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CFD Simulations of the Mexico Wind Tunnel and Wind Turbine
Réthoré, Pierre-Elouan Mikael; Zahle, Frederik; Sørensen, Niels
N.; Bechmann, Andreas
Published in:Proceedings
Publication date:2011
Document VersionPublisher's PDF, also known as Version of
record
Link back to DTU Orbit
Citation (APA):Réthoré, P-E. M., Zahle, F., Sørensen, N. N.,
& Bechmann, A. (2011). CFD Simulations of the Mexico WindTunnel
and Wind Turbine. In Proceedings European Wind Energy Association
(EWEA).
https://orbit.dtu.dk/en/publications/b2ab7f2a-4023-481e-b9e3-a24469aaebe9
-
CFD model of the MEXICO wind tunnel
P.-E. Réthoré∗, N. N. Sørensen, F. Zahle, A. Bechmann and H.
A. Madsen
Wind Energy Division, Risø DTU, DK-4000 Roskilde, Denmark.∗
[email protected]. tel: +45 21331293.
Abstract
The MEXICO Experiment is reproduced in CFD,including the
geometry of the wind tunnel and thewind turbine rotor. The wind
turbine is modelledboth as a full rotor and as an actuator disc.
Vari-ous questions regarding the wind tunnel effects onthe
measurements are investigated. As in a previ-ous work carried out
without modelling the windtunnel, the CFD methods are found to give
satisfy-ing agreement with the axial velocity deficit in thewake.
However, confirming the previous work, theblade loadings estimated
from CFD are found tobe consistently larger than the one estimated
frommeasurements. In order to investigate further thisissue, the
loadings estimated from measurement areused with an actuator disc
model. This approachgives a too small wake deficit in comparison
withthe measurements, which tends to agree with thefull rotor
computation results.
1 Introduction
The EU FP5 project ’Model EXperiments In COn-trolled COnditions’
or ’MEXICO’ [13] was a windtunnel experiment campaign leaded by the
Energyresearch Center of the Netherland (ECN). A 4.5 mdiameter
fully instrumented wind turbine was setup in the 9.5×9.5 m open jet
wind tunnel of theGerman Dutch Wind tunnel Organization (DNW).Some
’breathing slots’ were introduced behind thecollector to avoid flow
recirculation. The forcesacting on the blades were estimated using
pressuresensors at 5 sections on the blades. The flowupstream and
downstream of the wind turbine wasobserved using Particle Image
Velocimetry (PIV).The purpose of this experiment was to provide
highquality experimental data to validate various typesof wind
turbine models. An exhaustive literaturelist concerning the
experiment and the related
analysis is available on the MEXICO Experimentwebsite [1].
Risø DTU has developed a set of Wind EnergyCFD tools to model
the flow passing through windturbines [9, 15] or in complex terrain
[2, 14]. RisøDTU has therefore been interested from the begin-ning
to participate in the MEXICO Experimentin order to compare the
measurements with ourdifferent models.
When dealing with new types of wind tunnelexperiments many
questions arise on the possibleunexpected influence of the wind
tunnel on themeasurements. Some of the questions addressedin this
study are as follow. How far downstreamand upstream of the wind
turbine is the flowunaffected by the wind tunnel geometry? Do
the’breathing slots’ have a noticeable influence on
themeasurements? Is the flow influenced by the windtunnel room
geometry? In order to investigatethose questions the geometry of
the DWN windtunnel is used as a background mesh to the
CFDcomputation.
Moreover, previous CFD Full Rotor (FR)calculations where done by
Bechmann et al. [3]on the MEXICO wind turbine rotor,
withoutconsidering the wind tunnel. These results showthat it is
possible to obtain a satisfying fit withthe flow measurements,
while at the same time tohave loadings on the wind turbine blades
that aresignificantly larger than what the measurementspredict.
This could potentially indicate that theestimation of the wind
turbine blade loading havebeen underestimated during the
experiments. TheActuator Disc (AD) method [12], which takes
theloadings from the MEXICO experiment, is used inthis work to
investigate this issue.
