3D RANS Simulation of NREL Phase-VI and MEXICO …isromac-isimet.univ-lille1.fr/upload_dir/finalpaper17/...3D RANS Simulation of NREL Phase-VI and MEXICO Wind Turbines — 2/11 to

3D RANS Simulation of NREL Phase-VI and MEXICO Wind

Turbines

Dr.-Ing. Irfan Ahmed1*, Dipl.-Ing. Matthias Teich2, Univ.-Prof. Dr.-Ing. Martin Lawerenz3

ISROMAC 2017

International

Symposium on

Transport

Phenomena and

Dynamics of

Rotating Machinery

Maui, Hawaii, USA

December 16-21,

2017

AbstractIn the recent years, efforts have been given to perform experimental investigations on windturbine models to facilitate a database for a better understanding of the aerodynamic effectsof the different design aspects, and benchmark testing of the numerical model of wind turbineflow simulations. The authors present the result of 3D RANS simulations of two test cases,namely the Phase-VI [1] of NREL’s measurement campaign conducted under the (UnsteadyAerodynamic Experiments) UAE, and the EU project (Modelled Experiments in ControlledConditions) MEXICO [2] by the International Energy Association, under the IEA Wind Task 29,Mexnext Phase III. The commercial software FINE™/Turbo is used for this purpose. The resultsfrom the simulations are compared to the measurement data available from the experimentalinvestigations. The results will serve as a benchmark test for the numerical code, and will allowa better understanding of the development of flow field structures along the radial positions,specially near the tip region of the blade, thus facilitating improved design philosophies.

KeywordsHorizontal Axis Wind Turbine — Computational Fluid Dynamics — Experimental Investigation

1, 2, 3 Department of Thermal Power Engineering, Section of Turbomachinery, University of Kassel, Germany

*Corresponding author: [email protected]

NOMENCLATURE

ǫ turbulent dissipation rateϑ opening angle of conical sectionλ2 second eigenvalue of

(

S2 +Ω2)

~c absolute flow velocity vectorcz , cy, cz absolute flow velocity components~w relative flow velocity vectork turbulence kinetic energyl lengthlc chord lengthlturb turbulence length scalep static pressurer radial distancex, y, z Cartesian coordinate pointsCf skin friction coefficientCp coefficient of pressureCN normal force coefficientCT tangential force coefficientI turbulence intensityMT turbine torqueN rotational speedRtip blade radiusS rate of strain tensorΩ rate of rotation tensor

SUPERSCRIPTS

~A vector

INDICES

0 inlet station1 upstream station2 downstream station

INTRODUCTION

With the growing demand on renewable energy conver-sion, wind energy has undergone a recent boom. This istranslated into the increase in the installed power. Themodern wind turbines are upscaled in capacity whichhas resulted in an increase of the dimension of such tur-bines. Incentives are taken worldwide to increase theyield from the wind energy sections, and national en-ergy policy guidelines, along with market research haveattracted increasing involvement of scientific communitymembers in the development the sector.Such attempts require an understanding of the scienceof wind energy conversion. The flow around the windturbine can be predicted with the help of aerodynamicsimulations, which is very demanding in computationaleffort. The grid resolution required to resolve the flowstructures for the wind turbine flow using 3D NavierStokes equations is too high for the currently availablecomputational hardware. The less computationally de-manding Reynolds Averaged Navier Stokes solvers are

