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
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