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Int. J. Mech. Eng. & Rob. Res. 2014 S Prathiban et al., 2014

EFFECT OF AEROFOIL THICKNESS OVERPRESSURE DISTRIBUTION IN WIND TURBINE

BLADES

S Prathiban1*, M Manickam1, P Renuka Devi1, and T T M Kannan2

*Corresponding Author: S Prathiban, srprathiban88@gmail.com

Renewable energy is the energy extracted from the natural resources like wind, rain, etc. Sincewind power is growing at a rate of approximately 30% annually and now it has the capacity togenerate 430 TWh annually, which is about 2.5% of worldwide electricity usage. Although avariable source of power, the intermittency of wind seldom creates problems results wind powerto supply up to 20% of total electricity demand. Thus, much research is focusing to utilize thisvast adequate wind resource because the evolutions of serious problems like air pollution whichleads to global warming due to the consumption of more conventional fossil fuels, coal andnuclear energy resources. The study on the effect of aerofoil thickness in wind turbine blades isone of the effective methods to improve its performance. In this paper numerical simulation wascarried out to study the effect of aerofoil shape using commercially available CFD software. Themain scope is to increase the resultant force produced by each blade which is resolved as Liftand Drag. Lift is an aerodynamic force on the body in the direction normal to the flow direction,while Drag is an aerodynamic force on the body parallel to the flow direction. For a windmill tooperate efficiently the lift force should be high and drag force should be low. In this work NACA4415 is chosen as a wind turbine blade cross section. From this basic configuration thicknessof the cross section is changed and the effect of thickness is studied through lift curve slope.

Keywords: Wind turbine blades, Aerofoil thickness, Numerical simulation, CFD software

INTRODUCTIONWind turbine is a device which converts kineticenergy from air to mechanical energy. Furthermechanical energy converted into electricalenergy. This energy conversion is the result of

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Int. J. Mech. Eng. & Rob. Res. 2014

1 Department of Mechanical Engineering, Ponnaiyah Ramajeyam College of Engineering and Technology, Thanjavur, India.2 Department of Mechanical Engineering, PRIST University, Thanjavur, India.

several phenomena. The wind is characterizedby its speed and direction, which are affectedby several factors, e.g., geographic location,climate characteristics, height above ground,and surface topography. Although the

Research Paper

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Int. J. Mech. Eng. & Rob. Res. 2014 S Prathiban et al., 2014

performance of wind turbines is dominated byvarious factors, the performance of airfoils isthe most influential. The low Reynolds numberairfoils designed for small aircraft were widelyused in the early stage development of modernhorizontal axis wind turbines. Aerofoil designedfor aircraft are not always suitable, however,because the range of operational Reynoldsnumber is wide and because the inflow windis fluctuated. Atmospheric turbulence causesimportant fluctuating aerodynamic forces onwind turbines. Turbulence is an importantsource of aerodynamic forces on wind turbinerotors. Turbulence is an irregular motion of fluidthat appears when fluids flow past soil surfacesor when streams of fluid flow past or over eachother. However, the turbine geometry whichaffect the aerodynamic performance of thewind turbine as well as its power performance.The development of aerofoil series tailored fora specific wind turbine is not effective, it isconjectured that by varying the thickness of theairfoil cross section it become effective. In thisresearch, the thickness of the cross section ofthe wind turbine blade (aerofoil) is changedfor the specific wind turbine blade and theireffectiveness was verified by simulating CFDanalysis. The scope of this work was toprovide a preliminary evaluation of the effectof thickness on aerofoil performance. Thisblade-root aerofoil was designed to have ahigh maximum lift coefficient which is largelyinsensitive to surface-roughness inducedpremature transition. The results of threeconsidered wind-turbine aerofoil exhibit effectsof aerofoil thickness and maximum liftcoefficient on the sensitivity of the maximumlift coefficient to its performance. Because thedesign specifications for these aerofoil are notconsistent, however, the results cannot be

used to conclusively determine these effects.Accordingly, a matrix of three natural-laminar-flow aerofoil has been designed and analyzednumerically to quantify these effects.

The wing extends in the y direction (the spandirection). The free stream velocity V∞ isparallel to the XZ Plane. Any Section of thewing cut by a plane to the XZ plane is calledAerofoil.

Figure 1: Aerofoil Section

Figure 2: Nomenclature of Aerofoil

LITERATURE REVIEWPeter Fuglsang and Ioannis Antoniou (2003),studied the two aerofoil cross sections for thewind turbine blades by Numerically andExperimentally. Two-dimensional wind tunneltesting was carried out for the Risø-B1-18 andRisø-B1-24 aerofoil in the VELUX wind tunnelat Re = 1.6 x 106. The measurementscomprised both static and dynamic inflow.

