Download - AutoSim Nov2006a
Current and Continuing Issues in CFD
Althea de Souza
AutoSim, 3rd Technology Workshop,
Lisbon, November 2006
www.nafems.org
Current and Continuing Issues in CFD
2
The Issues
1. Accuracy
2. CAD geometry
3. Users
4. Application areas
5. Computational resources
6. New challenges
Source of data:
Personal view
NAFEMS CFD survey 2005
NAFEMS Technology Strategy Plan
www.nafems.org
Current and Continuing Issues in CFD
3
1. Accuracy
• Validation
• Turbulence
• Convergence
• Mesh quality
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Current and Continuing Issues in CFD
4
Accuracy - Validation
• Physical validation is often difficult and may be
impossible.
• Difficult for a non-expert to assess analysis quality.
• Even competent analysts can get a wide variety of
answers for the same problem.
• Uncertainty can be minimised by good model design
and understanding of flow fundamentals.
• No fixed level of credibility is appropriate to all CFD
simulations.
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Current and Continuing Issues in CFD
5
• Pressure recovery factor (efficiency) of a draft tube
Industrial Validation Exercise
Created by John Bergstrom, Luleå University of Technology
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0.8
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Experimental Result
With permission of: Professor Tony Hutton
www.nafems.org
Current and Continuing Issues in CFD
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• Results from the Workshop on CFD in
Ship Hydrodynamics, Gothenburg
2000
• Form factor prediction for the KRISO
300K tanker hull
• Form Factor = CT/CFO-1
• Variation Coefficient =
26.4%
• Different results from the
same code and turbulence
model
• Different results from
different turbulence models
• Variation increased at full
scale0
0.05
0.1
0.15
0.2
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0.35
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CTU
ECN
ECN
ECN
FLUENT
MARIN
-IST
SRI
SRI
SVA
-AEA
USDDC
SOTO
N
KRIS
OM
SU
EXP
ERM
ENTS
Typical Industrial Example
With permission of: Professor Tony Hutton
www.nafems.org
Current and Continuing Issues in CFD
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Accuracy - Turbulence
• The lack of universally applicable turbulence models renders CFD inherently uncertain.
• Direct Numerical Simulation (DNS) of flow over an aircraft would take years on the most powerful computers currently available.
• Even Large Eddy Simulation (LES) of flow over an aircraft not practical for some time.
• As a result models are needed for turbulence.
• Industrial practice will rely on Reynolds Averaged Navier-Stokes methods (RANS) for the foreseeable future.
• Such models can be trusted only within their limited range of calibration.
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Current and Continuing Issues in CFD
8
Accuracy - Convergence
• Improved solver robustness and stability means many simulations converge quickly and reliably.
• For complex models convergence may still be elusive.
• Divergence still happens.
• No universal agreement on ‘sufficient’ convergence.
• Default settings are often inadequate.
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Current and Continuing Issues in CFD
9
Accuracy – Mesh Quality
• Mesh independence
• Knowledge of the flow
• Understanding of the solver (use of the mesh)
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Current and Continuing Issues in CFD
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2. CAD geometry
• The problems
• The options
• Two way CAD transfer
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Current and Continuing Issues in CFD
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CAD geometry - Problems
• Inverse volumes (not the regions modelled in CAD)
• Removal of unnecessary features
• Small surfaces and CAD surface definition
• Assemblies
• Watertight geometry
• Computational domain extents
• Meshing tools that are too dependant on the
geometry
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Current and Continuing Issues in CFD
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Geometry issues - examples
• Filter housing with multiple sliver surfaces
• Assembly
• Seals / o-rings
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Current and Continuing Issues in CFD
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Geometry issues - examples
• Multiple parts from multiple sources
• CAD with overlapping surfaces, gaps
• No external domain
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Current and Continuing Issues in CFD
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CAD geometry - Options
• CAD user clean-up.
• CFD user clean-up.
• Different understanding of ‘simplification’ and
‘clean-up’.
• Different tools and capabilities
– CAD packages often focus on manufacturing needs.
– CFD pre-processors focus on meshing.
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Current and Continuing Issues in CFD
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CAD geometry – Data Transfer
• CAD to dedicated CFD mesher
• CAD to general mesher
• Design changes back to CAD
• Fundamental difference between geometry
definition requirements
– for meshing
– for manufacturing
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Current and Continuing Issues in CFD
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3. Users
2 Categories
• Dedicated CFD analysts
– New users
– Expert users
• Occasional users
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Current and Continuing Issues in CFD
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Dedicated Users - New
• CFD is inevitably a knowledge-based activity.
Sophisticated analysis codes are widely available, the
knowledge base is not.
• The result is a simulation rather than a solution which
must be interpreted from a position of considerable
knowledge.
• Thus practitioners must be skilled in CFD as well as
proficient in the engineering application under analysis.
• Different applications require different training.
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Current and Continuing Issues in CFD
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Dedicated Users - New
• How to recruit.
• How to train.
– Vendor courses
– Self-training: manuals, text books, tutorials
– Undergraduate courses
– Post graduate course
– Independent courses (NAFEMS)
• Stress office approach.– team of analysts at different levels?
• There is no established industry-recognised certification of CFD competency.
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Current and Continuing Issues in CFD
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Dedicated Users - Expert
Not much to say - similar issues to other fields, however:
• There are currently (2003) between 25,000 and 30,000 regular industrial users of CFD codes.
• The annual growth of CFD code sales ~15 to 20%
• The growth of CFD software sales exceeds the rate at which analysts can be trained.
• Several thousand new practitioners must be supplied each year - these cannot be fully trained experts.
Data courtesy of: Professor Tony Hutton
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Current and Continuing Issues in CFD
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Occasional Users
• Not experts
• Design Engineers
• Integrated software
• Use of wizards
• Guidelines to ensure underlying physics is capture
• Fully automatic ‘invisible’ meshing
• Checks on set up and solution
• Validation and confidence
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Current and Continuing Issues in CFD
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4. Application Areas
• Traditional fields
– Aerospace, Automotive, Process etc
• New fields
– Medical & surgical
– Manufacturing processes
– Environmental
– Specific individual geometry (people, landscapes)
– etc.
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Current and Continuing Issues in CFD
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5. Computational Resources
• Speed
• Memory
• Remote solving
– Confidentiality and export issues
• Licensing costs
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Current and Continuing Issues in CFD
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6. New Challenges
• Optimisation
• Integration
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Current and Continuing Issues in CFD
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New Challenges - Optimisation
• Mesh morphing
• Manufacturing variability
• DoE and Response surfaces
• Topology!!
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Current and Continuing Issues in CFD
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New Challenges - Integration
• PDM / PLM
• System simulation tools
• Multi-physics
• CAD and design suites
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Current and Continuing Issues in CFD
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NAFEMS activities in these areas
1. Accuracy– Booklets and Seminars
2. CAD geometry– Booklet (undergoing update)
3. Users– Seminars, Courses, Networking, Code vendors
4. Application areas– Involvement with code vendors
5. Computational resources– Exhibition areas at events
6. New challenges– UGMs, vendor involvement, interaction within NAFEMS
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Current and Continuing Issues in CFD
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Conclusions
• CFD is being used widely in industry and in a wide
range of applications.
• CFD can and is being carried out by non-experts in
some circumstances.
• There are still challenges and difficulties out there for
code developers and expert users.
• Many of the issues have already been encountered
by the stress analysis community but some are new.
• There is no established industry-recognised
certification of CFD competency.