A customer discussed a meshing job with me last week. I would like to share some information based on this real case:
Input model: CAD with axial symmetry. The shape of its cross section looks like a gear with a lot of small teeth.
The customer wanted to run Dual Domain (DD), because 3D does not provide residual stress.
A Dual Domain mesh was created by default settings. It has 46K triangles. Max aspect ratio (AR) 48, match ratio 76%.
Then the customer started to repair the mesh, the goal was: max AR < 6 and match ratio > 85%.
The customer spent a week to repair the mesh, and eventually max AR < 6, but match ratio is 78%, still far from 85%.
This process is not ideal. I gave the customer the following suggestions:
1. The first step is to check the model and see if it is suitable for DD analysis. If not, choose 3D.
2. After creating a DD mesh, look at the mesh closely, and see if edge length is appropriate.
If you see elements in curved areas are not smooth, you need to turn on Chord Angle control on CAD tab.
If you see triangles are too large in some areas, define local mesh density on those faces, or change to use smaller global edge length.
If you need to change any of the settings, delete the old mesh, and re-generate DD mesh with new parameters.
By optimizing control parameters, you can save time and have better mesh than manuall repair.
Manual repair is not the first option.
3. When you start to repair DD mesh, first check Free Edges, Ovelaps/Intersections. Those errors may happen when CAD model has defects. For DD analysis, they are not critical, but good to fix unexpected errors.
If analysis is 3D, those errors should be fixed. Otherwise, 3D mesh cannot be created. You can either fix the DD mesh, or fix your CAD model by using the mesh as a clue to find defects on CAD.
4. Then fix triangles with AR>20.
5. If Cool(BEM) is needed, check Closeness. Cool analysis may have problems when the centroids of 2 triangles are too close. Those triangles can be found by Closeness diagnostics. To fix AR down to 6 is not the right solution. It can happen even if max AR < 6.
6. When DD mesh match ratio is low, if DD analysis is still desired, check the mesh and see if it has potential to increase match ratio by manual editing. Sometimes, it may have unexpected nodes near edges. Merge some nodes or swap some triangles may increase nmatch ratio a little bit, but it is unlikely to increase match ratio significantly if the model itsself is bascically unmatched. You still can run DD analysis when match ratio is lower than 85%. The threshold is a general guideline, but not rigorous.
7. When editing meshes, diagnostics results will be updated after every editing. This can be too slow for large models.
The recommended workflow is: Find and zoom to the area that needs to be repaired, click "Ctrl D" to turn off diagnostics. Then edit. Click "Ctrl D" again after editing. When diagnostics is off, it will not be updated automatically.
Normally, you do not need to update diagnostics for every editing.
Hope this will help.
Shoudong Xu
Autodesk Moldflow Meshing
Shoudong.Xu@autodesk.com
