a study of the near wake structure of a wind turbine comparing measurements from laboratory and full...
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Pergamon
Solar Energy Vol.
56, No. 6, 621-633, 1996p.
Copyright
0 1996 Elsevier Science Ltd
PII: SOO 38-092X 96) 00019-9
Printed in Great Britain. All rights reser ved
003%092X/96 $15.00 + 0.00
A STUDY OF THE NEAR WAKE STRUCTURE OF A WIND TURBINE
COMPARING MEASUREMENTS FROM LABORATORY
AND FULL SCALE EXPERIMENTS
J. WHA LE,* K. H. PAPAD OPOU LOS,** C. G. AND ERSON,*** C. G. HELM IS**
and D. J. SKYNER*
*University of Edinburgh, Department of Mechanical Engineering, Kings Buildings, Mayfield Rd.
Edinburgh EH9 352, U.K., **University of Athens, Department of Applied Physics, Laboratory of
Meteorology, Ippokratous 33,10680 Athens, Greece and
***Aerpac Special Products B.V., P.O. Box 167.
7600AD Almelo, The Netherlands
(Communicated by DAVID MILBORROW)
Abstract Wake flow
measurements have been performed using the technique of particle image velocimetry
(PIV) at stations downstream from a model wind turbine rotor, and evaluated against experimental data
from two full-scale machines. Comparisons include both mean velocity and turbulent intensity cross-wake
profiles at a range of tip speed ratios. The application of PIV to the study of wind turbine wakes is
described in detail, including the steps required to ensure appropriate and accurate simulation of the flow
field conditions. The results suggest that the PIV method is a potentially useful tool in the investigation
of detailed wake flow, though significant differences are observed between wake velocity deficits at full-
and model scale. These are discussed with regard to scale effect, the influence of terrain, model similarity,
and the phenomenon of wake meandering and effective cross-wake smoothing. Copyright 0 1996 Elsevier
Science Ltd.
1. INTRODUCTION
The operation of a wind turbine produces a
downstream region of reduced wind speed, the
so-called wake. The wake constitutes an impor-
tant factor in determining the siting of turbines
in windfarms, for two principal reasons:
(1) mean wake characteristics, and their relation
to the incident wind field and the local topogra-
phy, influence the total energy resource at a
potential windfarm site, and (2) the turbulent
structure of the wake affects the fatigue loa ding
of downstream turbine rotors, thu s dictating the
minim um spacing of wind turbines within the
windfarm. The design of windfarms, therefore,
can benefit significantly from a detailed know l-
edge of these fundamental wake parameters.
The present article describes a recent investi-
gation into the properties of the near wak e
region of three-bladed wind turbines. Meas-
urem ents were obtained from two full-scale
machines, and from a replica model in the lab-
oratory, at approximately l/100 scale. The full-
scale data were derived from new experiments
carried out on the Greek island of Sam os, by a
research group at the University of Athens, and
from data previously recorded at the Na tional
Test Station for Wind Turbines, at Risa,
Denm ark. At full scale, wake data were obtained
from anemometry measurem ents. The small-
scale experiments were conducted by a research
group a t the University of Edin burgh , Scotland,
using the relatively recent techniqu e of particle
image velocimetry (PIV).
Field studies of wakes behind single turbines
or of multiple wakes in wind farms (H6gstrBm
et al. 1988; Taylor et al. 1988; Larsen & Velk,
1989; Nierenberg 1989; Elliot and Barnard,
1990 ) usually con centrate on the decay rate of
the velocity deficits in the far-wake region. This
is then related to powe r production optimisation
of wind farms. W hile confirming the qualitative
trends revealed by win d tunnel sim ulations, field
studies h ave indicated the necessity of further
mea surem ents, especially over complex terrain
(Van der Snack, 1989), in order to improve the
accuracy of theoretical wak e mo dels.
Wind tunnel studies have also demonstrated
that the simulation of the near wake region,
whic h is usually d escribed by a uniform velocity
profile in the so-called potential core, does not
represent the real-flow situation accurately
(Ainslie, 1987). This suggests that the effect of
the turbulence produced by the turbine is
improperly parameterised, and again the need
for more detailed measurements is indicated.
The use of PIV in wind turbine studies is a
relatively recent developm ent. Infield
et al.
(1994) have applied the technique both in the
wind tunnel, and to a full-scale wind turbine in
the field. Their stud ies concentrated on the
imm ediate vicinity of the blade, and produced
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622 J. Whale et al.
detailed profiles of bound circulation and the
tip vortex. Visualisation of the full wak e of the
rotor up to 4 rotor diameters (D) downstream
was achieved at Edinburgh (Whale and
Anderson, 1993 ) using a small-scale model, with
water, rather than air, as the flow medium.
One major objective of the present work w as
to assess the validity of small-scale PIV meas-
urements as a technique for investigating full-
scale wind turbine phenomen a. If successful,
there would be significant attractions in using
PIV, because of its ability to map the velocity
in the entire rotor wak e at a given instant. PIV
vector maps may be processed to yield both
bulk wak e measurem ents, such as velocity defi-
cits, or data relating to the detailed structure of
the wak e, e.g. vorticity contours. In the present
campaign, results were restricted to mean and
turbulent wak e charac teristics only, measu red
at distances of l.lD and 1.5D downstream of
the model turbine, i.e. in the near wak e, and
subsequently comp ared with full-scale data.
