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© 2011 ANSYS, Inc. May 8, 2012 1
Wind Farm Flow Modelling Using CFD
2012 Update Webinar Christiane Montavon, Ian Jones
ANSYS
© 2011 ANSYS, Inc. May 8, 2012 2
• 15:00 Introduction to the webinar and ANSYS
• 15.10 Presentation
– Why CFD Simulation?
– Validation and technical advances
– Overview of ANSYS CFD tools for wind farm flow modelling
• 15.45 Software Demonstration
– Automated workflow for wind farm modelling
• 16.05 Question and Answers
• 16.15 Close
Agenda
© 2011 ANSYS, Inc. May 8, 2012 3
• Carbon Trust, project CT090-091, Power from Onshore Wind Farms, and partners,
– RWE, Scottish Power, SgurrEnergy
• Carbon Trust OWA Phase 1 and partners
– (Dong, RWE, Scottish Power, SSE Renewables, Statoil, Frazer-Nash).
• SSE Renewables
• E.ON
• EU BREIN project
• Loughborough University
• University of Strathclyde...
• and many more
Acknowledgements
© 2011 ANSYS, Inc. May 8, 2012 4
Site selection, land and sea
Generator and shaft design
Wind farm configuration for optimal power generation
Blade design
Rotor sizing and acoustics
Tower design and FSI
Offshore Installation and certification
ANSYS: A Comprehensive Simulation Platform
Electric Machine
Transformer
Power Distribution
Speed Sensor
Electromechanical Component
Power Electronic
© 2011 ANSYS, Inc. May 8, 2012 5
More and more onshore sites developed in complex terrain and complex forestry environment.
Associated risks:
Separation, negative shear exponent factors
Increased turbulence
implications for turbine longevity and energy output
On such sites, standard industry tools (linearised models) used outside of the envelope where they are meant to operate
Large array losses, particularly so offshore
Empirical models tend to underestimate the losses for large arrays
Atmospheric stability significantly affects array efficiency (e.g. L. Jensen, EWEC proceedings, Milan 2007)
CFD models increasingly advocated to address these issues.
Need for fidelity of solution and reliability validation!
Need for automated solution for users without CFD background
Background
© 2011 ANSYS, Inc. May 8, 2012 6
N-S Solvers vs. Linearised models
Advantages of Navier-Stokes solvers as compared to linearized models:
Accurate prediction of turbulence: - flow turbulence is modeled or resolved using RANS/LES
Better prediction of multiple-wake effects - accurate geometry description and wake prediction from multiple installations - no limit to number of wind turbines considered
Separation/shade effects due to complex terrain - complex terrain is resolved - shading effects, recirculation and separation are captured
Ref. 2
Ref. 1
Ref. 1
1. Barthelmie, R.J et al., Modelling and measurements of wakes in large wind farms, Journal of Physics: Conference Series 75 (2007) 012049. 2. Barthelmie, R.J et al., Modelling Uncertainties in power prediction offshore, IEA, Risoe, March 2004.
© 2011 ANSYS, Inc. May 8, 2012 7
On range of sites
– Onshore: Blacklaw, An Suidhe, Nant y Moch, Harestanes
– Offshore: Horns Rev, North Hoyle
On range of issues
– Complex terrain
– Complex forestry
– Stability
– Wake interaction
Some done by our users
– Offshore: Burbo Bank, Gunfleet Sands, Barrow (DONG energy)
– Forestry: Loughborough University
Validation material
CFD delivers increased accuracy and insight in flow conditions
© 2011 ANSYS, Inc. May 8, 2012 8
Example – Blacklaw Power Prediction
Complex forestry
Significant wake effects
Good prediction of normalised power output
RMSE for power prediction over all turbines and over both masts is 8.5%
R. Spence, C. Montavon, I. Jones, C. Staples, C. Strachan, D. Malins, 2010, Wind modelling evaluation using an operational wind farm site, http://www.ewec2010proceedings.info/allfiles2/517_EWEC2010presentation.pdf
© 2011 ANSYS, Inc. May 8, 2012 9
Forestry model (resistive model)
– Variable forestry height
– Variable loss coefficient
Wake Model, large array losses
– Horns Rev, North Hoyle
Atmospheric stability accounted for via equation for potential temperature, buoyancy effect in turbulence model
– Harestanes, An Suidhe
– Horns Rev
Technical Advances
© 2011 ANSYS, Inc. May 8, 2012 10
Resistive ‘Canopy Model’ available:
– Svensson
– Lopes da Costa
– Katul
– Resistance in momentum only
Canopy Input Data
– From roughness data
– CFX Interpolation Table
variable tree heights
Forestry loss coefficient – Constant or variable with height
Forest Canopy Model
Max Forestry Loss Coef
Height Max
Galion Lidar data
CFD
Flow separation off forested region
C. Montavon, I. Jones, D. Malins, C. Strachan, R. Spence, R. Boddington, 2012, Modelling of wind speed and turbulence intensity for a forested site in complex terrain, EWEA 2012, Copenhagen.
