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OpenFOAM in Wind Energy
GOFUN 2018, Braunschweig
Matthias Schramm
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Fundamental research
Applied research
Fraunhofer IWES and ForWind
Oldenburg University started with wind physics
Research on wind fields, aerodynamics and turbulence
CFD is the link between the groups
Fraunhofer IWES transfers knowledge to industry
Teamwork
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Agenda
Airfoil aerodynamics
Rotor aerodynamics
Site assessment
Other topics
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2D-RANS
Steady 2D-RANS → computationally cheap
Fully resolved boundary layer
Hexahedral meshes
Use of different turbulence models
k–ω-SST
Spalart-Allmaras
γ-Reθt-SST
k–ω-SST-γ
Automatized polars via scripts for small and medium angles of attack (not 360°)
Lift and drag polars using improved coefficients w.r.t. the standards
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Stall Prediction in 2D-RANS
Calibrated coefficients for better stall prediction Tripped airfoil at Re=3·106 k–ω-SST
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Standstill with 2.5D-DDES
Source Cl / - Cd / -
IWES CFD 0.09 1.7
Experiments 0.11 1.9
DTU CFD 0.09 2.3
URANS not suitable for fully separated flow
DDES can improve accuracy for very high angles of attack (standstill)
360°-polars with DDES
DU 96-W-180, Re=2·106, 3 million cells
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Modelling of vortex generators in CFD
Better performance in blade root region Installed as Add-Ons on existing blades Not always included in design phase Use of BAY model (1999) Source term in momentum equation
www.högrehöjder.com
www.3m.com/wind
© micro Aerodynamics
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Modelling of vortex generators in CFD
www.högrehöjder.com
www.3m.com/wind
© micro Aerodynamics
Exp.
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Agenda
Airfoil aerodynamics
Rotor aerodynamics
Site assessment
Other topics
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BladeBlockMesher
Automatic, fast meshing tool for wind turbine rotor blades developed by IWES
Only airfoil coordinates and their position required
Based on hexahedral meshes (elliptic or hyperbolic equations)
Rahimi, H., Daniele, E., Stoevesandt, B., Peinke, J.: Development and application of a grid generation tool for aerodynamic simulations of wind turbines. Wind Engineering, 40(2), 148-172 (2016) doi: 10.1177/0309524X16636318
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Very few experiments exist
Limited wind tunnel size
Limited Reynolds numbers
Validation of CFD: Wind Turbine Rotors
NREL Phase VI turbine (10 m diameter) MEXICO turbine (4.5 m diameter)
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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’,
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m/s
at
30
%
Validation of CFD: NREL Phase VI
Stall regulated turbine (10 m diameter) Upwind and downwind measurements in NASA-Ames wind tunnel Experimental pressure distribution and loads for different sections available
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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
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Our FSI approach
FSI framework developed in Oldenburg
OpenFOAM and additional implementations
Steady-state or dynamic simulations
Runtime post-processing (AoA)
In-house grid deformation
Finite Element framework
Geometrically exact beam theory (GEBT)
Supports large deformations and torsion
6x6 section properties
+
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NREL 5 MW turbine under yawed inflow
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Clear effect on aerodynamic loading
Rigid CFD under-predicts forces
Blade deformations have clear effect
Tangential
NREL 5 MW turbine under yawed inflow
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Big drawback of downwind turbines: Blade-tower interaction
Idea: Use lattice structure towers instead of tubular towers
NREL 5 MW in downwind configuration
NREL 5MW Downwind turbine and tower shadow
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Time-accurate Delayed-Detached Eddy Simulation (DDES)
Comparison of sectional blade loading for both tower types
Fluid-structure coupling for blades for higher fidelity
NREL 5MW Downwind turbine and tower shadow
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Similar mean values (deviation < 1%)
Clear deviation in time resolved results
Wider tower shadow with lattice tower
Significant velocity drop for lattice tower
2D approach for lattice tower not valid
Downwind turbine and tower shadow
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Agenda
Airfoil aerodynamics
Rotor aerodynamics
Site assessment
Other topics
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Site assessment
Optimize the yearly energy output
Wind rose is site dependent (sometimes 360° necessary)
Consideration of Coriolis force, complexity of terrain, forests etc.
Thermal stratification needs to be considered (stability classes)
Chang, Chi-Yao, et al. "A consistent steady state CFD simulation method for stratified atmospheric boundary layer flows." Journal of Wind Engineering and Industrial Aerodynamics 172 (2018): 55-67.
stable neutral unstable
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Inclusion of forests
Automatic detection of forest from simple graphics file
Use of porous cells as sinks in the momentum equation
Also turbulence equations need to be considered
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Inclusion of forests
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Automatic detection of forest from simple graphics file
Use of porous cells as sinks in the momentum equation
Also turbulence equations need to be considered
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Simulation
Reference
Wind distribution
Prediction of wind distribution at other location
Validation by met mast data
Simulation of complex terrain
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Prediction at
different point
Wind distribution
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Reference
Prediction of wind distribution at other location
Validation by met mast data
Simulation of complex terrain
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In-house mesh generation (incl. smoothing)
Automatic mesh refinement
Modelling turbines as actuator disks
CFD of wind farms
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Agenda
Airfoil aerodynamics
Rotor aerodynamics
Site assessment
Other topics
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Other topics
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Shape optimization using the adjoint approach
Aeroacoustics
Turbulent inflow
Add-Ons
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Wave interaction
Multi-phase flow for offshore turbines
Tower design and load computation
Volume-of-Fluid approach (olaFoam)
20 million cells
7 m diameter
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OpenFOAM offers possibility to simulate every interesting CFD in wind energy
Airfoil & rotor aerodynamics
Aeroacoustics
Fluid-Structure Interaction
Turbulent inflow
Site assessment
Wave forces on turbine towers
Some in-house developments and extensions necessary
Conclusions
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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
Free and Hanseatic City of Hamburg
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