3D RANS Simulation of NREL Phase-VI and MEXICO Wind Turbines — 2/11

to be put to benchmark testing before they can be usedin the development of the future wind turbine designs.Recently, efforts have been given to acquire a betterunderstanding of the flow field around the wind tur-bine. One such extensive measurement program is car-ried out by the National Renewable Energy Laboratory(NREL) under the Unsteady Aerodynamic Experiments.The Phase-VI [1] of NREL’s measurement campaign con-ducted detailed aerodynamic field measurement for a 2-bladed wind turbine with rotor diameter 10.058m inthe 24.4m × 36.6m Wind Tunnel of the NASA Amesresearch center. A more recent experiment is carriedout under the EU project MEXICO [2] for a 3-bladedhorizontal axis wind turbine with a rotor diameter of4.5m in the 9.5m×9.5m wind tunnel of the Large Low-speed Facility (LLF) in the German Dutch Wind Tunnel(DNW). Detailed measurements of the aerodynamic flowfield around the wind turbine along with the blade sur-face pressure data are available from the measurementcampaign.It is to be mentioned that, apart from the availablefield-measurement data, both the test cases mimic differ-ent operating conditions of the respective wind turbines.The boundary conditions for the NREL Phase-VI windturbine are taken for a near-stall operating condition ofthe wind turbine, whereas, the Mexnext Phase III mea-surement data are taken for a normal operating condi-tion of the MEXICO wind turbine. Furthermore, field-measurement data at the inflow and the wake regions areavailable for the MEXICO wind turbine, allowing the op-tion for benchmark testing numerical codes in terms oftheir ability to resolve secondary flow structures in theflow field.In the following sections, an effort is made to simulatethe aerodynamic flow field around both the aforemen-tioned turbines, and the results are compared to per-form a benchmark testing. The simulations are carriedout using the commercial software FINE™/Turbo fromthe company Numeca™.

1. NUMERICAL METHODS

The aerodynamic flow field is simulated using the fi-nite volume solver Euranus from the commercial soft-ware FINE™/Turbo. The field variables are simulatedusing the Reynolds-Favre averaged Navier Stokes equa-tions. The turbulence properties are modeled using 2-equation model for the turbulence kinetic energy k andturbulent dissipation rate ǫ.An effort has been given here to test the performanceof the high Reynolds number (high-Re) turbulence mod-els in resolving the flow structures of the wind turbineflow field. The high-Re turbulence model [3] employswall functions to model the near wall region of the wallboundary layer. This allows one to employ a grid dis-cretization allowing lower computational effort.The solution algorithm employs a time-marching scheme

for resolving the flow field. The numerical scheme isequipped with a number of convergence acceleration meth-ods [4, 5]. A full multigrid approach is used to solvethe coarse grid, and successively uses the results to ini-tiate and update the fine grid results. Furthermore,implicit residual smoothing is employed for the Runge-Kutta schemes. In addition to that, local time steppingis used in the time marching scheme. Furthermore, asthe flow field Mach number lies within a low subsonicregime, suitable preconditioning [6] is used to acceleratethe time marching scheme.The boundary conditions are selected as to mimic theconditions prevailing during the measurement campaignsdocumented in [1, 2]. For the numerical simulation ofthe turbulence conditions, the turbulent kinetic energyk, and turbulent dissipation rate ǫ are prescribed at theinflow. The values are predicted using the turbulenceintensity values I, the wind tunnel flow velocity |~c0|,and turbulence length scale lturb values exerted fromthe measurement data. The computational domain isdefined as periodic. This allows one to model one bladepitch to represent the flow domain. The far-field bound-aries are selected as external boundary conditions. Atthe far-field, static pressure p, and flow velocity com-ponents are prescribed. The values are taken from themeasured quantities prevailing at the wind tunnel dur-ing the measurement campaigns.

2. COMPUTATIONAL DOMAINS

In the following sections, the preparation of the compu-tational domains for both test cases will be registered inbrief.

2.1 Test Case I: NREL Phase 6In a first step, blade geometry from the Phase-VI windturbine [1] of the Unsteady Aerodynamic Experimentscarried out by NREL is used. The Phase-VI measure-ment campaign dealt with a two-bladed upwind horizon-tal axis wind turbine. During the experiments, blade sur-face pressures were measured along with the inflow mea-surement with the help of blade-mounted 5-hole probe.Additionally, structural loads were measured along withthe generator power. The twisted and tapered blade isconstructed of S 809 airfoil profile developed by the Na-tional Renewable Energy Lab [7]. The blade pitch anglewas set at 3, and the cone angle was set as 0. The in-flow velocity of the chosen working point is 10.051m/swith a pure axial flow. The blade rotational speed wastaken as N = 72.14 rpm. Detailed description of thecampaign along with the following benchmark testing ofdifferent numerical codes can be found in [1, 8, 9, 10].