© 2011 ANSYS, Inc. May 8, 2012 11
Horns Rev Results at Hub Height – Sector 280
Horizontal velocity Turbulence intensity
Uref = 8 m/s at 70m, z0 = 0.0002m Wind direction: sector 280
© 2011 ANSYS, Inc. May 8, 2012 12
0
0.2
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0.6
0.8
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1.2
1 2 3 4 5 6 7 8 9 10
Norm
alis
ed P
ow
er
Turbine Group
10m/s 270° 2° bin
ANSYS CFD
0.2
0.4
0.6
0.8
1
1.2
1 2 3 4 5 6 7 8 9 10
Norm
alis
ed P
ow
er
Turbine Group
10m/s 270° 10° bin
ANSYS CFD
Data - UpWind
0.4
0.6
0.8
1
1.2
1 2 3 4 5 6 7 8 9 10
Norm
alis
ed P
ow
er
Turbine Group
10m/s 270° 30° bin
ANSYS CFD
Data - UpWind
Horns Rev Normalised Power Down a Row
Simulations by step of 1 degree, sector 270 – 285, averaged for three different bin sizes.
Reasonably good prediction
– Tendency for over-estimation of array losses
– Good prediction of slope down the row
Consistent for various bin sizes
Upwind data from “Wake Measurements Used in the Model Evaluation”. K.S. Hansen, R. Barthelmie, D. Cabezon and E. Politis. Upwind Wp8: Flow; Deliverable D8.1 Data. 18 June 2008.
© 2011 ANSYS, Inc. May 8, 2012 13
0
0.2
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1
1.2
1 2 3 4 5
No
rma
lis
ed
Po
we
r
Column
10 deg bin
ANSYS CFD
Measured Data
Upper 25%
Lower 25%0
0.2
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0.6
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1
1.2
1 2 3 4 5
No
rma
lis
ed
Po
we
r
Column
30 deg bin
ANSYS CFD
Measured Data
Upper 25%
Lower 25%
North Hoyle Normalised Power Down a Row
Very good agreement with power data for both bin sizes
Absolutely blind test case!
Uref = 10 m/s at 67m, z0 = 0.0001m, upstream TI = 7% Wind direction: sector 260
C. Montavon, S.-Y. Hui, J. Graham, D. Malins, P. Housley, E. Dahl, P. de Villiers, B. Gribben, 2011, Offshore Wind Accelerator: wake modelling using CFD, EWEA 2011, Brussels.
© 2011 ANSYS, Inc. May 8, 2012 14
Improved results with atmospheric stability
Atmosphere on average stable above boundary layer
Including this effect
– Changes the relative distribution of the wind speed between hill tops and valleys improves mast to mast cross prediction when masts located in different type of positions (i.e. hill tops vs valleys)
– Improves the prediction of the relative TI on site
Including surface stability effects significantly affects the prediction of array losses
C. Montavon, C. Staples, C. Weaver, 2011, Simulating the flow conditions over complex terrain with RANS models: sensitivity to a selection of parameters including atmospheric stability , EWEA 2011, Brussels. C. Montavon, I. Jones, D. Malins, C. Strachan, R. Spence, R. Boddington, 2012, Modelling of wind speed and turbulence intensity for a forested site in complex terrain, EWEA 2012, Copenhagen.
© 2011 ANSYS, Inc. May 8, 2012 15
4 masts on site, 3 near hill tops, 1 in a location with difficult flow conditions (valley, proximity to forestry)
Cross prediction done for 3 masts with long concurrent time series
Harestanes, Masts on site
All masts with data at 70m, 60m, 40m, 30m Hill top masts: Holehouse Hill Hareshaw Rig Valley mast: Bran Rig
Hareshaw Rig
Holehouse Hill
Bran Rig
© 2011 ANSYS, Inc. May 8, 2012 16
Maximum relative errors in wind speed cross predictions for three model configurations:
1. Purely neutral
2. Conventionally neutral i.e. stable conditions in free stream, with potential temperature gradient of US standard atmosphere, and neutral conditions at ground (adiabatic).
