at SciVerse ScienceDirect

Renewable Energy 42 (2012) 152e156

Contents lists available

Renewable Energy

journal homepage: www.elsevier .com/locate/renene

Wind tunnel test results for a 2/4.5 scale MEXICO rotor

Taehwan Cho*, Cheolwan KimKorea Aerospace Research Institute, 45 Eoeun-dong, Youseong-gu, Daejeon 305-333, Republic of Korea

a r t i c l e i n f o

Article history:Received 8 March 2011Accepted 18 August 2011Available online 13 September 2011

Keywords:Wind turbineMEXICO rotorWind tunnel testTransition

Abbreviations: MEXICO, Model EXperIments inKorea Aerospace Research Institute; BEMT, Blade Elem* Corresponding author. Korea Aerospace Researc

Youseong-gu, Daejeon 305-333, Republic of Korea. TeE-mail addresses: thcho7@gmail.com (T. Cho), cwk

0960-1481/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.renene.2011.08.031

a b s t r a c t

This paper presents the wind tunnel test results from MEXNEXT, an IEA wind task for analyzing themeasurements which have been taken in the EU project ‘MEXICO’. A 2/4.5 scaled model of ‘MEXICO’ rotorwas tested in the KARI low wind tunnel with 5 � 3.75 m2 open jet test section. The aerodynamicperformance of the blade which was represented by the torque was measured in the wind speed from0 to 30 m/s by using the torque sensor installed in the rotating axis. The rotational speed of the rotor wascontrolled by the electric motor to keep the prescribed blade tip speed from 50 m/s to 90 m/s. Twodifferent surface conditions, free and forced transition conditions were used for all blade tip speeds.Transition dots with 0.18 mm height were attached at the 5% chord line on both sides of the blade surfacefor the forced transition condition. The torque coefficients with respect to the wind speed coefficient forthe forced transition condition show same characteristics for all blade tip speed conditions except for thestall region. But, the torque coefficient for the free transition condition gradually increases as the bladetip speed increases until the tip speed reaches 76 m/s and it has the similar value above that speed. Thecomparison result between the free and the forced transition conditions at the blade tip speed 76 m/sshows that the torque coefficient for the former case is 30% higher than the latter case. The computa-tional results from ‘Rfoil’ and the BEMT method also shows that the aerodynamic performance of therotor for the forced transition condition is lower than the free transition one at the wind tunnel testcondition.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

This paper presents the results from MEXNEXT [1], an IEA windtask for analyzing the measurements which have been taken in theEU project ‘MEXICO’ [2]. A wind tunnel test for the wind turbinewith 4.5 m diameter rotor was conducted in the DNW large lowspeed wind tunnel with 9.5 � 9.5 m2 open jet test section. The 6-component forces and moments of the wind turbine system weremeasured by using the external balance installed in the bottom ofthe testing model. The aerodynamic performance of the rotorwhich was represented by the torque was studied by using the flowdirectional moment. But that moment influenced not only by therotor torque itself but also by the side force of the strut which wasinstalled between the nacelle and the external balance. The detailanalysis for the ‘MEXICO’ test will be presented in the MEXNEXTreport. This paper contains only the scaled model test results of theMEXICO rotor which was conducted in Korea Aerospace Research

Controlled cOnditions; KARI,ent Momentum Theory.h Institute, 45 Eoeun-dong,l./fax: þ82 42 860 2628.im@kari.re.kr (C. Kim).

All rights reserved.

Institute (‘KARI’) low speed wind tunnel with 5 � 3.75 m2 open jettest section.

The aerodynamic characteristics of the wind turbine blade forthe various rotational speeds and blade surface conditions werestudied by using the scaled down model of MEXICO rotor. Thetorque generated by the rotor was directly measured by using thetorque sensor installed in the rotating axis and the measured resultwas compared with the BEMT calculation result. The aerodynamiccharacteristics of the airfoils which were the input data for theBEMT were calculated by using the ‘Rfoil’ in the same condition asthe wind tunnel test.

This article is organized as follows. In Section 2, the experi-mental method including the model and test facilities is presented.The test result for various conditions is presented in Section 3 andthe comparison between the computational results and windtunnel test results is also presented in Section 3. Finally Section 4 isthe conclusion.