2.1.1 Computational Setup

The tower geometry was not considered during the simu-lation. Furthermore, the hub profile was modified. Theoriginal Phase-VI turbine had instrumentation on its


hub, which would have disrupted the inflow. Further-more, these parts are not rotation symmetric, and arethus hard to model in the available grid generation scheme.Instead, a hub profile is taken with cylindrical cross sec-tion. The nose section of the hub is taken to be of ellipti-cal shape. The tail section is modeled as a cone with anopening angle of ϑ ≈ 4. The resulting geometry is rota-tion symmetric. For the computations, one blade pitchis taken to represent the blade passage. The computa-tional domain is stretched around five times the rotorradius up- and downstream of the rotor, and about 1.43times rotor radius along the spanwise direction.

2.2 Test Case II: MEXICOUnder the EU project MEXICO [2], a 3-bladed hori-zontal axis wind turbine with a rotor diameter of 4.5mis investigated in the 9.5m × 9.5m test section of theLLF in DNW. In this case, the experimental investiga-tions corresponded to a more detailed measurement cam-paign. Blade surface pressure distributions were mea-sured, along with inflow and wake-field measurementwith the help of particle image velocimetry. The bladestructural loadings were also recorded. In addition, thegenerator torque was measured. Furthermore, far-fieldacoustic measurements were carried out for different com-binations of flow conditions and blade configurations.A comprehensive description of the measurement cam-paign can be found in [11, 12].

2.2.1 Computational Setup

For the numerical simulations, the tower geometry wasexcluded. The hub profile was modified at the aft sec-tion to facilitate a rotation symmetric geometry. Thecomputational domain was stretched by 3.3 times therotor radius along the inflow, and 5.8 times the rotor ra-dius along the wake region. The far-field is extended by2.98 times. The dimensions are so selected as to coverthe open section of the LLF facility.

2.3 Spatial DiscretizationThe spatial discretization is carried out with the help ofthe automatic structured grid generator Autogrid™. Fig-ure 1 depicts the blade skin meshes for the two test cases.Hence, the blades are seen from the top which gives anidea of the blades geometrical twist. The NREL Phase-VI blade is constructed of S 809 airfoil profile except inthe blade-root region (figure 1a). The MEXICO windturbine, has a tip section gradually converting from theairfoil section of NACA 6-series airfoil to elliptical profileat the tip (figure 1b). The flow direction is along the zaxis for both cases. The NREL turbine’s rotation direc-tion is counterclockwise, whereas the MEXICO turbinerotates along the clockwise directions, when observedfrom the inlet. For both cases, the blade blocks aremeshed using an O4H topology [13], with the blade skinresolved through an O block, with 4 H blocks placedsurrounding the O grid. The 3D grid is obtained by

stacking such blade-to-blade meshes along the radial di-rection.

x

z

(a) NREL Phase-VI wind turbine

x

z

(b) MEXICO wind turbine

Figure 1. Blade skin mesh for NREL Phase-VI andMEXICO wind turbines

Figure 2 depicts the results of the grid convergence stud-ies for both test cases. Turbine torques are plottedagainst the grid size. In both cases, the turbine torquesMT are normalized with the values obtained from thefinest grid MT,finest. Judging the rate of the changes inMT with grid refinements, the grid sizes of NREL Phase-VI was taken as of ≈ 7.29× 106 nodes, and that of theMEXICO wind turbine was taken as of ≈ 6.38 × 106

nodes. A further refinement in the grid size would havebeen computationally cost-inefficient for the respectivecases. Using the values of MT , a grid convergence index[14] of 0.004% has been achieved for the NREL Phase-VI turbine, whereas for the MEXICO wind turbine thevalue grid convergence index achieved is 2.46%.

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0 1 2 3 4 5 6 7 8

NREL

0 0.2 0.4 0.6 0.8

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0 1 2 3 4 5 6 7

MEXICO

MT/M

T,f

inest[−

]

GridSize[106]

Figure 2. Grid Convergence


3. RESULTS

In the following sections, results obtained from the nu-merical simulations for both test cases are presented. Aneffort is made to compare the results from the numericalsimulations with those obtained from the measurementcampaigns.