It is wor th noting that, despite their impor-
tance for understanding the basis of wind turbine
rotor performance, measurements at distances as
close as 1D downstream are rare. The few exam-
ples include the compreh ensive wind tunnel study
by Papaconstantinou and Berge les (1988 ), the
extended Nibe project (Taylor, 1990) which
included some experimental results at 1D and
the study of the near wake structure of a Darrieus
turbine by Strickland and Goldman (1981 ).
2. DESCRIPTION OF EXPERIMENTS
2.1. Ful l -sca le measurements
2 .1 .1 . Exper i m ent a l m eth od. At full scale,
windspeed measurements were made simulta-
neously upwind and downwind of the wind
turbines to determine velocity pro files in the
wake. The method has been widely used in the
wind turbine field. Windspe eds are record ed
using mast-mounted anemo meters, one ahea d
of, and one behind, the rotor. Mean values of
upstream windspeed are assumed to represent
the freestream velocity. W ake velocity profiles
are obtained by recording data over a range o f
incident wind directions, such that the down-
stream anemom eter is immersed in different
parts of the wake. The wake velocity ratio for
a given wind direction is normalised as the ratio
of downstrea m to upstream windspee d, suitably
time-averag ed. Wa ke velocity ratios are then
plotted aga inst upstream wind direction. On the
assumption that the wind turbine yaw system
tracks the wind direction accurately over the
averaging period, the data can be re-interpreted
as velocity ratio profiles obtained by a hori-
zontal traverse behind, and parallel to, the rotor.
Note that the wake profiles thus obtained are
inherently averag ed with respec t to short-term
variations of upstream wind direction. Although
no information on the yaw control of the experi-
mental machines was available, it is assume d
that on average the rotors were correctly aligned
with the upstream wind direction throughou t
the measurements. It is to be expected, however,
that rotor alignment lags behind changes in
upstream wind direction, leading to a degre e of
cross-w ake smoothing of the velocity profiles.
It should also be noted that by normalising
the wake velocity with respect to the upstream
anemom eter readings, it is implied that the
upstream values o f wind speed and ambient
turbulence are considered representative of the
actual flow which intersects the rotor. This
assumption has previously been justified on the
basis of experimental analysis (Helmis et al . ,
1995).
2.1.2. Sam os I s l and 19 m w ind u rb i ne . Samos
Island lies in the eastern region of the Aegean
Sea, and hosts a wind farm located 390 m above
mean sea level, on a saddle confined by the
island’s two major mountain ranges. The wind
farm comp rises nine three-bladed , horizontal-
axis, Vestas (formerly Windmatic) WM 19S wind
turbines, with rotor diameter of 19 m, hub-
height of 25 m, and output rating of 100 kW.
The WM 19S is stall regulated, achieving rated
power at a windspeed of 13 ms- ‘. Cut-in and
cut-out wind speeds are 3 ms-’ and 2 7 ms-’
respectively. The maximum pow er coefficient
C pmax is 0.38, attained in the windsp eed range
8-10 ms-‘, at a constant rotational speed of
48 r.p.m.
Measurements were made on a single wind
turbine using two met masts, one located 0.80
(where D is rotor diameter) upwind, the other
l.lD dow nwind, of the machine. The measure-
ments used for comparison with the laboratory-
scale PIV data were taken from cup anemome-
ters mounted at 12 m and 29 m above ground
level, on the upwind and downwind masts,
respectively; the anemom eter sampling r ate was
1Hz . The experimental layout is described fully
by Helmis et a l . (1995), and the measurements
were made over the period 16-24 August 1991.
At the given elevation (29 m), the downstre am
anemometer was above the centreline of the
rotor, clear from the influence of tower shadow.
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A study of the near wake structure of a wind turbine
623
This also meant, however, that the wake profiles
me asured by it were not on the rotor centreline,
but rather at an offset of 0.42R , where
R
is the
rotor radius. Th is was taken into account when
replica tests were carried out at small scale using
PIV (see Section 2.2.2.). Ano ther factor to be
accounted for in the comparisons was the com-
plexity of the terrain on Samo s, and an attemp t
was made to compensate the results for a non-
uniform inflow profile, based on the use of ‘non-
wake’ data, measured with the turbine stopped
(see Section 2.1.3.).
During the experiments, the mean w indspeeds
at the Sam os site ranged from 9 to 27 ms-‘.
The influence of wind speed on the wak e features
has been discusse d previously (Helm is et al.,
1995; Papadopoulos
et al . ,
1995). The mean
upstream turbulence intensity was 6%, while the
atmospheric stability w as estimated by near-
surface air tempe rature profiles to be effec-
tively neutral.
2.1.3. R i se 20 m w ind t u rb i ne . The second
wind turbine for which wake profile data were
obtained was a Vestas V20/100, three-blade,
stall regulated machine, located at the Rise
National Laboratory in Denm ark. The V20 has
a 20 m diameter rotor, rotor speed of 45.5 r.p.m.,
and rated output of 100 kW .
The measurem ents were made on the V20
prototype mac hine in 1988 , and have been fully
documented previously (Paulsen, 1989). The
upwind and downwind met masts were situated
at 0.680 and 1.5D, respectively. The anem ome-
ters were located at hub height (24.25 m) in
both cases; as a result of this, wake velocity
profiles traversed the rotor centreline. W ind-
speed data were sampled at 2Hz, with run
statistics based on 1 0 min averaging. The experi-
mental data used were measured in the mean
windspeed range 8-9 ms-‘; the turbulence
intensity was in the range 5-10% .
