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From CAD User Mechanical Magazine  Vol 18 No 06 - JUNE/JULY 2005

David Chadwick takes some of the mystique out of CFD – which Blue Ridge Numerics have already done, of course.

What’s so complicated about Computational Fluid Dynamics, anyway? If you are going to be designing something that has fluid sloshing about in it, you are halfway there to understanding it already. All you need is some software that looks at the shape of your model, understands the properties of the fluids that you are going to use – and that can set up a scheme for calculating the rate of flow into, out of and around the construction.

For fluid, it doesn’t even have to be the wet stuff! For fluids, also read heat, gas and air – the former a critical factor that has to be considered in the design and manufacture of electronic components – and the latter, the thing that tends to keep aircraft aloft as it flows across a wing section. Also used effectively in designing buildings. Fulham fans may care to know that the design of a new riverside stadium was compromised by a report that found out that wind flowing around the building would adversely affect sailing craft on the Thames!

Fluid behaviour, quite rightly, influences the design of a product. From setting out a pipeline, to creating the molten liquid filler paths in a mould, the number of kinks, curves, and other types of obstacle in the path of the fluid will slow down the rate of flow – which, in the case of the mould, could be quite disastrous.

And, because it involves some fancy mathematics, and the knowledge of where to apply them, the task of determining the behaviour of fluids was always shipped off, along with the design, to some fellows in white coats who seemed to understand the language!

But not any more! A company called Blue Ridge Numerics has produced a software package that gives design engineers the opportunity of extending the role of their model into the analytical arena. After all, parametric modelling incorporates within the model just those properties that the computational fluid dynamic software needs to perform its function. All it needed was the right sort of human interface that converted the CFD codes and practices into the methodology that the designer already understands – CAD!

Blue Ridge Numerics is a curious name. The sort of name – as Jim Spann, VP, Marketing, explained, that a small, provincial American company adopts to make it familiar to its geographical and human environment – until it discovers its’ product has caught the attentions of a far wider audience than it expected, and it has become the fastest growing CFD software developer on the Planet. It owes its success to the fact that its principal product, CFdesign, is aimed at the Design Engineer. It demystifies the analysis and puts it squarely where it belongs – in the hands of the designer. It resides in the CAD software, and uses the same type of commands to activate it – that the designer is already familiar with.

Of course, that’s a vast over-simplification. There are functions within CFD that the design engineer may not be familiar with, and which he has to learn, and to understand the implications involved in using them – and there are also many CFD applications that demand the undivided attentions and particular expertise of CFD professionals. It is also, probably, the case that the vast majority of products aimed at the market, can’t be released until they have been given the once-over by the experts. That would also be taking the bread out of their mouths and handing it directly to industrial claims lawyers.

What CFdesigngives design engineers, though, is confidence that their designs are heading in the right direction and that they will perform the function required of them in the most efficient manner.

CFdesign v8.0

Blue Ridge Numerics developed CFdesign knowing, right from the outset, that the users of the software are not going to be CFD professionals, and that they will not be using the software regularly. It has to be easy to pick up and use, therefore, when it is needed. The CFD tools incorporated within the software have to be automated as much as possible, to eliminate any chance of making unconsidered errors.

Functions that are common to MCAD and CFD are designed, therefore, to look the same – Feature Trees, component selection, mouse navigation controls, etc., - to provide the initial comfort factor. Cfdesign also uses native geometry from most of the leading MCAD systems, covering both mechanical and electrical sectors of the market, enabling models to be used, without translation, for CFD analysis.
Models aren’t actually imported. Cfdesign actually queries the MCAD model within the MCAD environment using very elaborate techniques to instantaneously produce a 3D mirror image of the assembly. This is truly a unique and superior approach driven by Blue Ridge’s philosophy that the MCAD model must drive the simulation, to accommodate iterative upfront simulation.


The starting point of all analysis is generating the mesh. CFdesign will automatically generate a mesh for the model, using many complicated algorithms to enhance or reduce the concentration of the mesh, depending upon the complexity of the model shape, to provide an optimum distribution of mesh elements throughout the model. Mesh density (and with it, accuracy of the results), can be defined by the user prior to generating the mesh, with the software counting the number of elements needed to ‘do the job’ and so that adequate computing resources can be allocated. CFD analysis is heavy on computing. With the same ends, part-by-part meshing can also be applied. In practice, however, most design engineers do not want to be bothered with generating the mesh, if it can be done transparently and effectively by the software.

CFD can also apply to 2D products! Sheet metal, for example, where thermal conductivity can be analysed by using 2D Surface parts, meshed by CFdesign. The same applies to very thin physical layers in electronic PCBs and components, where the surface parts can be used to apply heat to the components, and active resistors or contact resistance can be applied.

Material definition allows users to specify the type of material being used in the model, each of which, of course, has its own properties that affect the results. Leading on then, of course, to the actual CFD simulation – with a basic analysis module, a motion module that analyses models that involve some form of motion – and an advanced module for heat, compressible liquids and integration with FEA.

You know, the nice thing about this software, is that the progression from one step to the next is entirely linear and logical. The simplest of flow charts, where each step leads to the next, and where the user is guided along the analysis path – and, for design iterations and reviews, the fundamental purpose of the software, a simple loop back into the flow after the design has been modified or some parameters have been changed.

CFD Simulation Scope.

This is where the design engineer needs to understand the factors that affect fluid flow in order to perform one of the many types of analysis available. In the Base Analysis Package, for instance, the local wall roughness height allows users to specify the relative smoothness of parts. A sand casting on an intake manifold will be rougher than a plastic mould-injected part, and will influence fluid flow. CFdesign users can specify local areas of roughness. Another example - a Low Reynolds Turbulence Model will allow CFdesign users to simulate mixed flow turbulence – high speed air jets injecting air into a room in modern ventilation systems, for example.

Motion simulation includes Orbital, nutating disk, linear, angular and sliding vane pump motions. It also includes algorithms for analysing fluid and structure interaction and can handle externally driven models, or models that incorporate spring resistance.

Advanced Simulation uses an expanded Joule Heating model for analysing heat generation of conducting materials subject to voltage and current – a complex procedure, accomplished using varying resistivity based on local temperature throughout the part. It can also be used to determine the behaviour of compressible liquids by specifying the bulk modulus, using the Expanded Compressible Liquid model.

Simulation Speed

Back to the free ride! Which solver and turbulence model should the user specify for the new product design? Let CFdesign choose, of course! It will select the best one for the analysis – although advanced users can do the job themselves. CFdesign will also handle the analysis convergence, tuning itself as it runs to improve the accuracy and the robustness of the outcome, and telling the user when the solution is ready for review. Besides providing reliable simulations, optimising the solution also optimises the amount of computer usage.

Numerous other tools are available for enhancing the design review capabilities of CFdesign– the ability to handle non-planar cutting surfaces – vector information at the leading edges of impeller blades in pumps, for instance – even cutting surface morphing, a visualisation technique for transforming cutting surfaces between different geometric shapes – useful for turbo-machinery designers who want to assess the flow within a blade passage of a centrifugal gas compressor by sweeping between the hub and the shroud surfaces.

And more – but that last has given the game away, as CFdesign is not a piece of CFD software that has been compromised to suit the needs of the occasional MCAD user, but incorporates some of the most advanced techniques available within the technology, and which can be used for many demanding analytical tasks.

www.cfdesign.com

Review

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