3.1 Test Case I: NREL Phase 6The following section documents the performance of thenumerical solver in predicting the flow field around theNREL Phase-VI wind turbine. The results obtainedfrom the numerical simulation are compared to thoseobtained from the experimental investigations.The 3D flow field around the blade-tip is illustrated withhelp of streamlines in figure 3. The local flow inductionowing to bound circulation can be detected from theshape of the streamlines. This phenomenon changes thelocal inflow angle, which is generally not predicted inthe 2D solution algorithms [15]. Additionally, secondaryflow structures can be detected near the blade tip. Thepresence of the tip vortices disrupts the inflow stream-lines, thus creating high aerodynamic losses.

Figure 3. 3D flow near the blade tip

In a further effort, blade pressure contours are plottedalong with the isobars in figure 4. Hence, both the pres-sure (left), and the suction (right) sides are illustrated.The isobars show the rotational effect on the flow. Inaddition to that, the flow structures around the bladeroot geometry are also observed to have secondary flowstructures. The radial upwash might be originally ini-tiated from the near wall flow at the hub, but the factthat the operating conditions modeled in scope of thisstudy is that of a turbine at the onset of stall, makes ithard to quantify the effect of the radial upwash on thespanwise flow conditions.

Figure 4. Blade surface pressure distributions

The distributions of the coefficient of pressure alongthe airfoil suction and pressure sides are depicted in thefollowing sections. The results are extracted for the fiveradial stations r/Rtip = 0.30, 0.47, 0.63, 0.80, and 0.95.For these radial stations surface pressure measurementdata are available from the experiments [1].At the inboard section, (r/Rtip = 0.30), the coefficient ofpressure is over-predicted by the numerical solver com-pared to the experimental data, as seen in figure 5a. Ananalysis on the streamlines of the relative velocity fig-ure 5b shows that in the numerical model, flow separa-tion occurs past the mid-chord position along the airfoilsuction side. The simulation result indicates towards adifferent inflow induction, which is projected in the Cp

distribution near the leading edge.


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p[−

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x/lc[−]x/lc[−]

(a) Distribution of coefficient of pressure at r/Rtip = 0.30

(b) Airfoil streamlines at r/Rtip = 0.30

Figure 5. Flow characteristics at r/Rtip = 0.30

In a further step, the near wall flow for the next in-board radial station (r/Rtip = 0.47) is analyzed. Thecomparison of the distribution of coefficient of pressureis presented in figure 6a, and the streamlines of the rel-ative velocity past the airfoil section are illustrated infigure 6b. During the experiment, the flow near the lead-ing edge of the blade is separated at this radial station[16, 8]. In the simulation, the separation is delayed upto the mid-chord section.Figure 7 depicts the skin friction coefficient distributionfor the suction side along the investigated radial stations.The zoomed view of the distributions near the mid-chordregions are presented in the inset. For the inboard sec-tion, transition occurs around half chord length. Thetrends suggest that, the flow transition point wanderstowards the leading edge for the outboard section.

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rs1rs2rs3rs4rs5

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]

x/lc[−]

Figure 7. Distributions of blade suction side skinfriction coefficient Cf . (rs1 : r/Rtip = 0.30,rs2 : r/Rtip = 0.47, rs3 : r/Rtip = 0.63,rs4 : r/Rtip = 0.80, rs1 : r/Rtip = 0.95 )

The comparison of the Cp distributions for the re-maining three radial stations r/Rtip = 0.63, 0.80, 0.95are depicted in figure 8 and in figures 9a and 9b. Forthese radial stations, the predictions from the numeri-cal simulations are fairly in coherence to the results ob-tained from the experimental investigations. Also to benoted is that, the numerical model shows a good per-formance in predicting the distribution of Cp along theboth the airfoil suction and pressure sides despite thehighly loaded condition the blade is operating under.