The use of the V20 for comparison with the
W M1 9S was based on the similarity of the two
ma chines in terms of their configuration, i.e.
both three-bladed and stall regulated, and size
(100 kW rating in both cases, and rotor diame-
ters of 20 m a nd 19 m, respectively). It should
be noted, however, that the V20 has a somewhat
lower solidity than the WM 19S, a t 5.5% rather
than 9% . The po ssible influence of this on the
experimental results is discusse d in Section 3.
2.1.4.
Fu l l - s ca l e w ake p ro j i l es
a ) W M 19S, Samos I s l and .
As noted before an attempt was made to
correct for the influence of complex terrain on
the Samos Island wake data, and in particular
the existence of a non-uniform upstream profile.
To do this, measurem ents were taken with the
turbine in operation (the wake data set) and
stationary (the non-wake data set). A prelimi-
nary analysis of the non-wake data set was then
used to establish the backgro und correction to
be applied to operational data. The importanc e
of this procedure is seen from previous results
described by Helmis
et a l .
(1995) who highlight
the uncertainty introduced by estima ting wa ke
velocity deficits by com paring upstream and
downstream measurements using data recorded
only with the turbine in operation.
The non-wake velocity ratio was found to
depend significantly on win d direction (Fig.
1 ),
and less strongly on windspeed. Data for low
and high wind speeds were therefore used to
yield two correction curves, which gave the non-
wak e velocity ratio as a function of wind direc-
tion for each windspeed range. This was then
used to provide correction factors for the data
obtained during operation of the turbine: a
given velocity ratio o btained with the turbine
running was divided by the non-wake ratio
corresponding to the same upstream wind direc-
tion. In this way, the effects of topography and
wind shear were compensated for.
The corrected wak e data, i.e. with the turbine
operational, are shown for a range of wind
speeds in Fig. 2. Although full wake profiles are
not ava ilable for all win d speed ranges, the
results show a clear dependence on windspeed,
with the wake deficit (defined as one minus the
velocity ratio) increasing as a function of tip
9
al 0.7
E
I
Upstream speed range
Dooo 13-15 m/s
+++++ 15-17 m/s
ooooo 22-26 m/s
0.6 j
I I 1 I I I
310 320 330 340
350 360
370
Wind direction (deg.)
Fig. 1. Non-wake velocity ratios at l.lD for the WM19S
wind turbine (rotor stationary), as a function of upstream
wind direction and windspeed.
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624 J. Whale et al.
1.1
0
1.0
.;
E 0.9
h
ti
5 0.6
E
c
g 0.7
E
0.6
310
, 3io
3io 340
350 360
Wind direction (deg.)
3+0
Fig. 2. Wake velocity ratios at l.lD for the WM19S wind
turbine; downstream velocity is measured at an offset posi-
tion of 0.42R above the rotor shaft axis, and corrected for
non-uniform inflow conditions using factors derived from
Fig. 1.
speed ratio. It is assumed that the wake centre-
line correspo nds to the 350” wind direction, for
which the wind turbine is directly upwind of the
29 m measuring anemo meter. Centreline veloc-
ity ratios, at a radial distance of 0.42R above
the rotor shaft centreline, may be derived
directly from the data in Fig. 2.
The given w ake profiles are based on 1 min
averag es. Longitudinal and lateral coheren ce
considerations suggest that the relatively short
averaging time is approp riate (Taylor, 1990) .
Analysis of corresponding 15 min samples gives
almost identical results, though with a some-
wha t more ‘spiky’ appearan ce: this was attrib-
uted to change s in rotor orientation during the
15 min period caused by operation of the yaw
system. The graph s based on 1 min data are
nonetheless fairly smoo th. The statistical signi-
ficance of the results may be assume d greates t
for the more extended wake data sets. The
measu red standard deviation of wind direction
was of the order 5-6” , which translates into a
maximum cross-wake smoothing over f 5%D .
b) V20/100, Riss.
Unlike the Sam os Island site, the Rise Test
Station in Denm ark is situated on very flat
terrain. For this reason, no correction was made
to the measured velocity profile for the Vestas
V20 /100, which is shown in Fig. 3. The mini-
mum velocity ratio on the wak e centreline is
approxim ately 0.4. A point of note is that the
velocity ratio rises above unity at cross-w ake
1.1
0 1. 0
?
ti o. 9
. s
E 0.8
x
x
8 0. 7
?
3 0. 6
i s
c 0.5
:
0.4
0.3
.
:
. ’
.
.
.
I
. . .I RiSB data (A-5.6)
I I
I
I
I
245 265
285 305
Wind direction (deg.)
325
Fig. 3. Wake velocity profile measured for Vestas V20/100,
Riss Test Station. Downstream distance is 1.5D, tip speed
ratio 1= 5.6.
distance y/R > 1, i.e. the boundary prescribed by
the rotor radius. This indicates a speed-up of
the Ilow at the edge of the w ake.
2.2.
Laboratory-scale measurements
2.2.1. Experimental method. The experiments
at model scale were made using the technique
of particle image velocimetry (PIV). PIV is a
non-intrusive velocity measurem ent technique
which allows two-dimensional flow fields to be
capture d at a single instant. The basis of PIV is
to illuminate a two-dimensional plane of flow
containing small, neutrally buoyant, seeding
particles, using a stroboscopically repeating
light source. A double (or multiple) exposu re
photograph of this plane is taken, whereby the
spacing between the images of each particle on
the film gives the local flow velocity. The ph oto-
graph is then analysed to determine the flow
velocities across th e entire field.
