02 Para Gleici - item5-10

02 Para Gleici - item5-10

Copyright © 2014 by ABCMNovember 10-13, 2014, Belém, PA, Brazil

Proceedings of ENCIT 2014 15th Brazilian Congress of Thermal Sciences and Engineering

EXPERIMENTAL ANALYSIS OF AN S809 AIRFOIL

Anderson O. Gomes, anderson.o.gomes@ufv.br Henrique Márcio Pereira Rosa, henrique.rosa@ufv.br Julio Cesar Costa Campos1, Julio.campos@ufv.br

Alváro Messias B. Tibiriça, alvaro.tibirica@ufv.br Department of Industrial and Mechanical Engineering Federal University of Viçosa Viçosa, MG, Brazil – 36570-0

Rogério Fernandes Brito, rogbrito@unifei.edu.br Department of Mechanical Engineering Federal University of Itajubá, Advanced Campus Itabira, MG, Brazil – 35903-087

Pedro Casanova Treto, pcasanova2000@gmail.com University of Costa Rica, Engineering Research Institute (INII) Montes de Oca - Costa Rica

Abstract: This paper looks into the aerodynamic behavior of an S809 airfoil commonly utilized in wind turbines. Tests were carried out to measure drag coefficient profiles under high speed flows of up to 14 m/s, with Reynolds numbers ranging between approximately Re = 11400 and Re = 135400. The prototype was fabricated on a fused deposition modeling machine with ABS Plus thermoplastic. Several tests were carried out in a wind tunnel. Angles of attack ranging from 0° to 20° were tested in increments of two degrees in both the clockwise (leading edge above trailing edge) and counterclockwise directions (leading edge below trailing edge). Drag coefficient versus Reynolds number curves were obtained for the aforementioned angles. The airfoil drag coefficient was found to decrease as the Reynolds number increased for all the angles of attack analyzed. Airfoil dynamic stall was determined (maximum lift coefficient). In the tests, dynamic stall occurred at approximately 16° counterclockwise. This value is in agreement with the literature.

Keywords: S809 Airfoil, drag coefficient, Reynolds number, angle of attack

1. INTRODUCTION

wind energy generation, often listed as a requirement in current projects

Wind turbines were first used to produce electricity in the twentieth century, in response to accelerated economic growth and increased power consumption. Today, wind energy is highly desirable for being a non-polluting and theoretically inexhaustible mode of power generation. Another relevant factor is the strong ecological appeal present in

Researchers from all over the globe have been studying sustainable development and looking into alternative energy sources. Wind – and particularly wind turbines used for purposes of power generation – has been considered as one of the means to mitigate the use of fossil fuels. With that in mind, Alves (1997), Rasila (2003), and ChenWu et al. (2011) have studied wind turbines and analyzed the performance of rotors and variables such as torque, blade velocity control, and the effect of roughness on airfoil performance.

Burton (2001), Epaarachchi (2006), and Eggleston and Stoddard (1987) have focused their studies in the engineering design of wind turbines.

Ebert and Wood (1997) and Wright and Wood (2004) have found that when the Reynolds number increased from 1x104 to 1x105 in the initial stage of a 500W wind turbine, the angle of attack gradually decreased from 86 to 20 degrees. Wang (2005) reported this finding when analyzing the flight of insects.

Sheng and Galbraith (2009) reported results on the aerodynamics of the S809 airfoil in unsteady conditions. Zhou Y. (2010) discussed the NACA0012 airfoil for a wide range of angles of attack, and described a drop in lift and a jump in drag force when the Reynolds number was in the 104 range. The author further emphasized that the issues arising from a low Reynolds number have not been sufficiently addressed in the literature, much less when combined with high angles of attack.

Airfoil aerodynamics research appears to be more concentrated on conventional aircraft design with Reynolds numbers above 5x105 and below stall conditions.

Countless aerodynamic profiles have been proposed as a result of the intense efforts made by the scientific community to improve the efficiency with which wind power is used. However, special attention is required when the S809 airfoil is considered, as it has been designed to operate in turbines at low speeds.

1Author to whom correspondence should be addressed.

Copyright © 2014 by ABCMNovember 10-13, 2014, Belém, PA, Brazil

Proceedings of ENCIT 2014 15th Brazilian Congress of Thermal Sciences and Engineering

In this context, the development of an experimental project on the S809 airfoil is justified. The main purpose is to assess airfoil aerodynamic performance at various angles of attack.