1
-
The rest of the paper is organized as follow. First,a brief
description of the different methods usedin this work. Then, the
presentation of differenttest cases run and associated results.
Finally, a dis-cussion of different issues such as the wind
tunnelReynolds Number independence, the flow asymme-try, the wind
tunnel perturbations, the influence ofthe breathing slots and of
the jet oscillation. Fi-nally, the mismatch between blade loadings
andflow features is addressed.
2 Method
2.1 Flow Solver
The flow solver used in this work, EllipSys, is anin-house
general purpose CFD code developed atRisø DTU [14] and DTU MEK [7].
It is a flowsolver based on a finite-volume spatial and tem-poral
discretization of the incompressible Navier-Stokes Equations
formulated in general curvilinearcoordinates. The variables are
collocated in the cellcenters to enable computations using complex
geo-metrical meshes. The pressure correction method isbased on the
Rhie-Chow algorithm, which is mod-ified for the treatment of the
body forces of theactuator disc model [10] and is accelerated using
amultigrid technique. The flow is solved successivelyover three
levels of discretization in order to accel-erate the convergence
and to ensure a grid indepen-dent solution [7]. The equations can
be discretizedusing different schemes (see Section 2.3 & 2.4).
Forthe full rotor computation, the PISO [4] method isused to solve
the Navier-Stokes Equations system,while for the actuator disc
method, the SIMPLE [8]method is used. The flow equations are
discretizedusing a QUICK formulation [5] in both methods.The
Reynolds stresses are modelled in the Navier-Stokes equations using
the eddy-viscosity assump-tion and a k-ω-SST [6] turbulence
model.
2.2 Wind Tunnel Mesh
Most of the human work time of this project wasspent on meshing
the DWN wind tunnel. Theshape of the nozzle and collector are
confidentialand a Non Disclosure Agreement was signed withDWN in
order to obtain them. In order to be ableto freely publish the
results the decision was madeto not show the final shapes of the
nozzle andcollectors. All the figures illustrating those partsare
therefore whether slightly altered, obstructedor a free ’artistic’
representation (done prior to
Figure 1: Horizontal view of the mesh blocks.
Figure 2: Perspective view of the mesh blocks.
obtaining the confidential information), and shouldnot be
considered as realistic in any way. Thesimulations are, however,
carried out using thecomplete nozzle and collector shapes.
The wind tunnel is meshed using the soft-ware PointWise as 386
323-cell structured blocks(around 12M cells). The support structure
of thewind turbine rotor, as well as the support structureof the
collector are not considered in this study.These structures are
rather large blunt bodies,and should have an important impact on
the flowbehavior around these regions. It is thereforeassumed that
the region of interest and the overallloads on the wind turbine
blade are not influencedby these support structures. Some details
ofthe mesh structure are illustrated on Fig.1-3.Note that in these
figures the blocks illustratedhave not yet been subdivided into
323-cell elements.
The inlet and outlet of the domain are extended,and the cells
size are gradually stretched, expand-ing towards the inlet and
outlet (see Fig.1). This isdone in order to avoid to have some
influence fromthe boundary condition over the zone of interest,and
to enforce a numerical dissipation to avoid flowstructures to
create unstability while passing theoutlet. As these regions are
nowhere near the in-teresting locations their geometry should have
littleinfluence on the results.
The wall boundary conditions are taken as
2
-
Figure 3: Detail of the collector lips meshing. The’breathing
slots’ are visible on the right side of themesh.
slip condition (symmetry). This is done to avoidresolving the
boundary layer at the wall surface forspeeding up both the meshing
and computationalexpenses. The shortcoming of this approach is
thatthere is not any development of a boundary layeron the walls of
the room, nozzle and collector. Theassumption made is therefore
that this boundarylayer does not influence the region of the
flowwhere the measurements are made.
The “breathing slots“ located at the exit of thecollector are
taken into account in the mesh geom-etry (see Fig.3). In order to
assess their effect, avariation of the wind tunnel mesh is done
withoutthe slots (referred later as ”WT-no hole”).
