Environmental Engineering Reference
In-Depth Information
where K ( f ) (the shape of the spectrum) and C are determined empirically, U =
Ω
R ref and the peak frequency is given by:
16.6
0.7
U
( 28)
f
=
peak
h
R
The reference chord location is at 30% of the blade length from the tip of the blade
and the root mean square of the turbulence intensity is given by:
2/3
hw
ww VRw
2
2
r
=
(29 )
r
2
(
0.014
w
)
wr
r
where V w is the wind speed, h the height above the ground, and the turbulence
intensity is:
1.185
0.193 log
h
2
0.353
1/
10
(30 )
w
=
0.2(2.18
V h
)
r
w
Lowson [9] derived a model based on Amiet's work [7]. Contrary to Amiet, Low-
son has a single formula for the high- and low-frequency regimes, by introducing
a correction term:
2
22
3
3
2 7/3
r
cl
Δ
sM wk
(1
+
k
)
K
0IT
LF
L
=
10 log
+
58.4
+
10 log
p
10
10
2
1
+
K
r
(31 )
LF
where K LF is the low-frequency correction factor, k = (
π
fC )/ U , U includes
contributions from the wind speed as well,
, and M = U / c 0 .
2
2
U
=Ω +
(
R
)
(2/ 3)
V
w
6.2.7 Sample results
Moriarty and Migliore [26] used Category II models to predict the acoustic fi eld
generated by two-dimensional airfoils and from a full-scale wind turbine. Except
the infl ow turbulence noise the models from Brooks et al. [18] have been used. The
infl ow turbulence noise model was adapted from Lowson [9]. The tower
wake
interaction and propagation effects are neglected. As it regards the directivity, it
is assumed that all sources are propagated in the wind direction with a speed cor-
responding to 80% of the average wind speed. For the airfoil cross sections good
agreements were obtained for high frequencies (around 3 kHz), while at lower
frequencies the sound pressure levels were overpredicted by as much as 6 dB (see
Fig. 16). For the full-scale turbine it was observed that the turbulence inlet noise
had the largest contribution to the overall sound pressure level. The second most
important noise source was found to be the blunt trailing edge noise or the laminar
vortex shedding noise, depending on the wind speed.
6.3 Category III models
The models belonging to this category take into account the complex three-
dimensional and time-dependent distribution of the acoustic sources. A recent
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