Strategies to Achieve Reliable and Accurate CFD Solutions UK/staticassets... · Strategies to...
Transcript of Strategies to Achieve Reliable and Accurate CFD Solutions UK/staticassets... · Strategies to...
© 2010 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary© 2010 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary
Strategies to Achieve
Reliable and Accurate
CFD Solutions
Mark Keating
ANSYS UK
© 2010 ANSYS, Inc. All rights reserved. 2 ANSYS, Inc. Proprietary
Agenda
• Why develop a CFD strategy?
• Pre-Processing Strategies
• Solver Strategies
• Post Processing Strategies
• Full Process Strategies
• Summary
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Why Develop a CFD Strategy?
• Reliable results means a consistently accurate
result
• Using default settings is never optimised for each
application for speed/accuracy
• Process can be prone to user error
• Hone an optimised strategy for the application to
prevent deviation and maintain high quality
process
• Ensures repeatably accurate solution
• Allow full design space appreciation faster!
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Pre-Processing Strategies
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Pre Processing Considerations
• Think ahead about what you want to do and gain
from the CFD analysis
• What are the driving parameters?
• What zones need to be separate for constraints
or post processing?
• What fluids zones will be replaced?
• What level of geometric representation is
needed?
• Small changes can have large effects
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Mesh Quality Affects Accurate
and Reliable Result
• Geometry problems
– Small edge
– Gaps
– Sharp angle
• Meshing parameters
– Sizing Function On / Off
– Min size too large
– Inflation parameters
• Total height
• Maximum angle
– Hard sizing
• Meshing methods
– Patch conformal or patch independent tetra
– Sweep or Multizone
– Cutcell
Geometry cleanup in Design Modeler
or
Virtual topology & pinch in Meshing
Mesh setting change
Mesh setting change
Direct meshing can be used to
minimize remeshing time
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Mesh Quality Affects Accurate
and Reliable Result
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(max,avg)CSKEW=(0.912,0.291)
(max,avg)CAR=(62.731,7.402)
(max,avg)CSKEW=(0.801,0.287)
(max,avg)CAR=(8.153,1.298)
VzMIN≈-100ft/min
VzMAX≈400ft/min
VzMIN≈-90ft/min
VzMAX≈600ft/min
Large cell size
change
Mesh 2
Mesh 1
Mesh Quality Affects Accurate
and Reliable Result
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Parameterization in ANSYS
Meshing
Meshing controls can be parameterized
– Global controls and local controls
– Selection of parameter promotes the parameter to the
WB project page
– Geometry and Meshing parameters can be related using
expressions in the parameter manager
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Parameterization project
example
• Number of divisions
on the outlet pipe
equal to two times its
length
• Number of divisons
on the inlet pipe equal
to its length + 4
8+
4=
12
div
isio
ns
Inlet
Outlet
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Parameterization project
example
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Pre-Processing Scripting
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Systematic Errors
• Discrepancies remain, even if numerical and model errors are insignificant
• „Systematic errors‟:
– Approximations of:• Geometry
• Component vs. machine
• Boundary conditions (Turbulence, profiles, …)
• Unsteady-state flow behavior
• Fluid and material properties, …
• Try to „understand‟ application and physics
• Document and defend assumptions !
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Poor quality mesh
Reducing Errors:
Some Best Practice Guidelines
• Grid quality
– Grid angles 90° for hex
affects truncation error
– Recommendation• Good: 20° < α < 160°
• Fair: 5° < α < 20° & 160° < α <
175°
• Poor: α < 5° & α > 175°
• Skewness = f(α)
High quality mesh
not scalable scalable
90
90,
90
90minmaxmax
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• Use AMP, FLUENT and CFX to check mesh
Mesh Quality Affects Accurate
and Reliable Result
+--------------------------------------------------------------------+
| Mesh Statistics |
+--------------------------------------------------------------------+
Domain Name: Air Duct
Minimum Orthogonality Angle [degrees] = 20.4 ok
Maximum Aspect Ratio = 13.5 OK
Maximum Mesh Expansion Factor = 700.4 !
