By Shyam Kishor, GAMBIT Product Support Manager

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An increasing number of CFD users are now relying on the direct use of CAD models, created during the design stage, to streamline the CFD model building process. GAMBIT supports a wide range of options for data exchange with other CAD/CAE systems, and there are several fundamental issues affecting CAD interoperability. In particular, actions can be taken at the upstream end (the originating CAD system) to help streamline the process at the downstream end (GAMBIT). This article provides tips that will help make the CAD import and cleanup process easier from start to finish.

Example of an imported geometry with typical problems

CAD interoperability, or the ability to share a CAD model across different applications, has been a challenge for industrial engineers. Hidden errors and anomalies in the originating CAD system, as well as translation issues, often result in numerous problems and frustrations for the downstream users. While the emergence of STEP as a global standard, an increased focus on feature-based translation, and direct interfaces have all helped reduce some of these problems, true interoperability is still far from reality. Some of the issues that affect data exchange from one CAD system to another are:

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.

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.

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 (10^-3) 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 10^-6, 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.

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

There are several steps that can be taken in the CAD system to make the import process less problematic.

• Use a tighter tolerance (close to 10^-6) in the CAD system.


• Wherever possible, use basic geometry and primitive solids for model creation.


• Make all geometry visible and selectable. Simplify the model to remove unnecessary geometry, layers, and annotation.


• In general, solid or native CAD models are preferable to wire frame models for the data exchange.


• For assemblies, confirm that all component files are in the same directory.


• Use tools available in the CAD system to validate the geometry prior to export.


• Import the data back for a loop test to be certain that the original model can be recreated in the CAD system.

Helpful tips for the Downstream End

While fixing the problems upstream yields the best results, it is not always possible to do so. 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 2.2, several 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. Improved healing in GAMBIT 2.2 also supports automatic detection and removal of sliver faces and short edges during import. In addition, a smoothing function is available that can replace a bad geometry with a good one.

Result after using the cleanup tool in GAMBIT

TOLERANT MODELING: Tolerant modeling 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 2.2 has a semi-automated cleanup tool. The cleanup tool is actually a set of interactive tools that quickly identify, zoom in on, and highlight areas that cause connectivity and mesh quality problems.

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

To illustrate how the cleanup tool works, consider the cleanup of duplicate geometry as an example.

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

The clean up panel used for duplicate faces in GAMBIT

The cleanup duplicate geometry tool allows you to remove duplicate (coincident) vertices, edges, faces, and volumes in the model. The following steps are followed during the operation:

• Problematic entities (e.g. sets of duplicate entities within a specified tolerance) are automatically detected and listed. (A preset default is used to specify the tolerance, 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 short edges, cracks, holes, dangling edges, small faces, and fillets are also available.

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