The film is interpreted point by point over a
dense grid using a combination of optical and
digital analysis: this involves scanning successive
small regions of the negative with a probe laser
to produc e an interference pattern from the
multiple particle images in that area. The inter-
ference fringes are measured and recorded in
digital form and the data Fourier transformed
to yield the particle velocities at that point. The
whole negative is scanned in this way to build
up a flow velocity map, which forms the basis
of all subsequent analysis.
The technique of PIV was introduced to the
field of wind turbine aerodynam ics by Infield
et al. (1994) who conducted tests on a 0.9 m
diameter wind turbine in a wind tunnel, using
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A study of the near wake structure of a wind turbine 625
T
50
mm
l
.A
Tffles
water surface
T
FI
streamlined
I
tower
I
I
measurement zone
Fig. 4. Schematic diagram of PIV set-up in the laboratory
pulsed lasers. The tests established the applica-
bility and usefulness of PIV as a velocimetry
tool for wind turbines. The same resea rchers
subsequently made PIV measurem ents on a full-
scale wind turbine of 17 m diameter. These tests
were mainly concerne d with visualising the flow
around a localised region of the blade, ho weve r.
Visualisation of the entire wak e at full scale
using pulsed lasers presents obvious difficulties.
The PIV equipment at Edinburgh University,
as used in the present study, is applicable to
scale model rotors only, but is capable of captur-
ing images from the near w ake up to 4 down-
stream. The experiments were carried out in a
two-dimensional wate r flume, 10 m long by
400 mm wide (Fig. 4). The use of water, rather
than air, as the flow medium greatly facilitates
seeding and illumination. The flume has glass
walls and base, and is filled with wate r to a
depth of 750 mm. A steady current can be
established in the tank, driven by a wate r pump ,
and recirculated via an external pipe system.
A continuous wave (CW) laser was used in
conjunction with a scanning beam system of
illumination to produc e the laser sheet (Gray
et al. 1991). The laser sheet was directed
through the base of the flume, illuminating a
two dimensional cross-section of the flow. The
model turbine rig was placed in the tank, with
the rotor aligned normal to the upstream flow.
The water was seeded with conifer pollen of
average diameter 70 pm; concentrations were
maintained at a level that ensured a high density
of non-overlapping particles on the resulting
film record.
The model rotor (Fig. 5) was a l/lOOth scale
replica of the three-bladed Vesta s (Windmatic)
WM 19S. The blades were manufactured from
rigid plastic, using a numerically controlled
cutter. Despite the small scale, the model blades
were accurately profiled with a NACA -632XX
Fig. 5. Scale model of the WM19S rotor used in the labora-
tory test.
section, w ith twist, chord and thickness distribu-
tions based on the manufacturers’ original d raw-
ings. The turbine model was driven in the tank
by an electric motor, located on a frame above
the water level, and connected to the rotor shaft
by a toothed belt running inside a hollow tube,
effectively an inverted “towe r” . In order to
reduce the disturbance to the rotor w ake caused
by the tower, it was streamlined with a foam
plastic shroud of symmetric aerofoil cross-
section.
The image recording equipment consisted of
a rotating-mirror shifting system,
and a
Hasselblad large format cam era. The purpose
of the shifting system was to superpose a known
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626 J. Whale et al.
velocity component onto the record ed image in
order to (a) eliminate directional ambiguity in
the final vector maps, and (b) increase the
dynamic range of the system. The shifting
sequence was synchronised with an index pulse
from a position encode r, connected to the tur-
bine drive motor.
For reasons of geom etry, the image shift-
velocity was non-uniform over the flow field. A
correction therefore had to be applied to the
proce ssed results, base d on a calibration of the
shifting system. In order to separa te the effects
of image-shifting distortion from the flow
recording of the wake, a number of PIV photo-
graph s wer e taken of still wate r in the tank. The
still-water records w ere then averaged and
subtracted pointwise from the vector fields of
the turbine wak es in order to correc t for shift
velocities.
2.2.2.
Replicating full scale conditions.
The
experimen ts wer e aimed at reproducing the full-
scale measurements taken on Samos with the
Vestas WM19S . Thus, the model was positioned
with the rotor centreline at a distance of 0.42R
from the laser sheet. Assuming an axially sym-
metric w ake, data were thus recorded at an
equivalent offset position from the centre of the
wak e to the readings taken at full scale (see
Section 2.1.2.).
In order to control the ambient turbulence
level, turbulence manipulators wer e placed
upstream of the rotor. T hese consisted of a
parallel system of baffles comprising of an alu-
minium honeycom b section, a perfora ted plate
and a fine mesh (see Fig. 4). The honeycomb
acted as a flow straightener, with the perforated
plate serving to impose a particular upstream
profile. Final smoothing was provided by the
fine mesh screen.
From the Samos experiments, it was con-
cluded that the wind spee d and turbulence
intensity were fairly constant acros s the rotor
disk. The turbulence manipulators wer e there-
fore chosen to produce a uniform upstream
profile with low turbulence. A perfora ted plate
with 32 mm diameter holes and regular pitch of
38 mm w as placed 500 mm downstream of the
honeycom b section. A fine mesh screen o f 18
lines/inch was placed a further 800 mm down-
stream, and 1200 mm upstream of the rotor.
As noted above, an important feature of this
work was to investigate whether tests at model
scale could yield valid data regard ing the perfor-
mance of full-scale wind turbines. It was there-
fore decided to replicate as accurately as possible
the conditions pertaining to the full-scale meas-
urements on Sam os, with the obvious exception
of scale. In this way, any discrepancies between
the full-scale measurements and those obtained
from the PIV tests could be attributed either to
scale effect, i.e. Reynolds number, or tunnel
blockage , rather than an improperly charac ter-
ised experiment.
