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© 2009 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary
Application of CFD for
Simulation of Wind
Energy Converters
André Braune
ANSYS Continental
Europe
© 2009 ANSYS, Inc. All rights reserved. 2 ANSYS, Inc. Proprietary
Overview
• CFD simulations for
wind energy converters
– Blade design aspects
• Profile design
• Loads for FSI
• Turbulence
• Acoustics
– Siting & terrain modeling
– Cooling of generator
housing
© Siemens Wind Power
© 2009 ANSYS, Inc. All rights reserved. 3 ANSYS, Inc. Proprietary
Computational Fluid Dynamics for
Structural
blade design
Terrain modeling
Wind park
design
Tower design Housing & base
cooling
Acoustics
Aerodynamic
blade design
Generator
design
© Kato Engineering
© 2009 ANSYS, Inc. All rights reserved. 4 ANSYS, Inc. Proprietary
Blade Design
• Challenges
– Aerodynamic efficiency
across expected wind
speeds and wind profiles
– Determine integrity of
structures made of
complex composite
materials
– Minimize noise
– Maximize strength while
minimize weight
• Benefits of simulations
– Virtual prototyping of
initial candidate designs
– Reduced wind tunnel and
full scale testing
– Automation of design
process
– Fewer prototypes &
lower design costs
– Multi-physics
simulations
© 2009 ANSYS, Inc. All rights reserved. 5 ANSYS, Inc. Proprietary
Aerodynamic Blade Design
• Main aspects:
• Design of 2D profiles 3D blades– Advanced turbulence
modeling: • SST turbulence model
• Laminar to turbulent transition model
• Roughness effects
• Tip vortices
• Scale resolving simulation(LES, SAS …)
– Interaction with upstreamturbines
– Design studies & optimization
Photo © José Luis Gutiérrez, graphic courtesy of IMPSA S.A., Argentina
© 2009 ANSYS, Inc. All rights reserved. 6 ANSYS, Inc. Proprietary
Transition: 2D S809 Airfoil
• Laminar flow airfoil for wind turbine applications
• Rex = 2 106, = 0° 20°
• Experiment:– 2D: low-turbulence wind tunnel
@ Delft University of Technology, (Somers, 1989)
– 3D: profile for the NREL phase IV full wind turbine experiment, (Simms, 2001)
• ANSYS CFD– Transitional and fully turbulent
– Grid: 150 000 elements (2D)
– Max. y+ 1Sommers, D. M., 1989, “Design and Experimental Results for the S908
Airfoil”, Airfoils, Inc., State College, PA
Simms, D., Schreck, S., Hand, M, and Fingersh, L.J. (2001). “NREL
Unsteady Aerodynamics Experiment in the NASA-Ames Wind
Tunnel: A Comparison of Predictions to Measurements”, NREL
Technical report, NREL/TP-500-29494.
© 2009 ANSYS, Inc. All rights reserved. 7 ANSYS, Inc. Proprietary
Transition: 2D S809 Airfoil
Transition
Transition
Tu Contour
Transition
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Transition: 2D S809 Airfoil
Pressure (Cp) Distribution
AoA = 1°
AoA = 9°
AoA = 14°
AoA = 20°
© 2009 ANSYS, Inc. All rights reserved. 9 ANSYS, Inc. Proprietary
Transition: 3D NREL Wind Turbine
NREL 3D – Pressure Side
Transitional Turbulence
N = 72 rev/min
Separated flow
Turbulence production
Reattachment
Stagnation point
© 2009 ANSYS, Inc. All rights reserved. 10 ANSYS, Inc. Proprietary
Transition: 3D NREL Wind Turbine
Arrows indicate flow direction
Turbulent
Transitional
© 2009 ANSYS, Inc. All rights reserved. 11 ANSYS, Inc. Proprietary
3D Separation on Wind Turbine
• SST-SAS 3D CFD simulation– Combination of scale
resolving model (LES) and statistical model
– Resolves larger and medium scales, e.g. 3D shape of separation zones, turbulence structures etc.