2.3 Full Rotor Computation
In a previous work by Bechmann et al., the windturbine rotor was
mesh with an o-geometry. For anin-depth description of the method
used to meshthe surface of the rotor and compute the MEXICOrotor,
without wind tunnel, see [3].In the current work, the full geometry
of the MEX-ICO wind turbine rotor, excluding the spinner,hub, tower
and support structured, is meshedand is added to the wind tunnel
mesh throughan overset grid methodology [15]. Note that thesurface
mesh of the rotor is the same as usedin [3]. Three overset meshes
are used in additionto the wind tunnel to smoothly interpolate
fromthe very fine rotor mesh to the coarser backgroundmesh (see
Fig.4. One cylindrical mesh close tothe turbine that rotates with
the rotor mesh, andone coarser, which is fixed relative to the
windtunnel mesh and extends up to one rotor diameterdownstream. In
the case of the mesh withoutwind tunnel, a Cartesian background
mesh is used
Figure 4: Detail of the full rotor computation mesh.top: front
view. bottom: side view.
instead of the wind tunnel mesh. The rotor mesh isdesigned to
have a maximum cell size at the bladesurface so that y+ < 2. The
total mesh composedof the background wind tunnel mesh describedin
the previous section and the three successiverefinement meshes
represents in total 798 323-cellblocks (26M+ cells). The code runs
in transientmode at 7.5 hours per revolution on 160 CPUs.The result
shown are run for 10 revolutions, so 3.2days.
Fig.5 illustrates the full rotor computation of theMEXICO rotor
in the DWN wind tunnel.
2.4 Actuator Disc Model
The actuator disc model [12] is spreading the bladeforces over a
disc and applying them into the windtunnel background mesh. The
forces are treatedin a special way to avoid pressure-velocity
decou-pling [10]. In all the actuator disc simulations, theforces
are splined from the 5 wind turbine bladepressure sensor
measurements (see Fig.6).
Two types of solutions are considered, an un-steady Detached
Eddy Simulation (DES) solution
3
-
Figure 5: Full rotor computation of the MEXICOrotor in the
DWNwind tunnel. Vorticity iso-surfaceand axis velocity colour
contour in an horizontalplane at hub height. The wind tunnel
geometry isvisible in transparent.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1−100
0
100
200
300
400
500
600
Axi
al lo
adin
g [N
]
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1−20
0
20
40
60
80
100
120
Rotor radius r/R [−]
Tan
gent
ial l
oadi
ng [N
]
Measurements 10 m/s
AD 10 m/s
FR 10 m/s
Measurements 15 m/s
AD 15 m/s
FR 15 m/s
Measurements 24 m/s
AD 24 m/s
Figure 6: Description of the different wind turbineblade
loadings. The Actuator Disc (AD) loadingsare splined from the
measurements points, whilethe Full Rotor (FR) computations loadings
are es-timated from the simulation results.
and a steady state solution. The transient solu-tion is run with
a time step of ∆t = 0.01 sec, for2000 iterations using 6
subiterations, which takesabout 16h on 100 CPUs. The 2000
iterations onthe steady state computation only takes 2-3 hourson
100 CPUs.
3 Results
3.1 Runs
With the different wind turbine models and typesof mesh, 7
different types of runs are calculated atthe 3 different wind
speeds of the experiment (10,15 and 24 m/s).
• Wind tunnel mesh without wind turbinesteady state
(Tunnel).
• Actuator disc model with wind tunnel steadystate (AD-WT).
• Actuator disc model with wind tunnel withoutbreathing slot
steady state (AD-WT-noSlot).
• Actuator disc model without wind tunnelsteady state
(AD-noWT).
• Actuator disc model with wind tunnel un-steady DES
(AD-DES).
• Full rotor computation with wind tunnel un-steady RANS
(FR-WT).
• Full rotor computation without wind tunnelunsteady RANS
transient (FR-noWT).
3.2 Streamwise Sections
Fig.7 presents the axial velocity in an horizontalline passing
through the wind tunnel, withoutturbine, at hub height and z/D =
0.33 for variousinlet velocity.
Fig.8 shows the axial velocity in an horizontalline passing
through the wind tunnel at hub heightand z/D = 0.82132 for the
different wind turbinemodels, with and without the wind tunnel, for
dif-ferent inlet wind speed and compare them with therelevant wind
speed measurements.
