NREL/SR-440-6917 • UC Category: 1213 • DE97000105 Design and Experirltental Results for the S805';l~},f\.irfoil Dan M. Somers Aiifoils, Incorporated State College, Pennsylvania NREL technical monitor: James TangIer National Renewable Energy Laboratory 1617 Cole Boulevard Golden, Colorado 80401-3393 A national laboratory of the U.S. Department of Energy Managed by Midwest Research Institute for the U.S. Department of Energy under contract No. DE-AC36-83CHlO093 Work performed under task number WE7llll 0 January 1997 !"'II!!"'-.-"", OF THiS DOCUMENT IS UNUMiTED 1 NOTICE This report was prepared as an account of work sponsored by an agency of the United States government. 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Box 62 Oak Ridge, TN 37831 Prices available by calling (423) 576-8401 Available to the public from: National Technical Information Service (NTIS) U.S. Department of Commerce 5285 Port Royal Road Springfield, VA 22161 (703) 487-4650 ¥. '.~ Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. Foreword Validation of the Eppler Airfoil Design and Analysis Code has been a major goal of several NREL sponsored, two-dimensional investigations in the low-turbulence wind tunnel of the Delft University of Technology Low Speed Laboratory, The Netherlands. Initial validation of the code with respect to wind turbine airfoils was based on data acquired for a low maximum-lift-coefficient airfoil of a thin-airfoil family. This test was conducted in 1985 upon completion of the design effort for a thin-airfoil family for stall-regulated rotors. The primary airfoil of this family, the 13.5-percent-thick S805, was tested and the results showed that the Eppler Code predicted all the section characteristics well except the profile-drag coefficient. The drag coefficient was underpredicted as a result of underestimating the significance of the laminar separation bubbles, through which the laminar flow transitioned to turbulent flow. As a result of this test, an adjustment to the design methodology that accounted for this bias error was used to modify the S805 airfoil to alleviate the strength of the laminar separation bubbles. The resulting airfoil, the S805A, replaces the S805 airfoil. The use of the Delft University of Technology low-turbulence wind tunnel was essential to quantify the effect of the laminar separation bubbles on the section characteristics. In most wind tunnels, intense small scale turbulence generated by the fan suppresses the laminar separation bubbles. This results in optimistic performance measurements relative to those obtarned in the free atmosphere. Intense, small-scale turbulence that interacts with the boundary layer on the airfoil is not present in the free atmosphere. Atmospheric turbulence scales are orders of magnitude larger than the turbulence that exists in a wind tunnel. The Delft University of Technology low-turbulence wind tunnel is recognized worldwide as having the lowest (small-scale) turbulence level (less than 0.05 percent) of any commercially available wind tunnel and is thus better able to simulate free-atmosphere conditions. ~~.~ t1amesLTangler Wind Technology Division National Renewable Energy Laboratory 1617 Cole Blvd. Golden, Colorado 80401 USA Internet Address: [email protected] Phone 303-384-6934 FAX 303-384-6901 Contents Page Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Introduction ......................................................... 1 Symbols and Abbreviations ................................................ 2 Airfoil Design ....................................................... . 3 ExperiInental Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Discussion of Results 8 Concluding Remarks 12 AcknowledgIllents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 13 References .......................................................... 13 Table 1. Airfoil Design Specifications ........................................ 14 Table 2. S805 Airfoil Coordinates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 15 Table 3. Model Orifice Locations ........................................... 16 Table 4. Roughness Size and Location ....................................... , 18 List of Figures Page 1. lnviscid pressure distributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2. S805 airfoil shape ........................................... . 21 3. Delft University of Technology 1.80-x 1.25-m low-speed wind tunnel ..... 22 t •••••• 4. Model and wake rakes mounted in test section. All dimensions are in mm . . . . . . . . 23 5. Photograph of wake rakes mounted on strut . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 6. Wake rakes ............................................... . 25 7. Static-pressure, integrating, and total-pressure wake-rake tubes. All dimensions are inmm ................................................. . 26 8. Pressure distributions for R = 1,000,000. Arrows indicate direction of angle-of-attack change (for determination of hysteresis) ................... . Z7 9. Oil-flow photographs of upper surface for R = 1,000,000 .................. . 38 10. Oil-flow photographs of upper surface for R = 2,000,000 ................... . 44 11. Oil-flow photographs of lower surface for R = 1,000,000. ................. . 48 12. Oil-flow photograph of lower surface at u = 0.0 degrees for R = 2,000,000 ...... . 52 13. Transition location. Bars extend from beginning to end of transition. . . . . . . . . . . . . 53 14. Spanwise drag coefficients for R = 1,000,000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 15. Section characteristics. .. ............................ . . . . . . . . . . . . . 62 16. Effects of Reynolds number on section characteristics . . . . . . . . . . . . . . . . . . . . . . . 67 17. Effect of roughness on section characteristics ............................. . 68 18. Effect ofturbulators on section characteristics for R = 1,000,000 ............... . 73 19. Comparison of theoretical and experimental pressure distributions .............. . 74 20. Comparison of theoretical and experimental section characteristics with transition free . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 21. Comparison of theoretical and experimental section characteristics with transition fIxed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 22. Comparison of section characteristics of S805 and NACA 4412 and 4415 airfoils for R = 1,000,000 ..................................... . 87 23. Comparison of section characteristics of S805 and NACA 23012 and 23015 airfoils for R = 1,000,000 ..................................... . 88 24. Comparison of maximum lift coefficients of S805 and NACA 4412, 4415, 23012, and 23015 airfoils . . ..................................... . 