a study of the near wake structure of a wind turbine comparing measurements from laboratory and full...

8/19/2019 A Study of the Near Wake Structure of a Wind Turbine Comparing Measurements From Laboratory and Full Scale …

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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

621


2/13

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.


3/13

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.


4/13

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


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


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


5/13

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


6/13

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


7/13


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.

1.5

1.4

s1.3

\

3 1.2

6 1.1

.rl

1.0

k

&_0.9

.%S0.8

0.7

’ 0.6

f 0.5

2 0.4

0.3

0.2

I

I

I

I

-4

-3

I

-2

I

I

crosslLe OdistAce, y2/R

I

3 4

ooooooooooooo”ooooo

0

OOOO

000

00

ooooooooooooooo

~oo~noooooLm~~on~o

oooooo

+++++

00

oooooon~moon 0

00 +

+

++a

0

++

++++++++++

0

AA

A~A~~AA~~AAAAAAA+++nan+++AAAA

++++++

+++++++

xx

x xXxX

xxxxx~x~x A

+ 000

AAAAAAAAAA A

A,A

A xX

x

A +

+

XxXxX xxx x x x

+

Ax 7.1 xxxx

A

+++

x

A

xA

x

A

x

A

A

aA

A

x

x

ooooo),11.6

x

x

oo~h~2.7

x

xx

+++++ ~~3.2

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.


8/13

628

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.)


-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


9/13

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


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-


10/13

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


11/13


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;;


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


12/13

632

J. Whale

e t a l


lo**** PIV data (h=3.2)

5

m Full scale (h=3.3)

I 1.6

5

b

o.03803800


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


13/13


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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a study of the near wake structure of a wind turbine comparing measurements from laboratory and full...

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