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2008 GE. All rights reserved. 1
2008 International
ANSYS Conference
Implementation of a Hybrid Navier-Stokes / Vortex Panel Method for WindTurbine Aerodynamic Analyses in CFX
Mark E. Braaten1, Kevin Standish2, Slawomir Kolasa3,Emad Gharaibah4
1 GE Global Research, Schenectady, NY
2 GE Energy, Greenville, SC
3 GE Polska, Warsaw, Poland4 GE Global Research, Munich, Germany
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Outline of Talk
Overview of hybrid CFD method
Implementation in CFX Validation
Sensitivity Studies
Concluding Remarks
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Hybrid CFD for Wind Turbine Aero
Conventional CFD methods based on Navier-Stokes haveserious shortcomings for wind turbine analyses Very large domain required to enforce far-field boundary conditions
sufficiently far from blade Inlet and outlet conditions imposed many blade radii upstream
and downstream
Periodicity requires large sector (120for 3 blades) to be
modeled Results in very large meshes (> 10M nodes), long run times, difficult
post-processing
Prevents use of fine enough mesh near blade needed to capture
turbulence transition effects Unsteady CFD simply not practical on such large meshes
Wake behind blade dissipates too quickly due to numericaldissipation
Prior studies have shown need to resolve wake as far as 10-20blade radii downstream for accurate power predictions
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Full Domain CFD
Inlet
Outlet
50 m
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Hybrid Navier-Stokes/Vortex Panel method proposed bySchmitz, Chattot (UC Davis) looks very promising
Small Navier-Stokes region (~1 - 5 M nodes) around the bladecomputes near field
Vortex Panel method computes far field using Biot-Savart law
Effect of blade on far field represented by lifting line,
helicoidal paths from prescribed wake
Hybrid CFD (contd)
Small Navier-Stokes domain
Prescribedwake shape
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Hybrid CFD (contd)
Coupling between near and far fields
Navier-Stokes code computes circulation on polylines aboutspanwise blade sections needed by Vortex panel solver
Vortex panel method computes induced velocities on boundariesof NS region computed from Biot-Savart law
Effect of blade on far field represented by lifting line, helicoidalpaths from prescribed wake
These provide coupling between Navier Stokes and Vortex Panelsolvers
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Goals for Hybrid CFD for WindTurbine Aero Design
Goal is to develop design system using hybridmethodology that can be routinely used by GE engineersto design wind turbine blades for optimum aeroperformance
Scripting of meshing, pre- and post-processing is essential
Hybrid CFD analysis in CFX must be no more difficult to set upand run than conventional analysis
Best practices established and embedded in process
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The hybrid method requires:
The computation of the circulation on closed loops that
include all of the sources of vorticity The calculation of the induced velocities and their imposition
on the boundaries
These calculations are now done as part of a singleCFX solver run, instead of requiring a separateprogram to be run between successive runs of CFX
The vortex panel solver is implemented in CFX User
Fortran (CFX Version 11.0 and up)
Implementation of Hybrid CFD inCFX
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User Fortran Routines
CFX provides two types of User Fortran routines:
Junction Box routines, which are User Fortran routines that arecalled and executed at particular times during a CFX run.
CEL routines These evaluate a function in a similar fashion to CFX expression
language
We basically need routines to:
Read the user defined loops upon which to compute the circulation, andsave these in the CFX MMS (memory management system) for lateruse by the solver
Compute the helicoidal wake paths
Compute the circulation on the polylines at the start of each CFXiteration
Impose the induced velocities on the outer boundary as a prescribedvelocity opening boundary condition
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Basic Hybrid CFD Flow Chart
Iteration Loop
Note
User Fortran routines
shown in yellow
Initial Setup
Read user input
Compute circulation
Compute induced velocities
Solve equations
Output results file
data files Generate polylines
Compute helix
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Computing Circulation in CFX
The circulation is computed on closed loops (polylines) thatinclude all of the sources of vorticity The circulation calculation is made for a number of spanwise
positions along the blade This calculation is updated every iteration using the currentvalues of the velocity field
Polyline points are mapped to the closest mesh points
Velocity vector and gradient returned at mapped points User Fortran provides capability to recover gradient operator
Use Taylor expansion to interpolate solution from mapped point (mp)to polyline point (pp)
Vpp = Vmp + V * dr Circulation calculation requires parallel implementation
Simplest parallel implementation is sufficient, as the line integrals are1D calculations:
Routines use CFX Flow Parallel message passing calls for parallelcommunication (as does rest of CFX solver)
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Helicoidal paths are computed based onsimple actuator disk theory
Paths depend on estimated powercoefficient Paths are periodically updated during
CFX calculation based on latestcomputed power coefficient
Helicoidal paths computed redundantly oneach processor using a Junction Boxroutine
This allows each processor to
compute induced velocities in perfectlyparallel fashion
Computation of Helicoidal Paths
Winddirection
Navier-Stokesdomain
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Induced Velocities
The induced velocities are computed on the outer boundary ofthe domain, using the Biot-Savart law
This involves the line integration along a helicoidal path, starting out
from each spanwise location on the trailing edge Outer boundaries of Navier-Stokes domain are treated as
openings
Induced velocity on boundary is defined as an expression, which is
provided by a user CEL function Influence coefficients computed at beginning of run, and stored in
MMS
Subsequent updates of induced velocities use stored coefficients
Influence coefficients updated periodically to reflect updatedhelicoidal paths
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Induced Velocities (Biot-Savart Law)
Influence Coefficients
All equations from
Schmitz thesis
Equations for Induced Velocities
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Computation of Induced Velocities
Total storage requirement (6 influence coefficients) X (nbf
boundary faces) X (jx polylines)
For 100,000 boundary faces, 40polylines 100 MB storage
Typically represents about 20%additional storage for solver
Each processor needs only tocompute influence coefficients,induced velocities just for itsboundary faces
Naturally parallel !
