Design and Blade Optimization of Contra Rotation Double Rotor Wind Turbine

International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 11 No: 01 17 Design and Blade Optimization of Contra Rotation Double Rotor Wind Turbine Priyono Sutikno 1, Deny Bayu Saepudin 1 Institut Teknologi Bandung, Bandung, Indonesia, priyonosutikno@yahoo.com Institut Teknologi Bandung, Bandung, Indonesia, denibayu@yahoo.com Abstract-- The Intelligent Wind turbine (IWT) has two stages blades contra rotation. This kind of wind turbine has characteristic self regulated on the speed due to the difference torque between two stages horizontal axis wind turbine, than no need the pitch controller to control the speed and cut off the wind turbine due to the high wind speed. The research of IWT is designed first by optimize several important design parameters, as a blade section profile and the multiplier factor of the angle of attack. The design parameter results are the NACA 641 is selected as the optimum blade section profile and the optimum value of angle of attack multiplier factor is 0.5. The designed IWT has 3 blades for each front and rear rotor. The research intelligent wind turbine has 600 mm front diameter and 600 mm rear blade diameter. The characteristics of IWT were simulated by using Computational Fluid Dynamic (CFD) software, demonstrated the non entrainment of the contra rotation, each blades should have the same produced torque. Index Term- Intelligent Wind Turbine, Numerical Simulation, Contra rotation Wind Turbine I. INTRODUCTION The conventional wind turbines with large sized wind rotor generate high output in the moderately strong wind. The output of the small sized wind rotor is low such a wind rotor is suitable for weak wind. That is, the size of the wind rotor must be appropriately selected in conformity with potential wind circumstances. Besides, in general the wind turbines are equipped with the brake and or the pitch control mechanis ms, to control the speed due to the abnormal rotation and the overload generated at the stronger wind, and to keep the rotation of generator. In that sense, some studies present a good review of various invented the superior wind turbine generator, T. Kanemoto [1] has invented Intelligent Wind Turbine Generator (IWTG) composed of the large sized front wind rotor, the small sized rear wind rotor and the peculiar generator with inner and the outer rotational armatures, as the rotational speeds of the tandem wind rotor are adjusted pretty well in cooperation with the two armatures of the generator in response to the wind speed. The IWTG model is composed of tandem wind rotor using the flat blades, and demonstrated the fundamentally superior operation of the tandem wind rotor. In this paper, the effect of the blade profiles using NACA profiles on the turbine using numerical simulation on the turbine performances are investigated to optimized the rotor profiles. Nomenclature A Area a Axial induction factor a Radial Induction factor B Number of blade C D Drag coefficient C L Lift coefficient c Chord length C p Power Coefficient D Diameter F x Axial Force g Acceleration of gravity L Lift force P Power p pressure Q correction factor r local radius element rotor Re Reynolds number R Radius T Torsi T Thrust V o Absolute Velocity w Relative velocity u Tangential velocity x Local speed ratio α β e γ η λ ρ σ Ω angle of attack (AOA) stagger angle Ratio coefficient Lift and Drag pitch angel Efficiency Tip speed ratio density angle of attack relative Solidity Angular velocity II. OPERATION OF TANDEM WIND ROTORS In the IWTG both wind rotors start to rotate at low wind speed, namely cut in wind speed, but the rear wind rotor counter rotates against the front wind rotor. The increase of the wind speed make the both rotational speeds increase, and the rotational speed of rear wind rotor becomes faster than that of the front wind rotor because of its small size. The rear wind rotor reaches the maximum rotational speed at rated wind speed. With more increment of the wind speed, the rear wind rotor decelerates gradually and begins to rotate at the same direction of the front wind rotor so as to coincide with larger rotational torque of front wind rotor. Such behaviour of rear wind rotor is induced from the reason why the small sizes wind rotor must work as the blowing mode against the attacking wind because the wind rotor turbine mode can \not generate adequately the rotational torque corresponding to the front wind rotor. The 115301-7474 IJMME-IJENS February 011 IJENS