2. Experiments

The wind tunnel test for the scaled down MEXICO rotor wasconducted at KARI low speed wind tunnel with 5 � 3.75 m2 open

Nomenclature

Vwind wind speedVtip blade tip speedRblade blade tip radiusu rotational speedAdisc rotor areal tip speed ratio l ¼ Vtip

Vwind1.l wind speed ratio

s torque generated by the wind turbine rotorCs torque coefficientCP power coefficient

Fig. 1. The wind turbine blade test rig install in the open jet section of KARI low speedwind tunnel.

T. Cho, C. Kim / Renewable Energy 42 (2012) 152e156 153

jet test section. The turbulence intensity level of the facility is lessthan 0.4% and the flow angularity is less than 0.2�. The detail flowquality of the tunnel measured at 2007 is in the Table 1.

The original ‘MEXICO’ rotor which has the 4.5 m diameter wastested in the DNW LLF with 9.5 � 9.5 m2 open jet test section at2006. A 2/4.5 scaled down model was used in this test to keep thesame blockage ratio which is defined as the ratio between therotating disc area and test section area. The rotor was designed byusing 3 airfoils, the DU91-W2-250 airfoil up to 40% radius, RISO-A1-21 airfoil up to 62% radius and NACA-64418 airfoil up to the tip. Thedetail configuration of the rotor is described in reference 2. The testrig for the scaled down model was composed with 3 parts, blades,nacelle and strut. The rotational center of the blade is on the testsection center position (Fig. 1) and the flow directional distance ofthe rotational center is 0.7 m to the upstream direction from thestrut center. The electric motor (LG CN60A, 5 kW, Max. RPM 4500),1/3 reduction gear, rotating axis and torque sensor (HBM T20WN,50 Nm, accuracy 0.2%, Max. RPM 3000) are installed inside thenacelle with cylindrical shape cover which has 1.78 m length and0.25m diameter. The 3.2 m cylindrical strut with 0.16m diameter isused to support the nacelle system.

The manufacturing accuracy of the blade fabricated bycomposite material was check by using the 3 dimensionalmeasurement tools. More than 1300 points were measured andcompared with the design data. The measurement result showsthat themean error whichwas defined by the distance between themeasured value and designed value is less than 0.1 mm and thestandard deviation of the error is 0.08 mm. Most of the measuredpoints have the error less than 0.1 mm and some points in the innerpart of the blade have bigger error but less than 0.3 mm. The 3dimensional measurement method and results are shown in Fig. 2.

The rotational speed of the rotor was controlled by the electricmotor and kept the target RPM within þ/�0.5 RPM during the test.The same rotational speed conditions with the original MEXICOtest, 76m/s and 100m/s for the blade tip speed, were applied in thistest but the experiment for the 100 m/s case was conducted only atlow wind speed region for the model and sensor safety. Test resultfor the blade tip speed from 50 m/s to 90 m/s was presented in thisarticle. The Reynolds number with reference to the chord length at96% radial section for each rotational speed is in the Table 2.

To match the same surface condition with the original MEXICOtest, transition dot with 0.18 mm height was attached at 5% chordline on both pressure and suction sides and this test condition isdenoted as ‘forced’ transition condition in this article (Fig. 3). The

Table 1Flow quality of KARI low speed wind tunnel.

Dq/q (%) Angularity (�) DT (�C) Turbulence level (v/V) (%)

<0.5 <0.2 <0.5 <0.4

clean surface condition which is denoted as ‘free’ transitioncondition was also conducted for all rotational speed. The windspeed varies from 0 m/s to maximum 30 m/s for a given rotationalspeed conditions. The torque and wind speed data were acquiredfor 10 s with 10 Hz sampling rate for a given condition and theaveraged value was used in this article.

The rotor torque is roughly proportional to the square of rota-tional speed and the flow angle for the sectional airfoil is relatedwith the inverse of the tip speed ratio. So, non dimensional vari-ables for wind speed and torque were used to compare the variousrotational speed data. Lambda (l) was used for the wind speed andC_tau_tip (Cs) with a blade tip speed (Vtip) as a reference velocity

Fig. 2. Upper: The 3D measurement tools of the model blade. Lower: The 3Dmeasurement results of the model blade. Blue line indicates error level less than0.1 mm, cyan line 0.2 mm and green line 0.3 mm. (For interpretation of the referencesto colour in this figure legend, the reader is referred to the web version of this article.)

Table 2Wind tunnel test conditions.

Rotating speed (RPM) V_tip (m/s) Reynolds Num. @ 96% R (�105)

477 50 1.34572 60 1.61668 70 1.88729 76 2.05763 80 2.15859 90 2.42900 94 2.53

Fig. 3. Transition dots attached at 5% chord line for the forced transition condition.