Domain Name: Water Pipe
Minimum Orthogonality Angle [degrees] = 32.8 ok
Maximum Aspect Ratio = 6.4 OK
Maximum Mesh Expansion Factor = 73.5 !
Global Mesh Quality Statistics :
Minimum Orthogonality Angle [degrees] = 20.4 ok
Maximum Aspect Ratio = 13.5 OK
Maximum Mesh Expansion Factor = 700.4 !
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Solver Strategies
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Initialising
• Many options beyond simply by zone
• FMG-I (steady state single phase flows)
• Hybrid initialisation (poorer quality grids)
• Interpolation Files
– transfer data from one grid to any another (eg coarse
to fine mesh or different geometry)
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Pre-Processing in ANSYS CFD
Workflow Parameters
• Input parameters
– Associate with multiple
boundaries
– Manage in a single panel
• Output parameters
– Quantitative values
– Report all at once
• ANSYS Workbench
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Pre-Processing in ANSYS CFD
ANSYS Fluent
• Automatic Solution
Initialization and Case
Modification
• Automatically executed user
specified solution strategies
• Journal setup
• Spatial interpolation
files for better start
• Gradually ramp up
conditions
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Pre-Processing in ANSYS CFD
ANSYS CFX and CCL
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Choosing the solver
• Segregated solver remains default in FLUENT
• PBCS typically 5x faster, though can be orders of magnitude. Solves the continuity and momentum correction equations in a coupled implicit manner. Works by dampening out pressure-velocity decoupling errors inherent with segregated solver promoting faster convergence
• PBCS is much more stable on poor quality mesh (high skewness, high aspect ratio, jumps in cell size) and applicable for all flow regimes. Recommended for all but highly compressible flows.
• DBNS remains choice when there is a strong interdependence of momentum, energy and density
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Turbulent Flow over a Backward
Facing Step
Problem Description:
•ReH = 37,400
•Inlet height = 8H
•Outlet/Inlet area ratio = 1.125
•Standard k-w model
•EWT
•Inlet profiles for u,v,k,w
•21,750 quad cells
•Mesh weighted towards
walls and backstep
Reference:
D. M. Driver and H. l. Seegmiller. Features of reattaching turbulent shear layer in
divergent channel flow. AIAA Journal, 23:163-171, 1985.
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PBCS Solver
Settings:
•CFL = 200
•ERFs = 0.75
•PRESTO! for pressure
•MUSCL all other eqs
Contours of
Velocity
Magnitude
from PBCS
Skin Friction Coefficient (Cf*1000)
.vs.
Distance behind Step (X/H)
Pressure Coefficient (Cp)
.vs.
Distance behind Step (X/H)
Turbulent Flow over a Backward
Facing Step
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Solver Memory (MB)Time per
Iteration (s)
Iterations to
Convergence
Time to
Convergence (s)
Segregated 73.2 0.288 2677 771
PBCS 80.7 0.500 494 247
Results from the different solvers
An accurate result can be obtained in a fraction of the time
Turbulent Flow over a Backward
Facing Step
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Solver Options
to Improve Accuracy
• Steady state VOF. Use BGM instead for faster results but
comparable accuracy to geo-reconstruct.
• Transient Multiphase. Consider NITA or variable
extrapolation for faster transient results.
• High accuracy VOF solution maintained using new
compressive scheme and applied by zone or phase
• Conjugate heat transfer. Use W-cycle for energy with
BCGSTAB for stability and accuracy.
• Single Phase flows. Use F-cycle for flow and turbulence.
• Use NBG for high accuracy. More stable (reliable) than
cell based default gradient scheme
• DBNS has solution steering by regime
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RBF Morpher – An Example of a
Fast, Reliable Process
• Radial Basis Function Morpher
• ANSYS Partner
• Designed by Marco Biancolini @ Rome Uni
• Embedded in FLUENT
• Morph in parallel on clusters
• Zero File I/O between designs
• Fast convergence from previous design
• High levels of control on boundaries moving or not
moving
• Easily scripted and connected to optimisation codes, e.g.
iSight, ModeFrontier, ANSYS Design Explorer, etc...
