Acoustic noise associated with the MOD-1 wind turbine: its source, impact, and control Page: 6 of 262
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TR-1166
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stall), the operating angle of attack, and the leg-to-blade distance downwind
of the tower. The airfoil shape may be a primary contributor to the impulse
generation, particularly when the incidence approaches the stall angle. Fac-
tors in the impulse generation at the intersection of the blade and wake are
spatially organized turbulent perturbations that simultaneously affect the
lift generating portion of the blade span and incorporate length scales rang-
ing from less than a chordwidth to several chords.
Perturbation pressure distributions, resulting from vortex core pressure defi-
cits in the leg wakes, have been shown to adversely influence the blade lead-
ing edge pressure gradient, resulting in a transient separation of the blade
boundary layer as it passes through the wakes. This is a consequence of the
characteristic chordwise pressure distribution of the 44xx-series airfoil
shape. Because of a forward shift (towards the stagnation point) in the peak
negative pressure with an increasing quasi-steady incidence angle, transient
leading edge separation and reattachment, as well as airfoil hysteresis,
apparently become more severe when the upwash circulation and core pressure
deficits created by the embedded vortices in the leg wakes are encountered.
Wind tunnel tests have shown, and comparisons of full-scale field data have
confirmed, the existence of a critical turbulent scale defined by the reduced
frequency parameter k and covering the range 0.5 < k < % or, expressed in
terms of a perturbation length scale, 2nc > Xp > c (where c is the chord
dimension at 80% span). Turbulent structures in these ranges that are spa-
tially coherent in a direction parallel to the span impart the most severe
lift fluctuations through an interaction with the blade and account for not
only the intense, impulsive acoustic radiation but the generation of strong
aeroelastic stresses in the blade structure as well. Critical, unsteady aero-
dynamic parameters, which have been identified as exerting control over these
unsteady processes, include the reduced frequency k (or, equivalently, the
perturbation wavelength Xp ), the quasi-steady incidence angle "a , the per-
turbation spanwise coherence (with respect to the blade span), and possibly
the vortex core pressure deficit Apv , all of which are stochastic with nar-
rowband (critical) sensitivities.
An investigation into the role atmospheric propagation plays in the MOD-1
annoyance has shown that surface and ground propagation are negligible in com-
parison with a combination of terrain reflection and atmospheric refaction.
Strong focusing (25 dB or more) of the emitted MOD-1 acoustic impulses as a
result of these processes can account for local, far-field enhancements (caus-
tics ) .
Acoustic and seismic (vibration) measurements taken in two of the affected
homes near the MOD-1 site revealed that the structures had been undergoing
transient elastic deformation under the periodic acoustic loadings from the
turbine's operation. The excitation of lightly damped structural modes has
also been responsible for summarily exciting cavity (Helmholtz) and air volume
resonances within the rooms of the homes and producing secondary acoustic
emissions from loose objects. Possibly very important, however, are the
strongly oscillatory (harmonic), low-frequency pressure fields created within
the smaller rooms and their relation to annoyance of the residents. A
measurement of the indoor threshold perception (audible stimuli but no
sensation of vibration) at one of the homes undergoing excitation by the MOD-1
impulsive noise led us to suggest the design goal of limiting peak coherent
v
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Kelley, N. D.; McKenna, H. E.; Hemphill, R. R.; Etter, C. L.; Garrelts, R. L. & Linn, N. C. Acoustic noise associated with the MOD-1 wind turbine: its source, impact, and control, report, February 1, 1985; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc1095935/m1/6/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.