DUWIND, Delft University Wind Energy Institute 1 An overview of NACA 6-digit airfoil series characteristics with reference to airfoils for large wind turbine Nando Timmer DUWIND Delft University Wind Energy Institute The Netherlands

DUWIND, Delft University Wind Energy Institute 2 Outline Introduction Measurements in LTPT Comparison with RFOIL calculations (Maximum) lift Drag Roughness effects Conclusions

DUWIND, Delft University Wind Energy Institute 3 Introduction Large machines have blades performing at Reynolds numbers up to 9 to 10 million Many dedicated wt airfoils have not been tested at these Re-numbers Testing at these Re-numbers is relatively expensive If blade designers do not want to spend this amount of money they have to rely on the predictive value of codes like XFOIL and/or CFD

DUWIND, Delft University Wind Energy Institute 4 Introduction (cntd) NACA airfoils were tested in the Langley LTPT up to Re=9x10 6 and can be used to verify the predictions. Main question in this presentation is: How good are these data anyway and how well can we predict them with RFOIL. (as a typical example we investigate the 18% thick airfoil from the NACA 63 and 64 series)

DUWIND, Delft University Wind Energy Institute 5 LTPT measurements Test section 3x7.5 feet (0.914 m x 2.29 m) Model chord 2 feet (0.61 m) Maximum velocity at atmospheric pressure is 130 m/s Maximum Mach number during the tests was 0.17 Models were made of laminated mahogany Lift from the pressure reaction on the walls (over a length of 13 feet – 3.96 m), drag from a wake rake. Basic wind tunnel wall corrections were applied

DUWIND, Delft University Wind Energy Institute 6 RFOIL Basically XFOIL Improvement of the numerical stability by using the Schlichting velocity profiles for the turbulent boundary layer instead of Swafford’s the shear lag coefficient in Green’s lag entrainment equation of the turbulent boundary layer model was adjusted Deviation from the equilibrium flow was coupled to the shape factor of the boundary layer

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DUWIND, Delft University Wind Energy Institute 24 Chord (m) Span (m) C/S LM LTPT Test section top view wall Separation line chord ABC span LM wind tunnel test setup

DUWIND, Delft University Wind Energy Institute 25 T.E L.E Stall cells Pressure orifices

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DUWIND, Delft University Wind Energy Institute 27 Drag

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DUWIND, Delft University Wind Energy Institute 30 Roughness

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DUWIND, Delft University Wind Energy Institute 33 Roughness?

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DUWIND, Delft University Wind Energy Institute 35 Reduction 18% - 20%

DUWIND, Delft University Wind Energy Institute 36 Roughness configurations NACA wrap-around roughness (no. 60 grid distributed sparsely from 8% at the lower surface to 8% on the upper surface (worst case?) NASA roughness (no. 80 grid strips, 2.5 cm wide at both the upper and lower surface 8% chord stations Zigzag tape, various thicknesses and positions Fixed transition on the leading edge in calculations

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DUWIND, Delft University Wind Energy Institute 40 Conclusions The measured zero-lift angle of several NACA airfoils needs to be adjusted with absolute values ranging from 0.4 to 1 degree The maximum lift coefficients predicted with RFOIL match the LTPT data well at Re=3x10 6, but under predict the C l,max at 6x10 6 by 3.5% up to 6.5% at Re=9x10 6 Though it may be possible that the higher C l,max in the LTPT data partly originates from the wall pressure method, RFOIL also under predicts the maximum lift measured with surface pressures.

DUWIND, Delft University Wind Energy Institute 41 Conclusions (cntd) RFOIL consistently under predicts the drag coefficient with about 9% for a wide range of airfoils and Reynolds numbers NACA standard roughness causes a reduction in the lift coefficient of 18% to 20% for 18% thick airfoils from the NACA 64-series The effect on airfoil performance of various types of roughness has been measured in the past, but it is unclear what type of roughness may be expected, though wrap-round roughness may serve as a worst-case scenario

DUWIND, Delft University Wind Energy Institute 42 How to proceed? Roughness investigations in the wind tunnel at the appropriate Reynolds numbers and field tests with zigzag tape on the blades are necessary to be able to better quantify the effect of blade soiling on the rotor performance Side-by side tests are necessary to better understand the amount of soiling during operation.