Investigation of Aerodynamic Performances of NACA 0015 Wind Turbine Airfoil

 

 

International Journal of Engineering Research Volume No.5, Issue No.4, pp : 327-331

ISSN:2319-6890)(online),2347-5013(print) 1 April 2016

Investigation of Aerodynamic Performances of NACA 0015 Wind Turbine Airfoil a

 b

Karthick.M , Senthil Kumar. M   Department of Aeronautical Engineering, Kumaraguru College of Technology, Coimbatore-49, Tamil Nadu, India. a  b  b   E-mail:  [email protected]. [email protected]

 Abstract  Abst ract - Continuo Continuous us ri se of of ene nerg rgyy demand and and nee need ffo or cleaner environment emphasizes efficient conversion of energy  fr  fro om rene renewable so source urces. s. Wind ene nerg rgyy is th the e most viab viable  source  so urcess of of rene renewable ene nerg rgyy and it is env envir ir onme nmenta ntally frien fr iend dly alterr nati alte nati ve energy nerg y sour source ces. s. E nergy nerg y extr extract actii on from fr om wind energy is rapidly competitive to power production from other  source  so urcess lik like e coa coal. I n th this is paper, Comp Computa utational ional F luid D ynami ynami cs (CF (C F D ) i s used used to to pr pr edict the aer aer od odynam ynamii c eff effii ciency of wind win d tur turbine bine blade blades. s. A blade blade'' s aer aer od odyna ynamic mic eff effii cie ci ency is is expressed in terms of its lift-to-drag ratio. The design and analysis of blades is one of the critical areas of wind turbine de desi sigg n. I n this thi s pape paperr , a rev revii ew of aer aer od odynam ynamii cs of the two two dimensional NACA 0015 airfoil for vertical axis wind turbine (V A WT) WT ) i s atte attempted ted. The main main foc f ocus us of this thi s iinve nvestig stigat atii on is to analyze the flow behavior around the airfoil body and to calculate the performance coefficients at velocity 10.5 m/s and angle of att attac ackk ffrr om 0° to 20° 20°.. C omp ompariso ari sons ns of of the CF D r esults wi wi th nume numer i ca call predi predi ct ctii ons from fr om the XF OI L r esult and NACA report showed a good agreement.

direction, and operate efficiently in turbulent wind conditions (Fig1). Simple in configuration is the main advantage of VAWT. It is relatively simple with major moving components and complex parts like rotor, gearbox and generator are located at the  base of the wind turbine, therefore reducing the loads on the tower and facilitating the maintenance of the system [3]. VAWT do not require a yaw control system because of insensitive to the wind direction capability. capability. Manufacturing a VAWT is much simpler than a HAWT due to the constant cross-section blades. Because of the VAWTs simple manufacturing process and installation, they are perfectly suited for residential applications. The main drawback of VWAT is low self-starting capability and high starting wind speed.

Keywords - Aerodynamics, CFD, NACA 0015, VAWT, Wind Turbine I. Introduction Introduction

A wind turbine transfers fluid energy to mechanical energy through the use of blades and a shaft and converts that form of energy to electricity through the use of a generator. Depending on whether the flow is parallel to the axis of rotation or  perpendicular determines the classification of the wind turbine [1]. In the case of horizontal axis wind turbines (HAWT), blade rotates around the axis which is parallel to the flow while a vertical axis wind turbine (VAWT) rotates around the axis which is perpendicular to the flow direction. HAWTs are better suited for large scale energy generation while VAWT suited for smallscale micro power generation. HAWTs are currently exploited around the world because it offers several advantages over VAWTs. HAWT give a high efficiency at high tip speed ratio and high self-starting capability [2]. But in rece recent nt years, there has been a resurgence of interest in VAWT. In this paper, the lift and drag performances of NACA 0015 wind turbine airfoil is investigated and numerical results are compared with NACA report and XFOIL result.VAWTs are small, quiet, and easy to install, can take wind from any

