Evaluation on the Numerical Simulation Model ofWind Speed Distribution behind the Offshore Wind Power Generation
Used Observed Data
S. Shiraishi1, T. Nagai2, H. Hayashi3, K. Nishi4, H. Kume5 and S. Dohata6
1: Port and Airport Research Institute, Yokosuka, Japan, TEL: +81-468-44-5037, FAX: +81-468-44-0255,
shiraishi-s@pari.go.jp, 2: Port and Airport Research Institute, 3: Japan Weather Association, 4: Coastal
Development Institute of Technology, 5: Newjeck Inc., 6: Setana town, Hokkaido

1. Introduction
The amount of wind power generation aims for three millions kWh by 2010 in Japan. It is very few offshore
construction in Japan compared with European countries. It is generally said that offshore wind energy is greaterthan in land area, and few disturbance. In Japan, suitable construction sites for wind power generation in land are
decreasing, then, the construction of wind power generation in port and coastal area is strongly hoped.
The first offshore wind power generation in Japan was completed at the port of Setana, Hokkaido in 2004.
The size of wind power generation is 600 kW and the numbers of wind generators are two. Field observation onwind speed distribution was carried out by the Wind Power Generation Research Association for Port and Coastal
Area organized in Coastal Development Institute of Technology (CDIT), Japan. This paper describes on the windspeed distribution behind the offshore wind power generation by use of the filed observation carried out at theport of Setana and the result of numerical simulation on the wind speed distribution. Field observation was carriedout in February, 2004. The measuring of wind speed was carried out by use of the laser radar installed on the land.
The accuracy of measured data was calibrated by observed data by wind cup pattern anemometer installed on thebreakwater. The profiles of vertical wind speed distribution at the offshore site are examined by use of observed
data. And also the numerical simulation model and its parameter on the ratio of sheltering effect by the wind
power generation are examined.

2. Wind observation at the port of Setana
2.1 Offshore wind firm in the port of Setana
The first offshore wind power generation was completed at the port of Setana, Hokkaido, Japan. Photo 1
shows the wind power generation. Two wind power generators of 600 kW were installed behind the breakwater.
The foundation is the dolphin structure supported coupled piles. The diameter of rotor is 47m and the height of
hub is 40m. Cut-in and cut-off wind speeds are 4m/s and 25m/s, respectively.
Photo 2 shows the layout of the port, the arrangement of wind power generations and the observation systemof wind speed and wind direction. The number 1 in the figure is wind power generation, the number 2 and 4 arewind speed meter installed on the breakwater and in land, respectively. The number 3 is the position of laser
radar.

2.2 Method of observation

a) Observation system
Photo 3 shows the appearance of laser radar. The system is composed of a main unit, a sensor head, and acomputer [1]. The single wavelength laser of safe 1.5 μm to the eye is made to occur in the main unit. It istransmitted into the atmosphere through the fiber-optic cable from the sensor head. After it is collected by thesensor head, scattered light from aerosol in the atmosphere is sent to the body parts with a fiber-optic cable. Andthe Doppler shift of the transmission and reception light is detected in the main unit. The movement speed ofaerosol is calculated in the computer from Doppler shift, and the wind speed and direction is found. The
measured wind speed and direction by laser radar is good accordance with observed one by wind cup patternanemometer and arrow wing pattern wind vane [1].

Photo 1 Wind power generation

Photo2 Layout of the port of Setana

Photo3 Observation systemb) Observation layout
Figure 1 shows the area of wind speed observation behind wind generators. Wind speed is observed at thelocations of 2.5, 5.0, 7.5, 10.0 times of the diameter of rotor. Observed lines are behind lines of two windgenerators and the center line of two generators.
Figure 2 shows the layout of breakwater and wind power generator. It should be considered the interaction

W
S00C

Measurement point
on sea

225

5D

10D

Y25N

Y50N

Y75N
725

Wind power generator

DL +47.35

rated output600kW
Rotor diameter 47m

Y100N
Cup anemometer

B00C

DL+15.00

Y25C

Y50C

Y75C

Y100C

Y25S

Y50S

Y75S

Y100SFigure 1 Location of observed points

Lightwave rader

40.7
41.75

5°LineNW

225

9.5
35.5
10

Main Wind Direction

Center line of breakwater

effect of breakwater and wind power generators in the modeling of computation of wind speed distribution.