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RBF Morpher
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Solver Scripting
• The Fluent and CFX solvers have their own scripting that can be run interactively (open) or batch (closed)
• A journal file contains a sequence of TUI (Fluent) or CCL objects/commands (CFX), arranged as they would be typed interactively into the program or entered through the GUI.
• Fluent‟s GUI commands are recorded as Scheme code lines in journal files for re-play. FLUENT records everything you type on the command line or enter through the GUI.
• CFX, CFX-Pre, CFD-Post and TurboGrid commands invoked by Perl script.
• You can also create these scripts manually with a text editor. Comments can be included.
• Ensures to prevent any lost time due to incorrect setup
• Ramp up solver settings gradually
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Solver Scripting
• Anything you normally do can be written as a script. Some
typical examples are:
– Running a batch job or RSF submission
– Setting up complex material properties (alternative to read
boundary conditions)
– Setting up a simulation
– Data analysis/post processing
– Transient data analysis
– Automating a known convergence strategy
• Wild card support at R13 (Fluent post operations)
report>surface integrals “*outlet*”
• Combined with batch solve, whole process can be run
“hidden” and is very repeatable & controlled
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Employing CFD Strategy through
scripting
• Gradually ramp up conditions using staged process
• Rotating solid example process
– Start single phase with all fluid zones stationary and set
first order with conservative CFL
– Initialise using FMG-i and solve
– Switch on energy and enable thermal boundary conditions
then solve further
– Switch to second order/PRESTO/MUSCL and solve
– Change fluid to solids and solve further
– Change solid zone to MRF zone at N rpm and solve
– Switch to aggressive settings (CFL, AMG stabilisation)
and solve final section before reporting
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Sources of Solver Error
• Round-off errors
• Iteration errors
– Difference between „converged‟ solution and solution at
iteration „n‟
• Solution errors
– Difference between converged solution on current grid
and „exact‟ solution of model equations
– „Exact‟ solution Solution on infinitely fine grid
• Model errors
– Difference between „exact‟ solution of model equations
and reality (data or analytic solution)
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Iteration Error – Example
Residuals
Check for monotonic convergence
© Siemens PG
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Iteration Error – Example
Res=10-2
Iteration 35
Res=10-3
Iteration 59
Res=10-4
Iteration 132
Relative error:
0.18% 0.01%
Ise
ntr
op
ic E
ffic
ien
cy
Convergence criterion
Iteration Number
Iteration errors:
Difference between
„converged‟ solution
and solution at
iteration „n‟
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Discretisation Error – Example
• Compressor cascade
• Residual = 1 10-4
• 2nd order discretization scheme
Grid 1 Grid 3Grid 2
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Model Errors - Example
• Inadequacies of (empirical)
mathematical models:
– Base equations (Euler vs.
RANS, steady-state vs.
unsteady-state, …)
– Turbulence models
– Combustion models
– Multi-phase flow models
• Discrepancies between data
and calculations remain,
even after all numerical
errors have become
insignificant
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Model Error - Example
Model error: k-
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Solver Side Changes
ANSYS Fluent
• You need not always revert back to the geometry
or meshing to change the grid
• Extrude domain (3D)
• Separate face or cell zones
• Adapting grids (grid independence)
• Deactivate/Activate cell zones
• Delete/Append cell zones
• Mesh swapping in parallel (R13)
• Append case/data in parallel (R13)
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Post Processing Strategies
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Post-Processing in ANSYS CFD
Expressions, State & Session Files
• CFDPost Expressions (user defined outputs eg
pressure co-efficients) can be pre-defined and
called via CCL, session or state file for quick
analysis (like Custom Functions)
• CFDPost state files can be written and read to
allow same objects to be used on different
results file ensuring consistency
• CFDPost can record and replay a session file to
repeat repetitive operations and reduce user
error
• Fluent Post Journals
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Post-Processing in ANSYS CFD
Post object definition
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Post-Processing in ANSYS CFD
Case Comparison
Click to
activate
Select two of the loaded
cases or two timesteps
• Objects can be
locked across
models
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Workbench Integration of
CFDPost
• No need to worry about files
• No need to save/load state (auto-saved on
close)
• All project files saved in one shot, including CFD-
Post state
• Automatic refresh of files when they change
• Integration with ANSYS DX for Optimisation
• Automatic Report creation
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Full Process Strategies
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Pre-Processing in ANSYS CFD
Workbench Integration
• All in one simulation – huge saving of effort!
