Concurrent CFD Explained (Part IV)

Last time I promised I’d drill down into more detail about the difference between the CAD-integrated up-front approach to CFD and CAD-embedded Concurrent CFD. CAD-integrated CFD tools are essentially stand-alone tools launched from within the CAD system whereas Concurrent CFD is fully built into the CAD system. So what’s the real difference between the two?

CAD-integrated CFD tools use the same approach for all CAD systems, so perhaps the best way to answer this is to look at what happens when the CFD tool gets launched from within the CAD system: First the original CAD model gets converted into the format required by the solid modeller built into the CFD tool, e.g. Parasolid. This converted geometry is then imported as parts and assemblies into the CFD program. So far, so good – so what’s the snag?

The geometry that gets transferred from the CAD system into an up-front CFD tool comes over ‘warts and all’. The implications of this are not apparent when dealing with rudimentary demo geometries, since these have very few parts with simple mates. However, real world CAD geometry contains dozens, sometimes hundreds of unwanted (and often very small) voids resulting from modeling simplifications that are common practice in CAD design.

All geometry parts including all new dummy parts that represent actual flow cavities in the model get some default material property assigned, e.g. that of air. The user has to manually select all the parts that should not belong to the computational domain and mark them as unwanted (otherwise they get meshed and treated as flow or solid domains). This includes marking small voids as unwanted to ensure that only the few desired fluid regions (most often only one) remain. Meshing these small voids would otherwise cause very high additional and unnecessary cell count, and, if not properly meshed, extreme solver convergence problems. Finally, the user has to manually assign the correct material properties to all fluids and solids remaining in the model and apply boundary conditions.

Small voids in a valve body correctly ignored in Concurrent CFD

Small voids in a valve body correctly ignored in Concurrent CFD

This all appears very quick and easy when working on a simple demo case with just a few solid blocks to represent something more complicated like an automobile – if you blink you’ll miss it. For a large number of parts is very laborious, time-consuming, and error-prone, because all solids are listed even if no heat transfer considered. Obviously, the user has to be meticulous in manually working through all the listed parts in the model and assign materials correctly.

Consequently, when using CAD-integrated CFD it is often much more efficient (especially for flow-only tasks) just to create the required flow domain in the CAD system using CAD functions for Boolean operations (“Cavity” function) and then transfer only this one part to the CFD tool for meshing and solving. This is the ‘Create Cavity’ step shown in my ‘Concurrent CFD Explained (Part II)’ post and something that you never have to do in Concurrent CFD.

A second issue arises when the user wants to modify the geometry. Quite rightly, this has to be done in the CAD system. Substantive design changes are common early in design when very little of the design has solidified, and so CFD can bring the most benefit as changes can be made freely in the pursuit of improved performance, reduced cost, etc. Moving geometry around is no problem, but making substantial changes that alter the topology of the model, for example by creating different parts, or building the assembly differently basically means that in CAD-integrated CFD the whole process has to be repeated.

Hopefully that’s given you a broad understanding of the state-of-the-art in up-front CAD-integrated CFD, described as being “fully associative” with the CAD geometry. Concurrent CFD resides fully within the CAD system so does not suffer from these issues. Full CAD embedding gives access to details of the native CAD geometry. This has allowed us to develop proprietary technology that automatically ignores all cavities in a CAD model that are not intended to be a computational domain for meshing and solving. It automatically handles solids and fluids appropriately, consistent with the user’s choice about whether to include heat transfer in the project or not, with no need for the kind of manual user intervention required by the CAD-integrated CFD approach.

One of the latest developments in up-front CFD is the concept of a ‘design study’, allowing different designs to be compared. This emulates the project management system that’s available in all CAD packages – and something that has been standard in Concurrent CFD for many years as Concurrent CFD utilises the CAD system’s capabilities. Another key advantage of Concurrent CFD is that the results can be displayed on the actual CAD geometry to aid understanding. Again, this is hard to achieve but delivers real benefit to the user in interpreting the analysis results.

I hope this provides more detail on the differences between the CAD-integrated upfront CFD and the CAD-embedded Concurrent CFD approaches illustrated in the animation below.

You can also check out the joint MAD/CIMdata presented web seminar entitled ‘CFD Analysis for MCAD Designs – Better Products Faster’. If you want to see Concurrent CFD in action register now for next week’s web seminar ‘Flow, Pressure, Cavitation! Use ‘X-Ray Vision’ to Avert a Design Disaster’.