International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 11 No: 01 18 behaviour of the front and rear wind rotors also depends on the blade profiles and flow condition between both rotors, and will be discussed. The rotational direction and speed of the rotors are adjusted in response to the wind circumstance (see fig. ) Fig. 1. Drawing IWTG [1] Fig.. Operation of IWTG [1] The authors has proposed the optimized blades with adopted the NACA Air foils for rear and front blades of the contra rotation wind turbine. It is difficult, however to know the rotational torque but also to get optimized blades profiles, using the contra rotation model. In order to elaborate and to get the optimized blades, the model was separated from tandem to single isolated wind turbine, however the rear turbine has the velocity data s from the front wind turbine blade simulation. III. AIR FOIL AND ROTOR PERFORMANCE ANALYSIS Airfoil has made rotor possible to rotate in high speed and load, early aerodynamics of wind turbine has based on theory of air plane wings. However, aerodynamics of wind turbine has been required different idea, the accuracy of rotor performance analysis depend mainly on the treatment of the wake effect, because the wake of propeller type wind turbine is induced a large velocity in rotor plane. For Horizontal Axis Wind Turbine blades the aviation airfoils such as NACA series have been widely used. But these air foils have been recognized to be insufficient for requirements, such reduction of rapid stall characteristics, in-sensitivity to wide Reynolds number the range of between 5.10 5 to.10 6. Rotor performance analysis has been performed using several methods. The Blade Element Momentum (BEM) method is mainly employed as a tool of performance analysis because of their simplicity and readily implementation. Vortex wake methods can adequately treat the effect of wake vortices and have some advantages over BEM. 3.1 Blade Element Momentum Method Most wind turbine design codes are based on Blade Element Momentum (BEM) method [7]. The basic BEM method assumes the blade can be analyzed as a number of independent elements in span wise direction. The induced velocity at each element is determined by performing the momentum balance for an annular control volume containing the blade element. The aerodynamic forces on the element are calculated using lift and drag coefficient from empirical two dimensional wind tunnel test data at the geometric angle of attack (AOA) of the blade element relative to the local flow velocity. BEM method have aspect by reasonable tool for designer, but are not suitable for accurate estimation of effect of wake, complex flow such as three dimensional flow or dynamic stall because of their assumption. 3. Vortex Wake Method The induced velocity in the rotor plane of Horizontal Axis Wind Turbine (HAWT) is largely increased in heavy loading condition and the wake vortices of HAWT develop to the downstream constructing highly skewed vortex sheet in largely decelerated axial flow near rotor plane. Thus determination of the velocity induced by wake and wake geometry is one of the most important aspects in the rotor performance analysis. Vortex wake method directly calculates the induced velocity from the bound vortices of blades and the trailing vortex in wake which are represented by lifting line or lifting surface model [4]. The treatment of wake geometry can be classified roughly into two type, as a prescribed wake model and free wake model. In the former model the wake represented by a line a vortex or spiral vortices with fixed pitch. In later one a fractional step scheme is adopted and the configurations of the wake are calculated at every time step using local velocity including the components induced by wake and bound vortices. The free wake model is generally tackled with vortex lattice method which can fit on arbitrary blade shape with camber, taper and twist. Another method of the vortex wake methods is use of an asymptotic acceleration potential. Acceleration potential method is basis on the Laplace equation of pressure perturbation. The rotor blades are represented in the model as discrete surfaces on which a pressure discontinuity is present. The model implies the presence of span wise and chord wise pressure distributions, which are composed of analytical asymptotic solution for Laplace equation. More elaborate model makes it possible to calculate the dynamics load caused by dynamic inflow and yawed inflow situation [5]. 3,3 Computational Fluid Dynamic Recent development of the computational fluid dynamics (CFD) allows us to simulate overall flow around HAWT including tower and nacelle. In 1999 Duque et al. [6] 115301-7474 IJMME-IJENS February 011 IJENS

