Production CAD and analysis CAD are two completely different things. Engineers must be aware of several issues when bridging the gap and familiarise themselves with the latest tools for CAD import, repair and defeaturing. By Paul Schreier.

Designers are becoming increasingly aware of the value of CAE analysis to study flow, heat transfer, structural integrity or other aspects of a design with the help of modelling, simulation or virtual-prototyping packages. And while there is a strong trend towards integrating CAD and CAE in the same software, engineers still generally use separate packages. First of all, CAD and CAE have traditionally been performed in different departments with their own favourite tools. In addition, even though CAE packages often have some sort of basic CAD editor, for anything beyond a basic geometry users prefer to create them in CAD packages with advanced editing features. Whether performing an analysis on a new or a legacy CAD drawing, it's necessary to import the geometry into the CAE package. This task doesn't make headlines, but it's nonetheless a critical link that deserves detailed examination. For not only must you get an accurate geometry into a CAE package, there are issues about optimising the geometry for the most efficient analysis.

During geometry import, the CAD Import Module for COMSOL Multiphysics fixed two non-adjoining surfaces by adding a sliver object (upper left). The resulting mesh (upper right), however, is far denser than required. After defeaturing (bottom left) there is only one edge between the surfaces, leading to a mesh that is much easier to handle yet gives the same accuracy.

In fact, according to CAE solutions provider, CD-adapco, the automobile industry suggest that as much as 80% of the simulation effort spent in a typical CAE project is taken up in generating the computational model - the lion's share of which is surface preparation and repair.

Production versus analysis CAD

Nobody disputes that there is a big difference between production CAD and analysis CAD. Ed Fontes, VP of Applications at COMSOL, points out that many CAD packages create drawings using faces, but analysis software often needs a volume. Further, a CAD engineer doesn't necessarily want or need a volume, so although that person could use today's tools to create a geometry perfectly suited for analysis, there's little motivation to take the time to be absolutely precise.

Thus, between individual faces the designer might leave tiny gaps that aren't a problem if the goal is to create manufacturing drawings or documentation. Or perhaps a gap might be intentional, for instance leaving room for a welding bead. In either case, such a geometry won't work in a CAE package, which needs a 'watertight' volume.

This is changing, though: 'CAD packages that work with surfaces are a thing of the past, and modern parameterised CAD packages are solid based,' comments Ivo Weinhold, manager of CFD products at Flomerics/NIKA. He notes that in some industries, surface-based niche packages are still preferred. He adds that, especially with old CAD files, it is generally necessary to invest huge amounts of time to import and repair designs to create the volumetric objects needed for analysis. But even with modern solid-based CAD, there is no guarantee of a smooth connection between CAD and CAE.

When dealing with CAD imports, there are actually two broad issues: accurately importing the geometry, and optimising it for analysis. As for the first, often the original model is of poor quality, with missing parts or invalid definitions being common problems; these might be due to user error, numerical limitations of the CAD system or design requirements. CAD packages create a complex mathematical description of a surface, but the geometry kernel in a CAE package might not be able to interpret every aspect of it. Also, CAD systems often work with a relatively loose tolerance (1e-03), which is adequate for their primary purpose and that level also improves memory needs and speeds processing.

CAE packages, on the other hand, often use a higher tolerance of 1e-05 or 1e-06 to deliver their accuracy. This difference can result in a gap between adjacent entities or between a boundary curve and surface data. A related problem is that an analysis package might be able to read all the objects but might not be able to analyse them, such as in the case of two cubes slightly off face.

Further, geometric models don't always pass flawlessly between different file formats, due to the different representations they use. Sometimes the data types of a CAD system don't all have a 1:1 mapping with standard formats used by file translators, which must then, therefore, make approximations. Thus, the quality of a translation from a CAD model to a CAE geometry depends heavily on the file format. In reformatting files, there can be the loss of faces and edges. The smoothest way is, of course, to use the native format of the CAD system. If this isn't possible, contact the CAE supplier for recommendations on which formats work best with their software.

Although translators have gained a poor reputation in the past, they have improved in robustness according to Mark Keating, principle CFD engineer at Fluent Europe Ltd. He does add that multiple translations might be required, depending on the source/destination software. For instance, in some cases the CAD system might export the design in a neutral format such as STEP or IGES, and then a translator on the analysis pre-processor transforms the geometry into that package's required format.