Geom etric similarity between th e model and
the full-scale machine was attained by the use of
an accura te rep lica. Kinematic similarity was
achieved by running the model at an approp riate
range of tip speed ratios A, using the motor speed
controller. The wate r current velocity was main-
tained constant throughou t, with its value accu-
rately determined from the PIV analysis.
A further consideration was that, while the
full-scale results w ere based on time-averag ed
recordings, the PIV vector maps corresponded
to instantaneous wak e images. It was there-
fore necessary to introduce “equivalent time-
averaging” in the latter case. This was done by
repeating each PIV test a number of times, with
the camera exposure synchronised to a different
rotor position in each case, and taking a numeri-
cal averag e of the resulting vector maps . Ideally,
i.e. to achieve stationarity of the averag ed map,
this procedure would have been repeated a very
large number of times, with the rotor photo-
graph ed at positions evenly distributed around
the disk. In practice, the shifting sequence was
synchronised to photo graph the blades in just
6 azimuthal positions, 20” apart. For a
three-bladed rotor, this discretizes one whole
revolution. Post-analysis averaging of these six
exposures yields six vector m aps which,
averaged together, provide the equivalent of a
time-averaged wake image.
2.2.3. Results and analysis. The PIV photo-
graphs thus obtained were processed to yield
two-dimensional velocity vector maps of the
type shown in Fig. 6, whe re ea ch vector indicates
the velocity in the flow at that point. The figure
shows the wake behind the WM 19S model
operating at a tip speed ratio of 4.8; the area of
reduce d velocity behind the rotor is clearly seen
in this image. V elocity m aps we re obtained at
five tip speed ratios in the range 1.6-4.8.
The cross-w ake profile at l.lD (o ffset position
0.42R) downstream of the model rotor was
found by averaging four columns from the
vector map corresponding to downstream dis-
tances 1.0-1.20 from the rotor to account for
any uncertainty in the downstrea m position.
The results are shown in Fig. 7, as velocity ratio
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A study of the near wake structure of a wind turbine
621
Downstream distance, x(metres)
Fig. 6. PIV velocity vector map of wake behind WM19S model rotor at i. =4.8. The vertical cross-section
of the wake is evident as a downstream region of reduced velocity, and lies 0.42R from the centreline to
correspond to the full-scale experimental case.
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AAAAAh=4.2
x xxx x hz4.8
Fig. 7. Wake velocity ratio profiles at l.lD from PIV experiments. The curves for L=4.2, 3.2, 2.7 and 1.6
have been shifted upwards by 0.1, 0.2, 0.3 and 0.4, respectively, for clarity.
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J. Whale et al.
plotted against cross-wake distance. A single
averaged value of the upstream velocity w as
assum ed in calculating the velocity ratio for
each location: this was evaluated from the mean
flow statistics of the first column of vectors at
0.60 ups tream of the rotor, with corrections for
the distortion introduced by imag e-shifting
(see above).
The influence of tip speed ratio can be seen
clearly in Fig. 7. The wak e velocity ratio
decreases with increasing 2, with correspond-
ingly greater centreline deficits. The asymmetry
seen in the profiles at their outer edges (where
conditions approac h those in the freestream) is
attributed to the presence of the mod el suppo rt
structure, i.e. the “tower” of the wind turbine.
Desp ite its steamlined shroud , this evidently s till
introduced some non-uniformity behind the
rotor disc.
3. COMPARISON OF MODEL AND FULL-
SCALE RESULTS
3.1. M ean w ake p rope r t i es
a ) WM 19S Sam os I s l and ) da ta . Figure 8
contains a comparison of the full-scale and
model wake data for the WM19S wind turbine,
in the form of wake profiles at two tip speed
ratios selected from Figs 2 and 7. The cho ice
wa s dictated by the availability of a reasonab le
amount of cross-wake data from the full-scale
data set, and the similarity of the tip speed
ratios in the two cases. The wake profiles suggest
that the shapes of the model and full-scale wake
are somewh at different. The full-scale wak e is
wide and has a homogeneous central portion
whereas, in general, the PIV results produced
narrower profiles with deeper troughs.
The most likely reasons for the difference are
( 1) scale effect, and (2) mean dering of the full-
scale wa ke. The latter is caused by variation of
wind speed and direction during the averaging
period, causing smoothing of the experimental
profiles (Helmis
et al . ,
1995). The shape of the
full-scale wake profiles suggests significant
cross-wake mixing.
The effect of wak e m eandering is less likely
to be impo rtant in the near (as opposed to far)
wake and the relatively short averaging period
(1 min) should have ensured that gross changes
in wake direction were avoided. Nonetheless,
the minim a of the full-scale curves very often
do not coincide with the machine alignment of
350” and the influence of wake m eandering on
the experimental profiles cannot be discounted .
-1.0
1.2 ;
Cross-wake distance, y/R
-0.5 0.0
0.5 1.0
OeooO PIV data h=2.7)
= Full scale h=3.0)
0.7 I
a) ,
320
I I
330
I I
340 350
I
360
370 380
Wind direction deg.)
Cross-wake distance, y/R
-1.0
-0.5
1.2 ;
0.0
0.5
1.0
oe- PIV data h=3.2)
= Full-scale h=3.3)
,
9
0’
\
\
B.m,
Lf’
0.7l0
320
/
I
330 340
I
I
350 360
370 380
Wind direction deg.)