International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 11 No: 01 19 calculated aerodynamics of HAWT using RANS model and overset grids to facilitate the simulation of flow about complex configuration. Recently, some CFD s codes actively are developed of CFD analysis of rotor flow by three dimensional Navier Stokes code. Though the state of the art CFD is needed considerable computer power and validation for Navier Stokes model, CFD has potential advantage for detailed understanding of aerodynamic of the HAWT. IV. OPTIMAL ROTOR BLADE 4.1 The NACA series air foil The design model, which is composed of tandem wind rotor, designed based on Blade Element Momentum (BEM) method. The design is used the 4 (four) digit NACA airfoil and to be chosen among 7 (seventh) airfoil profile as shown at fig. 3. A : 1 st digit is the percent of chord B : nd digit is the ten percent of the chord C : 3 rd and 4 th digit is the percent of chord Beside the Lift and Drag Ratio, the camber to chord ratio can be influenced the Lift to Drag Ratio, and as shown at fig. 4. The NACA airfoil has been chosen, have a certain AOA at the maximum Lift to Drag Ratio. The number of blades at the front and rear rotor depend on the velocity to tip ratio as shown at table II [5]. The rotation of the front and rear rotor depend on the tip speed ratio, for tip speed ratio between three and more than four, the number of rotor is three. T ABLE II T HE NUMBER OF BLADE DEPEND ON SPEED TIP RATIO Λ λ B [number of blade] 1 8 4 6 1 3 3 6 4 3 4 More than 4 1 3 The rotor performance analysis of IWT has been calculated by the model Actuator Disc and Blade Element Momentum. This method has been modelled and developed by Glauert (Ingram, 005), the inflow near the rotational blade or disc as the induced velocity in the rotor plane is largely increased and represent by rotational inflow factor. The aerodynamic forces on element are calculated using the lift and drag coefficient from XFOIL software. Fig. 3. Airfoil Profile of 4 digits NACA XXXX The criteria of NACA airfoil to be implemented to the front rotor and rear rotor, the XFOIL software is used to simulated the Lift and Drag Coefficient at function of AOA, the criteria s are a. The airfoil has a good performance, should have as bigger as possible the ratio of the ratio the Lift and Drag coefficients as shown at Table I. b. The section of the airfoil has simple form possible, which has a flat suction in order to simply the blades manufacturing, see Fig. 3. T ABLE I T HE MAXIMUM LIFT AND DRAG RATIO OF NACA 5 AND 6 SERIES Fig. 4. The Lift to Drag ratio of the NACA XXXX series The optimum blade can be concluded by comparing the data on table I and performance of blade in the fig. 1 and 4 with respect to the criterion above, the chosen blade has thickness to chord ratio of 1%, the camber to chord ratio is 6% and the air foil NACA 641 is chosen as airfoil for front and rear rotor. 4. Opti mal rotor bl ade using GLAUERT-PRANDTL- XU model. The calculation is based on the Blade Element Momentum (BEM) method, this method is suitable for engineering development and there are two kinds of categories: fixed pitch and variables pitch rotor blade. The blade length is divided into several small elements for which the two dimensional airfoil theory can be applied. The dimensionless coefficient, C L and C D, the net force, power and torque caused by B blades, each of local chord c, are as follow [6]: 115301-7474 IJMME-IJENS February 011 IJENS

For torque: International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 11 No: 01 0 ΔQ 1 ρw r C sin C cosbcδc (1) L D For power: C sin C cosbcr P Q 1 W r () L D For thrust: C cos C sin Bcr T 1 W (3) L D where W r w = u sin cos Fig. 5. Local element velocities and flow angles [8] u V tan 0 1 a 1 a (6) r w r1 a' x1 a' r where, x =, is local speed ratio. At the end of the V 0 blades, r become R, and we find the most important parameter for wind turbine rotors, the tip-speed-ratio, R or X, using X, we can V 0 R 1 a write, tan, the two dimensional lift and rx 1 a' drag coefficients C L and C D are both function of angle of CD attack, Instead of using the average CL solidity, it s define a symbol called the blade loading Bcc coefficient, l, using and 8.r a cot (7) 1 a sin And a' 1 a' tan sin To obtain a single point optimum including the effect of drag, deriving a local power coefficient [6], (8) Based on actuator disc theory and Using dimensionless axial V 0 u w and radial induction factor, a and a' and V r Bc solidity, we find equation above became R a Fig. 6. Local elemental forces [8] R C L D 1 a 8r sin a' R C 8r 0 cos C sin sin CD cos sin cos L 1 a' Also we have (4) (5) C where, P W Bc( C sin C cos ) ' L D P (9) 3 1 3 V V 0 0 A dp ΩdQ C sin C cos 1 ρv Ωr Bc dr total and da = π dr by using : V U 1 a r 1 a and equation 6, then total equation 9 can be write C p L 1 a 1 cot 4xλ sinφ εcos (10) Then, eliminating λ using equation 7 and expanding 1/(cot + ε) in a Taylor s series of two terms, there results 4xa1 atan 1 tan (11) C p Since the optimum value of a is founded to be quite insensitive to changes in ε, this imp lies that C decreases monotonically as ε increases. By defining a local Froude efficiency (Eq. 1), we can relate the performance of each blade element to the ideal value of unity [6]. 7 F C p (1) 16 The correction factor for total losses can actually be quite well represented by Prandtl and Xu represent the tip losses and hub losses, the equation is quite simple but can give the D p 115301-7474 IJMME-IJENS February 011 IJENS

