CFD development for wind energy aerodynamics Hamid Rahimi, Bastian Dose, Bernhard Stoevesandt Fraunhofer IWES, Germany IEA Task 40 Kick-off Meeting 12.11.2017 Tokyo
Agenda BEM vs. CFD for wind turbine simulations High fidelity simulations of full rotors Improving BEM by CFD Yawed inflow Tower shadow modeling Conclusion 2
Aerodynamic Design-Tool: Blade Element Momentum (BEM) Main aerodynamic design method for wind turbines Reasonable results for most load cases Quick enough to run many design cases 3
Aerodynamic Design-Tool: BEM Simple theory is improved by using correction models Dynamic Stall Stall delay Tip correction Yawed inflow Tower shadow 4
Aerodynamic Design-Tool: BEM Rotor designs of large scale turbine fall outside the validated range of BEM Simplified 2D approach High Reynolds number Thick(er) airfoils Increased flexibility, non-linear aeroelastic behavior Possible active and/or passive flow devices Yawed inflow Tower shadow 5
Computational Fluid Dynamics (CFD) High fidelity No empirical corrections models required Complex phenomena like stall delay included in NSE However: Computational expensive Not suitable for calculation of DLCs CFD can be used to Recalibrate existing correction models Development of new models 6
Validation of CFD CFD requires validation Validation performed on several experimental turbines Current work: IEA Task 29 MexNext In depth validation for yawed and axial inflow OpenFOAM CFD code is used 7
Validation of CFD: NREL Phase VI 10m rotor diameter, stall regulated turbine Upwind and downwind measurements in NASA wind tunnel Pressure, loads for different sections as experimental data available 25m/s at 30% Rahimi, H., Medjroubi, W., Stoevesandt, B. and Peinke, J. (in press) Progress in Computational Fluid Dynamics, Navier-Stokes-based predictions of the aerodynamic behaviour of stall regulated wind turbines using OpenFOAM, 8
CFD for modern wind turbine rotors Wind turbines are getting larger Light weight blade design Blade flexibility increased Non-linear interaction between aerodynamics and structure Fluid-Structure Interaction (FSI) Coupling of flow and structural solver Source: Siemens 9
Our FSI approach FSI framework developed in Oldenburg Open source CFD toolbox OpenFOAM Steady-state or dynamic simulations Runtime post-processing (AoA) Inhouse grid deformation Finite Element framework Geometrically exact beam theory (GEBT) Supports large deformations and torsion 6x6 section properties + 10
Example: NREL 5 MW subjected to yawed inflow 11
Example: NREL 5 MW subjected to yawed inflow Clear effect on aerodynamic loading Rigid CFD underpredicts forces Blade deformations have clear effect Tangential 12
How to improve BEM? High fidelity CFD of full rotors possible How to reduce gained information to an engineering model? Isolation of investigated effect Simulation matrix Fitting of coefficients Examples: Yawed inflow Tower shadow 13
Skewed wake model and BEM NREL VI Average yaw misalignment between 2 and 10 degrees Azimuthal variation of the loads fatigue blade loads Wrong prediction of yawing moment 14
Skewed wake model and BEM Glauert model is only true at the tip 1 sin Many attempts are made in the past to improve the k function Root vortex also induces axial velocities which is not included in the current correction models Upwind side Downwind side 15
Results - INNWIND 6 m/s at 20 yaw 50% Note: Focus is on varition of indution and moment over azimuth, Level is irrelevant for this discussion 16
Results - NREL 5MW: U=6 m/s, Yaw=20 17
Downwind turbine: Tower shadow NREL 5MW How accurate are the current rotor tower interaction models in BEM? Significant discrepancies between different models Can we improve them by CFD? How about different tower types? 18
Downwind turbine: Tower shadow NREL 5MW Big drawback of downwind turbines: Blade-tower interaction Idea: Use lattice structure towers instead of tubular towers NREL 5 MW in downwind configuration 19
Downwind turbine: Tower shadow NREL 5MW Time-accurate Delayed-Detached Eddy Simulation (DDES) Comparison of sectional blade loading for both tower types Fluid-structure coupling for blades for higher fidelity 20
Downwind turbine: Tower shadow Mean values for power and thrust very similar - Deviation < 1% Clear deviation in time resolved results Tower shadow for lattice tower more wide Significant velocity drop for lattice tower 21
Downwind turbine: Tower shadow However: Fair comparison of geometries difficult BEM results generally show opposite trend 2D approach for lattice tower valid? Lattice tower shows strong vertical mixing Work in progress: Highly resolved simulation of isolated tower geometries 22
Future work Investigation of inflow turbulence on rotor performance Simulation of smart load alleviation methods 23
Conclusions CFD suitable to improve BEM engineering models High fidelity framework for full rotor simulations presented Fluid-structure coupling for large, flexible blades Improvement of skewed wake correction based on CFD Investigation of tower shadows (tubular vs. lattice) Results indicate that 2D tower shadow might not be valid 24
Acknowledgements Fraunhofer IWES is funded by the: Federal Republic of Germany Federal Ministry for Economic Affairs and Energy Federal Ministry of Education and Research European Regional Development Fund (ERDF): Federal State of Bremen Senator of Civil Engineering, Environment and Transportation Senator of Economy, Labor and Ports Senator of Science, Health and Consumer Protection Bremerhavener Gesellschaft für Investitions- Förderung und Stadtentwicklung GmbH Federal State of Lower Saxony 25 / 51
Thank You For Your Attention Any questions? Hamid.rahimi@iwes.fraunhofer.de Bastian.dose@iwes.fraunhofer.de 26 / 51