DL+5.00 DL+5.60

Figure 2 Breakwater and wind power generator

3. Result of observation
3.1 Wind speed and direction during observation
Figure 3 shows wind speed and direction observed by use of wind cup type anemometer and arrow wing
pattern wind vane during February 18th to 20th, 2004. Upper and lower figures show wind direction and windspeed, respectively. Wind speed in February 18th was greater than 10m/s and offshore wind (NW, 315degrees).
Wind speed during the mid night and early morning of February 19th became low and a land wind.
Wind Speed(m/s)
40Wind Direction(degree）
400
Wind Direction(degree）

35300

30200

25100
020Wind Speed(m/s)

15-10010-200

5-300

02/18

2/19

2/20

2/21-400

Date

Figure 3 Wind speed and direction during observation

Figure 6 Area of numerical simulation

3.2 Vertical profile of wind speed
The vertical wind speed distribution between DL+15m to DL+205m was observed on the sea at the 1000
meters distance from the land. The observation was carried out during 6 seconds for each elevation and repeatedalternating measuring points automatically. Figure 4 and Figure 5 shows wind speed distribution of February
18th and 20th, respectively. Normalized wind speed at DL+15 m were 12.6 m/s in February 18th and 4.6 m/s inFebruary 20th, respectively.
Elevation(DL.m)
250

Elevation(DL.m)
250
Observation
Calculationα=0.2
Calculation α=0.15200Observation
Calculation α=0.2

200150150100100505000.5

11.5

Ratio of wind speed U(z)/U(15m)

200.5

Calculation α=0.15

11.5

2

Ratio of wind speed U(z)/U(15m)

Figure 4 wind speed profile (Strong wind, Feb. 18th) Figure 5 Wind speed profile (Weak wind, Feb. 20th)
By comparing vertical profile of wind speed between measured one and computed by exponential law, theparameter agrees well in 0.15 to 0.2. And vertical distribution higher than DL+50m seems constant. Boundarylayer on the sea seems thin compared with in land. This mean offshore site is good at for wind powergenerations.

4. Numerical simulation of wind distribution
4.1 Numerical modelingIn the numerical simulation of wind speed distribution around wind power generations, the generators aretreated as obstacles in the space. The additional term Fi, which is related to reduction effect of wind speed behind
like as trees, is considered in the numerical simulation.
∂ ui
∂t+

∂ ui u j=−∂x jF i = η C fi u i∂ uj∂ ⎛ p∂ ⎧⎪ ⎛⎜ ∂ u i
2 ⎞⎜⎟+++k⎨ν t∂xi ⎜⎝ ρ∂x i
3 ⎟⎠ ∂x j ⎪ ⎜ ∂x j
⎩ ⎝uj⎞ ⎫⎪⎟ −Fi⎟⎬⎠ ⎪⎭(1)

2(2)

Where,<ui>is three component of wind speed, k is energy of flow, εis dissipation , <p> is pressure and ρ isdensity of air, η is parameter evaluating sheltering effect.
The area of simulation around wind power generations is shown in Figure 6 [2]. The horizontal mesh size inthe model is 22.5 meter each and the total grid number is 60 by 60. The mesh for vertical elevation is divided tosix layers of 10 meter interval between 5m to 55m from ground level and ten layers between 55m to 300m.
The wind energy generators are modeled 1 by 2 mesh in horizontal, and also the 4th to 7th layer in verticaldirection. The roughness of surface for sea and land is set 0.001m, 0.2m, respectively. The roughness of abreakwater and a small island in the port is also used the same value for land.

4.2 Case of computation
Two cases listed in Table 1 are computed in this research. Case 1 is corresponding to the strong wind andCase 2 to weak wind.
Table 1 Computation case
CaseTime

Wind Direction and Wind Speed (AboveBreakwater 15m)

1Feb. 18

th13:10-14:00

WNW

12 - 15 m/s

NNW-N

4-

15:20-15:30
2Feb. 19th 13:30-13:50

6 m/s

16:30-16:50

Observed wind speed ratio behind wind power generations are 0.93 to 1.0 for generator 1 and 0.88 to 0.98 forgenerator 2 in strong wind case. And the ratio is 0.77 to 0.84 for generator 1, 0.70 to 0.79 for generator 2 in weakwind case.