International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 11 No: 01 1 good matched on HWAT (Horizontal Axis Wind Turbine) [10], the Prandtl tip correction factor is cos Q exp f if f 7 Q f 1 tip tip tip tip tip 1 if f 7 B R r r sin tip (13) And for hub correction factor can be written as cos Q exp f if f 7 Q f 1 hub hub hub hub hub 1 if f 7 hub B r Rhub R sin hub (14) Early 001, Xu proposed the correction factor on hub losses by using the Prandtl correction factor as written above and the Xu correction factor for hub can be written as new 0,85 Qtip 0,5 Qtip 0,5 if 0,7 r 1 R Q new tip r 1Qtip r / R 0,7 1 if r 0.7 0,7R R Flowchart in fig. 7 explained the complete procedures of rotor turbine design. This flow chart refers to optimum design procedure of rotor blade and the source program is written in FORTRAN code, while XFOIL is used to obtain the Lift coefficient and Drag coefficient of airfoil data which is chosen for blade design. After obtaining the Lift and Drag Coefficients an interpolation is performed to justify Reynolds number and angle of attack (AOA) on calculation XFOIL or two dimensional flow over the airfoil by Fluent. (15) Fig. 7. Flowchart to calculate forces and power at the optimum performance Wind turbine rotor with three blade formed by several airfoil profile with s maller chord length from hub to tip every blade along the span. Fig. 8 displayed graphic of 115301-7474 IJMME-IJENS February 011 IJENS

International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 11 No: 01 can concluded the optimum performance is used the angle of attack with 0,6 to be chosen with regard of The Maximum efficiency is near of the working or design point at the rated rotation The produced torque has relatively high The values of the efficiency of the wind speed region ( until 1 m/s) are always relatively high and stable as shown at fig. 11. Fig. 8. Graphic of distribution of chord length and twist angle at rotor span chord length and stagger angle in function of angle of attack (AOA). Fig. 9 shown graphic of the torque and the efficiency curve versus the rotational speed and the fig. 10 shown graphic of the torque and efficiency versus rotational velocity results of the numerical simulation using the FLUENT software. Fig. 11 shown graphic of the efficiency as functions of the velocity source calculated manually and simulated three dimensional numerically using the FLUENT. Fig. 10. Simulation result using the Blade Element Momentum and Prandtl_Xu correction factor on efficiency versus rotational speed The optimum blade is NACA 641, blade has thickness to chord ratio of 1%, the camber to chord ratio is 6% and the angle of attack is 0,6 multiplier. Fig. 9. Graphic of torque versus rotational speed calculated and simulated numerically Fig. 8 to 9 shown the graphics of chord length versus span length of rotor, the torque versus rotational speed and the efficiency versus rotational speed respectively, these results has been calculated by PRANDTL-XU correction equation and simulated numerically using the FLUENT 6.3.6. We 115301-7474 IJMME-IJENS February 011 IJENS

International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 11 No: 01 3 Fig. 11. The efficiency versus wind speed V. DESIGN AND SIMULATION OF THE IWT 5.1 Design Procedures for Wind Turbine Rotor Flowchart in fig. 7 explained the complete procedures of rotor turbine design. This flowchart refers to optimum design procedure of rotor blade, and the source program is written in EXCELL code, while the XFOIL or FLUENT software is used to obtain lift coefficient (C L ) and drag coefficient (C D ) of airfoil data which is chosen for the blade design. After obtaining the lift coefficient (C L ) and drag coefficient (C D ), an interpolation is performed to justify Reynolds number and angle of attack on calculation. 5. Simulation Procedures for Intelligent Wind Turbine Front and Rear Rotors The simulation of Intelligent Wind Turbine front and rear rotors are using computational fluid dynamic (CFD) method through Fluent software. The simulation process consists in two parts, the two dimension model and three dimension models. Two dimension model is using FLUENT DDP to calculate lift coefficient (C L ), drag coefficient (C D ), pressure coefficient and flow characteristic through airfoil profile in two dimension, while Fluent 3D is used to calculate force components which rotor produced and flow characteristic in three dimension, especially flow behind the rotor which shown velocity decrease and wind energy, turbulence, and wake. The two dimension simulation proposed to obtain airfoil characteristics which will be used in blade design with angle of attack variation and Reynolds number variations, then served as an input on blade design by using interpolation. The airfoil profile has been calculated and simulated at section 4. Two dimension simulation process is completed by Gambit meshing around 66.000 cells and iteration using FLUENT DDP with assumption of compressible flow and coupled solver was used including energy calculation using absolute velocity formulation in steady condition. These assumptions are requisite in order to obtain accurate current model on airfoil surface by showing turbulence phenomenon, flow separation, boundary layer, and reversed flow. This flow phenomenon is their natural flow characteristic, where the decreasing of whole airfoil performance and rotor efficiency in extreme situation [9]. The result of calculation for the front and rear rotor can be shown as bellows: Fig. 1. Result of distribution of chord length (c) of the front and rear rotor span of IWT Fig. 13. Result of distribution of pitch angle of the front and rear rotor span of IWT 115301-7474 IJMME-IJENS February 011 IJENS

