Environmental Engineering Reference
In-Depth Information
Fig. 2.26 JONSWAP spectra for three wind speeds
The five parameters (
a
,
v
p
,
g
,
s
a
,
s
b
) are defined as follows:
a
The so-called Phillips “constant”, which has a fixed value of 0.0081 in the Pierson-
Moskowitz spectrum but in the JONSWAP spectrum depends on the significant
wave height (H
s
) and therefore fetch and time.
v
p
The angular frequency of the maximum of the spectrum (peak frequency)
(
v
p
¼
2
p
/T
p
, with the peak period T
p
, where T
p
¼
1.296
T
¼
1.408
T
0
).
g
Shape parameter for the enlargement factor with respect to the Pierson-Moskowitz
spectrum.
s
a
Characteristic dimension for the width of the spectrum to the left of the maximum
(usually
s
a
¼
0.07).
s
b
Characteristic dimension for the width of the spectrum to the right of the maximum
(usually
s
b
¼
0.09).
The following formats are specified in [11] for parameters
a
and
g
:
H
s
4
p
v
5
16
a ¼
C
ðÞ
g
2
C
ðÞ¼1 0
ð
:
287 ln
g
Þ
8
<
for T
p
=
p
5
H
s
<
3
:
6
for
15 T
p
=
p
6 T
p
=
p
g ¼
exp 5
:
75 1
:
H
s
3
:
H
s
5
:
for T
p
=
p
1
H
s
>
5
Here, C (
g
) is a normalising factor included to guarantee that the same significant
wave height
is used for the JONSWAP spectrum as for the Pierson-Moskowitz
spectrum.
When no measurement data from the site is available for the preliminary planning or
when the offshore wind turbine is to be designed to a standard wind turbine class (see
Table 2.8), the formulations according to [11] Appendix 4.E can be used. The