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doi : 10.17950/ijer/v5s4/425

Fig. 1 Horizontal and Vertical axis wind turbine

Fig. 2 Various concepts of vertical axis turbines

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International Journal of Engineering Research Volume No.5, Issue No.4, pp : 327-331 The VAWT rotor, comprised of a number of constant crosssection blades, is designed to achieve good aerodynamic qualities at various angles of attack. Unlike the HAWT where the blades exert a constant torque about the shaft as they rotate, a VAWT rotates Perpendicular to the flow, causing the blades to  produce an oscillation in the torque about the axis of rotation. This is due to the fact that the local angle of attack for each  blade is a function of its azimuthally location. Because each  blade has a different angle of attack at any point in time, the

ISSN:2319-6890)(online),2347-5013(print) 1 April 2016 A physical limit exists to the quantity of energy that can be extracted, independent of design. The energy extraction is maintained through reduction of kinetic energy and a consequential drop in velocity of the wind. The magnitude of energy harnessed is a function of the reduction in air speed over the turbine. 100% extraction would imply zero final velocity and therefore zero flow. The zero flow scenarios are not realistic and, hence, it is never possible to utilize the entire kinetic energy. This  principle is widely accepted accepted [4, 8] and indicates that wind turbine

average torque is typically sought as the objective function. Even though the HAWT blades must be designed with varying cross-sections and twist, they only have to operate at a single angle of attack throughout an entire rotation. However, VAWT  blades are designed such that they exhibit good aerodynamic  performance throughout an entire rotation at the various angles of attack they experience leading to high time averaged torque [4]. The blades of a Darrieus VAWT (D-VAWT) accomplish this through the generation of lift, while the Savonius-type VAWT (S-VAWT) produce torque through drag. Fig. 2 shows the various concepts of vertical axis turbines. Symmetric airfoils of finite thickness have similar theoretical lift coefficients; the lift coefficients would increase with increasing

efficiency cannot exceed 59.26%. This parameter is commonly known as the power coefficient Cp, where max Cp = 0.5926 referred to as the Betz limit [9]. The Betz theory assumes constant linear velocity. Therefore, any rotational forces such as wake rotation, turbulence caused by drag or vortex shedding (tip losses) will further reduce the maximum efficiency. Efficiency losses are generally reduced by [8, 9]: i) avoiding low tip speed ratios which increase wake rotation or ii) by selecting airfoils which have a high lift to drag ratio and specialized tip geometries.

angles of attack (α) and continue to increase until the α reaches 90°. The behaviour of real symmetric airfoils does indeed approximate this theoretical behaviour theoretical  behaviour at low α. The theoretical lift coefficient of a symmetrical airfoil is:

C l 

2   



(1)

There are significant differences between actual airfoil operation and the theoretical performance at higher α. The differences are due primarily to the assumption, in the theoretical estimate of the lift coefficient, that air has no viscosity. Surface friction due to viscosity slows the airflow next to the airfoil surface, resulting in a separation of the flow from the surface at higher α and then rapid decrease in lift. The lift coefficient of symmetric airfoil is about zero at an angle of attack zero and increases to over 1.0  before decreasing at higher α. The drag coefficient is usually much lower than the lift coefficient at α. It increases at higher α. Also, there are significant differences in airfoil behaviour at different Reynolds numbers [5, 6]. Fig. 3 shows the NACA 0015 airfoil. High rotor efficiency is essential for extraction of maximum energy from the wind [7]. Therefore, maximization of this  parameterr within affordable limits of production is of paramount  paramete paramount importance. Wind turbine rotor performance is given by its  power coefficient, Cp

C  p

 P  

1 2

 Rotor   Rot or   power 

 

