2.1 CFD PROJECT PLANNING 2006 ANSYS, Inc. All rights reserved.
2008 ANSYS, Inc. All rights reserved. 6-2 CFD PROJECT PLANNING Task definition Resources Timing and deliverables Design review Assumed part of bigger engineering project, but same principles apply to stand-alone consultancy
2008 ANSYS, Inc. All rights reserved. 6-3 TASK DEFINITION How can we break a CFD job into activities? How can work be shared? How do we measure progress? What are the inputs and outputs for each task? Where can bottlenecks occur?
2008 ANSYS, Inc. All rights reserved. 6-4 Generic CFD project plan CFD project activities are serial in nature Not all activities are full-time: share time across projects TASK RESOURCE TIME Define objectives Define model features / physics Define geometry Define inputs (prelim calcs) Import / build geometry Build mesh Specify physical model Specify boundary conditions Solve equations Post-process / export data Review results Write report Store / archive Consultative Modeler Engineer / consultative Engineer / modeler CAD op / modeler Modeler Modeler Modeler Modeler, part-time Modeler / consultative Eng / mod / cons Modeler Modeler
2008 ANSYS, Inc. All rights reserved. 6-5 Objectives - 1 Objective should never be to do a CFD model - unless you just want some colour pictures! Discuss the objectives with the modeler, explaining the engineering context and the accuracy required Get the modeler s agreement to provide the specific information that you need
2008 ANSYS, Inc. All rights reserved. 6-6 Objectives - 2 Clear and precise quantitative questions: What is the pressure change from A to B? What are the heat transfer coefficients on surface C? What are the forces acting on body D? What is the air-fuel unmixedness at plane E? Qualitative studies are OK, if you know what to do with the results!
2008 ANSYS, Inc. All rights reserved. 6-7 Define model features/physics Many physical modelling options provided in Fluent Selection by default adequate for many users Adopt cook-book approach for similar jobs Turbulence model, fluid properties should always be considered Non-standard physical models require UDFs - add greatly to project workload
2008 ANSYS, Inc. All rights reserved. 6-8 Geometric information Identify source of geometry to be modeled Select an appropriate calculation domain Omit irrelevant geometrical detail Do not omit details which you know to have an important effect in reality
2008 ANSYS, Inc. All rights reserved. 6-9 Inputs and preliminary calcs Good input data for CFD calculation are vital to success Flow rates, velocity distributions, boundary pressures, temperatures Usually necessary to make estimates or hand calculations for some quantities Turbulence data hardly ever known Discuss source of input data with modeler
2008 ANSYS, Inc. All rights reserved. 6-10 Geometry building Maybe the biggest single task in a CFD job CAD import can be effective, but clean-up can be timeconsuming Ask the CFD modeller to explain his requirements to the CAD operator before the CAD model is created
2008 ANSYS, Inc. All rights reserved. 6-11 Mesh generation Often the biggest single task, but increasingly automated Plan the mesh - type, overall structure, size requirements Decide how mesh is to be built up from geometry, and required sequence of operations
2008 ANSYS, Inc. All rights reserved. 6-12 Physics and boundary conditions Menu selection and data input process The most common source of errors in carrying out a job Good preparation essential - prepare data and do preliminary calculations before starting to input boundary conditions
2008 ANSYS, Inc. All rights reserved. 6-13 Solution Can be easy or time-consuming, but not normally a full-time job Solver optimised for standard flow equations on good quality mesh Large source terms in equations tend to make solution more difficult: eg. buoyancy, chemical heat release, radiation Resist temptation to tweak solver controls (under-relaxation) unless necessary
2008 ANSYS, Inc. All rights reserved. 6-14 Post-processing and data export Post-processing necessary to verify solution quality understand qualitative aspects of flow determine answers to objective questions produce material for reports Data can sometimes be exported directly for further analysis eg. boundary conditions for stress analysis
2008 ANSYS, Inc. All rights reserved. 6-15 Reporting Conform to requirements of QA system State objectives Identify source of data and preliminary calculations Identify code, version, modelling options selected and reasons why State important results Provide details of archive/data storage
2008 ANSYS, Inc. All rights reserved. 6-16 Storage/archive Media incompatibility a major challenge Magnetic media (tapes, disks, cartridges) have life limitations CD or DVD-ROM recommended current option Store one copy with project file