Storage is distributed acrossprocessors
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Specification of BCs in CFX
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Mesh Generation
The blade geometry is created inUnigraphics, and imported into ICEM-CFD for meshing
Restrictions on Navier-Stokes domain: Needs to contain polylines for computation of
the circulation
Exit plane should be orthogonal to wake path
A grid template with a C-H topology hasbeen developed to simplify the meshgeneration process
The blocking file can be imported to anyICEM project with the same parts andadapted to a new blade geometry
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Creating the polylines
The polylines over which the circulation is computed aregenerated using two utility programs:
BladeContours
A CFX-POST Power Syntax script file that extracts thecontours of the blade cross sections at the spanwise locations
where the polylines are desired These blade contours are then input to
PolyGen
A program that created closed polylines that are offset fromthe blade contours
The polylines, and the location of the leading and trailing edgelocations of the spanwise blade sections are written into files
that are input files for the CFX run
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Screen shot from BladeContours script,showing spanwise blade contours
Polylines for NREL wind turbineblade
Generation of the polylines
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Post-processing
Post-processing of the results is also performed using aCFX-Post Power Syntax script
Normal and tangential directions input for each spanwise bladesection
At each section, script calculates:
Normal and tangential forces Pressure coefficients
Torque force
Same post-processing script used for full-domain andhybrid CFD computations to facilitate comparisons
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Pressure distribution at 63% span 7 m/s wind speed
Hybrid CFD
NREL Validation Case
National Renewable Energy Laboratory (NREL) performed detailedwind tunnel experiments on 10m diameter wind turbine in NASA Ameswind tunnel
This was the test case used by Sven Schmitz (UC Davis) to help
develop the hybrid methodology Calculated torque values, pressure distributions match experiment very
well for low wind speeds 7 & 9 [m/s] cases are attached flow
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Navier-Stokes domain
Computed Mach Numbers (TSR=8)
PowerCoefficient
Tip Speed Ratio
Run Time Comparison(equal # iterations)
CPUtim
e(min)
0
50
100150
200
250
300
350
400
Full CFD 366Hybrid CFD 46.9
Solver time [min]
Pressure Coefficient (PC) Comparison
GE46 Validation Case
This was the first GE blade runusing the hybrid CFD method
Comparisons made to full-domain
CFD, other analytical methods Power predictions match well for
attached flow cases
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Sensitivity Study
Sensitivity studies are being performed to investigate theeffects of a number of parameters, and to establish bestpractices
Parameters under study:
Size of Navier-Stokes domain
Grid size
Number of polylines
Location, orientation of polylines
Results appear to be relatively insensitive
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(a) Full domain CFD(b) Hybrid (C=1)(c) Hybrid (C=)
(d) Hybrid (C=)
(a) (b)
(c) (d)
Vary size of Navier-Stokes domain, from
one chord (C=1)down to chord Vorticity field
starts to lookunphysical ifdomain is madetoo small
Vorticity at 20% Span
Example: Effect of Domain Size
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Future Work
Hybrid methodology allows fine enoughgrid in Navier-Stokes domain to allow
transition to be modeled Initial calculations with Langtry-Menter
transition model look very promising
Reduction in run time makes transientsimulations possible
Unsteady hybrid analysis capabilitycurrently under development (w/ UC Davis)
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Concluding Remarks
Hybrid CFD approach looks very promising for windturbine aerodynamic analyses
Grid size and run times much less than full domain CFD
Results similar to full-domain CFD Results reasonably insensitive to size of Navier-Stokes domain
and location of polylines
User Fortran in CFX solver and Power Scripting in CFX-POST allows the development of an integrated hybridCFD design system
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Hybrid CFD Represents a unique differentiatingtechnology that will be a prime enabler for nextgeneration quiet, efficient wind turbine blades
Hi-fidelity 3D aerodynamic design
Low noise design with CFD-based or direct CAAnoise prediction
Aero-elastic fully-coupled design methods
Impact