International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 11 No: 01 4 Blade 1 Blade Blade 3 Front rotor Rotor axis Axis of rotor Fig. 15. Position of the pickup velocities and pressures from the front rotor blade Fig. 14. The front at the left fig. and rear rotor at the right 5.3 The three Dimensional Model Simulation of the IWT front and rear rotor. The analyzed aerodynamic problem is flow detriment including wake around rotor, distribution of velocity and pressure decrease in axial direction. The first simulation is made to a front rotor with 60 cm diameter which placed in a cylinder wind tunnel with 150 cm diameter and 300 cm length. Flow condition is steady, front rotor speed constantly at 600 rpm and tip speed ratio of 3.14 wind condition for rear rotor can show at fig. 15. directions are assumed uniform velocity input before hits the rotor. The second simulation is made a rear rotor with 60 cm diameter, the boundary condition of the input rear blade are the velocity vectors output from the first simulation of the front blade. The pickup boundary Three dimension wind turbine rotor is produced using 3D Inventor modeling program (Inventor 008) version. Blade is made of several airfoil profiles along the span using blend method to form blade with twist pattern, previously these airfoil profiles were kept in *.sec format. Afterwards, the blade making result that produced by Inventor 008 are exported to Gambit in *.igs format. Fig. 16. Intelligent Wind Turbine, the front and rear blades in isometric and front view 115301-7474 IJMME-IJENS February 011 IJENS

International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 11 No: 01 5 Modeling process in Gambit is making meshing around 6.0 million cells (TGRID) and defining boundary conditions. Modeling in Gambit taking the wind tunnel analogy as boundary conditions, and there is only one volume control around rotor as rotating frame. In Fluent, the finishing process is using segregated solver model with relative velocity formulation or multiple reference frames (MRF) model and steady conditions. It is important to do the relative velocity formulation because the volume control that used is rotating frame (non inertia) [], in order to analyze relative velocity impact to a rotor and exposed current flow behind the rotor (wake) [9]. The expected result in 3D simulation is to get far flow around rotor, not just only at the rotor surface. The applied viscous model is the same model that applied in D simulation wh ich is viscous k-ε model [8], [10]. VI. RESULT AND DISCUSSION Two dimension and three dimension rotor turbine are analysis using optimum blade design and calculated with BET PRANDTL-XU methods or designed and simulated by 3D Fluent indicates a good results and have same similitude. If we compare both analyses result by fluent and by BET PRANDTL-XU methods, it turned out that there is only small difference on calculation results of resultant velocity. It is showed by calculation result of velocity resultant distribution along the blade shown at fig. 9 and 10, where the torque is 17 Nm and the efficiency is 35% at 500 rpm and by using numerical simulation Fluent, the torque is 0.14 Nm and the efficiency is 30% at 500 rpm. The same way the efficiencies calculated by both methods has a same tend. The BET-PRANDTL-XU method has been used for the front and rear rotors optimum design condition and produced the front and rear rotor blades as shown at fig. 14 above. The numerical simulation used FLUENT to get the performance shown at fig. 17 is the numerical simulation result give the torque and the efficiency curves in function of rotation speed of the both rotor, front and rear rotor blades. The simulation is conducted by separate the front rotor as a single wind turbine. To get the result of rear rotor numerical simulation, the boundary condition should be setup from the output of the front rotor numerical simulation. The boundary condition for the rear rotor has been taped as shown at fig. 15, there are several pick up data s in the radial direction and data s at direction of flow in the upstream and downstream as we can see at z 1, z and z 3. The pickup data at radial direction are indicated by raw r 1 until r 5. The 3 dimensional IWT design can be seen at fig. 16, the front rotor has 3 blue blades and the rear blade rotor has green color. The result of numerical simulation using the FLUENT has results as shown at fig.18, the efficiency curve of IWT versus wind velocity and fig. 19 shown the characteristic of the rotational velocity relative of the front and rear blades depend on the wind velocity. Fig. 18. Efficiency Curve of IWT Fig. 19. IWT Rotational speed versus wind speed Fig. 17. Simulation result of the torque and efficiency curve of the front and rear rotor IWT In the classical wind turbine, there are two ways in controlling the output of wind turbine power, they are: 115301-7474 IJMME-IJENS February 011 IJENS