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Figure 8. Distribution of coefficient of pressure atr/Rtip = 0.63


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(a) Distribution of coefficient of pressure at r/Rtip = 0.47(b) Airfoil streamlines at r/Rtip = 0.47

Figure 6. Flow characteristics at r/Rtip = 0.47

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Figure 9. Distribution of coefficient of pressure at r/Rtip = 0.80, 0.95


The distribution of the normal force coefficient is de-picted in figure 10. The values are plotted for the fiveradial positions investigated. At the innermost bladestation, large difference is observed between the simula-tion and the experiment. This can be traced back to thedifference in the resolution of airfoil flow-field observedin figures 5a and 5b . The simulation result reproducesthe CN value derived from the experimental data for theblade middle and upper section. At the upper most sec-tion, however, simulation result suggests a higher valueof CN than the experimental investigation.

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CN[−

]C

N[−

]

r/Rtip[−]r/Rtip[−]

Figure 10. Radial distribution of normal forcecoefficient CN

The distribution of the tangential force coefficient for thefive radial stations investigated is depicted in figure 11.The values match for the outboard section, whereas forthe inboard sections, the values of CT differs, which canbe traced back to the flow characteristics observed infigures 5 and 6.

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Figure 11. Radial distribution of tangential forcecoefficient CT

3.2 Test Case II: MEXICOIn the following section, results from the investigationson the flow field around the MEXICO wind turbine arepresented. The test case taken in scope of this study isfrom the New MEXICO experiment [11]. For the sim-ulation, the inflow velocity was taken as 10m/s. Theblade rotational speed was taken as N = 425.1 rpm.Figure 12 depicts the blade surface pressure contours.Additionally, isobars of the blade surface pressure val-ues are plotted. The blade suction side is depicted onthe left, and the pressure side is shown on the righthand side. The operating condition simulated herewithwas meant for an attached flow condition. The inboardsection shows a clean flow pattern.

Figure 12. Blade surface pressure distributions

The distributions of the coefficient of pressure alongthe airfoil suction and pressure sides are depicted in fig-ure 13. During the experiments, blade surface pressuremeasurements were carried out at five radial stationsr/Rtip = 0.25, 0.35, 0.60, 0.82, and 0.92. These are com-pared with the results obtained from the numerical sim-ulations. The inboard section values deviate the most,but it was reported that owing to the uncertainties of thepressure sensors used, the values taken from the exper-imental investigations are most probably of low quality,and the values from the numerical simulations are ratherto be trusted [11, 17].


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Figure 13. Comparison of coefficient of pressure distributions


In the following sections, an effort is made to com-pare radial distribution of the flow velocity componentsat the inflow and the wake of the wind turbine. Dur-ing the New MEXICO measurement campaign, particleimage velocimetry measurements were carried out alongplanes situated at 0.133 × Rtip distance up- and down-stream of the rotor plane. The particle image velocime-try measurement planes were traversed along the radialdirection. Measurements were carried out for the bladesat thirteen azimuthal positions covering 27% of the cir-cumference [11]. The values of these thirteen azimuthalpositions are taken and averaged to represent the circum-ferential averaged radial distributions of the flow veloci-ties. As mentioned before, the simulations were carriedout for one blade pitch. The flow field variables are aver-aged along the pitch direction and the results obtainedare depicted in figures 14 to 16.The comparison of the axial velocity components areshown in figure 14. The simulation predicts a differentflow distribution near the tip region. Obviously, numer-ical performance is affected by employing steady statemodeling of the flow field.

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]

cz[m/s]cz[m/s]

s1s1s2s2e1e1e2e2

Figure 14. Comparison of axial flow velocity cz up- anddownstream of the blade. (sk: simulation, ek:experiment. k = 1: inflow, k = 2: wake)

Figure 15 illustrates the comparison of the radial flowvelocity distributions obtained from the simulation withthose from the particle image velocimetry measurements.The values are fairly in agreement between the simula-tion and the measurement. Although the difference atthe wake is still there, it is not as pronounced as for theaxial velocity near the tip region.

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Figure 15. Comparison of radial flow velocity cy up-and downstream of the blade. (sk: simulation, ek:experiment. k = 1: inflow, k = 2: wake)

The comparison of the circumferential velocity com-ponents are shown in figure 16. The flow inductionnear the leading edge is captured by both the simula-tion and the experimental investigations. At the wake,however, the values differ over the spanwise extent. Theexperimental result registered higher flow turning thanthat found from the simulation. The difference is about0.4 [m/s] for mid-span.

