AEROELASTIC TOOLS FOR 2D AEROFOILS WITH VARIABLE GEOMETRY FOR WIND TURBINE APPLICATIONS

1. AEROELASTIC TOOLS FOR 2D AEROFOILS WITH VARIABLE GEOMETRY FOR WIND TURBINE APPLICATIONS • A. González1*, X. Munduate1, R. Palacios2, J.M.R. Graham2 • Wind Energy Department, CENER, Ciudad de la Innovación, 7, Sarriguren, Spain, 31621 • Department of Aeronautics, Imperial College, London, SW72AZ, UK • *e-mail: agonzalez@cener.com

2. Introduction Aeroelastic tools Results Conclusion and future work

3. Introduction Rotor diameter increase • New technical challenges: • Increased loads • Aeroelastic performance Passive load alleviation Rotational speed control Blade pitch control INSUFFICIENT Ref. UpWind Project Solution: Smart rotor control Blade sections of changing geometry (flaps, variable camber…) • Effective load control: • Aerodynamic efficiency • Blade cost problem Lack of aeroelastic tools!

4. Introduction CENER ICL Development of unsteady aeroelastic models of aerofoils, including variable geometry + • Levels of modelling of aerofoils with variable geometry: • Classical thin aerofoil theory: • Engineering tools of with very good computational efficiency for preliminary purposes • In-house code AdaptFoil1D (validated) • Panel methods: • Good balance between computational efficiency and accuracy • In-house code AdaptFoil2D (currently in development) • CFD: • Detailed representation of aerofoil and flow, but much higher computational cost

5. Aeroelastic tools • AdaptFoil1D: • Thin aerofoil theory – Linear kinematics • Based on the work of Peters * • Attached flow • Prescribed deformation: • Large 3-dof rigid-body motion allowed (small effective angle of attack) • Modifications of the meanline geometry under the small displacements assumption • Body dynamics: • 3-dof system of springs and dampers at a single point for rigid body motion • Additional deformation of LE or TE flap, using a static Euler-Bernoulli approach * Peters, D.A., Johnson, M.J., Finite-State airloads for deformable aerofoils on fixed and rotating wings, AD-Vol. 44, Aeroelasticity and Fluid Structure Interaction Problems. * Peters, D.A., Hsieh, A., and Torrero, A., A state-space airloads theory for flexible aerofoils, In Proceedings of the American Helicopter Society, 62nd Annual Forum, Phoenix, AZ, USA, 2006.

6. Aeroelastic tools • AdaptFoil2D: • Panel methods * - Surface panel code for a thick aerofoil section • Piecewise constant doublets and sources in each panel • The Neumann and Dirichlet conditions are combined • Kutta condition: tangential velocity difference between the upper and lower panels at the TE = shed vorticity • The wake is a doublet panel attached to the TE, transformed into discrete vortices downstream • Free wake and time-stepping method to calculate the wake roll-up • Wake vortices implemented with a lamb vortex core to avoid numerical problems • Attached flow • Prescribed deformation & body dynamics: Idem AdaptFoil1D * Katz, J., Plotkin, A., Low-speed aerodynamics, Cambridge Aerospace Series, 2001.

7. Results 1. Steady aerodynamics • NACA 0015, α=6º Excellent agreement between AdaptFoil2D and XFoil * * Drela, M., XFOIL: An analysis and design system for low Reynolds number airfoils, Conference on Low Reynolds Number Airfoil Aerodynamics, University of Notre Dame, 1989.

8. Results 2. Unsteady aerodynamics • Flat plate (NACA 0003) performing a sudden acceleration, α=5º. Comparison between AdaptFoil2D, AdaptFoil1D and a lumped and discrete vortex methods * Excellent comparison. Convergence to the steady values. Inaccuracies for lumped vortex method. * Katz, J., Plotkin, A., Low-speed aerodynamics, Cambridge Aerospace Series, 2001.

9. Results 2. Unsteady aerodynamics • Flat plate (NACA 0003) performing a sudden acceleration, α=5º

10. Results 2. Unsteady aerodynamics • NACA 0012, combined pitching and oscillating TE flap • (specific case: ωα=0.021, ωβ=0.042, δ=59º) …compared with experimental data * Good overall agreement for a combined pithing and oscillating TE flap * Krzysiak, A., Narkiewicz, J., Aerodynamic loads on aerofoil with trailing-edge flap pitching with different frequencies, Journal of aircraft, 43(2):407-418, 2006.

11. Results 2. Unsteady aerodynamics • NACA 0012, combined pitching and oscillating TE flap • (specific case: ωα=0.021, ωβ=0.042, δ=59º)

12. Results 3. Aeroelastic modelling • Plunge-pitch flat plate (NACA 0003): aerofoil radius of gyration (rα)2=0.25, aerodynamic centre a=-0.3, inverse mass ratio, κ=0.05) …compared with data given by Zeiler * Good agreement. Divergence not calculated by Zeiler. Minor deviations for χα=0.2 at high ωh/ ωα * Zeiler, T.A., Results of Theodorsen and Garrick revisited, Journal of Aircraft Engineering Notes, 37(5):918-920, 2000.

13. Conclusions and future work • Conclusions: • Development and validation of AdaptFoil2D, new panel code for deformable aerofoils • Mostly successful validation • Steady and unsteady aerodynamic and aeroelastic computations • Fast and reliable tool for evaluation of the aeroelastic performance of 2D aerofoils • AdaptFoil1D and AdaptFoil2D are suitable for design of aerodynamic control on wind turbine blades • Further work: • Extenssion of AdaptFoil2D to unsteady • separated flow and dynamic stall conditions

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