Methodology for Assessment of Wind Turbine Noise Generation Author: Kelley, N.D.; Hemphill, R.R.; and McKenna, M.E. [Abstract] The detailed analysis of a series of acoustic measurements taken near several large wind turbines (100 kW and above) has identified the maximum acoustic energy as being concentrated in the low-frequency audible and subaudible ranges, usually less than 100 Hz. These measurements have also shown any reported community annoyance associated with turbine operations has often been related to the degree of coherent impulsiveness present and the subsequent harmonic coupling of acoustic energy to residential structures. Thus, one technique to assess the annoyance potential of a given wind turbine design is to develop a method which quantifies this degree of impulsiveness or coherency in the radiated acoustic energy spectrum under a wide range of operating conditions. Experience has also shown the presence of annoying conditions is highly time dependent and nonstationary, and, therefore, any attempts to quantify or at least classify wind turbine designs in terms of their noise annoyance potential must be handled within the proper probabilistic framework. A technique is described which employs multidimensional, joint probability analysis to establish the expected coincidence of acoustic energy levels in a contiguous sequence of octave frequency bands which have been chosen because of their relationship to common structural resonant frequencies in residential buildings. Evidence is presented to justify the choice of these particular bands. Comparisons of the acoustic performance and an estimate of the annoyance potential of several large wind turbine designs using this technique is also discussed. Characteristics of Large Wind Turbine Noise Figure 1 summarizes the acoustic pressure spectrum associated with large wind turbines and indicates the dominate noise sources as a function of frequency. Not all wind turbines will exhibit the features of the spectrum shown. The ultimate cause of aerodynamically generated sound is the unsteady loading of the blades. The degree of this un- steadiness, for the most part, is responsible for the distribution of acoustic energy across the spectrum of Fig. 1. Kelley-Fig1 Conventional classifications of rotor noise include rotational, broadband or vortex, and impulse noise. Rotational noise is characterized by the large number of discrete frequency bands which are harmonically related to the blade passage frequency. The amplitude of these bands is determined by the sum of the steady load, which is a function of the commanded level of operation of the machine, and the unsteady loading at any moment arising from such sources as inflow turbulence and upstream wakes. Broadband or vortex noise results from the slightly viscous interaction of the unsteady lift and the blade boundary layer and is responsible for such mechanisms as flow separation and tip-and trailing’ edge vortex shedding. Broadband noise, which is described as the “swishing” sound associated with the turbine operation, is characterized by largely incoherent radiation over a wide frequency range with a spectral “hump” sometimes found at relatively high frequencies. Recent measurements of the MOD-2 turbine have found just such a “hump” in the region shown in Fig. 1. Impulsive noise, such as has been found with the MOD-1, is identified with short, transient fluctuations in the radiated acoustic field which can contain considerable energy. The dashed lines in the region transcending the rotational and broadband regions of the spec- trum in Fig. 1 are indicative of impulsive behavior and reflect the very large number of harmonics necessary to describe the blade loading spectrum which are the sources of the radiation. Impulsive noise tends to be the most annoying because it dominates all other sources due to a high degree of coherence and radiation efficiency. From Fig. 1, the highest levels of acoustic energy can be seen to reside in the low-frequency and subaudible (
Posted on: Wed, 18 Sep 2013 20:00:48 +0000
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