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466 V
OLUME
20JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY
q2003 American Meteorological Society
Offshore Wind Turbine Wakes Measured by Sodar
R. J. B
ARTHELMIE
,* L. F
OLKERTS
,1F. T. O
RMEL
,#P. S
ANDERHOFF
,* P. J. E
ECEN
,#O. S
TOBBE
,@
AND
N. M. N
IELSEN
*
*Wind Energy Department, Risoe National Laboratory, Roskilde, Denmark
1Ecofys bv, Utrecht, Netherlands
#Wind Energy Department, Energy Research Centre of the Netherlands, Petten, Netherlands
@Ecofys Energieberatung und Handelsgesellschaft mbH, Cologne, Germany
(Manuscript received 14 February 2002, in final form 18 September 2002)
ABSTRACT
A ship-mounted sodar was used to measure wind turbine wakes in an offshore wind farm in Denmark. The
wake magnitude and vertical extent were determined by measuring the wind speed profile behind an operating
turbine, then shutting down the turbine and measuring the freestream wind profile. These measurements were
compared with meteorological measurements on two offshore and one coastal mast at the same site. The main
purposes of the experiment were to evaluate the utility of sodar for determining wind speed profiles offshore
and to provide the first offshore wake measurements with varying distance from a wind turbine. Over the course
of a week, 36 experiments were conducted in total. After quality control of the data (mainly to exclude rain
periods), 13 turbine-on, turbine-off pairs were analyzed to provide the velocity deficit at hub height as a function
of the distance from the turbine. The results are presented in the context of wake measurements at other coastal
locations. The velocity deficit is predicted with an empirical model derived from onshore measurements based
on transport time dependent on surface roughness. The measurements are closer to those predicted using an
onshore rather than an offshore roughness despite the relatively low turbulence experienced during the exper-
iments.
1. Introduction
Offshore wind energy developments are under way
in many European countries (DEA/CADDET 2000)
with planned projects of several thousand megawatts to
be installed in the first decade of the new millennium
(Barthelmie 1999a; Barthelmie et al. 2000). In this con-
text, the term ‘‘offshore wind farm’’ indicates that the
wind turbines are erected with their foundations in wa-
ter. While experience gained through the offshore wind
farm demonstration projects currently operating (Bar-
thelmie et al. 1996), (Lange et al. 1999) is valuable, a
major uncertainty in estimating power production in
large offshore wind farms lies in the prediction of the
dynamic interaction between the atmosphere and wind
turbines. Planned offshore wind farms consist of up to
80 turbines giving complex wake effects (defined as the
velocity decrease, turbulence increase downstream of a
wind turbine rotor). Crespo et al. (1999) describe the
development of wind turbine wakes and their decay
downwind of the turbine rotor. Outside of the near-wake
region the main parameter in determining wake decay
Corresponding author address: Dr. R. J. Barthelmie, Dept. of Wind
Energy and Atmospheric Physics, Risoe National Laboratory, Ros-
kilde 4000, Denmark.
E-mail: r.barthelmie@risoe.dk
is ambient turbulence. Given that turbulence offshore is
typically lower than onshore, it is hypothesized that
wake effects are propagated over larger distances down-
stream offshore than over land and that atmospheric
stability will play a larger role in determining the down-
stream wake decay. The likely result is that in order to
optimize power output, offshore wind farms will require
larger distances between turbine rows than is common
in the design of onshore wind farms. This has a major
economic disadvantage because undersea grid connec-
tions and connections between turbines are proportion-
ally more expensive than their cost and installation at
land sites. Model simulations suggest that power loss
due to reduced wind speeds at the turbine rotor can be
more than 5% in large wind farms. Improved under-
standing of wake propagation offshore is an essential
part of design to minimize these losses.
The measurements of wind speed profiles in an off-
shore wind farm described in this paper are designed to
assist understanding of wake development offshore.
Measurements of wind speed profiles at fixed distances
from the wind turbines have been made at the wind
farm over a long period on meteorological masts (Bar-
thelmie et al. 1996; Frandsen et al. 1996), but the dis-
advantages are that the measurements extend only 12
m above the turbine hub height and the variability of
A
PRIL
2003 467BARTHELMIE ET AL.
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IG
. 1. Location of the Vindeby wind farm in Denmark. SMS and SMW are the southern and western meteorological
sea masts, respectively; LM is the land-based meteorological mast.
the wake with distance to the turbine cannot be assessed.
Hence an experiment was conducted to provide addi-
tional data for offshore wakes (especially in near-wake
situations for which very few datasets are available even
for land sites) and to form part of the evaluation process
for wake models within the Efficient Development of
Offshore Windfarms (ENDOW) project (Barthelmie et
al. 2001).