4.3 Effect of parameter of wind sheltering

The parameter of sheltering effect is discussed by alternating the parameter. Figure 7 shows the wind speed
distribution behind wind power generator. In the figure, the result obtained when the parameter is set 0.01, 0.03,0.05, and 0.1, respectively. In the case of strong wind the result by the parameter 0.005 is additionally shown in
the figure. In case of weak wind, the wind speed distribution behind wind power generator is decreased to about
0.75 and recovered to about 0.9 at the 100 meter behind the generators when the parameter is 0.01. It is agree
well to the observation. In case of strong wind, the wind speed ratio remains less than 0.8, so the parameter shallbe set 0.005.

1.00
0.80

0.80
0.005
0.01
0.03
0.05
0.1

0.60
0.40
0.20

wind speed ratio

wind speed ratio

1.00

0.60
0.01
0.03
0.05
0.1

0.40
0.20

0.00
020

40

60

80

100

120

distance(m)

0.00
050

100

150

200

250

300

distance(m)(1) Strong wind(2) Weak wind

Figure7 The relation of parameter and wind speed distribution

4.4 Wind speed distribution behind wind generator
Figure 8 shows horizontal wind speed distribution obtained by numerical simulation at 45m from sea level,
where is close to the height of hub. The parameter of sheltering effect is set for 0.005 for strong wind case andfor 0.01 for weak wind case. In strong wind case, the wind speed reduction behind wind generators is more than
0.8. The wind speed reduction by wind generator is not so large, but the effecting area is relatively large behind
wind generators. In the case of weak wind, the wind speed reduction around the wind generators decreased to 0.7,
but the sheltering effect is limited to the area around wind generators.

ANCS(1) Strong wind(2) Weak wind

Figure 8 Horizontal wind speed distribution (45m above the sea level)
Figure 9 shows the comparison of horizontal wind speed by numerical simulation and observation for thelines of N, C, S, respectively. Figure 10 shows the vertical profile of wind speed distribution for the line A,which is SSE wind direction. In strong wind, the result of numerical simulation agrees well to observed data. Inweak wind, the result of numerical simulation agrees well at neighbors of wind power generator. There are large
differences at the area apart from the generators. The wind ratio, however, becomes around 0.8 in the line A.
Then, the data obtained by numerical simulation seems almost same as observed one.
1.4

1.4

1.2

N-cal
N-obs
C-cal
C-obs
S-cal
S-obs

1.0

0.8

wind speed raitio

w in d s pe e d raio n

1.2

N-cal
N-obs
C-cal
C-obs
S-cal
S-obs

1.0

0.8

0.6

0.6

0.4

0.4
-50

050

100

150
200
distance(m)250300350-50400050(1) Strong wind100150200distance(m)250300350400(2) Weak wind

Figure 9 Wind speed ratio behind wind generator
100height(m)80a-1
a0a1a2a3

60
40
2000.4

0.6

0.8
1.0
wind speed ratio

1.2

1.4

Figure 10 Vertical profile of wind speed

5. Conclusion
The conclusions in this research are as follows.
1) The vertical profile of wind speed is obtained around the breakwater in the port. The parameter inexponential law agrees well in 0.15 to 0.2. And vertical distribution higher than DL+50m is constant.
2) The parameter of numerical model of wind speed distribution around offshore wind power generation is
examined by means of field observed data. The model can evaluate the reduction of wind speed behind the windpower generation.
3) The model shall be applied the planning of layout and the arrangement of wind power generators.

References[1] K. Nishi, S. Shiraishi, M. Hamano, Y. Umeki, H. Kume: Survey report of offshore wind conditions inprospect of offshore wind energy conversion systems in Japan, Proc. of OCEANS’ 04 MTS/IEEETECHNO-OCEANS ’04, pp.1830-1834, 2004.11[2] S. Shiraishi, T. Nagai, H. Hayashi, K. Nishi H. Kume, S. Dohata: Evaluation on the wind speed distribution
behind offshore power generation used field data obtained at the port of Setana, Annual Journal of Civil
Engineering in the Ocean, JSCE, Vol.21, pp.1059-1064, 2005.7 (in Japanese)