Dr J, Hampton Court

Post Author

Posted March 12th, 2010, by

Post Tags

, , , , , , , , ,

Post Comments

1 Comment

About John Parry’s Blog

A mixed bag of things that interest me professionally -CFD technology and its use in education, cooling technologies and the place of thermal design in the overall design flow. John Parry’s Blog

Comments

One comment on this post | ↓ Add Your Own

[…] If you’re working in a fast-paced design environment and need a robust tool that design engineers can use I would encourage you to check out the breadth of applications that FloEFD covers despite what some “CFD Comparison Charts” might say about it, which includes complex physical phenomena like non-Newtonian fluids, compressible flow, combustion and cavitation. FloEFD is CAD-embedded, allowing you to evaluate the product’s performance concurrently with changes to the design – something we term “Concurrent CFD”. To get a feel for how Concurrent CFD is different to Traditional CFD approaches and Upfront CFD, take a look at these blog posts… Concurrent CFD Explained (Part I) Concurrent CFD Explained (Part II) Concurrent CFD Explained (Part III) Concurrent CFD Explained (Part IV) […]

Add Your Comment

Archives

May 2014
  • Ramping Up for THERMINIC! Abstract Deadline 28th May 2014
  • February 2014
  • FREE Exhibition at SEMI-THERM 30 Conference, San Jose March 9-13
  • August 2010
  • Lies, Damned Lies, and “CFD Comparison Charts” – Part IV
  • Lies, Damned Lies, and “CFD Comparison Charts” – Part III
  • Lies, Damned Lies, and “CFD Comparison Charts” – Part II
  • Lies, Damned Lies, and “CFD Comparison Charts” – Part I
  • July 2010
  • Mechanical Analysis Products Now in Mentor’s Higher Education Program
  • Sony Vaio laptop in mass ‘recall’
  • June 2010
  • Nearly Back to Business as Usual
  • April 2010
  • The Debate about Liquid Cooled Data Centers
  • MicReD Technology Wins Highest Technical Honor
  • March 2010
  • Sticking Plaster and Light beats Skin Cancer
  • IBM Work to take Moore’s Law to 2025
  • Concurrent CFD Explained (Part IV)
  • Roundup of SEMI-THERM, FloTHERM IC launch and JEDEC
  • February 2010
  • NEW ElectronicsCooling Magazine Website
  • Liquid Cooling – Are We There Yet?
  • Foresight and X-Ray Vision or Hindsight and Regret?
  • FREE Exhibition at SEMI-THERM, Santa Clara February 23-24
  • Force Prediction with Concurrent CFD
  • January 2010
  • Champcar Exhaust Analysis
  • Concurrent CFD Explained (Part III)
  • Stop Press: New Electronics Cooling Community
  • Interested in Indy Car?
  • Concurrent CFD Explained (Part II)
  • Concurrent CFD Explained (Part I)
  • December 2009
  • Cool Youtube Video
  • The Secret’s Out!
  • Wanna Know a Secret?
  • Thermal Design: Who’s Job Is It Anyway?
  • November 2009
  • More on Concurrent CFD in Product Design
  • Concurrent Design and Thoughts on ‘Flows’
  • Mind Your Head(room) Again
  • Solutions Expos – Going MAD in the UK
  • Mind Your Head(room)
  • October 2009
  • Solutions Expos – Just Back
  • Web slashes and missing polar ice
  • Going MAD at European Solutions Expos
  • What’s black and stuck on a PCB?
  • September 2009
  • Try the latest thing in CFD – Free!
  • How to survive a recession
  • ‘Simulating and Optimizing’ – A Series of Free Web Seminars
  • August 2009
  • Fluid Dynamics = Fun (Just back from holiday!)
  • Engineers Spend 60-80% of Work Time Changing Existing Designs
  • Hayfever: Stopped by a Red Light
  • Free Thermal Management Design Guide
  • July 2009
  • Where did CFD come from?
  • Formula 1 and KERS
  • A True Market Leader
  • Mind Your Thermal Management To Improve Reliability
  • Tennis – it’s a rough sport
  • Fluid Dynamics and BBQs
  • June 2009
  • Air – Is it Running Out of Gas?
  • The Deal with Electronics Cooling CFD – Meshing
  • The Deal with Electronics Cooling CFD: Geometry (Lots!)
  • May 2009
  • What’s the Deal with Electronics Cooling CFD?
  • The Start of A New Chapter