For the first time, a sodar mounted on a ship was used
to measure wind speed profiles in the turbine wake in
an offshore wind farm. While sodar has a long history
of use at land sites (Crescenti 1997) and has many ad-
vantages (Vogt and Thomas 1994) for boundary layer
and air pollution studies, the instrument has had less
exposure in marine-based studies (e.g., Otterstein et al.
1974; Petenko et al. 1996) and has not been extensively
employed in wind energy work (see, e.g., Hogstro¨m et
al. 1988). Although minisodars have been used to assess
wind energy sites, there is little documentation avail-
able. In the experiment described here, selective oper-
ation of turbines in different conditions reflecting wind
speed and direction (influencing the fetch variation) al-
lowed the direct impact of turbine operation on wake
effects to be measured at varying distances from the
turbine (here 1.7–7.4 rotor diameters, D). Use of a sodar
provided wind speed profiles to hub and rotor heights
of offshore wind turbines currently being developed,
which were supplemented by ongoing measurements on
meteorological masts.
The main objectives of the experiment were to eval-
uate whether the sodar could operate successfully when
mounted on a ship and whether the noise from a wind
turbine would interfere with the sodar wake measure-
ment. If successful in these regards, the priorities of the
experiment were to measure near-wakes at a range of
distances from the turbine and, assuming favorable wind
directions, single and double wakes.
2. Experimental details
a. Experiment design
The measurement campaign was conducted at the 5-
MW offshore wind farm at Vindeby, Denmark (Fig. 1).
Note that the wind farm is relatively close to the coast
(about 2 km). The site was chosen because it is one of
very few operating offshore wind farms and has three
monitoring masts [two offshore, referred to hereafter as
sea mast south (SMS) and sea mast west (SMW), and
one at the coast, hereafter land mast (LM)] providing
detailed meteorological measurements to 48-m height.
The wind farm, which has been operating since 1991,
consists of 11 BONUS (Bonus Energy A/S, Brande,
Denmark) 450-kW turbines in two rows oriented toward
the southwest (prevailing wind direction). The hub
height is 38 m and the rotor diameter is 35.5 m. Layout
of the wind farm and the two offshore masts is also
shown in Fig. 1. The site has the advantage of shallow
water (2–5 m) with relatively low wave heights and
swell compared to more exposed sites.
April was chosen for the experiment to avoid periods
of very high wind speeds (which mainly occur in
winter). However, to measure wakes, wind speeds also
have to be above turbine cut-in wind speeds of 4 m s
21
making summer months less attractive. The mean wind
speed measured at 10 m above mean sea level at SMW
in April for the period 1996–99 inclusive is 7.4 m s
21
with mean air temperatures of 5.98C and a mean water
temperature of 5.88C.
The sodar was mounted on the Seaworker, which is
a highly stable ship with regard to both tilt and position.
The Seaworker is equipped with four anchors although
during most experiments only three were used. The two
front anchors were used to position the ship in the direct
wake of the turbine. By moving one (the rear) anchor,
the ship could be repositioned at different distancesfrom
468 V
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20JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY
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. 2. Photograph of the sodar on the Seaworker behind
a wind turbine.
the turbine providing a transect for the measurements.
Given that a large proportion of the experiment time
was spent positioning the ship, being able to measure
at three wake distances over a distance of 150–200 m
without having to take up all three anchors was an ad-
vantage. Further requirements were 1) that the equip-
ment used to operate the sodar could be protected from
the weather and high waves, 2) that sufficient power
was available when the ship’s engine was turned off (to
reduce noise that interferes with the sodar signal), and
3) that structure of the ship was low enough not to
provide a barrier causing noise reflections. Although the
Seaworker is equipped with several cranes, these were
lowered onto the deck during measurements. The rear
boom could not be lowered to the deck but lowering it
a few meters proved adequate. The Seaworker also had
sufficient room for the sodar on the deck (Fig. 2) and
lastly had an extremely low draft of about 1.5 m, which
is important at Vindeby since the water depth is around
2 m to the south of the wind farm. The level of the deck
is close to the water line and so the height of the sodar-
measured wind speeds have not been corrected with
respect to mean sea level. The first three range gates
(15, 20, and 25 m) were excluded because of the poor
number of returns. This may be related to the structures
on the boat but was also noted during a sodar experiment
on an offshore platform (Coelingh et al. 2000).
During the experiment period 21–28 April 2001 wind
speeds were lower than expected allowing the Sea-
worker to sail each day. However, measurements were
limited by periods of rain (wind speed profiles measured
during rain had unacceptably large standard deviations).
The experiment was also limited by periods in which
wind speeds at the turbine hub height dropped below 4
ms
21
. In this situation turbines stop operating andthere
are therefore no wakes. A further disadvantage of lower
than expected wind speeds was the directional vari-
ability of the wind. While the Seaworker could be ac-
curately positioned in the direct wake of a wind turbine,
shifts of wind direction sometimes occurred during mea-
surements, which meant that the sodar was no longer
measuring in the direct wake. Since repositioning the
boat in the direct wake took approximately 40 min, this
directional variability limited the number of experi-
ments that could be conducted each day.