2. MATHEMATICAL FORMULATION Çengel (2007) described the following equations to calculate the Reynolds number and the drag coefficient:

FC d d

Where is the density of the air at ambient temperature, V is air velocity, c is the length of the chord, the dynamic viscosity of the air, Fd the drag force, and A the planform area of the airfoil. The planform area of the airfoil is calculated as follows:

Where lis the width of the airfoil.

3. MATERIAL AND METHODS

The profile presented in this paper belongs to S809 airfoils used in wind generators. Profile definitions, sizes, specifications, and curves have been described by Somers (1994). Figure 1 illustrates the profile of an S809 airfoil. An airfoil with a 150-m chord was used in this study. This chord size was chosen due to wind tunnel size limitations.

Figure 1. The profile of an S809 airfoil.

Modeling package SolidWorks was used to define the curves in the S809 profile – Fig. 1 – along with the width of the prototype set at approximately 100 m. Figure 2 shows the curve generated with the aid of the SolidWorks software package.

Figure 2. Model generated with the aid of the SolidWorks package.

In order to analyze the effect of weight on the model depicted in Fig. 2, a hollow model was built to assess airfoil cost-effectiveness (Fig. 3).

Figure 3.Model of the prototype shell

Copyright © 2014 by ABCMNovember 10-13, 2014, Belém, PA, Brazil

Proceedings of ENCIT 2014 15th Brazilian Congress of Thermal Sciences and Engineering

The S809 model was fabricated on the prototyping machine available at the Department of Industrial and Mechanical Engineering. The model is shown in Figure 4.

Figure 4. Airfoil equipped with wind tunnel attachment pin (chord and width measurements).

The model was built with thermoplastic ABS PLUS – a material 40% stronger than regular ABS. The properties of this material are described in the prototyping machine literature.

This paper aimed to measure drag coefficients as a function of Reynolds numbers for an S809 airfoil profile in a range of angles of attack in the clockwise and counterclockwise directions, for wind speeds between 0 and 14 m/s with a standard error of 1m/s. The blockage factor in the wind tunnel was 0.092. Wind tunnel turbulence could not be measured, as a hot-wire anemometer was not available. The Aerostream Wind Tunnel shown in Figure 5 was used for the purposes of this study.

Figure 5. The wind tunnel located at UFV.

The S809 airfoil model was attached to the wind tunnel with the aid of a free moving pin (Fig.5). Test sessions were carried out with angles of attack ranging from 0° to 20°. The angle of attack was reset in increments of two degrees in the clockwise and counterclockwise directions. In the clockwise direction, the leading edge was above the trailing edge, while the opposite was true for the tests run in the counterclockwise direction.

Measurements were made with alternating angles of attack, i.e., drag forces were first measured in one direction at a certain angle and then in the opposite direction at the same angle. This method precludes the occurrence of testing systematic error. The angles were measured with a Mitutoyo goniometer, with a minimum resolution of 1°. Figure 6 illustrates the measurement procedure.

Figure 6. Method used to measure airfoil angles of attack (clockwise direction).

Figure 7(a,b) shows how the angle of attack was reset by adjusting the free moving pin and the airfoil front and upper views in the wind tunnel test.

Copyright © 2014 by ABCMNovember 10-13, 2014, Belém, PA, Brazil
(a)(b)

Proceedings of ENCIT 2014 15th Brazilian Congress of Thermal Sciences and Engineering Figure 7. Angle of attack directions: (a) counterclockwise, (b) clockwise.

4. RESULTS AND DISCUSSION

The results presented in this paper refer to the drag coefficient versus Reynolds number plots at various angles of attack. These experimental results were obtained with the aid of a wind tunnel.

Figure 8 shows the behavior of experimental drag coefficient Cd as a function of angle of attack α, for Re=121,200. It also depicts the dependence between drag coefficient and angle of attack. The horizontal axis of the graph in this

Figure represents a clockwise angle of attack with a positive signal. In the graph, the maximum value for Cd is approximately 0.12.

The experimental results of the Cd versus α plot for Re = 106 presented by Gharali and Johnson (2012) were similar to the results reported in Figure 8. The maximum value found for Cd was observed at an angle of 16o.

Figure 8. Drag coefficient versus angle of attack, Re=121200.

Figure 8 reveals that when the Reynolds number is high, Cd values tend to manifest along a horizontal straight line. Therefore, high-speed wind flows tend to make the drag coefficient constant regardless of airfoil angle of attack. This can be verified for α values from 2° to 8°, followed by a slight oscillation at angles between 8 and 10 degrees, only to become constant again in the 10° to 14° range, then some more oscillation from 14° to 16°, and finally a constant drag coefficient from 16° to 20°.