– Combination with automatic wall treatment, transition & wall roughness possible
© Siemens Wind Power
© J. Laursen, P. Enevoldsen, S. Hjort: 3D CFD rotor computations of a
Multi-megawatt HAWT rotor , EWEC 2007
NACA 63618, ACA 10
SAS simulation snapshot
© 2009 ANSYS, Inc. All rights reserved. 12 ANSYS, Inc. Proprietary
Tip Vortex: NACA 0012 Wing
• NACA 0012 with
rounded wing tip tip
vortex
• Re = 4.6 106
• Experiment:
– Bradshaw et al (1997)
• ANSYS CFD:
– Grid: 5.5 million elements
– Max. y+ = 1 (on airfoil)
© 2009 ANSYS, Inc. All rights reserved. 13 ANSYS, Inc. Proprietary
Tip Vortex: NACA 0012 Wing
• Models resolving
streamline curvature
• Eddy viscosity ratio:
– Lower turbulence in
vortex core region
reduced production of
turbulent kinetic energy
– Better prediction of
swirling velocities and
turbulence levels in
vortex core
SST SST-CCPlane X/C=0.67
© 2009 ANSYS, Inc. All rights reserved. 14 ANSYS, Inc. Proprietary
Structural Blade Design
• Fluid Structure Interaction - FSI
– 1-Way Fluid Structure Interaction
• ANSYS Mechanical ANSYS CFD (deformations)
• ANSYS CFD ANSYS Mechanical (pressure
loads, …)
– 2-Way Fluid Structure Interaction
• Full unsteady-state interaction between
aerodynamic loads and structural response
© 2009 ANSYS, Inc. All rights reserved. 15 ANSYS, Inc. Proprietary
1-Way- and 2-Way-FSI
Geometry model
CFD mesh CSM mesh
Operating points
LoadsCFD calculation
CSM calculation
Stresses, deformations
2-way coupling
© 2009 ANSYS, Inc. All rights reserved. 16 ANSYS, Inc. Proprietary
Aero-Acoustic Simulations
• Challenges– Aero-acoustic noise
based on unsteady-state phenomena
– Coupling of different noise sources and transmission processes
– Large differences in time and length scales!• Small sound pressure
fluctuations & acoustic energies, compared to aerodynamic pressure differences!
• Benefits of simulations
– All aero-acoustic sources
of noise can be simulated
(e.g. inherent turbulent
fluctuations
quadrupoles)
– Different acoustic models
allow for balancing
between computational
efforts & accuracy /
details
© 2009 ANSYS, Inc. All rights reserved. 17 ANSYS, Inc. Proprietary
Aero-Acoustic Source Classification
Monopole (simple source)
Dipole (2 monopoles)
Quadrupole(2 dipoles)
Unsteady mass
injection
Acoustic ~ U 3M
Power
Unsteady external
forces
Acoustic ~ U 3M 3
Power
Unsteady turbulent
shear stresses
Acoustic ~ U 3M 5
Power
Monopole and dipole sources dominant at low Mach numbers
m = m(t) psurface = psurface(t) = (t)
Flow FlowFlow
© 2009 ANSYS, Inc. All rights reserved. 18 ANSYS, Inc. Proprietary
CFD Approaches to Aeroacoustics
• Steady-state RANS based noise source modeling
– Empirical correlations estimate acoustic radiation
• Modal Analysis
– Linearized Navier-Stokes-Equations with super-imposed pertubations
– Resonant frequencies and mode shapes
• Acoustic Analogy modeling
– CFD calculate source field
– Analytical solution propagate sound from source to receiver location
• Coupling of CFD and specialized acoustics codes:
– Acoustic sources determined with CFD, but acoustic waves not tracked with CFD
– Account for external scattering & reflections
• Direct Computational Aero-Acoustics (CAA)
– Resolve the acoustic pressure fluctuations as part of the CFD solution
Incre
asin
g c
om
pu
tatio
na
l effo
rt
Incre
asin
g a
ccu
racy
© 2009 ANSYS, Inc. All rights reserved. 19 ANSYS, Inc. Proprietary
Sensors
downstream
the mirror:
10 100 1000Frequency [Hz]
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
SP
L [
dB
]
Freestream Velocity = 140 km/h
Experimental data
SAS model
Sensor 121
10 100 1000Frequency [Hz]
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
SP
L [
dB
]
Freestream Velocity = 140 km/h
Experimental data
SAS model
Sensor 122
10 100 1000Frequency [Hz]
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
SP
L [
dB
]
Freestream Velocity = 140 km/h
Experimental data
SAS model
Sensor 123
Example: Generic Car Mirror
© 2009 ANSYS, Inc. All rights reserved. 20 ANSYS, Inc. Proprietary