1/Lambda

C_t

au_t

ip*1

000

0 0.1 0.2 0.3 0.4 0.50

0.5

1

1.5

2

2.5

3

3.5V_tip = 50m/sV_tip = 60m/sV_tip = 70m/sV_tip = 76m/sV_tip = 90m/s

Fig. 5. Test result for the free transition condition. Torque (C_tau_tip*1000) versuswind speed (1/Lambda). Rotational speed (V_tip) is varies from 50 m/s to 90 m/s.

T. Cho, C. Kim / Renewable Energy 42 (2012) 152e156154

was used for the torque. Thewind speed ratio (1.l) and 1000 times

of Cs are used in this article to compare the torque characteristicsfor various rotational speeds.

1.l ¼ Vwind

Vtip¼ Vwind

uRblade(1)

Cs ¼ s0:5� r� V2

tip � ADisc � Rblade

1/Lambda

C_t

au_t

ip*1

000

0 0.1 0.2 0.3 0.4 0.50

0.5

1

1.5

2

2.5

3

3.5

Test #1Test #2

Fig. 4. Repeatability test result: V_tip ¼ 76 m/s, forced transition condition.

CP ¼ su0:5� r� V3

windADisc¼ Csl

3

3. Results and discussion

3.1. Experimental results

The repeatability test result in Fig. 4 shows that the deviation forCs � 1000 between two tests is less than 0.01 in the wind speedbefore the stall region, 1=l < 0:23 and 0.04 in the after stall region.

Test result for the free transition condition in Fig. 5 shows thatthe aerodynamic performance (Cs � 1000) for the givenwind speedratio increases as the rotation speed increases up to 70 m/s for theblade tip speed and it has the similar value above the blade tipspeed 76 m/s. Test result for the forced transition condition in Fig. 6shows that the aerodynamic performance for the givenwind speed

1/Lambda

C_t

au_t

ip*1

000

0 0.1 0.2 0.3 0.4 0.50

0.5

1

1.5

2

2.5

3

3.5V_tip = 50m/sV_tip = 60m/sV_tip = 70m/sV_tip = 76m/sV_tip = 90m/s

Fig. 6. Test result for forced transition condition. Torque(C_tau_tip*1000) versus windspeed (1/Lambda). Rotation speeds (V_tip) are varies from 50 m/s to 90 m/s.

1/Lambda

C_t

au_t

ip*1

000

0 0.1 0.2 0.3 0.4 0.50

0.5

1

1.5

2

2.5

3

3.5

V_tip = 76m/s : forcedV_tip = 76m/s : free

Fig. 7. Torque comparison between two transition conditions. Torque (C_tau_tip*1000)versus wind speed (1/Lambda). Rotation speeds (V_tip) is 76 m/s.

alpha

CL

-5 0 5 10 15-0.5

0

0.5

1

1.5

CD

CL

0 0.01 0.02 0.03 0.04 0.05-0.5

0

0.5

1

1.5

FreeXtr:Re0.2FreeXtr:Re0.5Xtr5% : Re0.2Xtr5% : Re0.5

Fig. 8. Aerodynamic performance of the NACA-64418 airfoil for the Reynolds number0.2 � 106 and 0.5 � 106. Xtr5% means forced transition at 5% chord and FreeXtr meansfree transition condition. Calculated by using Rfoil.

alpha

L/D

-5 0 5 10 150

50

100

150FreeXtr:Re0.2FreeXtr:Re0.5Xtr5% : Re0.2Xtr5% : Re0.5

Fig. 9. Lift to drag ratio of the NACA-64418 airfoil for Reynolds number 0.2 � 106 and0.5 � 106. Xtr5% means forced transition at 5% chord and FreeXtr means free transitioncondition. Calculated by using Rfoil.

T. Cho, C. Kim / Renewable Energy 42 (2012) 152e156 155

ratio has similar value for all blade tip speeds except for the 1=lw0.24 region. The stall wind speed which is similar concept as thestall angle in the airfoil gradually increases from 0.2 to 0.24 as therotational speed increase in Fig. 6. The aerodynamic performancefor two surface conditions, forced and free transition, wascompared in Fig. 7. The torque coefficients for the free transitioncondition are about 30% larger than the forced transition conditionin low wind speed region, 1=l < 0.26. And they have similar valuewhen the blade is in the full stall condition, 1=l > 0.28.