Harestanes Mast to Mast Cross Prediction (wind speed)
3 masts (70m only) 3 masts (all heights)
Model BRAN-HOL BRAN-HAR HOL-HAR BRAN-HOL BRAN-HAR HOL-HAR
neutral 11.8% 13.3% 1.8% 24.0% 24.0% 3.2%
stable 0.4% 4.1% 5.4% 2.9% 6.5% 6.2%
Improved results with atmospheric stability
© 2011 ANSYS, Inc. May 8, 2012 17
Ongoing validation exercise together with end users, see e.g. our joint EWEA publications at
Copenhagen 2012, available from:
https://docs.google.com/open?id=0B6Cp_fvx8o5Dei0tenVqelZZQzQ
Brussels 2011, available from:
https://docs.google.com/leaf?id=0B6Cp_fvx8o5DMjBkMzQxNWMtOTkxYy00ZmFiLWFlNzMtZGFhM2U0NWQ1ODFh&hl=en_GB
Previous years, available from:
https://docs.google.com/leaf?id=0B6Cp_fvx8o5DZmEzYzYxMDAtZGYxZi00YmI0LWIwMjYtZWM3NmViYWM2NDI3&hl=en_GB
Validated Tools
© 2011 ANSYS, Inc. May 8, 2012 18
Beaucage, P., Robinson, N., Brower, M., Alonge, C., Overview of six commercial and research wake models for large offshore wind farms, Proceedings EWEA 2012, Copenhagen.
Garza, J., A. Blatt, R. Gandoin and S.-Y. Hui (2011) Evaluation of two novel wake models in offshore wind farms . Proceedings from the EWEA Offshore conference, 29 Nov. - 1 Dec 2011.
Desmond, C., Sayer, A., Watson, S., Hancock, P., Forest Canopy Flows in Non-Neutral Stability , EWEA 2012, Copenhagen. (poster award).
Clive, P., Dinwoodie, I., Quail, F., Direct measurement of wind turbine wakes using remote sensing, Proceedings EWEA 2011, Brussels.
ANSYS CFD Featured In Independent Publications
© 2011 ANSYS, Inc. May 8, 2012 19
WindModeller: set of tools wrapped around ANSYS standard CFD products:
Allow non-CFD experts to perform wind farm analyses in
automated way
Drive ANSYS CFX or FLUENT flow solver
Allow advanced user to encapsulate their own expertise (access to customised setup and post-processing scripts which can easily be altered by the user to further develop the tools )
WindModeller Tools for Automated Solution
© 2011 ANSYS, Inc. May 8, 2012 20
Tools for Automated Solution
Objective – Automation of Analysis and Data
extraction
– Map Mesh CFD Report in
one step
CFD solution + automated post-processing
Wind data transposition module (cross prediction and energy assessment)
© 2011 ANSYS, Inc. May 8, 2012 21
WindModeller: Simulation Process
Wind farm simulation process from user perspective
– Set up analysis on desktop computer (either via GUI or command line)
– Submit job to:
– Run possible large number of cases on the local machine or on a remote server
– Postprocess results to automatically generate reports/summary data files
– Possibility to perform additional post-processing on individual results files using CFD Post
Setup on desktop Run on local or remote computer Report as html file
© 2011 ANSYS, Inc. May 8, 2012 22
Current recognised terrain format – SRTM, Shuttle Radar Topography
Mission, freely available, 90m resolution (finer resolution in the US)
– NTF, National Transfer Format, contour data (UK)
– .map files (WAsP format)
– Generic point data file (.csv)
Terrain converted to tesselated format (STL)
Meshing with custom tools – Fixed mesh structure, hexahedral mesh
(5 or 9 blocks), aimed at process automation
– Template mesh morphed onto STL terrain representation
– Variable mesh topology for elongated/twin wind farms
Meshing Approach
© 2011 ANSYS, Inc. May 8, 2012 23
Varying Mesh Topologies
Compact wind farm
Elongated wind farm
Twin wind farms
© 2011 ANSYS, Inc. May 8, 2012 24
User can prescribe:
– horizontal resolution in central region
– Rate of horizontally expansion outside
– first layer cell heights in vertical
Good control of mesh resolution in lower heights (ensures appropriate resolution is achieved in forested regions)
Smooth vertical expansion above
Meshing Controls
© 2011 ANSYS, Inc. May 8, 2012 25
Mesh Adaption on Wind Turbine Rotor
Improve resolution by automatically refining mesh around the turbine location, from the specification of the rotor location and actuator disk parameters only
Automatically enabled if wake model is used.