• Project schematic can be stored for re-use
• Customised schematics can be generated
(ensuring process consistency for quality and
reducing user deviation/error)
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Design Updates in ANSYS Workbench
1. Change geometry dimensions and/or
boundary conditions
2. Generate updated results with the click
of a button.
1. Change the geometry in the CAD
system
2. Export a STEP, Parasolid, or ACIS file
from the CAD system
3. Import the STEP or other file into
geometry tool
4. Reclean/re-simplify the geometry, often
from scratch
5. Recreate the mesh, often from scratch
6. Export the mesh
7. Import the new mesh into CFD solver
8. Re-apply the physics setup
9. Calculate the new CFD solution
10.Redo post-processing
ANSYS Workbench Workflow Traditional CFD Workflow
This is an enormous time
savings for even the most
trivial geometry changes!!!
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Save Project as Custom System
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Pre-Processing in ANSYS CFD
Scripting Overview
• ANSYS 12.1 fully supports Workbench journaling and scripting – Project concepts & operations
– Parameter management
– Native applications• Project Schematic, Design Exploration, Engineering Data
– File management and data models
• Python-based scripting language– Object-oriented
– Platform-independent
• Fully documented & supported
• Works “hand-in-hand” with application-level scripting– DesignModeler, Meshing,
Mechanical, Mechanical APDL,FLUENT, CFX, etc.
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Pre-Processing in ANSYS CFD
Workbench Journaling
• Workbench operations are recorded in a journal file
• Each session creates a new journal file
• Playing back the journal recreates the session
• Two types of Workbench journals
– Automatically recorded session journals
– Manually recorded journals
• Tools -> Options… -> Journals and Logs
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Post-Processing in ANSYS CFD
Plane Creation in CFD-Post
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Spreadsheet Controller
A simple spreadsheet can be used to control or set up
workbench workflows thanks to IronPython. (Iron Python is
the journaling language of workbench but also allows you
to program and link to other applications through the .NET
framework.).
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Data Entry Tab
Data entry
including analyst
and simulation ID
describes the
simulation
Status/progress is
is reported here
Numerical reports
retrieved and
displayed on
completion
Do the stuff buttons
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Recorded simulations
On completion of the run the background script will
record all your settings and the results on the next
worksheet
Hyperlinks takes you to a
detailed automatically
generated HTML detailed
report with graphics
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Summary: CFD Strategy
• Walk before you can run
• Think about what you want to gain in advance of
setting up the simulation
• Is process repeatable?
• Do I need to have a constrained process?
• What extensibility tools can I use? (UDF‟s, CCL)
• Look to combine strategies
• Look beyond the defaults
• Construct a reliable and accurate process for
your application
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Summary: Reducing Errors
• Representative mesh
• Define target variables:
– Pressure loss
– Efficiency
– Mass flow rate
– …
• Select convergence criterion (e.g. residual)
• Plot target variables as a function of convergence criterion
• Set convergence criterion such that value of target variable
becomes „independent„ of convergence criterion
• Check for monotonic convergence
• Check convergence of global balances
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Summary
• Quality assurance is essential for industrial use of CFD
• Ensure all details captured suitably
• Accept and understand the sources of error
• Quantify and reduce numerical errors by deterministic and
rational procedures
• Quantify model and systematic errors by validation work
• Resources:
– ERCOFTAC SIG: „Quantification of Uncertainty in CFD‟
– CFD Best Practice Guidelines for CFD Code Validation for Reactor-
Safety Applications
– ANSYS CFD Best Practice Guidelines
– Your helpful ANSYS support office