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Static inflow covered inflow angles from 50 to300. Dynamic inflow was obtained by pitchingthe aerofoil in a harmonic motion aroundvarious mean angles of attack. The test matrixinvolved smooth flow, various kinds of leadingedge roughness, stall strips, vortex generatorsand Gurney flaps. Franck Bertagnolio made aNumerical Study of the Static and PitchingRISØ-B1-18 Aerofoil. The objective of thisreport is the better understanding of thephysics of the aero-elastic motion of windturbine blades in order to improve thenumerical models used for their design. In thisstudy, the case of the RISØ-B1-18 aerofoilwhich was equipped and measured in an openjet wind tunnel is studied. Two and three-dimensional Navier-Stokes calculations usingthe k–€ SST and Detached Eddy Simulationturbulence models are conducted. Anengineering semi-empirical dynamic stallmodel is also used for performing calculations.Computational results are compared to theexperimental results that are available both forthe static aerofoil and in the case of pitchingmotions.

Mac Gaunaa (2006) studied the unsteady2D Potential-flow Forces on a Thin VariableGeometry Aerofoil Undergoing ArbitraryMotion. In this report analytical expressionsfor the unsteady 2D force distribution on avariable geometry aerofoil undergoingarbitrary motion are derived under theassumption of incompressible, irrotational,inviscid flow. The aerofoil is represented byits camber line as in classic thin-aerofoiltheory, and the deflection of the aerofoil isgiven by superposition of chord wisedeflection mode shapes. Jeppe Johansenand Niels Sorensen (2006) made a study on

Profile Catalogue for Aerofoil Sections Basedon 3D Computations This report is acontinuation of the Wind Turbine AerofoilCatalogue which objective was, firstly toprovide a database of aerodynamiccharacteristics for a wide range of aerofoilprofiles aimed at wind turbine applications,and secondly to test the two-dimensionalNavier-Stokes solver EllipSys2D bycomparing its results with experimental data.In the present work, the original two-dimensional results are compared with three-dimensional calculations as it was surmisedthat the two-dimensional assumption might bein some cases responsible for discrepanciesbetween the numerical flow solution and theactual fluid flow, and thereby the incorrectprediction of aerofoil characteristics. Inaddition, other features of the flow solver, suchas transition and turbulence modeling, andtheir influence onto the numerical results areinvestigated. Conclusions are drawnregarding the evaluation of aerofoilaerodynamic characteristics, as well as theuse of the Navier-Stokes solver for fluid flowcalculations in general. The aim of their workis to evaluate the prediction capabilities ofthe computational code EllipSys3Dcompared to its two-dimensional versionEllipSys2D on one side, and the experimentalresults (when available) on the other side. Inorder to perform this study, the flow aroundthree-dimensional blade sections for chosenaerofoil profiles will be computed. Severalaspects of the numerical code areinvestigated. The derived aerofoilcharacteristics show that the maximum liftcoefficient at the tip is low and that themaximum lift coefficient is high at the rootcompared to 2D aerofoil characteristics. The

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use of the derived characteristics inaeroelastic calculations shows goodagreement with measurements for power andflap moments. Furthermore, a fatigue analysisshows a reduction in the loads of up to 15%from load calculations with the derivedaerofoil characteristics compared with acommonly used set of aerofoil characteristics.The numerical optimisation is based on boththe 3D CFD computations and measurementson a 41-m rotor with LM 19.1 and LM 19.0blades, respectively. The method requiresmeasurements or CFD calculations of powerand loads from a turbine and is promisingsince a set of lift and drag curves is derivedthat can be used to calculate mean values ofpower and loads. The maximum lift at the tipis low and at the root it is high compared to2D aerofoil characteristics. In particular the

power curves were well calculated by use ofthe optimised aerofoil characteristics. In thequasi-3D CFD computations, the aerofoilcharacteristics are derived directly. ThisNavier-Stokes model takes into accountrotational and 3D effects. The model enablesthe study of the rotational effect of a rotorblade at computing costs similar to what istypical for 2D aerofoil calculations. Thedepicted results show that the model iscapable of determining the correct qualitativebehavior for aerofoils subject to rotation. Themethod shows that lift is high at the rootcompared to 2D aerofoil characteristics. Thedifferent systematic methods show theimportance of rotational and 3D effects onrotors. Furthermore, the methods show highmaximum lift coefficients at the inboard partof the blade and low maximum lift coefficients

Figure 3: Model of Computational Domain and Their Corresponding Boundary Conditions

PressureOutlet

Aerofoil asVelocity Inlet

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at the outboard part of the blade comparedto 2D wind tunnel measurements.