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Figure 16. Comparison of circumferential flow velocitycx up- and downstream of the blade. (sk: simulation,ek: experiment. k = 1: inflow, k = 2: wake)


The findings from the comparisons of the differentflow velocities seen in figures 14 to 16 raise a critical ques-tion regarding the performance of the numerical model-ing, particularly, considering the fact that the simulationresults substantially reproduce the measured blade sur-face pressure distributions (figure 13).To further understand the flow field predicted in the nu-merical simulation, the λ2 criterion [18] is plotted in fig-ure 17 for one blade pitch at about 0.266× Rtip distanceaft of the rotor plane. To facilitate the investigation onthe flow structures past the blade tip, the domain is ex-tended up to the far field. Hence the λ2 values of therelative flow velocity ~w is illustrated with the help ofcontour plot. Additionally, iso-contour lines are plot-ted to detect the vortical flow structures. The blade issituated in the 90 position. Secondary flow structuresare observed along the circumference near the hub. Atthe inboard section, traces of vortical structures are de-tected. Presence of trailing vortices are seen originatingfrom the blade inboard section under transition from cir-cular to airfoil profile. Secondary flow structures are alsodetected near the blade leading edge. Traces of vorticalflow structures are also detected in the outboard section.The blade tip shear layer is clearly visible, whose effectextends past the tip radius. The blade tip vortices aredispersed in the 3D domain, giving a region of highlymixed secondary flow structures whose extend is almost1.5 times the blade tip radius.

λ2(~w)

Figure 17. λ2 criterion of the the relative flow velocity~w at the wake

4. SUMMARY

The present contribution presents an effort to bench-mark the commercial flow solver FINE™/Turbo for thesimulation of horizontal axis wind turbine flow usingRANS simulation. For this purpose, two state of the artresearch wind turbines are selected. The flow field turbu-lence is resolved using high Reynolds number turbulencemodel. The results give a valuable insight on the perfor-mance of the numerical model. For both test cases, thesimulation results imitate the results obtained from theexperimental investigations with fair accordance. Thenumerical code predicts the blade surface pressure distri-butions quite accurately when compared to the results

obtained from the measurement campaigns. Investiga-tions on the NREL Phase-VI turbine shows differencein the blade surface pressure distributions obtained fromthe simulation and those from the experiment for the in-board sections. The MEXICO test case, being the mostrecent research project in this field, offers a substantialsource of data for testing the numerical code. An ef-fort has been given to compare flow-field velocity compo-nents at the inlet and the near wake region of the blade.The comparison between the circumferentially averagedvalues obtained from the simulation and particle imagevelocimetry measurements presented a good match forthe inflow region. However, notable differences were ob-served for the values at the wake region. In an effort tobetter understand the underlying cause of the discrepan-cies, the near wake plane was investigated further. Theinvestigation shows that, in the numerical simulation,traces of secondary flow structures are detected near theblade at the inboard and outboard sections, which arespread out along the blade pitch. The results suggestthat these secondary flow structures were not fully cap-tured by the particle image velocimetry measurements.As such intensive measurement campaign requires hugecomputational effort, and are cost-intensive, the employ-ment of high fidelity computational method is thus jus-tified. The results obtained from these investigationswill play a major role in understanding the physics ofthe development of the aerodynamic flow field aroundthe blade. Resolution of the near wall flow and the sec-ondary flow structures around the blade would facilitatea better understanding of the aerodynamic loss mecha-nisms, thus allowing an efficient design of future windturbines. Further investigations have to be made in thisline to deeply investigate the flow structures leading upto and in the wake of the turbine.

5. ACKNOWLEDGMENTS

The authors would like to express their gratitude to theIEA Wind Taskforce 29 for Mexnext Phase III, speciallyDr. J. G. Schepers and Dr.ir. K. Boorsma from the En-ergy Research Center of Netherlands for providing uswith the geometry and the measurement data availablefor the MEXICO test case. A special thanks goes to Mr.Bastian Dose from the University of Oldenburg, Ger-many, for helping with the issues regarding the MEX-ICO wind turbine geometry.

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