Initial mesh 1st refinement
2nd refinement
Final mesh
© 2011 ANSYS, Inc. May 8, 2012 26
Outer surface divided into 24 regions
– 12 for inlet b.c. (Dirichlet on velocity)
– 12 for outlet b.c. (entrainment conditions with prescribed static pressure)
Setup automated to run for e.g. 12 wind directions
Selection of surfaces defining inlet/outlet automated in script running cases for various wind directions
meshing done only once
Setup
© 2011 ANSYS, Inc. May 8, 2012 27
Flow Modelling in WindModeller
Atmosphere modelled as: – incompressible fluid (Air at 15C) – assuming neutral stability – solving for steady state RANS
Turbulence modelled via two-equation model – Shear Stress Transport (SST) turbulence model or k- ε.
Ground modelled as rough wall (spatially variable roughness)
Inlet boundary conditions – Classical constant-shear ABL profiles (Durbin & Petterson Reif ):
Additional physics: – Forest canopy model (resistive term in momentum equation + additional source
terms in turbulence model) – Multiple wake model (actuator disk model) – Atmospheric stability as beta feature
2/1
2
*
C
uk u
uln
z
z * ( ) 0 z
u
3
*
© 2011 ANSYS, Inc. May 8, 2012 28
Hierarchy of Wake Models available in ANSYS CFD
Resolved blade models
Virtual Blade Models
Simple Actuator Disk Models
– Provide practical model for calculations with many turbines
– Input is turbine thrust curve, turbine diameter, turbine hub height
– Provides momentum sink in cylindrical volume surrounding each turbine
– Basis of Models for WindModeller
Wake Modelling
© 2011 ANSYS, Inc. May 8, 2012 29
Simple Wake Model
Wind turbine represented by
– momentum sink
– constant thrust per volume within
– identified rotor disk.
Wind turbine orientation parallel to wind direction at inlet
Works on any type of mesh, although it is expected that the best results will be obtained with resolution that captures the wind turbine disk reasonably well
User input:
– Coordinates of hub location
– WT diameter
– WT thrust and power curve
© 2011 ANSYS, Inc. May 8, 2012 30
As part of the automated approach WindModeller can generate: – Plots of streamlines
– Identification of recirculation zones
– Plots at constant height AGL and profiles at wind turbine/mast locations for quantities such as normalised velocity, turbulence intensity, shear exponent factor
– Exported data tables of similar quantities at wind turbine/mast locations
– Export to Google Earth (.kml files)
– Automated report in html format
Post-Processing in WindModeller
© 2011 ANSYS, Inc. May 8, 2012 31
As part of the automated approach WindModeller can generate: – Plots of streamlines
– Identification of recirculation zones
– Plots at constant height AGL and profiles at wind turbine/mast locations for quantities such as normalised velocity, turbulence intensity, shear exponent factor
– Exported data tables of similar quantities at wind turbine/mast locations
– Export to Google Earth (.kml files)
– Automated report in html format
Post-Processing in WindModeller
© 2011 ANSYS, Inc. May 8, 2012 32
As part of the automated approach WindModeller can generate: – Plots of streamlines
– Identification of recirculation zones
– Plots at constant height AGL and profiles at wind turbine/mast locations for quantities such as normalised velocity, turbulence intensity, shear exponent factor, flow angles
– Exported data tables of similar quantities at wind turbine/mast locations
– Export to Google Earth (.kml files)
– Automated report in html format
Post-Processing in WindModeller
© 2011 ANSYS, Inc. May 8, 2012 33
As part of the automated approach WindModeller can generate: – Plots of streamlines
– Identification of recirculation zones
– Plots at constant height AGL and profiles at wind turbine/mast locations for quantities such as normalised velocity, turbulence intensity, shear exponent factor , flow angles
– Exported data tables of similar quantities at wind turbine/mast locations
– Export to Google Earth (.kml files)
– Automated report in html format
Post-Processing in WindModeller
© 2011 ANSYS, Inc. May 8, 2012 34
As part of the automated approach WindModeller can generate: – Plots of streamlines
– Identification of recirculation zones
– Plots at constant height AGL and profiles at wind turbine/mast locations for quantities such as normalised velocity, turbulence intensity, shear exponent factor , flow angles
– Exported data tables of similar quantities at wind turbine/mast locations
– Export to Google Earth (.kml files)
– Automated report in html format
Post-Processing in WindModeller
© 2011 ANSYS, Inc. May 8, 2012 35
As part of the automated approach WindModeller can generate: – Plots of streamlines
– Identification of recirculation zones