Further matters which had to be considered concerned
the operation of the sodar accounting for the movement
of the ship and accurately determining the position and
angle of the ship. The positions of the ship and turbines
were measured using a GPS to an accuracy of 64m.
As in Fairall et al. (1997) recordings of the tilt and yaw
were made. Data were discarded if the tilt angle ex-
ceeded 648. A major consideration was whether the
wind turbines themselves would distort the sodar signal
due to noise during their operation. As indicated by the
Danish Wind Energy Industry Association (available
online at www.windpower.dk), at a distance of 3 rotor
diameters (D), the sound level of a wind turbine is ex-
pected to be below 45 dB (A) and even at 1 Dis less
than common noise levels in cities. Crescenti (1998)
indicates noise levels less than 50 dB(A) are acceptable
for sodar measurements. Hogstro¨m et al. (1988) found
good agreement between sodar measured wake velocity
deficit and data from a nearby tower or kites suggesting
that noise was not an issue at 2 D. Although the recorded
noise level increased during the near-wake experiments
(1–2 D) described in this paper, it remained below ac-
ceptable levels and the sodar gave wind speed profiles
with standard deviations that were not distinguishable
from the freestream measurements behind the nonop-
erating wind turbine. In contrast to the experience of
Fairall et al. (1997) the performance of the sodar on the
ship experiment at Vindeby was not significantly dif-
ferent than during land-based or offshore fixed-base ex-
periments, which had been conducted previously with
the same instrument (Coelingh et al. 2000). This may
be because the requirement during the Vindeby exper-
iment was to maintain a fixed position (with engines
turned off) rather than to operate under sail.
b. Sodar specifications
The main characteristics of the sodar used at Vindeby
are shown in Table 1. This Aerovironment 4000 mini-
sodar uses sound to measure wind speed and wind di-
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2003 469BARTHELMIE ET AL.
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ABLE
1. Characteristics of the Aeroenvironment 4000 minisodar
operated during the Vindeby experiment.
Characteristic Specification
Max sampling altitude
Min sampling altitude
Height resolution
Transmit frequency
Tilt angle of the beams
Averaging interval
Wind speed range
Wind speed accuracy (wind speed .2ms
21
)
Wind direction accuracy (wind speed .2ms
21
)
Power output (acoustic average)
200 m
15 m
5m
4500 Hz
168
1–60 min
0–35ms
21
,0.50 m s
21
658
40 W
F
IG
. 3. The minisodar and its sound beams.
T
ABLE
2. Instrumentation at the Vindeby masts in spring 2001.
Heights are above ground or above mean sea level.
LM SMW
Min time resolution 1 min 1 min
Heights of instrumentation (m)
Wind speed
Wind direction
Temp
Temp difference
47, 30.9, 9.8
20.9, 33.9
9.7
46.5–9.7, 19–9.7
47.5, 28.7, 10
18.5, 42.8
10
47–10, 22.8–10
rection over a range of heights (15–200 m) with a res-
olution of 5 m. The instrument consists of an antenna
with a speaker array with 32 piezoelectric speakers, an
acoustic signal processor, an audio amplifier, and a con-
trolling PC. The speaker array emits a sound pulse about
once per second at a frequency of 4500 Hz, which is
amplified and transmitted into the air. By a phased array
technique the sound beam is alternately steered in one
of three directions u,
y
,or w, where the wdirection is
vertical whereas the uand
y
directions have an incli-
nation of 748(tilt angle of 168) and are mutually sep-
arated by a 908azimuth angle (see Fig. 3). Sound pulses
are backscattered to the antenna by air density differ-
ences mainly caused by temperature variations. Air ve-
locities along measurement paths are measured by fre-
quency shifts between the transmitted and received sig-
nals (Doppler effect) and the range to sound-reflecting
air parcels is detected by the time lag between pulse
emission and return signal. The time distribution of the
returned pulse enables velocity estimates for several
heights above the ground, and measurements along the
three paths are translated into a three-dimensional wind
vector.
1) O
RIENTATION OF THE SODAR
The sodar was mounted on the Seaworker with the u
beam oriented sideward toward the port side and the
y
beam forward toward the bow. By positioning the ship
with the stern into the wind (using its three anchors)
the
y
beam is oriented along-wind and the ubeam is
oriented crosswind. The major advantage of this ori-
entation is that the main part of the wind speed can be
obtained from one beam, the
y
beam. The uand the
y
beams sample separate volumes of the wake. Because
of the tilt angles of these beams (168of vertical), the
shift in the point of actual sampling increases with
height, with about 10.5 m (0.3 D) at hub height. This
distance has been added to the distances determined
using GPS. In terms of the accuracy of the distance
between the rotor and the sodar measurements, 4.9 m
should be added to the uncertainty [calculated as the
position shift over half the rotor diameter (535.5/2 3
sin16)]. Since the
y
beam is aligned with the mean wind,
the measured wind speed profile still accurately repre-
sents the velocity deficit downwind of the turbine.