2008 ANSYS, Inc. All rights reserved. 6-17 Resources Human resources CFD skills and experience Management Computers Software
2008 ANSYS, Inc. All rights reserved. 6-18 What makes a good CFD modeler? Good fluids engineering background recognises and understands fluid behaviour relates to engineering objectives and context able to read and interpret drawings/cad! Basic understanding of CFD principles Able to think and work in 3D Trained and experienced in use of software
2008 ANSYS, Inc. All rights reserved. 6-19 Who can do CFD? Many activities do not require specialist Software improvements and on-line support are aimed at de-skilling CFD Definition of problem and analysis of results requires fluid dynamics know-how Use of CAD operators and stress analysts is feasible, but close supervision by fluid dynamicist is almost essential
2008 ANSYS, Inc. All rights reserved. 6-20 Management of CFD CFD is not an exact science: Basic equations correct, but turbulence modelling approximate Finite mesh size limits accuracy of solution Experience and know-how needed to interpret solution and judge quality Important role for technical management Make effective use of Fluent Support and other external resources
2008 ANSYS, Inc. All rights reserved. 6-21 Computers Affordable computer hardware now up to the job Clusters, workstations, top end PCs (Win-32, Win-64, Linux) 1M+ computational cells the norm Typical need is 1Gb RAM for 1M cells BUT REMEMBER Design the job to fit the computer!
2008 ANSYS, Inc. All rights reserved. 6-22 Software The choice is obvious...
2008 ANSYS, Inc. All rights reserved. 6-23 Timing and Deliverables Time estimation Milestones Managing the processes The unforeseen Job completion
2008 ANSYS, Inc. All rights reserved. 6-24 Time estimation Time-consuming activities are skills-based plan according to the individual Base estimates on experience of similar job Uncertainties usually occur due to geometry creation and mesh generation Slippage most frequently results from rework due to inadequate, incomplete or changing data
2008 ANSYS, Inc. All rights reserved. 6-25 Milestones Major milestones used to measure progress Data and geometry complete Mesh complete Solution converged Project complete Final deliverables Fluent data files Answers to objective questions Report
2008 ANSYS, Inc. All rights reserved. 6-26 Managing the processes - 1 Attempt to define standard approach QA procedure, best practice guidelines Resist temptation for modeler to try the effects of different options! Data matching encourages this tendency Grid refinement, validation and calibration should be carried out separately, not as a normal part of an engineering project
2008 ANSYS, Inc. All rights reserved. 6-27 Managing the process - 2 Get the modeler talking to the engineers explanation uncovers errors Manage data exchange between the modeler and other team members use engineering co-ordination memo or similar configuration control system Don t get blinded by science!
2008 ANSYS, Inc. All rights reserved. 6-28 The unforeseen Turbulence models are inexact Fluid dynamics is non-linear chaotic behaviour can result! Even the best software can contain bugs impossible to test all combinations of options Critical changes can occur at a late stage so allow reasonable contingency - it is often necessary to repeat parts of work
2008 ANSYS, Inc. All rights reserved. 6-29 Job completion Reporting and archiving essential tasks generally high turnover of CFD staff difficult to pick up someone else s model without notes Typically 20% of project remains after converged solution obtained post-processing, report, review, presentation
2008 ANSYS, Inc. All rights reserved. 6-30 DESIGN REVIEW Purpose Content Reviewers
2008 ANSYS, Inc. All rights reserved. 6-31 Purpose of Design Review To verify that: calculations were fit for purpose (ie. appropriate to original objectives) model conforms to intent, and free from obvious errors results have been interpreted correctly To review and ratify engineering implications of results
2008 ANSYS, Inc. All rights reserved. 6-32 Design review content Presentation of model Physical models, critical areas of mesh, boundary conditions Presentation of results Overall fluid behaviour, key objective results Summary of engineering implications Critical discussion
2008 ANSYS, Inc. All rights reserved. 6-33 Reviewers Engineering and CFD know-how required hard in many organisations to provide both key questions relate to quality, uncertainty, and best practice engineering judgement has important role ANSYS Fluent can provide resource to contribute to either side of review process
2008 ANSYS, Inc. All rights reserved. 6-34 Summary Guidelines offered for project managers in handling CFD tasks CFD is not fundamentally different from many other technically challenging tasks beware of the specialist! ANSYS support and consultancy services are available to help