Environmental Engineering Reference
In-Depth Information
Distributed Source Effects
Because of their large rotor diameters, some wind turbines exhibit distributed source
effects relatively close to the machines. Only when listeners are at distances from a turbine
that are large in relation to its rotor diameter does the rotor behave acoustically as a point
source. As indicated in Figure 7-22, distributed source effects are particularly important in
the upwind direction. In this figure, sound pressure levels in the 630-Hz, one-third-octave
band are presented as a function of distance in the downwind, upwind, and crosswind direc-
tions. In the downwind and crosswind directions, the measured data agree well with the
solid curves representing spherical spreading and atmospheric absorption. In the upwind
direction, however, the measured data fall below the solid curve, which indicates the pre-
sence of a shadow zone.
An improvement in predicting upwind sound pressure levels is obtained when the noise
is modeled as being distributed over the entire rotor disk. Each part of the disk is then
considered to be a point source, and attenuation is estimated by means of the empirical
model shown in Figure 7-21. The resulting predictions are shown as the dashed curve of
Figure 7-22 and are in good agreement with the sound measurements upwind of the turbine.
In the downwind and crosswind directions, point-source and distributed-source models result
in identical calculations of sound pressure levels.
Figure 7-22. Measured and calculated sound pressure levels in three directions from
a large-scale HAWT. (one-third-octave band = 630 Hz, rotor diameter = 78.2 m)
[Shepherd and Hubbard 1985]
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