In the analysis of the sodar data, two different meth-
ods were compared to check the secondary relevance
of the ubeam in this orientation. The standard method
is to calculate the wind speed as the vector mean using
the three beams. The alternative method, valid with the
y
beam aligned with the mean wind, is using only the
y
beam and the wbeam (vertical), and ignoring the
crosswind ubeam. In this method, small misalignments
of the sodar were corrected using the wind direction
from SMW. The two methods yield nearly identical re-
sults, confirming the marginal importance of the ubeam.
c. Mast data
Instrumentation installed at the Vindeby masts used
here (LM and SMW) is shown in Table 2. All three
masts are purpose-built meteorological masts of 45 m
standing on bases of approximately 2.5 m above mean
sea level or 2 m on land. Heights in Table 2 are above
ground or above mean sea level. Additional instrumen-
tation at LM includes a barometer, net radiometer, and
a precipitation detector, while water temperatures are
470 V
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ABLE
3. Details of single wake experiments. Relative velocity deficit is calculated from sodar wind speed profiles using a height of 40
m. Freestream wind (U) at 48 m and direction (dir) are measurements from the meteorological mast, Lis the Monin–Obukhov length, and
xis the distance to the turbine expressed as number of rotor diameters D. Max disp is the largest distance from the center of the wake due
to the directional variability of the wind during each experiment (expressed as a fraction of rotor diameters). Here C
T
is the thrust coefficient
for the specific wind speed (characteristic of the wind turbine at Vindeby) and tis the transport time. Wake-generated turbulence I
added
calculated from Frandsen et al. (1996) is compared with ambient turbulence I
0
(%).
No. DU/UU(m s
21
)
at 48 m Dir (8)L(m) DMax
disp. C
T
t(s) I
0
(%) I
add
(%) I
total
(%) No.
wake
No.
non-
wake
1*
2*
3*
4
5
6
7
8
0.36
0.13
0.53
0.37
0.30
0.21
0.24
0.32
10.54 60.30
8.76 60.43
8.76 60.43
5.74 60.20
5.74 60.20
5.74 60.20
6.37 60.25
3.76 60.33
336.4 60.9
341.2 62.3
342.8 61.9
226.6 61.1
226.6 61.1
226.6 61.1
152.2 63.1
133.1 64.8
2156
23088
23088
130
130
130
668
332
3.8
6.5
4.1
2.8
3.6
4.5
3.4
4.1
0.3
0.75
0.3
0.3
0.5
0.5
0.5
0.5
0.55
0.67
0.67
0.85
0.86
0.86
0.82
0.93**
11.8
24.9
14.4
15.2
20.4
26.0
17.3
35.9
5.8
8.0
7.6
4.2
4.2
4.2
7.7
5.3
3.2
1.6
3.6
8.7
5.2
3.4
5.6
4.4
6.6
8.2
8.4
9.7
6.7
5.4
9.5
6.9
32
40
40
27
27
27
31
22
32
41
44
36
26
21
30
18
9
A
B
C
D
0.44
0.35
0.11
0.27
0.22
6.90 60.59
7.54 60.45
6.12 60.74
8.19 60.46
8.19 60.46
219.6 62.3
205.8 63.3
207.8 63.2
221.9 63.0
221.9 63.0
380
1103
231
900
900
1.7
2.9
7.4
3.4
5.0
0.3
0.5
0.5
0.3
0.5
0.82
0.76
0.76
0.70
0.70
7.9
12.2
33.5
13.0
19.7
7.7
9.0
15.1
8.7
8.7
25.6
7.3
1.4
4.9
2.4
26.7
11.6
15.2
10.0
9.0
22
17
17
35
35
34
22
21
29
31
* Reference wind speed and turbulence for these experiments is LM but wind direction and Lare taken from SMW. In all other cases data
are from SMW. Some expt pairs use the same turbine-off measurements, e.g., expts 2 and 3, 12 and 13.
** The turbine is not operating at this wind speed so the thrust coefficient is estimated based on winds of 4 m s
21.
also measured at SMW. Data from LM and SMW are
synchronized and are available as 1-min averages. Data
from SMS have not been used because they are not time
synchronized with the other masts. All times are Danish
standard time (UTC 11).
d. Data processing and quality control
1) S
ODAR DATA
The sodar data were recorded on a 1-min basis and
have been processed to give averages for each experi-
mental period with a minimum of 15 min. The following
steps were taken to exclude selected individual profiles
from the average.