International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol: 11 No: 01 6 1. Blade pitch controlled wind turbine. Stall controlled wind turbines; passive stall controlled wind turbines and active stall controlled On the Intelligent Wind Turbine (IWT) with contra rotation rotor blades has speed adjustment depend on the wind speed as shown at fig. 17. The IWT both rotors start to rotate at low wind speed, namely cut in wind speed, but the rear rotor contour rotates against the front rotor. The increase of the wind speed make the both rotational speeds increase, and the rotational speed rear rotor become faster than that of the front rotor. At wind speed of 4 m/s the rotational of front rotor is 400 rpm and rotational speed of rear rotor is -400 rpm and until wind speed of the 6 m/s, the rotational of front rotor is 600 rpm and rotation of rear rotor is -500 rpm, that means the relative rotational velocity is 1100 rpm and IWT has maximum efficiency of 7%. At the wind speed more than 7 m/s, the rotational speed of rear rotor decreased until the wind speed 11.5 m/s, the rotation speed direction of both rotor, front and rear rotors has a same direction but the relative rotational speed remain same is 1100 rpm. [5] Hasegawa Y., et al., Numerical Analysis of Yawed Inflow Effect on a HAWT Rotor. Proc. of 3 rd ASME/JSME. Joint Fluid Engineering Conference FEDSM 99-780. 1999. [6] Duque, E.P.N., et al., Navier-Stokes Analysis of Time Dependent Flow about Wind Turbine, Proc. Of 3 rd ASME/JSME Joint Fluid Engineering Conference, FEDSM99-7814, 1999. [7] Priyono Sutikno, Numerical Optimization of Wind turbine blades. Proceedings of the International Conference on Fluid and Thermal Energy Conversion 003. [8] Verdy Kohuan, Priyono Sutikno., Aerodynamic Design and Analysis of Wind Turbine Blade Propeller Type with Power 500 kw, Proceedings of the International Conference on Fluid and Thermal Energy Conversion 006, FTEC 006, Jakarta, Indonesia, December 10 14, 006, ISSN 0854 9346 [9] Moriarty P., Hansen A., (005). Aero Dynamic Theory Manual, National Renewable Energy Laboratory, NREL/TP-500-36881. [10] Fluent 6.3.6 Documentation User Guide (008) VII. CONCLUSION The IWT which composed of tandem rotors and contra rotation has characteristic superior as the conventional wind turbine, than no need pitch control or stall control to controlling the rotational speed when wind speed became too high. The IWT can start rotate on weak wind speed. At moderate wind speed IWT can rotated relatively on adequate rpm, because the IWT has contra rotation rotor. When the wind speed increased, the relative rotational speed remain constant, event at high wind speed the relative rotational speed remain constant about 1100 rpm, the rear rotor has been entrainment by the front rotor and rotated at same direction. The numerical simulation was demonstrated the direction of the rotation of both front and rear rotor should have a same order torque. The method to get the optimum blade profile and the numerical simulation can be used as preliminary design and to get the estimated characteristic of contra rotation blade span. ACKNOWLEGMENT This works was supported by Riset Unggulan 010 LPPM (Research and Service to the Community Institute) INSTITUT TEKNOLOGI BANDUNG. REFERENCES [1] Toshiaki Kanemoto. and Ahmed Mohamed Galal. 006. Development of Intelligent Wind Turbine Generator with Tandem Wind Rotor and Double Rotational Armatures, Series B, Vol. 49 No, JSME International Journal. [] Dahl K. S., et al., Experimental Verification of the new RISO-Al Airfoil family for wind turbine, Proc of EWEC 99, 1999, pp 85-88 [3] Wilson, R.E., and Lissaman, P.B.S., Applied Aerodynamics of Wind Power Machine, NTIS PB 38594, Oregon State University, 1974 [4] Afjeh, A.A., and Keith Jr. T.G. A Vortex Lifting Line Method for the analysis of Horizontal Axis Wind Turbine. Transaction of ASME, Journal of Solar Energy Engineering, Vol. 108, 1986, pp. 303-309 115301-7474 IJMME-IJENS February 011 IJENS

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