Noise White Paper Outline - Minuteman Wind, LLC

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nature, but tonal components may occur due to blunt trailing edges, or flow overslits and holes.Figure 7: Schematic of Flow around a Rotor Blade [Wagner, 1996].Table 1: Wind Turbine Aerodynamic Sound Mechanisms [Wagner et al., 1996]Type or indication Mechanism Main characteristics & importanceLow-frequency soundSteady thickness noise;steady loading noiseUnsteady loading noiseInflow turbulence soundAirfoil self-noiseTrailing-edge noiseTip noiseStall, separation noiseLaminar boundary layernoiseBlunt trailing edge noiseNoise from flow overholes, slits andintrusionsRotation of blades or rotation oflifting surfacesPassage of blades through towervelocity deficit or wakesInteraction of blades withatmospheric turbulenceInteraction of boundary layerturbulence with blade trailingedgeInteraction of tip turbulence withblade tip surfaceInteraction of turbulence with bladesurfaceNon-linear boundary layerinstabilities interacting with theblade surfaceVortex shedding at blunt trailingedgeUnstable shear flows over holes andslits, vortex shedding fromintrusionsFrequency is related to blade passingfrequency, not important at currentrotational speedsFrequency is related to blade passingfrequency, small in cases ofupwind rotors, though possiblycontributing in case of wind farmsContributing to broadband noise; notyet fully quantifiedBroadband, main source of highfrequency noise (770 Hz < f < 2kHz)Broadband; not fully understoodBroadbandTonal, can be avoidedTonal, can be avoidedTonal, can be avoidedWind Turbine Noise Issues Renewable Energy Research Laboratory Page 12
Infrasound from Wind TurbinesWhen discussing infrasound from wind turbines, it is particularly important to distinguishbetween turbines with downwind rotors and turbines with upwind rotors. Some earlywind turbines did produce significant levels of infrasound; these were all turbines withdownwind rotors. The downwind design is rarely used in modern utility-scale windpower turbines.Upwind rotors emit broad band sound emissions, which include low frequency sound andsome infrasound. Note that the “swish-swish” sound is amplitude modulation at bladepassing frequencies of higher frequency blade tip turbulence and does NOT contain lowfrequencies.One example of low frequency sound and infrasound from a modern turbine is shown inFigure 8 . The magnitudes of these are below the perception limits of humans, which areshown in Figure 5.Figure 8: Example of 1/3 octave sound spectra downwind of a Vestas V80. Theinfrasound levels (range marked by the arrow) are below human perception levelSound Reduction Methods for Wind TurbinesTurbines can be designed or retrofitted to minimize mechanical sound. This can includespecial finishing of gear teeth, using low-speed cooling fans and mounting components inthe nacelle instead of at ground level, adding baffles and acoustic insulation to thenacelle, using vibration isolators and soft mounts for major components, and designingthe turbine to prevent sounds from being transmitted into the overall structure. Efforts toreduce aerodynamic sounds have included [Wagner, et al., 1996] the use of lower tipspeed ratios, lower blade angles of attack, upwind rotor designs, variable speed operationand most recently, the use of specially modified blade trailing edges.Recent improvements in mechanical design of large wind turbines have resulted insignificantly reduced mechanical sounds from both broadband and pure tones. Today, theWind Turbine Noise Issues Renewable Energy Research Laboratory Page 13
Page 1: Wind Turbine Acoustic NoiseA white
Page 7: human ear's response to changes in
Page 10 and 11: There is no reliable evidence that
Page 14 and 15: sound emission from modern wind tur
Page 16 and 17: Figure 10: Summary of NREL study of
Page 18 and 19: A discussion of complex propagation
Page 20 and 21: speeds of 8 m/s because the backgro
Page 22 and 23: In the U.S., although no federal no
Page 24 and 25: o Small turbines: national standard
Page 26: Manwell et al., Wind Energy Explain

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