•Noise level. A noise level was recorded for every 1-
min wind profile. Higher noise levels could be traced
to disturbances recorded in the logbook, such as the
proximity of another ship. In the analysis a threshold
was set to exclude data above a certain noise level
defined using the background noise level for the day’s
experiments.
•Inclination. The angle of inclination of the ship was
recorded during the experiment. A maximum ampli-
tude of 628in the ship’s movement during a minute
is used to exclude periods of larger swing. The ex-
ception was for the first experiment when the angle
was not measured and during experiments 2 and 3
during periods of higher wind speed when the cutoff
was set to 648.
•Directional variation. During wake measurements
(turbine on), variation in the wind direction meant that
the sodar profile was taken at various distances from
the center of the wake. In the analysis a gate was set
to only include profiles near the center of the wake.
The maximum displacement from the wake center al-
lowed is given in Table 3.
Resulting wind profiles were composed by averaging
over each single height in the 1-min profiles. The re-
sulting standard deviation on the averaged wind speeds
consists of both actual variation in the wind speed as
well as deviations resulting from the measuring tech-
nique.
2) M
ETEOROLOGICAL DATA
During parts of the experiment the anemometer at 30-
m height at Vindeby SMW operated intermittently.
These periods were removed from the database. For
comparison with the sodar a dataset of 1-min averages
was prepared. Stability information was required and
this was provided by calculating the Monin–Obukhov
length (L) from wind speed and temperature data using
the method of Beljaars et al. (1989). The Monin–Obu-
khov length is initially estimated using the temperature
profile at SMW, and then an iteration procedure is used
to calculate the friction velocity and temperature scale
including the wind speed measured at one level. During
the experimental period, conditions were more stable
than neutral (only observed during the first three ex-
periments) at both the LM and SMW as expected at this
site in April (Barthelmie 1999b). A median of the 1-
min data is given for each experiment in Table 3. There
can be small shifts in the median Monin–Obukhov
length between the wake and nonwake periods, but these
are of the order 30 m and do not affect the broad stability
class. The meteorological data are used to provide an
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2003 471BARTHELMIE ET AL.
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. 4. Measurement of the (a) freestream wind and (b) offshore wake profile by sodar compared with mast data
(expt 1 from Table 3). Error bars shown are 1 std dev on each side of the mean.
absolute reference for sodar-derived wind speedsduring
both wake and nonwake experiments. The measure-
ments of freestream wind speeds are taken from LM if
SMW is in the wake of the wind farm (directions from
3208to 1108over north, first three experiments) and
from SMW if the wind direction is between 1108and
3208(remaining experiments).
3) D
ATA CORRECTION
Slight shifts in wind speed between the wake and
nonwake periods had to be accounted for. Assuming that
the freestream wind at 50 m measured on one of the
meteorological masts (U
mast
) was the baseline, a correc-
tion factor (CF) between the two (wake and nonwake)
periods was determined:
CF 5U/U.
mast (nonwake period) mast (wake period)
(1)
This correction factor was then applied to the nonwake
wind speed measured by the sodar. The velocity deficit
was then calculated using the corrected wind speedpro-
file.
3. Results: Specific experiments
In the first experiment, the freestream wind was mea-
sured for comparison with data from one of the mete-
orological masts. In the remaining experiments turbine-
on, turbine-off experiments (of ø30 min) were con-
ducted (see Table 3). All times (meteorological masts
and sodar) are synchronized to the nearest minute. At
the beginning of the experiment, the wind direction was
north-northeasterly, wind speeds were fairly high, and
conditions tended toward neutral. Subsequently, the di-
rection changed to south-southwesterly and the wind
speed dropped giving slightly stable conditions. In ex-
periments 4–6, conditions were strongly stable and the
ambient turbulence intensity was very low. In the re-
maining experiments, turbulence intensity was between
5% and 15% at 48-m height. Two specific experiments
are described in detail below and the results of all the
experiments are shown in Table 3.
Table 3 shows the relative velocity deficit calculated
for all the single wake pairs of experiments where both
freestream (nonwake) and wake wind speed profiles are
available from the sodar and the masts. Velocity deficit
DUis defined in Hogstro¨m et al. (1988) as
DU5U2U,
freestream min
(2)
where U
min
is the minimum wind speed at any height.
Here U
freestream
is the wind speed measured by the sodar
at 40-m height with the turbine switched off and the
velocity deficit is defined at 40 m, which is the closest
measurement to the turbine hub height of 38 m:
DU5U2U.
freestream wake
(3)
As indicated in Table 3 the distance (x) to the turbine
was measured using GPS and is expressed in number
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. 5. Corrected freestream and wake profiles and relative velocity
deficit measured by sodar (expt 1 in Table 3). Error bars shown are
1 std dev on each side of the mean.
of rotor diameters Dwhere the rotor diameter of the
turbine is 35.5 m.
a. Freestream and one wake measurement
The first experiment (1 in Table 3) consisted of a com-
parison of the sodar wind speed profile with the mea-
surements from SMW and LM. The Seaworker was an-
chored close to SMW during a period of fairly strong
winds and the wind speed profile was measured for 40
min. During this period the wind direction remained
steady with a direction 340.9861.18. As shown in Fig.
