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Institute of Aerodynamics and Gasdynamics
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 [email protected]
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Institute of Aerodynamics and Gasdynamics
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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
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Institute of Aerodynamics and Gasdynamics
3/24
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
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Institute of Aerodynamics and Gasdynamics
4/24
Development in wind turbine size
Figure: UpWind – Final report, March 2011, www.upwind.eu
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Theory of similarity
Empirical scaling rules for wind turbines based on a similar turbine layout (tip speed ratio, profile selection, etc.)
Parameter Proportionality Power ~R2
Thrust ~R2
Rotor mass ~R3
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
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Active trailing edge flaps
• Reduction of dynamic load variations due to: • Tower shadow • Atmospheric boundary layer and turbulence • Yawed inflow
• Basic functioning:
α
wind
Ω
Undisturbed inflow
Disturbed inflow
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
wind
Ω
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Institute of Aerodynamics and Gasdynamics
7/24
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
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Institute of Aerodynamics and Gasdynamics
8/24
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
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Institute of Aerodynamics and Gasdynamics
9/24
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
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Institute of Aerodynamics and Gasdynamics
10/24
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 Deformation algorithm based on
hermite polynomials [2] 3. RBF Flap Deformation algorithm based on
radial basis functions [3]
Airfoil grid
Flap grid
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Institute of Aerodynamics and Gasdynamics
11/24
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
+ 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
Airfoil
Overlap
Flap
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Institute of Aerodynamics and Gasdynamics
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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
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Institute of Aerodynamics and Gasdynamics
13/24
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
Airfoil
Flap
Airfoil
Flap
Surface discritisation
+ Fewer amount of grid cells + No inaccuracies due to CHIMERA - Grid topology restrictions (HP flap)
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Institute of Aerodynamics and Gasdynamics
14/24
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
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Institute of Aerodynamics and Gasdynamics
15/24
2D Results - Comparison of cp trailing edge flap case
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Institute of Aerodynamics and Gasdynamics
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2D Results - Comparison of cp 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
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Institute of Aerodynamics and Gasdynamics
17/24
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
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Institute of Aerodynamics and Gasdynamics
18/24
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
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°
• 4 different grids: blade, spinner, nacelle and background
• Turbulence model: Menter SST
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Institute of Aerodynamics and Gasdynamics
19/24
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
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Institute of Aerodynamics and Gasdynamics
20/24
3D Results of the RBF flap - DTU 10 MW reference wind turbine
• Reduction of generated power caused by a to high deflection angle (increase of cw)
• Vortex caused by change in circulation (red circle)
• Computation time: ~ 50,000 CPUh
Parameter without
flap with flap
Power [MW] 11.02 10.86
Thrust [MN] 1.77 1.89
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Institute of Aerodynamics and Gasdynamics
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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
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Institute of Aerodynamics and Gasdynamics
22/24
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.
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Institute of Aerodynamics and Gasdynamics
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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
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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
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Institute of Aerodynamics and Gasdynamics
Thank you for your attention. Questions?