3.2. Discussion

The aerodynamic performance of the blade is roughly decidedby the lift to drag ratio of airfoils used in it. So, the aerodynamiccharacteristics of the airfoil were studied by using the computa-tional analysis tools, Rfoil [4] which was the improved version of

1/Lambda

C_t

au_t

ip*1

000

0 0.1 0.2 0.3 0.4 0.50

0.5

1

1.5

2

2.5

3

3.5

BEMT: ForcedBEMT: Free

Fig. 10. Aerodynamic performance of the 2/4.5 scaled down MEXICO rotor from thecomputational result by using BEMT method.

Lambda

Cp

0 2 4 6 8 100

0.5Exp. : FreeBEMT: Free

Fig. 11. Power coefficients of the 2/4.5 scaled down MEXICO rotor: Comparisonbetween experimental result (‘Exp’) and computational result (‘BEMT’) for the freetransition condition.

T. Cho, C. Kim / Renewable Energy 42 (2012) 152e156156

the Xfoil [5]. The lift and drag polar of NACA-64418 airfoil which isused at 62%e100% radius of the rotor is shown in Fig. 8. Reynoldsnumber 0.2 � 106 and 0.5 � 106 represent the scaled down modelcondition and the original MEXICO model condition respectively.The Re¼ 0.5�106 case has higher lift slope and lower drag than theRe¼ 0.2 � 106 case for both free and forced transition conditions inFig. 8. So, the lift to drag ratio in Fig. 9 for the Re ¼ 0.5 � 106 ishigher than the Re ¼ 0.2 � 106 case, which means that the aero-dynamic performance of the scaled down model would be lowerthan the original one.

The aerodynamic performance for the forced transition condi-tion is lower than the free transition condition at same Reynoldsnumber in Fig. 9. For the conventional airfoils like NACA0012, thelift slope increases slightly and the drag decreases as the Reynoldsnumber increases [3]. But, the laminar flow airfoil which is usednormally in the wind turbine blade to increase the aerodynamicperformance by increasing the lift to drag ratio has a differentcharacteristics. They keep the laminar boundary layer until the50%w60% chord line for the suction and pressure side. When theforced transition method is used in this kind of airfoils, the lift slopedecreases and the drag increases about 50% in the low Reynolds

number region (Fig. 8). So the aerodynamic performance of theblade is much lower downed than the free transition case. This isthe reason why the aerodynamic performance for the forced tran-sition condition in Fig. 7 is lower than the free transition condition.

The aerodynamic performance of the MEXICO blade wascalculated by using the BEMT method with 40 elements [6].Aerodynamic data of the airfoils which is the input data of theBEMT method was calculated by using the Rfoil. The calculationresults in Figs. 10 and 11 show good agreements with the experi-mental results especially for free transition condition.

4. Conclusions

In summary, the wind tunnel test for the 2/4.5 scaled downMEXICO rotor which has 2 m diameter was conducted with variousrotational speeds. The aerodynamic performance of the bladerepresented the by torque with the free and forced transitionconditions was compared. The test results for the free transitioncondition show that the torque coefficient for a given wind speedratio increases as the blade tip speed increases and it has the similarvalue for the cases above 76 m/s. But the torque coefficient for theforced transition condition has similar value for all blade tip speedconditions. And the aerodynamic performance of the rotor waslower downed when the forced transition method was used. Thecomputational analysis results by using the Rfoil and BEMT showgood agreement with the experimental results.

Acknowledgments

This work was supported by the New & Renewable Energy R&Dprogram (20103010020011) of the Korea Institute of Energy Tech-nology Evaluation and Planning (KETEP) grant funded by the Koreagovernment Ministry of Knowledge Economy.

References

[1] Schepers G, Boorsma K, Snel HIEA. Task 29 mexnext: analysis of wind tunnelmeasurements from the EU project Mexico; 2010. Torque 2010.

[2] Snel H, Schepers G. Mexico project: the database and results of data processingand interpretation; 2009. AIAA-2009-1217.

[3] Barlow JB, Rae WH, Pope A. Low speed wind tunnel test. 3rd ed. John Wiley &Sons; 1999.

[4] van Rooij RPJOM. Modification of the boundary layer calculation in RFOIL forimproved airfoil stall prediction; 1996. TU Delft, Report IW-96087R.

[5] Drela M, Youngren H. XFOIL 6.94 user guide. Cambridge: Massachusetts Insti-tute of Technology; 2001.

[6] E.T.G. Bot., Blade optimization tool; 2009. ECN, ECN-E-09e092.