1, when winds are from the north, both SMW and SMS
are in the shadow of the wind farm. Wind speed and
direction measurements from LM represent the regional
wind field with more accuracy. For this 30-min period
the stability (as determined by the Monin–Obukhov
length based on the temperature profile) was near neutral
and this is confirmed by the log-linear relationship shown
by the wind profiles. Turbulence intensity (
s
U
/U) as de-
termined at 48 m was 5.8%. After measurements close
to SMW, the Seaworker was repositioned at 3.8 Din the
direct wake of turbine 1W and the wake measured with
the sodar for approximately 30 min. During this period
the wind direction was 342.9861.98and the stability at
LM was near neutral. Turbulence intensity was 5.9%.
Figure 4 shows the average wind speed profiles for these
two periods. Error bars shown are one standarddeviation
on either side of the mean value, showing little variability
in the wind speed measured either by sodar or on the
masts. While there is good agreement between the sodar
and the mast measurements at 30 m (Fig. 4), the sodar
gives rather higher wind speeds than the mast measure-
ments at 48 m in the first period. The freestream and
wake profiles are shown in Fig. 5, together with the cor-
rected relative velocity deficit DU/U
freestream
, which has a
maximum at 40 m.
b. One wake turbine-on, turbine-off measurements
During this experiment (7 in Table 3) the ship was
positioned approximately 100 m (3.4 D) behind turbine
6E in light southeasterly winds. The wind direction was
146.50863.68at LM and 145.7862.48at SMW. Mea-
surements at all three masts are affected by the land
fetch to the south from this direction as shown in Fig.
1 and the sea masts give a better representation of the
freestream wind speed. The distance to SMS is approx-
imately 1.6 km and to SMW about 2.3 km, hence the
distance from the coast to the ship was about 1.9 km.
Turbulence intensity at 48 m at SMW was 5.9%. Sta-
bility was slightly unstable at LM (L;440 m) and
slightly stable at SMW (L;668 m). The height of the
internal boundary layer is expected to be close to 90 m
[calculated using the formula in Bergstro¨m et al.
(1988)]. However, it is not evident from the measured
wind speed profiles (Fig. 6).
As expected wind speeds at LM are lower than at the
sea masts. There is good agreement between wind
speeds measured at the sea masts and between the sea
mast and sodar data during the nonwake measurements.
The ratio between the measured wind speeds at 48 m
between the masts and the sodar is 0.96 at SMW, and
there was no measurable wind speed shift during the
two periods at either LM or SMW. Figure 7 shows the
corrected relative velocity deficit profile with a maxi-
mum between 35 and 40 m.
c. Single wake experiments
In total 13 wake experiments were conducted where
both freestream and wake measurements passed quality
controls. Figure 8 shows the relative velocity deficit
profiles and details are given in Table 3. In Fig. 8 the
velocity deficit profiles have been grouped according to
distance from the turbine (expressed as number of rotor
diameters D). Out of the three near-wake experiments
(4 and 9) two show a distinct minimum at the height
of the turbine nacelle and maximum at the midpoints
of the blades (29 and 48 m). This is not so evident in
the third experiment (A), which was also at less than 3
D. Of the five experiments between 3.3 and 3.9 D, all
except experiment 5 show a similarly shaped profile
with a maximum velocity deficit close to 40-m height.
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2003 473BARTHELMIE ET AL.
F
IG
. 6. Wind speed profiles for the (a) turbine-off, (b) turbine-on experiment at turbine 6E at Vindeby (expt 7 in
Table 3). Error bars shown are 1 std dev on each side of the mean.
However, there is quite a large variation in the velocity
deficits and the two experiments conducted in near-neu-
tral conditions (1 and 3) have the highest velocity def-
icits. Theory predicts that wake recovery should be fast-
er in near-neutral conditions. Three experiments were
conducted at distances of 4.1–5.0 Dand these show a
fairly flat profile. There are two ‘‘far wake’’ experiments
(D.6), which show good agreement in the velocity
deficit profile.
The velocity deficit profile depends on a number of
factors including wind speed profile for wake recovery,
the wind speed–related thrust coefficient of the wind
turbine, ambient (mechanical and thermal) and turbine-
generated turbulence, and the possible presence of an
internal boundary layer or nonequilibrium conditions as
flow adjusts in the coastal area. Hence it is difficult to
analyze the data further without use of wake/meteoro-
logical models. In the next section, the experimental
data are compared with results from an empirical model
that predicts the velocity deficit based on the transport
time.
