2008 Int ANSYS Conf Implementation Hybrid Navier Stokes

8/6/2019 2008 Int ANSYS Conf Implementation Hybrid Navier Stokes

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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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2008 GE All rights reserved. 2

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



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



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



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



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



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)



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



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



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, 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



Specification of BCs in CFX



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



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



Screen shot from BladeContours script,showing spanwise blade contours

Polylines for NREL wind turbineblade

Generation of the polylines



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



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



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



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

Documents

2008 Int ANSYS Conf Implementation Hybrid Navier Stokes