COMSOL's Fontes adds, software should be able to save a geometry in a generic format. Assume, for instance, that you have optimised the profile of a shaft using an analysis starting with a CAD file. The user should be able to save this optimised geometry in a generic format, so that the CAD file doesn't have to be redrawn after optimisation. CAD repair and unnecessary detail

When importing a CAD file, CAE tools generally let you set a tolerance and the software handles any features smaller than this amount. Many CAD objects contain small anomalies: short edges, holes, overlapping edges or gaps you want to remove, join, or knit together. Otherwise they can lead to large meshes or even failure of mesh generation. To avoid these problems, use the CAE package's repair feature and set an appropriate import tolerance.

However, some small details might actually be needed, so most import utilities allow you to manually select which specific details should be removed or even detached to form a new solid body. Flexibility in this and other areas is needed if users want to optimise a geometry for a specific CAE task.

Even after automatic or semiautomatic repair, there could still be many unwanted small edges, thin surfaces, slivers and spikes that later on create a huge number of mesh elements but add no accuracy to the analysis. And even correctly imported features might be unnecessary.

These include holes, gaskets, fasteners, bolts, threads or even lettering or numbering. The goal is to get a dense mesh only where modelling requires extra resolution or accuracy, because running a simulation that takes every small detail into consideration would result in an enormous mesh, leading to massive memory requirements and solution times. The step of eliminating features that have no effect on the analysis is known as defeaturing.

Today most CAD systems have tools that automatically suppress or remove those unwanted features, but users must keep in mind that, which aspects to defeature depends on the analysis type. COMSOL's Fontes gives the example of a cell phone, where the holes for the buttons are often rounded so that the entire phone's exterior consists of several hundred surfaces. For heat analysis you could probably assume the holes are rectangular, but for an accurate structural analysis (if I sit on my new mobile phone, will the pressure be enough to crack its large screen?) they must remain round.

Another option is to keep all the details of the geometry, but then do 'sloppy' meshing and disregard details below a selected size, such as approximating the hood-ornament angel on a Rolls Royce as a sausage.

Classes of import/repair methods

To help engineers with surface preparation, CAE vendors have created a variety of approaches and tools. Most customers these days are new to CFD and analysis, says Flomerics' Weinhold, and their expectation is that they can handle CAD geometries with minimal user interaction. Depending on the method of interconnectivity, that can be the case.

Mark Keating from Ansys prefers to categorise the various connectivity approaches in four levels, listed in increasing levels of communication with the CAD system and thus a reduced likelihood that the model could be incorrectly transferred:
1. Translation, the conversion of files from one format to another. This can result in corruption or a loss of data or changes, such as gaps or intersections appearing.
2. Direct read, where a pre-processor uses a CAD kernel, one popular choice among CAE vendors being the Parasolid kernel, so it sees the geometry exactly as the CAD system sees it and is thus more accurate. However, the user might be limited as to what new manipulations can be done.
3. File- or API-based connectivity (also known as a 'live interface') where, once the CAD geometry has been imported into the CAE system, both packages are running and can exchange information. This is available in both 1- and 2-way associativity. In a 1-way link, any user-changes to the CAD model are automatically pulled into the CAE model; in a 2-way link, the CAE code can also export performance data or optimised model parameters to the CAD system, which updates its geometry to reflect the new design based on the design goals for the CAE analysis. This 2-way associativity with major CAD packages makes a strong tool for CAD interfacing without being fully embedded.
4. CAE capabilities fully embedded in the CAD system; the analysis model and the CAD part are one and the same geometry. This, the most convenient method, is only now starting to gain momentum and be implemented in commercial products.

One interesting variation on the geometry import/repair cycle is an automatic geometry repair tool, within products from firms including CD-adapco and Ansys, which utilises 'surface wrapping' technology. The user first imports the geometry and sets a base size that determines the level of feature resolution in the final surface. The technology works by 'shrink-wrapping' a triangulated surface mesh onto the geometry, closing holes and joining disconnected and overlapping surfaces.

It automatically discards surfaces that are outside the calculation domain to instantly eliminate unnecessary detail. And while this method generates a surface mesh for flow analysis, an additional meshing step can create a volume mesh. This scheme has been received so well, notes Stephen Ferguson, senior consultant engineer at CD-adapco, that Ford Motors in Germany has decided to use this tool to create meshes for all of its underbody and internal components and create a database of objects for use in a variety of CAE tools.

In considering the overall area of CAD import/repair, says Flomerics' Weinhold, the most important thing is to start thinking about CAE during the CAD process. 'It doesn't cost any more, it just means having the discipline to follow good design guidelines. If the CAD design considers analysis requirements, we can reach the ideal of 'pushbutton' simulation.'