4. Comparison of sodar data with previous wake
measurements and an empirical model
One of the major concerns regarding offshore wind
farm development is that wakes will decay more slowly
offshore due to lower turbulence requiring larger spac-
ing of offshore turbines than is the case on land. This
has major financial implications due to the cost of un-
dersea cabling. Hence it is of interest to examine wake
decay with distance from the turbine and to compare
this with similar data from land sites. Wake measure-
ments have been made previously at offshore wind
farms at Vindeby and at Bockstigen, but since these are
mast measurements they are at specific, fixed distances
from the turbines. One of the main objectives of using
the ship for the sodar measurements was to determine
the velocity deficit at different wake distances.
Magnusson and Smedman (1994) summarized wake
data from a number of coastal (onshore) and inland sites.
Rather than repeat this exercise we show here (Fig. 9)
the relative velocity deficit against Dfrom the sodar
experiment with a regression line estimated from the
data in Magnusson and Smedman (1994) as
DUx
51.03 . (4)
20.97
12
UD
freestream
Relative velocity deficits from the sodar experiment (Ta-
ble 3) are also shown in Fig. 9. Regression of these data
give the following fit:
DUx
50.98 . (5)
20.96
12
UD
freestream
Correlation coefficient for this fit (relative velocity def-
icit vs distance in rotor diameters) is 0.79. Although the
velocity deficits from the sodar experiment are smaller
474 V
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20JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY
F
IG
. 7. Corrected freestream and wake profiles and relative velocity
deficit measured by sodar (expt 7 in Table 3). Error bars shown are
1 std dev on each side of the mean.
than those from Magnusson and Smedman (1994) (for
the same distance), the difference is small compared to
the uncertainty in the measurements. The agreementbe-
tween the distance decay of the velocity deficit from the
offshore Vindeby experiment and the onshore data from
Magnusson and Smedman (1994) may also partly reflect
the coastal location of the majority of the measurement
sites used in that study.
There does not seem to be any particular feature that
determines whether the relative velocity deficit for each
experiment lies close to the predicted value. For ex-
ample, the three higher wind speed cases (experiments
1–3) lie on either side of the line; the three most stable
cases (experiments 4–6) lie close to the line, but the
experiment B with a similar Monin–Obukhov length
does not. Nor can the predictability of the experimental
results be grouped in terms of ambient or total turbu-
lence.
A simple model defined by Magnusson and Smedman
(1996) gives the decay of the relative velocity deficit
DU/U
freestream
as
DUt
0
50.4 ln 1C,t.t(6)
T0
12
Ut
freestream
where C
T
is the thrust coefficient, t
0
is a timescale, and
tis the transport time defined as x/U
freestream
where xis
the distance in meters.
Equation (6) cannot be used for large tif the absolute
value of 0.4 ln(t
0
/t).C
T
.
The timescale t
0
can be defined for near-neutral con-
ditions by
1HR
t5ln , (7)
0
12
fzH
0
where fis the rotational frequency, Ris the radius of
the rotor, His the hub height, and z
0
is the roughness
length.
The parameters for the Vindeby wind farm are f5
0.59 Hz, R517.8 m, H538 m. To provide the first
estimates of t
0
,z
0
is assumed to be 0.05 m over land
giving t
0
55.3 s and z
0
50.0002 m over sea giving
t
0
59.7 s. [Note these are similar to those given for
Alsvik in Magnusson and Smedman (1996) ;5.25 s.]
From rearranging Eq. (6) the calculated relativevelocity
deficit (in near-neutral conditions) is
DUt
0
2C50.4 ln . (8)
T
12
Ut
For the same turbine (known C
T
), a velocity deficit
can then be determined for on- and offshore situations
(where the difference is the surface roughness) and these
are shown against transport time in Fig. 10. To compare
this with the results of the sodar experiment, a transport
time was calculated for each relative velocity deficit and
this is also shown in Fig. 10. The results lie close to
the curve derived from onshore roughness—implying
that the wake decay at Vindeby can be predicted using
a roughness more typical of coastal than offshore en-
vironments.
For the period 1996–2001, average turbulence inten-
sity at 48-m height at LM is 12% compared with 10%
at SMW. Although the turbulence level at SMW is in-
creased by the presence of the wind farm (removing
these sectors gives a turbulence intensity of 9%), these
results suggest that the turbulence level in the first few
kilometers from the coast is not significantly lower than
at coastal sites. Turbulence level calculated using data
from the same period at 48-m height at a similar mast
11 km offshore from the coast was 6.5%. However, the
turbulence intensity at SMW during the sodar experi-
ments was 9% or less in all but one case, lower than
would be expected for a land site.
