CFD studies of a 10 MW turbine equipped with trailing edge flaps

CFD studies of a 10 MW turbine equipped with trailing edge flaps 10th EAWE PhD Seminar on Wind Energy in Europe 28-31 October 2014, Orléans, France Dipl.-Ing. Eva Jost e.jost@iag.uni-stuttgart.de

Overview 1. Introduction 2. Objectives 3. Realisation of active flaps in CFD 1. CHIMERA technique 2. Grid deformation 4. 2D simulation and results 1. Trailing edge flap 2. Leading and trailing edge flaps 5. 3D simulation and results 6. Conclusion and outlook 2/24

Overview 1. Introduction 2. Objectives 3. Realisation of active flaps in CFD 1. CHIMERA technique 2. Grid deformation 4. 2D simulation and results 1. Trailing edge flap 2. Leading and trailing edge flaps 5. 3D simulation and results 6. Conclusion and outlook 3/24

Development in wind turbine size Figure: UpWind Final report, March 2011, www.upwind.eu 4/24

Theory of similarity Empirical scaling rules for wind turbines based on a similar turbine layout (tip speed ratio, profile selection, etc.) Parameter Proportionality Power ~R 2 Thrust ~R 2 Rotor mass ~R 3 Problem: Realisation of 10 to 20 MW turbines is hardly possible based on simple scaling Demand of new technologies to reduce loads, mass and load variations: Structure Control Aerodynamics Active trailing edge flaps 5/24

Active trailing edge flaps Reduction of dynamic load variations due to: Tower shadow Atmospheric boundary layer and turbulence Yawed inflow Basic functioning: Undisturbed inflow Disturbed inflow Ω Ω α wind wind approach velocity c, wind velocity v, rotational velocity u=ωr Figure top right: Joachim Heinz; Investigation of Piezoelectric Flaps for Load Alleviation using CFD ; M.Sc. Thesis; Riso DTU; 2009 6/24

Overview 1. Introduction 2. Objectives 3. Realisation of active flaps in CFD 1. CHIMERA technique 2. Grid deformation 4. 2D simulation and results 1. Trailing edge flap 2. Leading and trailing edge flaps 5. 3D simulation and results 6. Conclusion and outlook 7/24

Overall objectives CFD simulation of 10 MW wind turbines equipped with active trailing edge flaps to: validate the potential to reduce the dynamic turbine loading in a 3D CFD environment gain knowledge of optimal flap position and sizing research the needed deflection angles investigate unsteady effects in 2D and 3D apply control concepts based on the determination of loads or inflow measurement 8/24

Overview 1. Introduction 2. Objectives 3. Realisation of active flaps in CFD 1. CHIMERA technique 2. Grid deformation 4. 2D simulation and results 1. Trailing edge flap 2. Leading and trailing edge flaps 5. 3D simulation and results 6. Conclusion and outlook 9/24

Possibilities of active flap modeling in FLOWer Based on the CHIMERA technique [1]: 1. CHIMERA-flap Overlap of two seperate grids for airfoil and flap Based on grid deformation: 2. HP-flap Airfoil grid Flap grid Deformation algorithm based on hermite polynomials [2] 3. RBF Flap Deformation algorithm based on radial basis functions [3] 10/24

CHIMERA flap Two completely seperate grids, which are overlaped in the transition area from airfoil to flap Small gap between airfoil to geometrically realise the movement Introduction of overlap part in the airfoil grid to insure accurate CHIMERA interpolation Airfoil Overlap Flap + Movement can easily pescribed - Two grids with a high amount of cells Computation time - CHIMERA overlap on the airfoils surface Accuracy is reduced in a highly important area 11/24

Grid deformation: HP and RBF flap Deformed surface generated through rotation of the flap part around defined hinge point Computation of the final grid deformation based on undeformed and deformed surface Difference between both algorithms: HP: Deformation of grid nodes calculated based on i,j,k index of structured grid grid topology restrictions RBF: Deformation of grid nodes calculated based on x,y,z coordinates no restrictions even applicable for unstructured grids 12/24

Grid deformation - Distribution adaption at transition airfoil to flap Gap in surface discritisation due to rotation Unaccurate results in transition area from airfoil to flap Redistribution of nodes based on two sided stretching functions + Fewer amount of grid cells Airfoil Flap Flap + No inaccuracies due to CHIMERA Airfoil - Grid topology restrictions (HP flap) Surface discritisation 13/24