Fig. 8. Comparison of velocity ratio profiles at l.lD for the
WM19S rotor, comparing PIV and full-scale (FS) data: (a);
R,s=3.0, ,,=2.7, (b); I,s=3.3, l,,v=3.2.
Com parison of Figs 2 and 7 shows that at I=
4.4 the full-scale data contains a minimu m veloc-
ity ratio of about 0.68 at a cross-wake position
of 340” ; this is mu ch closer to the mod el result.
The complex terrain may also be a factor in
explaining the shape of the profiles a t full-scale.
Desp ite attem pts at similarity, the scales of
turbulence in the atmosphere may have been
different from those in the water tank and have
varied according to stability. Large-scale
inhomogeneities of the terrain im pose energetic
turbulent mo tions with characteristic scales of
the size of the wake (and even larger), leading
to sme aring of velocity gradients of the kind
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A study of the near wake structure of a wind turbine 629
found in the centre of the wake (see model wake
profiles).
From the given wake profiles, centreline
velocity ratios at an offset position of 0.42R
may be obtained. As noted above, the wake
ratios measu red at full scale (Fig. 2) incorporate
directional smoothing, because of the variation
in upstream wind direction during the averaging
period. To account for this at model scale, the
centreline velocity at l.lD downstrea m was
averaged over a cross-wake distance based on
the variance of the wind direction in the
full-scale tests. In practice, this involved averag-
ing togethe r the velocity vectors either side of
the centreline i.e. the two adjacent (cross-w ake)
values at l.lD downstrea m.
The resulting centreline velocity ratios from
the model tests are shown as a function of tip
speed ratio in Fig. 9, togethe r with the corre-
sponding data from the Sam os Island measure-
ments. The error bars attached to the model-
scale results are based on the standard error of
the cross-w ake averag e centreline velocity ratio
over four downstre am positions in the vicinity
of 1.1D , as noted above. For low 1, the compari-
son is promising. How ever, the two curves devi-
ate significantly at high /1.
Flow blockage in the tank could be a signifi-
cant factor in explaining the divergence of the
two curves tow ards high i. Another possibility
is that the experimental profiles are displaced
because of wake meandering (see above), and
that their velocity minima do not correspo nd
1.0 -j
\ .
b
. . * . . 95 confidence intervals
-PlV data
x_XXXY Full-scale data
A&AM MInimum ratios for full scale data
o~21~
r---. -I
/
,
3.0 3.5 4.0 4.5 5.0
Tip speed ratio h
(h&3
0.3
I I
I I
I
245
265
265 305 325
Wind direction (deg.)
Fig. 9. Comparison of centreline velocity ratios at l.lD for Fig. 10. Comparison of PIV wake velocity profile at 1=4.8
full-scale and PIV data. Best fit lines are drawn through the
with full-scale measurements from the V20/100 at Riser, at
corresponding points. The crosses are median values for the 1=5.6. The Rise, data have been processed to represent a
full-scale data; the triangles are subjectively chosen mini-
profile at centreline offset 0.42R (axisymmetric wake
mum values of the same data.
assumption).
to the assumed upstream wind direction of 350”;
accordingly, the “true” centreline velocity ratio
could be better represen ted by using the mini-
mum velocity ratio found at each A . This is
done in Fig. 9, whe re it is seen that the “cor-
rected ” curve f or full-scale data lies closer to the
laboratory results than the original.
The error bars in Fig. 9 also reflect the level
of turbulence existing at the location of interest
in each case. Note that the turbulence appears
to increase towards both high and low tip speed
ratios, w ith a minimum existing in between; this
feature is further discussed in Section 3.2.
b) V20/100 Rise) data . Figure 10 shows a
comparison of a PIV wake velocity profile at
2=4.8 with full-scale measurem ents from the
Vestas V20/100 at Rise, at a mean tip speed
ratio of A= 5.6. Again, model- and full-scale
data have been chosen on the basis of reason-
ably similar tip speed ratios. Note that the Ris0
profile show n here differs from the original data
in Fig. 3, which have been proce ssed to obtain
a velocity profile at an offset of 0.42R from the
rotor centreline. This is necessary because, as
noted in Section 2.1.2, the Riss ane mom eters
were set up at hubheight, to measure centreline
wake profiles.
In processing the Riser data, it has been
assume d that the centreline wak e profile is axi-
symme tric, and that a profile at arbitrary offset
distance d may be interpolated according to
the relationship JJ’~
y2 -d2
where y’ is the
Cross-wakedistance, y/R
-2.1
-1.05
0.0 1.05
2.1
1.1
I
I I
I
I
lr-
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630 J. Whale et al.
translated value of cross-wake distance y. The
high 2 values, with a minim um at an intermedi-
corresponding win d direction ((3)at 1.5D down-
ate value of A= 3.3. This effect of som ewh at
stream w as computed using the relation y=
lower turbulence levels at intermed iate 2, dis-
3R tan 0. Whatever the m erits of this assum p-
cussed above, is seen clearly in F ig. 12.
tion regarding the resulting profile shape, it
Moreov er, the flat profiles of Fig. 11(b) proba-
seems a legitimate method of extracting at least bly reflect the wake meandering and cross-wake
the wake minimum velocity ratio at the required smoothing of full-scale data. The local maxima
offset position, This is found to have a value of
of wa ke turbulence at approximately
I v /R I = 0 .6
0.44, compared with 0.39 for the small-scale PIV
(Fig. 11 (b)) are related to tip-vorticity induced
measurement. turbulence.
