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In many cases, CAD models can be dirty and thus do not import well into CFD-GEOM. Repairing these geometries can often take considerable time and effort. Many times, duplicate points, lines, and surfaces are created when importing CAD geometries. Along with duplicate entities, gaps in the geometry also cause problems. These issues almost always prevent the model from being "water tight", thus leading to grid generation problems. Figure 1 shows a gap between two geometries.
Figure 1. Geometry with gap between two surfaces.
When opening CAD models in CFD-GEOM, the default precision is 1E-06, i.e. if the gap between two entities is less than 1E-06, one of the entities will be filtered out of the model. Using a hard coded precision may not always be the best option for importing a model. Starting in CFD-GEOM V2008.0, all geometry file formats (such as IGES, SAT, etc.) have an option for setting the model relative precision, as shown in Figure 2. When using this option, the precision is a function of the size of the model. To obtain the precision, the diagonal of the bounding box is multiplied by the Model Relative Precision factor to obtain a precision that should be appropriate for the model being imported. This option will often times lead to a much cleaner model in CFD-GEOM.
Figure 2. Model Relative Precision option on the File --> Import Dialog
Another option for cleaning an imported CAD model is to use the Filter Points Globally and Filter Curves options, as shown in Figure 3 and Figure 4.
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| Figure 3. Global Point Filtering tool |
Figure 4. Global Curve Filter tool
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For example, in the geometry in Figure 1, the two lines are separated by a distance of 1E-05. If precision for filtering points globally is set to a value larger than 1E-05, one of these points and one curve will be filtered, as shown in Figure 5.
Figure 5. Geometry after setting the point filtering to 2E-05
As you can see, CFD-GEOM has made many improvements to CAD import. The next time you import a CAD model in CFD-GEOM, try using some of these options to make cleanup easier.
Regards,
Hans-Wilfried Mindt
ESI CFD Support Team - Germany
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