Turbine Site Selection &
Wind Park Modeling
• Challenges
– Turbine efficiency and
operation stability
depends on turbine
placement
• Steep terrain, mountains
• Off-shore installations
– Impact of turbine-turbine
wake effects for varying
wind directions and
speeds
• Benefits of simulations
– Optimize turbine output
and placement
• Wind speed & turbulence
prediction over
complex terrain
• Account for wake effects
– Upfront prediction of
power output as a
function of wind speeds
and direction
© 2009 ANSYS, Inc. All rights reserved. 21 ANSYS, Inc. Proprietary
Terrain Modeling
© 2007 swisstopoIsosurface of
high turbulence
© 2009 ANSYS, Inc. All rights reserved. 22 ANSYS, Inc. Proprietary
Wind Park Modeling
© From: Th. Hahm,
F2E Fluid & Energy Engineering GmbH & Co. KG
Velocity contours showing wake shading effect &
turbulence structures
ANSYS FLUENT: LES simulation with sliding
rotor meshes
© From: O. Röglin,
TÜV NORD SysTec GmbH & Co. KG
© 2009 ANSYS, Inc. All rights reserved. 23 ANSYS, Inc. Proprietary
• Central Scotland
– Operated by Scottish Power Renewables
– Largest operating wind farm in the UK (Jan 2006) with 54 turbines
– Total installed power capacity of 124 MW (2.3 MW each)
– Small height variations (170 m) across farm
– Measurements availablehttp://www.bbc.co.uk/britainfromabove/stories/rewinds/blacklaw.shtml
Example: Black Law Wind Farm
Map Image: Ordnance Survey © Crown Copyright 2008, License number 100048580
© 2009 ANSYS, Inc. All rights reserved. 24 ANSYS, Inc. Proprietary
Example: Black Law Wind Farm
Multiple Wakes
Wind speed at hub height, wind direction 210
Without wind turbines With wind turbines
© 2009 ANSYS, Inc. All rights reserved. 25 ANSYS, Inc. Proprietary
Wind Park Power Output Estimation
Montavon, C., 1998, „Simulation of atmospheric flows over complex terrain for wind power potential assessment‟, PhD thesis no. 1855, EPF
Lausanne, Switzerland.
© 2009 ANSYS, Inc. All rights reserved. 26 ANSYS, Inc. Proprietary
Housing & Generator Cooling
• Challenges– Ensure effective cooling
under all environmental conditions
– Complex geometries & many details
– Many parameters • Fan positions & number
• Positions of electrical devices
• Air flow blockage
• Outside temperature & incoming sun radiation
• Benefits of simulations– Virtual prototyping of
different cooling solutions & layouts• Fan locations & number
• Air guidance
• Device positions
– Less trial & error
– Reduce thermal peak loads on generator, gear, transformer, structures etc.• Pre-identify “problem”
regions (hot spots)
© 2009 ANSYS, Inc. All rights reserved. 27 ANSYS, Inc. Proprietary
Heat Transfer: Aspects
• Turbulence:
– Reliable turbulence models
– Near wall treatment of boundary layers
– Advanced turbulence models (SAS, Transition, …)
• CHT:
– Coupled simulation of heat transfer in fluid and
solid regions
• Radiation:
– Between surfaces
– Sun radiation
© 2009 ANSYS, Inc. All rights reserved. 28 ANSYS, Inc. Proprietary
Turbulence Models: Diffuser Flow
k- model
SST model
k- model
SST model
No separation
Separation
© 2009 ANSYS, Inc. All rights reserved. 29 ANSYS, Inc. Proprietary
Velocity
Inlet
Constant
Heat Flux
Turbulence Models: Comparison
• Experiment
– Baughn et al.
(1984)
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Example: Cooling in Electric
Motor / Generator
© 2009 ANSYS, Inc. All rights reserved. 31 ANSYS, Inc. Proprietary
Example: Tower Base Cooling
• Simulation procedure:– Geometry import &
simplification
– Geometry parameterization for some parts (e.g. fan openings)
– Parametric meshing of fluid and solid domains regions
– Simulation with fluid & solid regions (heat losses defined by energy sources)
© Nordex
© 2009 ANSYS, Inc. All rights reserved. 32 ANSYS, Inc. Proprietary
Computational Fluid Dynamics for
Structural
blade design
Terrain modeling
Wind park
design
Tower design Housing & base
cooling
Acoustics
Aerodynamic
blade design
Generator
design
© Kato Engineering