Environmental Engineering Reference
In-Depth Information
⎛
6*
2
2
⎞
Ut s
Δ
sin ( ) sin (
q
y
)
L
=
10 log
n
+
K f
(
)
+
C
( 22 )
⎜
⎟
pA
10
b
2
6
⎝
r
(1
+
M
cos
q
)
⎠
with the peak frequency:
0.25
U
(23 )
f
=
peak
*
t
+
d
/4
If
t
*
/
d
*
< 1.3
⎛
⎞
5*
2
2
Ut s
Δ
sin (
q
/ 2) sin (
y
)
L
=
10 log
n
+
K f
(
)
+
C
⎜
⎟
pA
10
[
]
b
(24)
2
3
2
r
(1
+
M
cos
q
)
1
+
(
M
−
M
) cos
q
⎝
⎠
c
and peak frequency:
0.1
U
(25 )
f
=
peak
*
t
The spectral shapes
K
(
f
) are empirical functions.
Brooks
et al.
[18] used a single model for the trailing edge bluntness noise:
⎛
⎞
⎛
⎞
⎛
*
5.5
⎞
*
*
tM
Δ
sD
t
t
St
L
=
10 log
+
K
,
y
+
K
,
y
,
⎜
⎟
⎜
⎟
⎜
⎟
(26 )
p
10
1
2
2
*
*
St
⎝
r
⎠
d
d
⎝
⎠
⎝
⎠
avg
avg
peak
*
*
*
where
y
is the angle of the trailing edge and
d
= . (
05
d
+
d
)
.
avg
p
s
6.2.6 Noise due to atmospheric turbulence
The atmospheric turbulence is highly dependent on weather conditions, the geom-
etry of the landscape and ground roughness. As a consequence, the noise gener-
ated by the interaction of onfl ow turbulence and the blades is the most diffi cult to
model. Indeed, the models used for the turbulence effects are very sensitive to the
choice of appropriate turbulence scales. As an example, Glegg
et al.
[ 25 ] reports
that using turbulent length-scales from the atmospheric boundary layer leads to an
over-prediction of the noise levels, while assuming the turbulent length-scales to
be of the order of the blade chord gave much better results.
Amiet [7] did a pioneering work deriving a theoretical expression for the far-
fi eld acoustic spectral density produced by an airfoil obtaining good predictions of
experimental data.
One of the earliest models for the noise generated by infl ow turbulence for wind
turbines is developed by Grosveld (cited in [4]). This model is valid only for low
frequencies because it is based on the assumption that the noise is generated by a
point source located at the hub. Furthermore, neutral stability conditions and nega-
tive temperature gradient are assumed for the atmosphere. The sound pressure
level is given by:
⎛
2
⎞
4
2
2
n
sin ( )
f
r
wU CR
b
L
=
10 log
+
K f
(
)
+
C
⎜
⎟
(27 )
p
10
⎜
2
2
0
⎟
r
c
⎝
⎠
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