   AV 

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Fig. 4 Lift and Drag vectors The lift force L arises in a direction that is perpendicular to the air stream caused by the Bernoulli’s principle that lowers the  pressure on top of the airfoil compared with the pressure at its  bottom. The curvature on the top leads to a higher stream velocity than at the bottom and hence a lower pressure. The drag force is parallel to the direction of motion. Fig. 4 shows the lift and drag vector for an airfoil [10]. In this study, numerical analyses are performed with 10.5 m/s wind velocity (v). Lift and drag coefficient of NACA 0015 airfoil at different α between 0° and 20° are measured. Lift and drag coefficients for two dimensional solutions are given as  below: 2 L

Lift coefficient:

C l 



Drag coefficient:

C d 



 

2

  v c

 

(3)

(2)



3

Fig. 3 NACA 0015 airfoil.

  power in the wind  

doi : 10.17950/ijer/v5s4/425

2 D 2   v c

(4)

 

 

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International Journal of Engineering Research Volume No.5, Issue No.4, pp : 327-331 Where L and D, are lift and drag force respectively, Cl  and Cd  are lift and drag coefficient of airfoil respectively, c is airfoil chord length, v is velocity of wind and ρ is density of air.   II. Numerical Numerical Evaluation of Airfoil

CFD is a numerical method used to simulate physical problems with use of governing equations. In this numerical analysis, C mesh was used as shown in Fig. 5 and 6. The top bottom and left

ISSN:2319-6890)(online),2347-5013(print) 1 April 2016 of attack is increased to get an increment in the lift of the airfoil  but however it increases drag also tto o a considerable amount but α cannot be increased beyond a cer tain tain limit as the lift begins to decrease. The particular α at which a first decrement of lift appears is known as the stalling angle. Stall angle is a function of the Reynolds number. Drag coefficient is highly dependent on Reynolds number

 boundaries were placed at a distance of 12.5 chords from fro m airfoil whereas the right boundary was placed at 20 chords. The mesh is constructed to be very fine at regions close to the airfoil and with high energy, and coarser farther away from the airfoil. For this airfoil, a structured quadratic mesh was used. A grid independence study was performed to verify that the solution would not change subsequent additional refinements and 10750 grids number suitable for our model [11, 12]. Fig. 6 Static Pressure at 8° angle of attack

Fig. 5 Structure of C mesh us using ing numerical analysis analysis Fig. 7 Velocity Contour at 8° angle of attack

Fig. 5 Mesh around the airfoil Fig. 8 Velocity Vector at 8° angle of attack

III. Results Results and Discussion

Fig.7 to 14 shows the contours of pressure, velocity magnitude, velocity vector and pressure coefficient of NACA 0015 airfoil, there is a region of high pressure at the leading edge (stagnation  point) and region of low l ow pressure on the upper surface of airfoil. air foil. It can be seen that the velocity at the upper region has the higher magnitude, also at bottom region has lower magnitude. The coefficient of pressure is analyzed in the upper and lower surface of the airfoil for α varies from 0° to 20°.   The variation of Cl and Cd for the various α has been simulated. The airfoil was computationally tested for different α. The angle

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Fig. 9 Pressure Coefficient at 8° angle of attack

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International Journal of Engineering Research Volume No.5, Issue No.4, pp : 327-331

Fig. 10 Static Pressure at 16° angle of attack

ISSN:2319-6890)(online),2347-5013(print) 1 April 2016

Fig. 14 Cl Vs angle of attack

Fig. 11 Velocity Contour at 16° angle of attack Fig.15 Cd Vs angle of attack

Fig. 12 Velocity Vector at 16° angle of attack

Fig. 16. Cl and Cd Vs angle of attack

Fig. 13 Pressure Coefficient at 16° angle of attack The conventional NACA 0015 airfoil stalls α of 16°. The maximum Cl of NACA 0015 is 1.1058 at 14° α. The maximum Cl/Cd  ratio attained at the velocity of 10.5 m/s is 25.69 at α of 8°. As the angle of attack increases the C l  increases till α = 14° but after α = 14°, the airfoil stalls and the lift starts to decrease and drag increases rapidly. Below shown plots are comparisons of C l, Cd, and Cl/Cd for an airfoil  NACA 0015. The CFD results are compared with XFOIL data and NACA report [13, 14].

