Environmental Engineering Reference
In-Depth Information
than
d
,
the distance to the receiver. The frequency-dependent scaling factor
K
a
,
in Figure
7-15, has been determined empirically from measured frequency spectra of rotor noise
caused largely by inflow turbulence.
Noise from the Interaction of the Turbulent Boundary Layer and the Blade Trailing Edge
Noise is generated by the convection of the blade's attached turbulent boundary layer
into the wake of the airfoil. This is a major noise source for helicopter rotors, and the stu-
dies on this subject by Schlinker and Amiet [1981] have been adapted to wind turbine
rotors. The resulting expression [Grosveld 1985] for one blade airfoil is as follows:
R
SPL
1/3
(
f
) = 10 log
10
ò
F
b
dr
+
K
b
(7-5a)
0
-
4
4
1.5
S
S
max
S
S
max
F
b
=
V
r
B D
d
+ 0.5
(7-5b)
d
2
sin
2
(q/2)
(1 +
M
cos q)[1 + (
M
-
M
c
) cos q]
D
=
(7-5c)
M
=
V
r
/
a
0
(7-5d)
d = 0.37
c
/
N
0.2
R
(7-5e)
N
R
=
V
r
c
/n
(7-5f)
S
=
f
d/
V
r
(7-5g)
where
V
r
=
resultant velocity at a blade segment (m/s)
D
=
directivity factor
q
= angle between the segment-to-receiver line and its vertical projection in
the rotor plane (rad)
M
=
blade segment Mach number
M
c
=
convection Mach number = 0.8
M
d
= boundary layer thickness (m)
c
=
segment chord (m)
N
R
=
segment Reynolds number
v
= kinematic viscosity (m
2
/s)
d
= distance from the segment to the receiver (m)
S
= segment Strouhal number
S
max
=
0.1
dr
=
spanwise length of the blad segment (m)
K
b
=
constant scaling factor = 5.5 dB
Sound pressure levels for the rotor are obtained by integrating contributions of all acoustic
sources over the length of each blade and adding the results.
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