Environmental Engineering Reference
In-Depth Information
Figure 7-8. Typical variation in noise spectra with power output and wind speed,
measured 200 m from a large-scale HAWT.
(78.2-m diameter, downwind rotor)
[Shepherd, Willshire, and Hubbard 1988]
Figure 7-8 and the upper spectrum in Figure 7-4 both represent the acoustic output of
the same wind turbine. The higher sound pressure levels in Figure 7-8 are the typical result
of increasing the frequency bandwidth selected for analyzing the acoustic output.
Figures 7-9 and 7-10 contain measured data for several HAWTs of various sizes and
configurations [Shepherd, Willshire, and Hubbard 1988]. In Figure 7-9 [Hubbard and
Shepherd 1984], measured far-field data for several upwind-rotor turbines are adjusted to
a distance 2.5 rotor diameters from the base of the tower and are plotted as one-third-octave
band spectra. The disk power densities (in W/m
2
) and tip speeds for all of these machines
are comparable, and the spectra (adjusted for distance) are in general agreement except at
the lower frequencies. Comparable data are presented in Figure 7-10 for several downwind
rotors [Shepherd
et al.
1988; Hubbard and Shepherd 1982; Shepherd and Hubbard 1981;
Lunggren 1984]; the results are similar. The variations in noise levels in Figure 7-10 can
be related to the variations in rotor tip speed noted in the legend. A reference gradient of
-10 dB per decade is included to indicate roughly the rate at which the broadband noise
levels decrease as frequencies increase.
Effects of HAWT Yaw Error
Horizontal-axis turbines sometimes operate such that the wind direction is not aligned
with the rotor axis. The effects of nonalignment, or
yaw error
,
on the generated noise have
been evaluated for a large-scale HAWT with a downwind rotor. In Figure 7-11, data are
shown for yaw errors of 0, 20, and 31 deg. The band levels plotted are arithmetic aver-
ages of measured values in the upwind and downwind quadrants. The obvious result is that
sound pressure levels at low frequencies are reduced as yaw error increases. This would
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