RESULTS AND DISCUSSIONNumerical Simulation on AerofoilsThe free-stream air is assumed to beapproaching the aerofoil with free-streamvelocity and pressure with angle of attack .Convergence criteria are set such that thenormalized residuals for each parameter areless than 10E-6. From the simulated flow field,Normal and Axial Force magnitudes aredetermined. From that it’s Coefficients Cn and

Table 1: Cl and Cd for NACA 4415

Figure 4: Variation of Cl, Cd, Cl/Cd, Cp for NACA 4415

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Table 2: Cl and Cd for NACA 23012

Figure 5: Variation of Cl, Cd, Cl/Cd, Cp for NACA 23012

Table 3: Cl and Cd for NACA 63-413

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Figure 6: Variation of Cl, Cd, Cl/Cd, Cp for NACA 63413

Figure 7: Comparison Graph for All Cases

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Ca is calculated. By Using the well knownrelation derived in chapter 2, the value of Liftforce (L) and Dreg force (D) and from that it’sCoefficients Cl and Cd is found. Figure 3 a-cRepresents variation of Cl with angle of attackfor the three considered cross sections isshown. And its numerical values are tabulatedfrom Tables 1 to 3.

Similar kind of C-Grid is used for the threeconsidered sections having fine mesh near thesurface because we need to calculate theproperties of the flow near the Surface of theAerofoil. Flow is carried out for Different angleof attack () ranging from –4 to 25 deg andthe corresponding values of Cn, Ca, Cl, and Cdare calculated and their values are tabulatedand graphically shown below.

CONCLUSIONAerofoil of various thickness has been studiedand the parameters like coefficient of lift, dragwere plotted with various angle of attack. Alsothe aerodynamic ratio L/D were consideredand discussed for the different cases ofaerofoil. From the above data it is very clearthat Aerodynamic efficiency of Case I (NACA4415) is very high approximately 14 whencompared to other cases. Also the value ofmaximum coefficient of lift for the case I islarger around 1.4.but the stall angle for case Iis around 14 degrees. This is small whencompared to other cases (for case II it is around18 degrees and for case III it s around 25degrees). So Based on the flow separationpoint of view, Case III is better but based onthe magnitude of force case I provides aOptimum benefit. Since in the wind turbineblade designs, every cross section is twistedto around 40 degrees. Thus optimum

performance at higher angle of attack isrequired, so based on that point modern lowspeed aerofoil series case III is the optimumchoice.

ACKNOWLEDGMENTWe express sincere thanks to our BelovedPrincipal Prof.M.Abdul Gani Khan,Ponnaiyahramajayam college of Engineering andtechnology,Thajavur for given valuablesuggestions and Motivate this Efforts.

REFERENCES1. Chalothorn Thumthae and Tawit

Chitsomboon (2006), “Optimal Pitch forUntwisted Blade Horizontal Axis WindTurbine”, The 2nd Joint InternationalConference on Sustainable Energy andEnvironment.

2. Chiristian Bak (2005), “Know-Blade Task-3.3 Report for Rotor Blade Computationswith 3D Vortex Generators”, RisøNational Laboratory, Roskilde, Denmark.

3. Danmei Hu, Ouyang Hua and Zhaohui Du(2006), “A Study on Stall-Delay forHorizontal Axis Wind Turbine”, Journal ofRenewable Energy, Vol. 31, pp. 821-836.

4. Franck Bertagnolio (2004), “NumericalStudy of the Static and Pitching RISO-B1-18 Airfoil”, Riso National Laboratory,Roskilde, Denmark.

5. Frederik Zahle (2005), “Operation andControl of Large Wind Turbines and WindFarms”, Riso National Laboratory,Roskilde, Denmark.

6. Jeppe Johansen and Niels N Sorensen(2006), “Aerodynamic Investigation ofWinglets on Wind Turbine Blades Using

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CFD”, Riso National Laboratory,Roskilde, Denmark.

7. Mac Gaunaa (2006), “Unsteady 2DPotential Flow Forces on a Thin VariableGeometry Aerofoil Undergoing ArbitraryMotion”, Riso National Laboratory,Roskilde, Denmark.

8. Ole Sangill (2005), “Profile Catalogue forAerofoil Sections Based on 3D

Computations”, Riso National Laboratory,Roskilde, Denmark.

9. Peter Fuglsang and Ioannis Antoniou(2003), “Wind Tunnel Tests of Risø-B1-18anRisø-B1-24”, Pitney BowesManagement Services Danmark A/S.

10. Sorensen N N, Johansen J and ConwayS (2005), “Know-Blade Task-3.2 ReportTip Shape Study Revised Version”, RisoNational Laboratory, Roskilde, Denmark.