Overview 1. Introduction 2. Objectives 3. Realisation of active flaps in CFD 1. CHIMERA technique 2. Grid deformation 4. 2D simulation and results 1. Trailing edge flap 2. Leading and trailing edge flaps 5. 3D simulation and results 6. Conclusion and outlook 14/24

2D Results - Comparison of c p trailing edge flap case 15/24

2D Results - Comparison of c p trailing and leading edge flap case Measurements: B. Lambie; Aeroelastic Investigation of a Wind Turbine Airfoil with Self- Adaptive Camber ; Doctoral thesis; TU Darmstadt; 2011 16/24

Overview 1. Introduction 2. Objectives 3. Realisation of active flaps in CFD 1. CHIMERA technique 2. Grid deformation 4. 2D simulation and results 1. Trailing edge flap 2. Leading and trailing edge flaps 5. 3D simulation and results 6. Conclusion and outlook 17/24

3D Simulation of the RBF flap - DTU 10 MW reference wind turbine First functionality test in 3D to validate the setup with regard to the grid deformation and resolution in the flap area High deflection angle Large flap extension 4 different grids: blade, spinner, nacelle and background Turbulence model: Menter SST Inflow velocity v [m/s] 11.4 Rotational speed Ω [rpm] 9.6 Reynolds number Re[- ] 6.15 mio. Amount of grid cells N [-] 23.7 mio. Timestep in azimuth dφ 2 18/24

3D Simulation of the RBF flap - DTU 10 MW reference wind turbine Geometry of trailing edge flap: Spanwise extension: 82 to 95 % radius Chordwise extension: 10 % local chord length Static deflection angle: 10 Closed gaps between rotor blade and trailing edge flap 19/24

3D Results of the RBF flap - DTU 10 MW reference wind turbine Parameter without flap with flap Power [MW] 11.02 10.86 Thrust [MN] 1.77 1.89 Reduction of generated power caused by a to high deflection angle (increase of c w ) Vortex caused by change in circulation (red circle) Computation time: ~ 50,000 CPUh 20/24

Overview 1. Introduction 2. Objectives 3. Realisation of active flaps in CFD 1. CHIMERA technique 2. Grid deformation 4. 2D simulation and results 1. Trailing edge flap 2. Leading and trailing edge flaps 5. 3D simulation and results 6. Conclusion and outlook 21/24

Conclusion Different possibilities to realise active trailing or leading edge flaps have been examined in a 2D case: based on the CHIMERA technique: CHIMERA flap based on grid deformation (hermite polynomials): HP flap based on grid deformation (radial basis functions): RBF flap Both possibilities based on grid deformation needed enhancement in the intersection area from airfoil and flap to achieve accurate results. The RBF flap is considered favorable: less computational time than the CHIMERA flap greater variability regarding grid topology than the HP flap A 3D functionality test has been performed on the DTU 10 MW turbine in a 120 degree model in which all expected effects could be observed. 22/24

Outlook Extension of the redistribution algorithm for 3D cases 1st focus point: Load variations caused by the tower shadow Full 3D computations without flaps Determination of the variations in angle of attack 2D simulations to determine optimal flap deflection (steady and unsteady) Application of control algorithms Flap deflection angle as function of azimuth Based on monitoring points on the airfoil (Pitot tube, pressure taps) Based on the determination of loads 23/24

Sources [1] J.A. Benek; J.L. Steger; F.C. Dougherty; P.G. Buning, Arnold Engineering Development Center; Arnold AFS TN.; Chimera: A Grid-Embedding Technique ; Defense Technical Information Center; 1986 [2] K.-H. Hierholz; S. Wagner; Simulation of Fluid-Structure Interaction at the Helicopter Rotor ; ICAS 98 Proceedings; International Council of the Aeronautical Sciences; Stockholm; Sweden; and AIAA; Reston, VA; 1998 [3] M. Schuff; P. Kranzinger; Manuel Keßler; E. Krämer; Advanced CFD-CSD coupling: Generalized, high performant, radial basis function based volume mesh deformation algorithm for structured, unstructured and overlapping meshes ; 40th European Rotorcraft Forum; Southampton; UK; September 2014 24/24

Thank you for your attention. Questions?