In most cases, importing CAD files in CFD-GEOM results in a clean geometry that can be directly used for further processing and/or meshing. However, for complex models, it is possible that the imported geometry has missing, untrimmed or defective surfaces that need to be manually repaired.

These problems can have many sources:

• Self-Intersecting loops describing holes or boundaries.
• Loops intersecting other loops.
• Loops that are not on the surface.
• CAD system uses a different precision for filtering points and curves as compared to CFD-GEOM.
• Unintended gaps and overlaps in the original geometry.

CFD-GEOM offers simple and easy-to-use tools to repair such geometry faults with minimal manual effort:

• The Model Relative Precision will sometimes remove many problems from a non-closed model (see User Tip "Geometric Precision and Filtering in CFD-GEOM").
• Average Points, Global and Local Point Filters may help in eliminating duplicate and/or unnecessary points.
• The Average Curve/Curve Sets, Global and Local Curve Filters are also quite useful and, in many cases, necessary to repair the given geometry.

This user tip describes another way of repairing bad geometries: Using the “Equate Point” and "Surface Trimming" tools. An example is used for illustration purposes.

Example: Impeller geometry in *SAT format.
The Impeller geometry file can be downloaded from here.

When ‘impeller.sat’ file is imported/opened in CFD-GEOM, it prompts a ‘SAT File import Errors’ window, and the resulting geometry is shown in figure 1.

Figure 1.  Impeller Geometry in Front View with ‘SAT File Import Errors’ window

The brown colored surfaces in figure 1 above are the untrimmed surfaces that need to be trimmed. The following procedure describes the various steps needed to repair the geometry.

1. Hide All Entities using the ‘All Off’ icon in the Entity Bar.

2. Open the Level Editor from ‘Edit menu -> Levels’ (see figure 2). Drag the Level Editor away from the center of the screen so that you can see the Graphics area clearly.

   

   Figure 2.  Level Editor showing failed geometry surfaces

3. In the Level Editor, select the first failed surface, named “sat_failed_surface_construction 0”, and click on the “Show” button. The displayed geometry can be seen in figure 3.

   

   Figure 3.  Failed surface #1
   

4. Select the ‘Trim’ option from ‘Geometry -> Trimmed Surface Modification Options’. Click on the untrimmed surface and select all the lines/curves (do Crtl+a). The resulting set of curves is not closed, as indicated by the geometry points highlighted in green (see figure 4). If you zoom in on a point, you can clearly see that there are actually 2 points, and that the curves are disjoint.

   

   Figure 4.  End points of disjoint set of lines/curves

5. Use the ‘Equate Points’ tool from ‘Geometry -> Point Creation Options’, select the first set of disjoint points using the selection box (Shift + mouse left button), and click apply. Repeat for the second set of points. Now you can create the desired surface (see step 4), and the result is shown in figure 5.

   

   Figure 5.  Corrected surface #1

6. From the Level Editor, click on “hide” and then “delete” (this only deletes the surface from the Editor, not the actual surface).

7. Repeat steps 4 through 6 for failed surfaces #2 to #14.

Failed surface #15 requires a special treatment because there are singular points at the tip of the blades (see figure 6). To fix this failed surface, you will need to delete the untrimmed surface first, and then break down the geometry into several regions. Each region is the area in between the blades. Finally, use the ‘Fit Through Curves’ option from ‘Geometry -> Trimmed Surface Creation Options’ to create the surfaces (see figure 7).

Figure 6.  Singularities in failed surface #15

Figure 7.  Corrected geometry with multiple surfaces

Now you can click on ‘All On’ to show the whole, now water tight model, presented in figure 8. Note that for this model, using the "Model Relative Precision" option with a value of 1e-5 will take care of all surfaces except the last one (failed surface #15).

Figure 8.  Fixed water tight impeller geometry

If you have any questions about this tip or would like us to discuss other topics in the future, please let us know.

Regards,
Shivakumar G.T.
ESI CFD Support Team