2008 International ANSYS Conference...2008 International ANSYS Conference Implementation of a Hybrid Navier-Stokes / Vortex Panel Method for Wind Turbine Aerodynamic Analyses in CFX

© 2008 GE. All rights reserved. 1

2008 International ANSYS Conference

Implementation of a Hybrid Navier-Stokes / Vortex Panel Method for Wind Turbine Aerodynamic Analyses in CFXMark E. Braaten1, Kevin Standish2, Slawomir Kolasa3, Emad Gharaibah4

1 GE Global Research, Schenectady, NY2 GE Energy, Greenville, SC3 GE Polska, Warsaw, Poland4 GE Global Research, Munich, Germany

© 2008 GE All rights reserved. 2

Outline of Talk

• Overview of hybrid CFD method • Implementation in CFX • Validation • Sensitivity Studies• Concluding Remarks


Hybrid CFD for Wind Turbine Aero

• Conventional CFD methods based on Navier-Stokes have serious 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 (120° for 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 numerical dissipation• Prior studies have shown need to resolve wake as far as 10-20

blade radii downstream for accurate power predictions


Full Domain CFD

Inlet

Outlet

50 m


• Hybrid Navier-Stokes/Vortex Panel method proposed by Schmitz, Chattot (UC Davis) looks very promising– Small Navier-Stokes region (~1 - 5 M nodes) around the blade

computes 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 (cont’d)

Small Navier-Stokes domain

Prescribed wake shape


Hybrid CFD (cont’d)

• Coupling between near and far fields– Navier-Stokes code computes circulation on polylines about

spanwise blade sections needed by Vortex panel solver– Vortex panel method computes induced velocities on boundaries

of NS region computed from Biot-Savart law– Effect of blade on far field represented by lifting line, helicoidal

paths from prescribed wake– These provide coupling between Navier Stokes and Vortex Panel

solvers


Goals for Hybrid CFD for Wind Turbine Aero Design

• Goal is to develop design system using hybrid methodology that can be routinely used by GE engineers to design wind turbine blades for optimum aero performance – Scripting of meshing, pre- and post-processing is essential– Hybrid CFD analysis in CFX must be no more difficult to set up

and run than conventional analysis– Best practices established and embedded in process


• 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 single CFX solver run, instead of requiring a separate program 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 in CFX


User Fortran Routines

CFX provides two types of User Fortran routines:• Junction Box routines, which are User Fortran routines that are

called 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, and

save these in the CFX MMS (memory management system) for later use by the solver

– Compute the helicoidal wake paths– Compute the circulation on the polylines at the start of each CFX

iteration– Impose the induced velocities on the outer boundary as a prescribed

velocity opening boundary condition


Basic Hybrid CFD Flow Chart

Iteration Loop

Note

User Fortran routinesshown in yellow

Initial Setup

Read user input

Compute circulation

Compute induced velocities

Solve equations

Output results file

data files Generate polylines

Compute helix


Computing Circulation in CFX

• The circulation is computed on closed loops (polylines) that include 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 current

values 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 are 1D calculations:

– Routines use CFX Flow Parallel message passing calls for parallel communication (as does rest of CFX solver)


• Helicoidal paths are computed based on simple actuator disk theory

• Paths depend on estimated power coefficient– Paths are periodically updated during

CFX calculation based on latest computed power coefficient

• Helicoidal paths computed redundantly on each processor using a Junction Box routine– This allows each processor to

compute induced velocities in perfectly parallel fashion

Computation of Helicoidal Paths

Winddirection

Navier-Stokesdomain


Induced Velocities

• The induced velocities are computed on the outer boundary of the 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 updated helicoidal paths


Induced Velocities (Biot-Savart Law)

Influence Coefficients

All equations from Schmitz’ thesis

Equations for Induced Velocities


Computation of Induced Velocities

• Total storage requirement– (6 influence coefficients) X (nbf

boundary faces) X (jx polylines)– For 100,000 boundary faces, 40

polylines 100 MB storage– Typically represents about 20%

additional storage for solver

• Each processor needs only to compute influence coefficients, induced velocities just for its boundary faces– Naturally parallel !– Storage is distributed across

processors


Specification of BCs in CFX


Mesh Generation

• The blade geometry is created in Unigraphics, 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 has been developed to simplify the mesh generation process– The blocking file can be imported to any

ICEM project with the same parts and adapted to a new blade geometry


Creating the polylines

• The polylines over which the circulation is computed are generated using two utility programs:– BladeContours

• A CFX-POST Power Syntax script file that extracts the contours 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 from the blade contours

• The polylines, and the location of the leading and trailing edge locations of the spanwise blade sections are written into files that are input files for the CFX run


Screen shot from BladeContours script,showing spanwise blade contours

Polylines for NREL wind turbine blade

Generation of the polylines


Post-processing

• Post-processing of the results is also performed using a CFX-Post Power Syntax script– Normal and tangential directions input for each spanwise blade

section– At each section, script calculates:

• Normal and tangential forces• Pressure coefficients• Torque force

• Same post-processing script used for full-domain and hybrid CFD computations to facilitate comparisons


Pressure distribution at 63% span – 7 m/s wind speed

Hybrid CFD

NREL Validation Case

• National Renewable Energy Laboratory (NREL) performed detailed wind tunnel experiments on 10m diameter wind turbine in NASA Ames wind 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


Navier-Stokes domain

Computed Mach Numbers (TSR=8)

Pow

er C

oeffi

cien

t

Tip Speed Ratio

Run Time Comparison(equal # iterations)

CPU

tim

e (m

in)

050

100150200250300350400

Full CFD 366Hybrid CFD 46.9

Solver time [min]

Pressure Coefficient (PC) Comparison

GE46 Validation Case

• This was the first GE blade run using the hybrid CFD method

• Comparisons made to full-domain CFD, other analytical methods– Power predictions match well for

attached flow cases


Sensitivity Study

• Sensitivity studies are being performed to investigate the effects of a number of parameters, and to establish best practices

• Parameters under study:– Size of Navier-Stokes domain– Grid size– Number of polylines– Location, orientation of polylines

• Results appear to be relatively insensitive


(a) Full domain CFD(b) Hybrid (C=1)(c) Hybrid (C=½)(d) Hybrid (C=¼)

(a) (b)

(c) (d)

• Vary size of Navier-Stokes domain, from one chord (C=1) down to ¼ chord

• Vorticity field starts to look unphysical if domain is made too small

Vorticity at 20% Span

Example: Effect of Domain Size


Future Work

• Hybrid methodology allows fine enough grid 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 transient simulations possible– Unsteady hybrid analysis capability

currently under development (w/ UC Davis)


Concluding Remarks

• Hybrid CFD approach looks very promising for wind turbine 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 hybrid CFD design system


Hybrid CFD – Represents a unique differentiating technology that will be a prime enabler for next generation quiet, efficient wind turbine blades– Hi-fidelity 3D aerodynamic design– Low noise design with CFD-based or direct CAA

noise prediction– Aero-elastic fully-coupled design methods

Impact

Documents

2008 International ANSYS Conference...2008 International ANSYS Conference Implementation of a Hybrid Navier-Stokes / Vortex Panel Method for Wind Turbine Aerodynamic Analyses in CFX