AutoSim Nov2006a

Current and Continuing Issues in CFD

Althea de Souza

AutoSim, 3rd Technology Workshop,

Lisbon, November 2006

www.nafems.org


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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

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1. Accuracy

• Validation

• Turbulence

• Convergence

• Mesh quality

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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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• Pressure recovery factor (efficiency) of a draft tube

Industrial Validation Exercise

Created by John Bergstrom, Luleå University of Technology

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Experimental Result

With permission of: Professor Tony Hutton

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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

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ECN

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ECN

FLUENT

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-IST

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SVA

-AEA

USDDC

SOTO

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KRIS

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ENTS

Typical Industrial Example

With permission of: Professor Tony Hutton

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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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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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Accuracy – Mesh Quality

• Mesh independence

• Knowledge of the flow

• Understanding of the solver (use of the mesh)

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2. CAD geometry

• The problems

• The options

• Two way CAD transfer

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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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Geometry issues - examples

• Filter housing with multiple sliver surfaces

• Assembly

• Seals / o-rings

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Geometry issues - examples

• Multiple parts from multiple sources

• CAD with overlapping surfaces, gaps

• No external domain

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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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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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3. Users

2 Categories

• Dedicated CFD analysts

– New users

– Expert users

• Occasional users

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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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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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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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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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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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5. Computational Resources

• Speed

• Memory

• Remote solving

– Confidentiality and export issues

• Licensing costs

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6. New Challenges

• Optimisation

• Integration

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New Challenges - Optimisation

• Mesh morphing

• Manufacturing variability

• DoE and Response surfaces

• Topology!!

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New Challenges - Integration

• PDM / PLM

• System simulation tools

• Multi-physics

• CAD and design suites

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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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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.

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1. Accuracy

2. CAD geometry

3. Users

4. Application areas

5. Computational resources

6. New challenges

Documents

AutoSim Nov2006a