These velocity ratios are relatively close,
though again the mo del-scale result corres-
ponds to a greater w ake deficit than that seen
at full scale. It is not possible to draw any firm
conclusions from this, i.e. with regard to the
influence of scale, as the PIV model was not a
replica of the V20 wind turbine, and the
solidity of the two rotors was somew hat
different (see Section 2.1.3). No netheless, con-
sidering the large discrepanc ies in results at
high I for the WM 19S comparisons, it is a
promising result.
The spectral analysis of the full-scale data has
already revealed fundam ental variations of the
turbulent structure of the near wak e as the tip
speed ratio varies (Papadopoulos
et al . ,
1995);
not only is the wake intensity small at low il
values, but furthermore, the expected turb ulent
characteristics of the wake are absent. The PIV
results demonstrate a large growth in turbulent
energy in the centre of the wake with increasing
2. This ap pears to be caused by the strong tip
vortex structure.
3.2.
Turbu l en t p rope r t i es o f the w ake
Figure 11(a) presents laboratory results for
the turbulence intensity ratios which involve
four column averages as before. The ratio is
defined as the ratio of the four-column average
I, to the turbulence intensity v alue 0.60
upstream of the rotor. A significant in crease in
turbulence intensity is seen as tip speed ratio
increases, with maximu m turbulence in the
region of the wake centreline. There is also some
evidence that wake turbulence does not increase
indefinitely with increas ing ;1, but goes through
a minim um at some intermediate value.
An explanation for this may be that at low
tip speed ratio the rotor is heavily sta lled, and
the turbulence is caused by the separated flow
behind the individua l blades of the mo del rotor;
this may be referred to as “local” turbulenc e. At
high tip speed ratio the blades are likely to be
largely unstalled, with smooth (unseparated)
flow over their surfaces; however, by now the
wa ke itself is highly turbu lent on a large scale,
because of the strong vorticity being transmitted
into it from the rotor. At some intermediate
value of tip speed ratio, the blades may be
operating out of stall, but w ith a relatively we ak
vortex pattern in the wak e. The turbulence
peaks seen at low A outside the rotor circumfer-
ence (specifically
y / R >
1) may be caused by the
wake of the supporting “tower”.
A comparison of individual cross-wake pro-
files of turbulent intensity ratios (Fig. 12) reveals
a proximity of laboratory and full-scale values
in the central part of the wak e. However, the
shape of the full-scale profile again implies the
effect of wak e mean dering. Figure 13 presents
the comparison for the centreline wak e turbu-
lence as a function of the tip speed ratio. Us ing
spline interpolation, a smoo thed curve is plotted
through the PIV data. The comparison is good
for A14 (Fig. 13(a)). The discrepancy for the
highest A value could be related to the limited
amount of full-scale data. Figure 13(a) was
reconstructed to include points of maxim um
ratios for both full- and laboratory-scale data.
The full-scale data is seen to lie more closely to
the PIV curve (Fig. 13(b)), suppo rting the possi-
bility that measurements from the wind park
are displaced because of wake meandering.
3.3.
Region o f accel e ra ted j ow
A further observ ation, comm on to both the
full-scale and laboratory results, is that a region
of accelerated flow exists outside the wake
boundary. Referring again to Fig. 7, the velocity
ratio clearly rises above unity for
I v / R I
1, with
the effect becom ing more pronounce d as L
increases. A t an experimental scale this tendency
may be exaggerated by a blockage in the tank,
but it is clear from Fig. 2 that it also occurs at
full scale. Similar findings have also been
reported by Taylor (1990 ).
The full-scale data, shown in Fig. 11 (b), also
reveal an increase of wake turbulence towards
A simp le explanation for the region of
accelerated flow is that the rotor partially
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A study of the near wake structure of a wind turbine
631
8 0
"07.0
k-
d 6.0
3
21
5.0
h
c,
.I-
; 4.0
aJ
2 3.0
%2.0
2
k
fz
1.0
0.0
-4
w h=1.6
m h=2.7
-ttcH x=3.2
A-A- h=4.2
M h=4.8
I
I I
I
-3
I
2
I
1 0
I
I
dista:ce, yZ/H
3
4
cross-wake
3.0
w h=3.0
w h=.3.;5
(b)
2
t-+--cf-+ h=4.0
o.03*F-T---1
340 360
3;;
Wind direction (deg.)
1
380
Fig. 11. (a) Wake turbulent intensity ratio profiles at l.lD from PIV experiments for five i, values. The
curves for 1= 2.7, 3.2, 4.2 and 4.8 have been shifted upwards by 1, 2, 3 and 4 respectively to separate them
from each other. (b) Wake turbulent intensity ratio profiles at l.lD from full-scale data for three A values.
obstructs the airflow, as a solid object would.
In order to conserve mass flow (at constant
pressure), the air must speed up around the
obstacle. Alternatively, as the air in the wake
slows down and expands, so the air outside it
must speed up to flow through a more confined
space. The effect is consistent with the idea of a
helical vortex structure in the wak e, wh ich
retards the air inside it, but accelerates the air
outside, with respect to freestream.
4. CONCLUSIONS
A comparison has been presented of PIV
wake measurements from a three-bladed model
wind turbine with data captured in the wake of
two full-scale machines. The shape of the PIV
velocity profiles differed in significant respects
from the measurements obtained from the
WM19S machine on Samos Island. In general,
the PIV data yielded narrow, deep velocity
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632
J. Whale
e t a l
Cross-wake distance, y/R
lo**** PIV data (h=3.2)
5
m Full scale (h=3.3)
I 1.6
5
b
o.03803800
Wind direction (deg.)
