower (Marmo and Carruthers 2010) and amplified. The generated noise is propagated cylindrically because of tower’s height. The authors showed in (Mollasalehi et al. 2012) that most of vibration energy is concentrated in lower frequency bands, i.e., below 20 Hz, using 24 accelerometers on the 10.2 m tower of a 2.4 kW wind turbine. The first three natural frequencies were excited more often than the higher modes while blade rotational speed varied up to 250 rpm. This work aims to estimate noise level produced by the tower using fluid/acoustic-structure interaction model, and conclude whether it is significant. The main reason to determine the noise level computationally is that the overall noise levels are low and are generated by multiple sources, which are difficult to separate without using microphone arrays and sophisticated signal processing (Bowdler and Leventhall 2011).
2 Model Develo
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