Troubleshooting Meshing Issues for AEC Applications

Although the meshing technology in Autodesk Simulation CFD is very robust and has been refined over years of development, obstacles will still likely be encountered which prevent the generation of the desired mesh or cause the mesher to fail entirely. This is a serious roadblock since without a mesh, there is no CFD solution.

Clean CAD equals Clean Mesh

The majority of meshing issues can be traced back to the CAD geometry; this is why proper CAD preparation (idealization) of the geometry is a vital aspect of the simulation process.  Most major CAD platforms have tools to evaluate geometric details such as short edges, small surfaces, small clearances, interfering parts or invalid surfaces.

For example, in a simulation of a 500,000 square foot warehouse space, the CAD model should not have edges that are only 0.001 inches long.  For more information on CAD idealization and examples of common CAD issues, please review the CAD module.

Diagnostic Tips

If the CAD geometry has checked out OK and no issues are easily seen, then there are some other techniques to try and locate the underlying problem.

Refine the mesh

  • Add more mesh refinement, particularly to areas of finer detail, and try meshing again.  If this works, the issue was that the mesh was too coarse (not enough elements) to fit larger elements into smaller spaces.

Disable mesh enhancement temporarily

  • Turn off mesh enhancement and try meshing again.  If meshing succeeds, then this typically indicates the presence of thin fluid channels that fluid elements are being “squeezed” into.  Either fix the area in CAD or target more mesh refinement in those areas.

NOTE:  Make sure to enable mesh enhancement once the repair has been made.  Never rely on results from a simulation model with enhancement disabled.

Check edge and surface diagnostics to locate potentially problematic areas

An Autodesk webinar discussing common meshing errors can be found here.

Nodal Aspect Ratio

A characteristic of mesh quality is the nodal aspect ratio of the elements.  For a particular element, this is defined by dividing the longest element edge by the shortest edge of the same element. A perfect aspect ratio is a value of 1.0, where all edges have the same length.  Aspect values should be kept below 100 for best results;  values in excess of this will have a noticeable influence on results and run times.

The ideal element shape has equal length edges (left).  Element shapes that start to look like long needles (right) are less than ideal.

Nodal aspect ratios can be evaluated, once a mesh has been generated, as follows:

  • The maximum nodal aspect ratio is written near the bottom of the mesh.log file which can be viewed with any text editor.  The file is located in your design study folder: C:\yourCFDfolder\studyname\designname\scenarioname\mesher\mesh.log
  • Nodal aspect ratios can be visualized the same as other results (e.g., velocities, pressures, temperatures) using the process detailed here: visualizing nodal aspect ratios.

TIP: Visualizing areas of highest nodal aspect ratios can help locate areas of the model that have CAD issues (e.g., very small surfaces or thin gaps) or need more mesh refinement to make the elements smaller to improve their shape.