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Fig. 17 Comparison of C l and Cd Vs angle of attack

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International Journal of Engineering Research Volume No.5, Issue No.4, pp : 327-331

ISSN:2319-6890)(online),2347-5013(print) 1 April 2016  NACA 0015 airfoil shows suitable option for VAWT and hence it is planned to conduct 3D flow analysis to analyze the flow  behavior over VAWT blade with appropriate turbulence model in future. References i. 

 Hau, E, “Wind Turbines, Fundament Fundamentals, als, Technologies, Technologies,  Application,  Applicati on, Economics”, Second edition, Springer, Berlin, Germany, Fig.18 Comparison of Cl Vs angle of attack

2006.  Kishinami, K, K, “Theoretical and experiment experimental al study on the ii.  aerodynamic characteristics of a horizontal axis wind turbine”,  Energy 2005, 30, 2089 – 2100. 2100. iii.   David A. Spera, “Wind turbine technology, technology, Fundamen Fundamental tal

concepts of wind turbine engineering”, second edition, ASME, Three  Park Avenue, New York, NY 10016, USA. iv.  Gorban, A.N, Gorlov, A.M, Silantye v, V.M, “Limits of the

turbine efficiency for free fluid flow”, J. Energy Resource. Technol. Trans. ASME 2001, 123, 311 – 317. 317.

Fig. 19 Comparison of Cd Vs angle of attack

Fig. 20 Comparison of Cl /Cd Vs angle of attack IV. Conclusion Conclusion This paper showed the behavior of the NACA 0015 airfoil at various α. The k -epsilon -epsilon turbulent viscosity model is used, which had a good agreement with the validation after investigation for a wider range of o f α. The coefficient of lift and drag is calculated for NACA 0015 for the α varies from 0° to 20°. The coefficient of pressure is analyzed in the upper and lower surface of the airfoil for α varies from 0° to 20°.  Numerical lift and drag coefficients of the NACA 0015 airfoil results are compared with NACA report and XFOIL results.

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v.   J. F. Manwell, J.G. McGovan, A.L.Rogers, Wind energy explained, University of Massachusetts, Amherst, USA, John Wiley and Sons Ltd, September 2002. vi.   Anderson, John D. Jr, “Computat “Computational ional Fluid Dynamics basics with Applications”, Fourth edition, Tata McGraw -Hill  Publishing Company Ltd, 2012. Sathyajith Mathew, “Wind Energy Fundamentals, Resource vii.   Analysis and and Economic Economics”, s”, Spring Springer  er -Verlag -Verlag Berlin Heidelberg 2006. viii.  Gasch, R, Twele, J, Wind Power Plants; Solar praxis: Berlin, Germany, 2002. ix.   Burton, T. Wind Energy Handbook, John Wiley and Sons Ltd, Chichester, UK, 2011.  x.   Anderson, John D, Jr, “Fundamen “Fundamentals tals of Aerodynami Aerodynamics”, cs”,  Fifth edition, Tata Tata McGraw-Hill Publishing C Company ompany Ltd, 2010.  xi.  http://www.fluent.com/software/ http://www.fl uent.com/software/sf_mesh_and_tutorials/tutori sf_mesh_and_tutorials/tutori al_airfoil.htm  xii.  http://www.mh-aerotools.de/airfoils/jf_applet.htm  İzzet Şahin and Adem Acir Numerical and Experimental Experimental  xiii.   Investigations of Lift and Drag Performances of NACA 0015 Wind Turbine Airfoil, International Journal of Materials, Mechanics and  Manufacturing, Vol. 3, No. No. 1, February 2015  xiv.   Eastman N. Jacobs et al, The characteristics of 78 related airfoil sections from tests in the variable density wind tunnel, NACA report.

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