– Plots at constant height AGL and profiles at wind turbine/mast locations for quantities such as normalised velocity, turbulence intensity, shear exponent factor , flow angles
– Exported data tables of similar quantities at wind turbine/mast locations
– Export to Google Earth (.kml files)
– Automated report in html format
Post-Processing in WindModeller
© 2011 ANSYS, Inc. May 8, 2012 36
As part of the automated approach WindModeller can generate: – Plots of streamlines
– Identification of recirculation zones
– Plots at constant height AGL and profiles at wind turbine/mast locations for quantities such as normalised velocity, turbulence intensity, shear exponent factor , flow angles
– Exported data tables of similar quantities at wind turbine/mast locations
– Export to Google Earth (.kml files)
– Automated report in html format, including the above
Post-Processing in WindModeller
© 2011 ANSYS, Inc. May 8, 2012 37
As part of the automated approach WindModeller can generate: – Plots of streamlines
– Identification of recirculation zones
– Plots at constant height AGL and profiles at wind turbine/mast locations for quantities such as normalised velocity, turbulence intensity, shear exponent factor , flow angles
– Exported data tables of similar quantities at wind turbine/mast locations
– Export to Google Earth (.kml files)
– Automated report in html format, including the above
Post-Processing in WindModeller
© 2011 ANSYS, Inc. May 8, 2012 38
Wind Data Transposition Module
Simulations establish climatological relationships between wind conditions at
mast (reference site) and WTG (predicted site).
Simulations are performed independently from the data collection at the mast.
Data collected at mast
Wind conditions predicted at WTG
Wind data transposition module
© 2011 ANSYS, Inc. May 8, 2012 39
Energy Assessment/Cross Prediction
Wind data for input:
• Time series or Frequency tables
Allows for multiple masts and with / without wake calculations
Can deal with heterogeneous wind farm
Masts data collected before or after wind turbines installed
Possibility to do a post-mortem on an existing wind farm to understand performance issues of single WT if mast still measuring when wind farm is operational
Note: masts must be within simulation domain (no MCP)
© 2011 ANSYS, Inc. May 8, 2012 40
Output from Data Transposition
Tables of Capacity Factors (by directions and overall) summarising the average annual energy output at each WT.
Wind speed distributions (WAsP .tab files) at WT and masts.
Resource file (WAsP .rsf) at WT locations.
Summary table with average wind speed at masts from cross prediction.
Summary tables of mean and representative turbulence intensity by wind speed classes at masts and WT locations. (when working from time series, including the wind speed standard deviation as input).
© 2011 ANSYS, Inc. May 8, 2012 41
Customised tools based on standard ANSYS CFD software under continuous development
Driven by customer and project demands
• Used on many cases
Made available to customer on ‘service based approach’ via two stage process:
• 1st phase: demonstration of capability on terrain chosen by customer
• 2nd phase: Technology Transfer. Tools made available to customer, also includes one-to-one training, support and maintenance of the tools after delivery.
Develop features on request.
Availability of WindModeller Tools
© 2011 ANSYS, Inc. May 8, 2012 42
OWA Phase II: integration of atmospheric stability and associated effects on large array losses
Carbon Trust POWFARMM project: complex terrain, forestry, wakes, comparison with masts and Galion LIDAR data
Various consultancy projects recently completed for customers
• Pollution transport
• Integration of buildings
• More complex stability conditions (e.g. strong inversions, coastal low level jets)
Diversification into modelling of marine arrays (TideModeller)
Ongoing Projects
© 2011 ANSYS, Inc. May 8, 2012 43
• CFD delivers increased accuracy for
– Complex terrain/complex forestry cases
– Estimation of large array losses
• Atmospheric stability improves accuracy compared to neutral cases, which tend to be used by other CFD packages in the industry
• ANSYS has a suite of tools (WindModeller) that helps you automate the simulation process, with state of the art models for
– Forestry
– Wakes
– Atmospheric stability
• Validation material available to attest this.
Summary
© 2011 ANSYS, Inc. May 8, 2012 44
Upcoming ANSYS Events
• Webinar - Advanced Multi-body Hydrodynamics and Motion Analysis Using AQWA Software - 2012 Update – Friday, 11th May 2012
– http://www.ansys.com/aqwawebinar2012
• All Energy in Aberdeen on May 23 & 24 - stand C111.
ANSYS Events
Scheduled for 2012
Information: http://www.ansys.com/events
• ian.jones@ansys.com
Question, Comments, Inquiries…
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