Figure 9 shows the Cd versus Reynolds number plot with α ranging from 4° to 16°. The graph indicates that, regardless of α, Cd decreases as the Reynolds number increases. Additionally, at high Reynolds numbers Cd remains constant, as seen in Fig.8. Another noteworthy finding is that the drag coefficient remains close to constant when the

Reynolds number is greater than 100x103, possibly characterizing the transition between laminar and turbulent flow.

(a)(b)

Re C d

Cd

Re

( g r a u s ) Cd

Copyright © 2014 by ABCMNovember 10-13, 2014, Belém, PA, Brazil
(c)(d)

Proceedings of ENCIT 2014 15th Brazilian Congress of Thermal Sciences and Engineering Figure 9. Drag coefficient versus Reynolds number, (a) α=12o; (b) α=16o;(c) α=4o; (d) α=8o.

5. CONCLUSIONS

The aerodynamic characterization of airfoil profiles should include smaller angles of attack, particularly of 20 degrees. Aerodynamic performance may be significantly affected, as important drag forces act on the airfoil in this angle of attack range. This was described by Zhou Y. (2010).

The drag coefficient increases as the angle of attack is increased in the clockwise or counterclockwise direction.

However, as previously described, drag gain is higher in the counterclockwise direction. For a fixed angle, the drag coefficient drops even as drag forces increase. This occurs when the Reynolds number increases, i.e., as speed increases.

As the Reynolds number increases, the drag coefficient tends to manifest along a horizontal straight line. Therefore, high-speed wind flows make the drag coefficient tend to a constant value, regardless of the airfoil angle of attack. This characterizes the transition between laminar and turbulent flow zones.

6. ACKNOWLEDGEMENTS The authors would like to thank FAPEMIG for the financial support provided.

7. REFERENCES

Alves, A. S. G. Análise do Desempenho de Rotores Eólicos de Eixo Horizontal, Dissertação de Mestrado,

Universidade Federal do Pará, Brasil, 1997.

Burton, T., Sharpe, D., Jenkins, N., Bossanyi, E. Wind energy: handbook. Ed. John Wiley & sons, Inglaterra, 2001. ChenWu, HUANG, Ke, Yang, Qiang, LIU, Lei, ZHANG, JingYan, BAI, JianZhong, XU. A study on performance influences of airfoil aerodynamicparameters and evaluation indicators for the roughness sensitivity on wind turbine blade. Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China, 2011.

Çengel, Yunus, A., Cimbala, John, M. Mecânica dos fluidos: Fundamentos e aplicações. 4ª Ed. McGraw Hill, 584p, 2007.

Ebert, P.R., Wood, D.H., 1997. Observations of the starting behavior of a small horizontal-axis wind turbine. Renew.

Energy 12, 245–257.

Eggleston, D. M. and Stoddard, F. S. Wind Turbine Enginering Design, Van Nostrand Reinhold Company, New York, 1987.

Epaarachchi, J. A., Clausen, P. D., The development of a fatigue loading spectrum for small wind turbine blade. Journal of Wind Engineering and Industrial Aerodynamics, p. 207-223, 2006.

Gharali, K., Johnson, David A., Numerical modeling of an S809 airfoil under dynamic stall, erosion and high reduced frequencies. Applied Energy 93 p. 45–52, 2012

Rasila, M., “Torque and Speed Control of a Pitch Regulated Wind Turbine”, Thesis for the Master of Science Degree,

Department of Electric Power Engineering, Chalmers University of Technology, Göteborg, Sweden, 2003.

Sheng, W, Galbraith, McD., Coton, F. On the S809 airfoil's unsteady aerodynamic characteristics, Wind Energy, 2009. Sommers, D. Design and Experimental Results for S809 Airfoil, NREL/SR-440-6918. 1994. Zhou, Y., Md., Mahbub, Alam, Yang, H.X., Guo, H., Wood, D.H. Fluid forces on a very low Reynolds number airfoil and their prediction. Schulich School of Engineering, University of Calgary, Canada, 2010.

Wang, Z.J., 2005. Dissecting insect flight. Annu. Rev. Fluid Mech. 37, 183–210. Wright, A.K., Wood, D.H., 2004. The starting and low wind speed behavior of a small horizontal axis wind turbine. J. Wind Eng. Ind. Aerodyn. 92, 1265–1279.

8. RESPONSIBILITY NOTICE The author(s) is (are) solely responsible for the printed material included in this paper.

Re C d

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Proceedings of ENCIT 2014 15th Brazilian Congress of Thermal Sciences and Engineering Copyright © 2014 by ABCM November 10-13, 2014, Belém, PA, Brazil

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