The question arises whether the turbine added tur-
bulence I
added
is more significant to the wake develop-
ment than the ambient turbulence I
0
. The turbulence
experienced at each turbine I
total
can be estimated using
the following equation from Frandsen et al. (1996):
222
I5I1I.
total 0 added
(9)
The turbine added turbulence intensity is determined
using the thrust coefficient and turbine separation [see
Frandsen et al. (1996) and Eq. (10)] and ambient tur-
A
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2003 475BARTHELMIE ET AL.
F
IG
. 8. Relative velocity deficit profiles for each of the 13 experiments grouped by distance of the measurements to
the turbine (expressed as number of rotor diameters). Numbers shown refer to the experiment designations given in
Table 3.
F
IG
. 9. Relative velocity deficit by distance (shown here as
number of rotor diameters D). The relative velocity deficit is de-
fined as (U
ambient
2U
hub height
/U
ambient
). The dashed line shows a
regression fit to data from Magnusson and Smedman (1994). Num-
bers shown refer to the experiment designations given in Table 3.
F
IG
. 10. Relative velocity deficit and transport time calculated for
two different roughnesses [representing onshore (0.05 m) and off-
shore (0.0002 m) and from the sodar data]. Numbers shown refer to
the experiment designations given in Table 3.
476 V
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20JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY
bulence intensity is calculated as the 10-min standard
deviation of the wind speed divided by the 10-min av-
erage wind speed. Hence the total turbulence intensity
is given by
1.2C
T
2
I5I1, (10)
total 0 2
!
s
t
where I
0
is the ambient turbulence calculated as
s
y
/U
and
s
U
is the standard deviation of wind speed and s
t
is the turbine separation (expressed in rotor diameters).
Given that the ambient turbulence is known, the equa-
tion is applied here for each experiment and the results
shown in Table 3. The added turbulence is significant
when Dis less than ;4.
The analyses presented indicate that the wake decay
measured at Vindeby (coastal offshore) is not substan-
tially different than that measured at other coastal (on-
shore) sites despite observed lower ambient turbulence.
This requires further investigation using both datasets
from onshore wind farms and by conducting further
sodar measurements at a far offshore site.
5. Conclusions
The main objectives of the sodar experiment de-
scribed here were to evaluate the operation of the sodar
when mounted on a ship offshore and to investigate
whether the noise of the operating wind turbine im-
pacted the vertical profile of wind speed as measured
by the sodar. These objectives were met with the sodar
operating well. Vertical profiles measured by the sodar
gave good agreement with the mast data. Sodar wind
speeds were within 2%–15% of mast measured wind
speeds (at 48-m height). In the situation of the Vindeby
wind farm in relatively shallow water and with lowwave
heights, it proved possible to measure wind speed pro-
files that clearly showed a deficit in wind speed close
to hub height during turbine operation, which disap-
peared when turbine operation was stopped. The noise
of the wind turbine did not prove a barrier to successful
monitoring of the wind speed profile up to 100-m height.
Wake profiles were clearly measured as a reduction in
the wind speed (centered on the hub height of the wind
turbine). The relative centerline velocity deficit was cal-
culated and the decay of the wake (with distance or
transport time) was shown to be similar to those deter-
mined by other wake studies in coastal (onshore) en-
vironments despite the relatively low turbulence. Com-
pared with an empirical model of relative velocity deficit
versus transport time, results from the sodar experiment
were closer to predicted velocity deficit over a rough-
ness of 0.05 m than to predictions using an offshore
roughness of 0.0002 m. This may indicate that offshore
wind farms in coastal environments (up to 3 km from
the coast) can be effectively modeled with wake model
designed for onshore wind farms. The results of the
sodar experiment are now being compared with physical
wake models [e.g., based on Ainslie (1988)], which can
also account for the variation in turbulence intensity.
The experience from the experiment may prove useful
in current plans to use sodar operated from floating plat-
forms for wind monitoring in the North Sea. Although
wave heights were comparatively low during the ex-
periment, it was possible to measure the longitudinal
and transverse tilt angles at high time resolution, which
could potentially be used to correct the sodar signal in
future experiments.
Acknowledgments. Financial support for this research
was given in part by the European Commission’s Fifth
Framework Programme under the Energy, Environment
and Sustainable Development Program. Project refer-
ence: ERK6-1999-00001 ENDOW. Meteorological
measurements at Vindeby are funded by SEAS. We
would also like to acknowledge PM Diving’s Seaworker
crew Carsten Johannesen, Gregers Glensdorf, Lars Jen-
sen, and Flemming Carlsen and Bent Christiansen of
Vindeby. Weather forecasts were provided by the Na-
tional Environmental Research Institute of Denmark
with thanks to Bjarne Jensen. Dr. Sara Pryor of Indiana
University is acknowledged for useful comments on the
manuscript. The comments and suggestions of three
anonymous reviewers have improved the clarity of the
paper.
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