Fig.9 presents a comparison between differentsteady state
actuator disc runs with different kindof meshes (without wind
tunnel, and win tunnelwith and without breathing slots).
4
-
−10 −4.07 −1.77 0 2.98 6.03 10
−5
0
5
10
15
20
25
30
35
40
Centerline x/D [−]
U−v
eloc
ity U
/Uin
[−]
Velocity centerline z/R=0.82132
AD − WT − 10 m/s
AD − WT − 15 m/s
AD − WT − 24 m/s
FR − WT − 10 m/s
FR − no WT − 10 m/s
Experiment 10 m/s
Experiment 15 m/s
Experiment 25 m/s
Figure 8: Comparison of different runs with respective
measurements. With wind tunnel (WT) and without(no WT). With a
steady state actuator disc (AD) and with an unsteady full rotor
computation (FR).
Figure 7: Wind tunnel with wind turbine of con-stant CT at
different wind speeds, collapsing whennormalized. The wind speed
indicated is the inflowwind speed at the inlet boundary. They are
nor-malized with a constant ratio that would give unityat the wind
turbine position, when no wind turbineis present.
−10 −4.07 −1.77 0 2.98 6.03 10
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Centerline x/D [−]
U−v
eloc
ity U
/Uin [−
]
Velocity centerline z/D=−0.33
Tunnel & DiscTunnel & No discNo tunnel & DiscTunnel
(No Hole) & Disc
Figure 9: Comparison of different steady state actu-ator disc
runs with respective measurements. Withand without wind tunnel.
With the ’breathingslots’ and without (no hole)
5
-
Figure 10: Transverse sections at different positionupstream and
downstream of the wind turbine fromdifferent types of
simulations.
Figure 11: Actuator disc simulation of the MEX-ICO rotor
(AD-WT). Vorticity colour contour in anhorizontal plane at hub
height.
3.3 Transverse Sections
Fig.10 shows the axial velocity along transversesections at
different positions downstream and up-stream of the wind turbine
(animated only in thedigital version), including regions of
perturbed flow.Different steady-state actuator disc simulations
arepresented: actuator disc with and without windtunnel, actuator
disc with a wind tunnel withoutbreathing slots (no hole) and a
simulation with awind tunnel without actuator disct.
3.4 Contourlines
Fig.11 illustrates the vorticity contouts at hubheight of a
steady state actuator disc model withan inflow wind speed of 10
m/s.
Figure 12: Actuator disc simulation of the MEX-ICO rotor. Axial
velocity colour contour on a hor-izontal plane at hub height.
Frames from the DESanimation [11].
3.5 DES animation
A video of the unsteady animation (AD-DES) isavailable online
[11]. An extract of it is also visiblein the digital version of
this article (Fig. 12).
4 Discussion
4.1 Reynolds Number independence
Fig.7 shows that the wind tunnel simulation isReynolds Number
independent. This shows thatthe jet development is not dependent of
the inflowvelocity. However, this mesh has a slip boundarycondition
on the wall, and therefore no boundarylayer is developing. A mesh
with a no-slip bound-ary condition might not be Reynolds number
inde-pendent.
4.2 Flow Asymmetry
The horizontal asymmetry of the wind tunnel ge-ometry (visible
in e.g. Fig.1) seems to generate awider recirculation zone on one
side than on theother. This effect could explain why the wake
en-tering the collector seems to be bended in Fig.11.It could also
explain some lateral forces recorded onthe wind turbine support
structure.
4.3 Wind Tunnel Perturbations
The difference between the actuator disc simula-tions with and
without wind tunnel seem to indi-cate that the wind tunnel
collector can influence
6
-
the wake measurements as soon as 1.5 rotor diame-ter downstream
(see Fig.9-10). Similarly, the nozzlehas an influence on the flow
that cannot be ignoremore than 1 rotor diameter upstream. Those
ob-servations are also visible while looking at the fullrotor
computations, Fig.8.