Fig. 12. Comparison of wake velocity ratio profiles at l.lD
between full-scale (FS) and PIV data for 1,s = 3.3, A,,, = 3.2.
profiles whereas
measurements from the
WM 19S produced wider profiles with homogen-
eous central portions. Larg e discrepancies in
centreline velocity de ficit between the PIV data
and the WM 19S results occurred at high tip
speed ratio. Closer agree ment in velocity d eficit
and profile sha pe was found when the PIV
results were compared with measurements made
on the flat terrain of the Vestas V 20/100 site
at Ris0.
A number of factors have been discussed to
account for the discrepancies at high A, of which
the difference in scale is the most obvious candi-
date. The Reynolds number of the PIV tests is
lower than full-scale by a factor of 1000; as a
consequenc e, the boundary layer flow on the
model blades will differ from th at at full-scale,
particularly regarding the stall angle and the
transition to turbulence (Galbraith et al., 1987 ).
The influence of the Reynolds number on the
near wa ke prop erties is not well understood,
how ever, and their sensitivity to scale may well
be less significant than blade flo w. Certainly the
bulk properties of the wake further downstream,
which is fully turbulent, are less sensitive to
Reynolds number.
The discrepancy between model and full-scale
results may also be attributable to (a) blockage
in the water tank, or (b) uncertainties pertaining
to the full-scale experiment. In the latter c ase,
for instance, the comp lex terrain of Sam os Island
may produc e large-scale inhomogeneities that
affect the wak e properties. It is possible that a
highly turbulent wak e (such as the turbulent
4.0
P
I
OoooO
PIV data
b3.5
x x x x x Full-scale data
0) 2.0 -
i
2
1.5 -
;
4
U 1.0 -
d
.*
z
s 0.5 -
G
”
(a)
0.0 I
I I
I / I
1.0 1.5
I I
2.0 2.5
I
3.0 3.5 4.0 4.5 5.0
Tip speed ratio, h
4.0
5
Om
PIV data
_ .5
xxxxx
Full-scale data
.$
,m
3.0
/p
h I /
.‘;
; 2.5 -
z
.d
x
; 2.0 - x
5
‘j
P 1.5 - 0
;
x
OX
g 1.0
_d
6
‘9 0.5 -
r
@I
0.0
I I
I
1 I I I
I
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Tip speed ratio, h
Fig. 13. Centreline turbulence intensity ratios at l.lD for
full-scale and PIV data, WM19S wind turbine; (a) using
spline interpolation, a smoothed curve is plotted through
the PIV data; in (b) the maximum values of the ratios have
been plotted for both the PIV and full-scale data.
wake state with large areas of recirculating flow)
may remain stable in the laboratory environ-
ment but could not exist in the field. Full-scale
wake profiles have been shown to be displaced
from the PIV profiles and wake meandering has
been suggested as a reason for the offset.
Estimating the degree to which e ach of the
factors contributes to the observed discrepancy
is not trivial. In the case of wak e meandering,
some insight is gained by re-plotting the profiles
using minimum and maximum ratios for the
full-scale data. The influence of Reynolds
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A study of the near wake structure of a wind turbine
633
number and tank blockage require further
Technology for financial support of the Samos experimen-
investigation.
tal campaign.
Although it would be unwise to draw too
Finally, both the teams at Athens and Edinburgh would
like to express their thanks to the British Council, for
many firm conclusions at this stage, bearing in
funding their ongoing collaboration.
mind the difference in scale of the two experi-
ments and the limitations of the metho d in each
REFERENCES
case, the further use of PIV in this field seem s
to be clearly indicated. It is inferred that a more
careful assessmen t of the effect of turbulence, in
terms of its spectral content rather than its
integral levels, on the wak e properties is
necessary.
A number of investigations suggest them-
selves. In particular, the study of the change in
wak e properties in the transition from low to
high windsp eed (i.e. high to low tip speed ratio)
is of interest. This w ork is planned, and will be
based on further analysis of the turbulence
content of the measu red wak es. Analysis of
wak e vorticity, readily available from the PIV
vector maps, is being undertaken. In this case
it may be possible to investigate the properties
of the wake under conditions where simple
analytic models for wind turbine rotors, e .g.
actuator-disc theory, fail, for example where the
rotor is heavily stalled, or whe re th e thrust
coefficient exceeds unity.
In these re spects, it is hope d that furthe r
compa risons of PIV measurem ents and full-
scale data will be forthcoming shortly.
D
R=D/2
YIR
d
u
UO
0
1
cv
1,
O
C
pmax
NOMENCLATURE
turbine rotor diameter, m
turbine rotor radius, m
non-dimensional cross-wake distance
offset distance from wake centreline, m
air flow velocity in the wake, m s-’
upstream wind speed, s s-i
wind direction
tip speed ratio
standard deviation of wind speed, m s-’
turbulent intensity (uu/U)
turbulent intensity of the upstream flow
maximum value of turbine power coefficient
Acknowledgements-The authors would like to extend their
thanks to the following people: Jean-Baptiste Richon and
Iain Morrison of Edinburgh University Physics Department,
for design of the image shifting system and assessment of
image shifting errors, respectively, and John Korsgaard of
LM Glasfiber A/S, and Tom Pedersen of Vestas A/S, for
supplying details of the WM19S wind turbine and rotor
blades.
The University of Athens research group would also like
to thank the Greek Ministry of Industry, Energy and
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