89 25. Comparison of drag coefficients at lift coefficient of 0.7 of S805 and NACA 4412,4415,23012, and 23015 airfoils ............................... . 90 Design and Experimental Results for the S805 Airfoil t Dan M. Somers October 1988 Abstract An airfoil for horizontal-axis wind-turbine applications, the S805, has been designed and analyzed theoretically and verified experimentally in the low-turbulence wind tunnel of the Delft University of Technology Low Speed Laboratory, The Netherlands. The two primary objectives of restrained maximum lift, insensitive to roughness, and low profile drag have been achieved. The airfoil also exhibits a docile stall. Comparisons of the theoretical and experimental results show good agreement. Comparisons with other airfoils illustrate the restrained maximum lift coefficient as well as the lower profile-drag coefficients, thus confirming the achievement of the primary objectives. Introduction The majority of the airfoils in use on horizontal-axis wind turbines today were originally developed for airplanes. The design requirements for these airfoils, primarily National Advisory Committee for Aeronautics (NACA) and National Aeronautics and Space Administration (NASA) airfoils (refs. 1-6), are significantly different from those for wind-turbine. airfoils. Accordingly, two sets of airfoils were designed, using the method of references 7 and 8, specifically for horizontal-axis wind-turbine applications. (See ref. 9.) The most prominent difference between the two sets is the relatively low ("restrained") maximum lift coefficients specified for the primary and tip airfoils (0.75 and 0.95 blade radial stations, respectively) of the second set. In conjunction with this effort, the primary airfoil of the second set was selected for experimental verification. In 1985, an investigation was conducted in the low-turbulence wind tunnel of the Delft University of Technology Low Speed Laboratory (ref. 10), The Netherlands, to obtain the basic, low speed, two-dimensional aerodynamic characteristics of this airfoil. The results have been compared with the predictions from the method of references 7 and 8 and also with data from another low-turbulence wind tunnel for other airfoils. The specific tasks performed under this study are described in Solar Energy Research Institute (SERI) Subcontract Number HK-4-04148-01. t President, Airfoils, Incorporated, State College, Pennsylvania 1 Symbols and Abbreviations Values are given in both SI and U.S. Customary Units. Measurements and calculations were made in SI Units. C pressure coefficient p c airfoil chord, mm Cd section profile-drag coefficient c section lift coefficient 1 c section pitching-moment coefficient about quarter-chord point m d streamwise distance from model trailing edge to tips of wake-rake total-pressure tubes, mm DFVLR Deutsche Forschungs-und Versuchsanstalt fUr Luft- und Raumfahrt max maximum (subscript) NACA National Advisory Committee for Aeronautics R Reynolds number based on free-stream conditions and airfoil chord T transition (subscript) V free-stream velocity, m1s wakerake wake rake (subscript) x airfoil abscissa, mm y span station, mm z airfoil ordinate, mm c:t angle of attack relative to chord line, deg 2 Airfoil Design Objectives and Constraints Two primary objectives are evident from the design specifications for this airfoil (table 1). The first objective was to achieve a maximum lift coefficient that is relatively low (restrained). A requirement related to this objective was that the maximum lift coefficient not decrease with transition fixed near the leading edge on both surfaces. The second objective was to obtain low profIle-drag coefficients over the range of lift coefficients from 0.5 to 0.9 for a Reynolds number of 1.0 x 106• Two major constraints were placed on the design of this airfoil. First, the zero-lift pitching-moment coefficient must be no more negative than -0.05. Second, the airfoil thickness should fall within the specified range. Philosophy Given the above objectives and constraints, certain characteristics of the design are evident. The following sketch illustrates the desired polar that meets the goals for this design. 1.2. .9 B .5 A o Cd. Sketch 1 The desired airfoil shape can be related to the pressure distributions that occur at the various points in the sketch. Point A is the lower limit of the laminar bucket; point B, the upper limit. The values of the drag coefficients at both points are nearly equal and are determined by the extents of laminar flow on the upper and lower surfaces. The drag increases very rapidly outside the laminar bucket because the transition point moves quickly toward the leading edge. This feature results in a rather sharp leading edge that produces a suction peak at the higher lift coefficients. This peak limits the maximum lift coefficient and assures that transition will occur very near the leading edge. Thus, the maximum lift coefficient occurs with turbulent flow along the entire upper surface, and, therefore, the addition of roughness at the leading edge should 3 have little influence on the boundary-layer development along the upper surface and, accordingly, the maximum lift coefficient. This outline of the desired section characteristics is not sufficient to design the airfoil, however, primarily because of the inexactly specified airfoil thickness. Accordingly, the thickness was selected to be equal to that of the S801 airfoil (ref. 9), 13.5-percent chord. Because the selected airfoil thickness allows a wider laminar bucket to be achieved than that specified, point A, which occurs at a lift coefficient of 0.5, should not correspond to the lower limit of the bucket but, instead, to a point near the middle of the bucket. From the preceding discussion, the pressure distributions at points A and B can be deduced. The pressure distribution at point A should look something like this: o .5 /.0 X/c Sketch 2 To achieve low drag, a favorable pressure gradient is desirable along the upper surface to about O.Sc. Aft of this point, a short region of slightly adverse pressure gradient ("transition ramp") is desirable to promote the efficient transition from laminar to turbulent flow. Thus, the initial slope of the pressure recovery is relatively shallow. This short region is followed by a steeper concave pressure recovery that produces lower drag and has less tendency to separate than the corresponding linear or convex pressure recovery (ref. 11). A slightly favorable pressure gradient is desirable along the lower surface to about 0.65c to achieve low drag. The initial slope of the pressure recovery is very shallow in order to inhibit the formation of significant laminar separation bubbles. The amounts of pressure recovery on the two surfaces are determined by the pitching-moment constraint and the airfoil thickness. 4