4.4 Breathing Slots Effect
From analyzing the results presented in Fig.9 and10, the
’breathing slots’ do not seem to have a sig-nificant effect in the
region where the wind tunneldoes not affect the flow (x/D ∈ [−1,
1.5]).
4.5 Jet Oscillation Effect
In the DES animations [11] there is a clear oscilla-tion of the
jet interface going in and out of the col-lector lips. These
oscillations seem to have a signif-icant effect on the wake
dynamics (see also Fig.12).Nonetheless, while looking at the full
rotor com-putation unsteady-RANS animation, this jet inter-face was
not oscillating. The difference betweenthe two simulations is that
the unsteady-RANS al-lows a build up of the eddy-viscosity and the
tur-bulence at the jet interface that could damp andaccount for
those oscillations. In the DES simula-tions, there is a limit put
on the turbulent length-scale size that effectively prevent an
increase ofeddy-viscosity. Nonetheless, it remains unclear ifthose
oscillations did actually appear in the exper-iments and if they
indeed had such a significantimpact on the wake dynamics. More work
needs tobe done on this issue to answer this question.
4.6 Blade Loading and Flow Features
The flow characteristics found with the actuatordisc model,
based on the forces from the measure-ments, do not give satisfying
results in comparisonwith the measurements. They systematically
un-derpredict the wake deficit in comparison with boththe
measurements and the full rotor computations.This seems to indicate
that the loadings estimatedfrom the pressure sensors are not large
enough toobtain the measured velocity deficits. This tends toagree
with the results of Bechmann et al. [3] thatindicates that the full
rotor computations obtainsignificantly higher loadings (see also
Fig.6).The full rotor computation carried out with andwithout the
wind tunnel give very close results inthe region of comparison.
This furthermore indi-cates that the wind tunnel effect cannot
explain the
mismatch between the forces measured on the bladeand the axial
velocity measurements.
5 Conclusion
The open wind tunnel of the MEXICO experimentwas successfully
modelled in CFD using two typesof wind turbine models (Full rotor
computation andactuator disc).The results show that the effect of
the wind tunnelover the region of comparison in the wake shouldnot
be significant. Similarly, the breathing slotsare not found to have
a significant influence in theregion of interest.However the DES
analysis showed that there couldbe an oscillation of the wind
tunnel jet interfacethat could potentially create a significant
oscilla-tion of the wake. It is unclear if this effect canbe
observed in the MEXICO measurements and ifthey had an impact on the
quality of the experi-ment. More work should be focussed on this
ques-tion, as it raises a potential issue with future openwind
tunnel experiments of wind turbine wakes.Finally there seems to be
a mismatch between theblade loadings measured and the axial
velocity mea-surements. Both the full rotor computation and
theactuator disc model cannot match both results atthe same time
satisfyingly. This could potentiallyindicate an under estimation of
the blade loadings.
Acknowledgement
The authors are grateful for the IEA projectMEXNEX and the DSF
Flow center for financingthis work, DNW for providing the wind
tunnel ge-ometry and J.G. Schepers, K. Boorsma and H. Snelfrom ECN
and for providing the wind turbine mea-surements.
References
[1] MEXICO experiment web
site.http://www.mexnext.org/resultsstatus.
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Complex Terrain. PhDthesis, DTU-MEK, Denmark, 2007.
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the MEXICO Rotor Wake.Wind Energy (in revision).
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8
http://www.risoe.dtu.dk/rispubl/reports/ris-phd-53.pdfhttp://www.ewec2008proceedings.info/statscounter.php?id=5&IDABSTRACT=225http://windenergyresearch.org/?p=81http://www.ecn.nl/docs/library/report/2007/e07042.pdf
1 Introduction2 Method2.1 Flow Solver2.2 Wind Tunnel Mesh2.3
Full Rotor Computation2.4 Actuator Disc Model
3 Results3.1 Runs3.2 Streamwise Sections3.3 Transverse
Sections3.4 Contourlines3.5 DES animation
4 Discussion4.1 Reynolds Number independence4.2 Flow
Asymmetry4.3 Wind Tunnel Perturbations4.4 Breathing Slots Effect4.5
Jet Oscillation Effect4.6 Blade Loading and Flow Features
5 Conclusion
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