By Shyam Kishor, GAMBIT Product Support Manager

More and more CFD users are relying on the direct use of CAD models, created during the design stage, to streamline the CFD model building process, saving time and money. GAMBIT now supports a wide range of options for data exchange with other CAD/CAE systems, and in this article, several fundamental issues affecting CAD interoperability are reviewed. In addition, tips are provided to help you at the upstream end (the originating CAD system), and to introduce you to the CAD repair tools available at the downstream end (GAMBIT), including the new cleanup tool in GAMBIT 2.1.

This figure illustrates the detection of a hole in the model using the cleanup tool; a simple mouse click will patch this hole by creating a surface from the boundary edges listed in the picture

What goes wrong during CAD import?

CAD interoperability, or the ability to share a CAD model across different applications, remains one of the biggest challenges facing industrial engineers today. Hidden errors and anomalies on the upstream side (where the CAD model is created), as well as translation issues, often result in numerous problems and frustrations for the downstream (where the model is received) users. Some of the issues that affect data exchange from one CAD system to another are:

1. Model quality in the originating CAD system

Many times the original model itself is of poor quality. Common problems include missing parts, invalid definition, and lack of connectivity. These problems could be due to user error, numerical limitation of the CAD system, and/or design requirements. Many CAD models work fine for design and drafting, but they do not have the quality required for CFD meshing operations.

2. Semantics

Each CAD system does some customization or adds local flavors to enhance its primary objectives. This leads to differences in the way a data type is interpreted by each package. Thus, when a model is moved from one system to another, inaccuracies can be introduced due to mismatches or poor communication.

3. Differences in tolerance

Geometry data are often in parametric form, accurate to the order of the specified tolerance. Differences in tolerance introduce gaps and overlaps in the model. CAD systems often use a loose (1e-03) tolerance since it is usually good enough for their primary purpose, and improves speed and memory requirements. GAMBIT, on the other hand, uses a tolerance of 1e-06, since it needs precise accuracy for Boolean operations and splits. This difference can result in a gap between adjacent entities or between the boundary curve and surface data.

4. Limitations of translation

Inaccuracies are also introduced by translation errors. Often all the data types of a CAD system do not have a one-to-one mapping with the standard formats used by translators, so approximations need to be made. Approximations are also applied when converting data from the standard format of the translator to the format used by the receiving system.

Helpful tips for the upstream end

• Tighter tolerance: If possible, tighter tolerance (~1e-06) should be used in the CAD system. It will significantly improve the quality of data after import into GAMBIT.
• Solid models: Solid models are always better to use than wireframe models, since they store connectivity information. The use of STEP, ACIS, PARASOLID and native CAD formats (if available) are preferable to other options since they all support data exchange using geometric solids.
• Simplifications: Simplifying the model in the originating CAD system can save a great deal of time and effort downstream. If possible, extra details not needed for CFD analysis should be removed, and flow volumes should be generated before exporting the model.

Helpful tips for the downstream end

While fixing the problems upstream yields the best results, doing so is not always possible. CFD analysts usually do not have control over how a model is first created, so they are forced to deal with problematic CAD files created without any thought given to their subsequent use by others. In GAMBIT 2.1, several new tools are available to make repairs to imperfect CAD models:

• Healing: Healing is designed to automatically detect and repair geometric and topological inaccuracies in the imported model by performing the following operations: (1) simplifying data by converting spline surfaces to analytic surfaces (e.g. a cylinder or sphere) wherever possible; (2) correcting topological problems by stitching; and (3) bridging gaps between boundary curves and surface data by recomputing intersections after extending the surfaces.
• Tolerant Modeling: Tolerant modeling in GAMBIT 2.1 increases the scope of the data that GAMBIT can import. It solves problems associated with inaccurate data or “leaky” models (with poor connectivity between neighboring elements, such as surfaces) and provides the framework for model healing and data translation. Since poor connectivity may be an issue when a small tolerance is used, this tool increases the tolerance in problem spots, generating less precise, yet connected geometric elements. The less precise geometry can then be used to create valid topologies for mesh generation. Tolerant modeling does not assume (or require) that the geometry agrees with the topology, and takes the geometric error in the topology into consideration during modeling operations and calculations.

Note: Both healing and tolerant modeling options are available during import. They should be used if normal import does not produce the desired results. The model should always be examined (using visual checks as well as the check commands in GAMBIT) after using these options to verify that the improvements are consistent with your expectations.

• The Cleanup Tool: In addition to the automated tools described above, GAMBIT 2.1 has a semi-automated cleanup tool. The cleanup tool is actually a set of interactive tools that quickly identifies, zooms in on, and highlights areas that cause connectivity and mesh quality problems.

A selection of operations and tools is available in GAMBIT for geometry cleanup

The clean up panel used for short edges in GAMBIT

Some of the common problems in an imported file that can adversely affect meshing include:

• Short edges
• Sliver faces
• Faces with small area
• Cracks
• Holes
• Faces with sharp angles
• Dangling edges

To illustrate how the cleanup tool works, consider short edge removal as an example. The following steps are followed during the operation.

• Problematic entities (e.g. all the edges shorter than a specified value) are automatically detected and listed. (A preset default is used to specify the limiting value, which can be changed by the user.)
• The user selects an item in the list, and it is automatically highlighted in the graphics window and on the list.
• Options become available to automatically zoom into (and out of) the selected region. Controls for local visibility and shading are available for better visual diagnostics.
• An appropriate fix for the problem is selected and presented to the user, who can then accept the default fix or switch to an alternative method and/or other options.
• After repairing the area, GAMBIT shows the result. Users can then move directly to the next item for repair. An option is also available to process the entire list in one step by applying the default fix to all areas.

Similar tools to fix other problems like cracks, holes, dangling edges, and small faces are also available.

Want to learn more?

Check out the User Services Center, http://www.fluentusers.com, to read more about CAD import. In addition, your support engineer at Fluent will gladly answer any questions you may have about CAD import and cleanup in GAMBIT.

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