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Documentation

How to Find Faults in the CAD Model?

Objective

CAD Cleanup is mostly different for any CAD so there is no “golden rule” which guarantees success, but there are some important rules and tricks. This article describes some general rules/standards required for CFD simulations and best practices on how to find faults in cad models so that you have a starting point.

This content provides an overview of everything that is necessary for preparing a CAD for simulation.

Approach

There are two main rules which are crucial for any flow simulation:

  1. Your CAD needs to be watertight: There must not be any holes except inlets and outlets.
  2. Your CAD must only contain solid parts. Shells or Sheets must not appear. The following picture demonstrates, what the geometry tree should look like after the upload:
how should the uploaded geometry look like in simscale
Figure 1: Geometry symbols to recognize between solid bodies and shell bodies

SimScale highlights the regions where the CAD is faulty. The picture below shows an example:

cad model of airplane with faulty edge and degenerated geometries cad faults simulation
Figure 2: CAD model with degenerated geometries and a faulty edge

More helpful rules are that you should avoid intersecting parts (this is crucial for CHT simulations), warped surfaces, too small entities in general.

Expected Outcome

The result should be a CAD model that consists of only solids. The less solids the better so merge as much as possible.

Best Practices

Tip 1: Merge Everything

If you have more than one part and they are supposed to touch / intersect, then try to merge them. CAD tools normally highlight regions where it fails to merge. These are most likely the regions which are causing the problem.

Generally merge as much as you can. This simplifies the simulation.

Tip 2: Convert Sheets to Solids

Try to fill the gaps. In most of the cases the sheets already show how the solid would look like, but there are holes which need to be closed in order to get a solid part.

The following list provides basic workflows for how to convert “surface to solid” in some of the most commonly used CAD tools. It might not be accurate for specific functionalities.

  • SpaceClaim: Stitch the surface to convert it to a solid part
  • Onshape: Add “Thickness” to the surface and merge them
  • SolidWorks: Have a look at this video.
  • CATIA: Have a look at this video.
  • Autodesk Inventor: Have a look at this video.

Tip 3: Watch out for Model Tolerance

A model tolerance comes directly from the CAD kernel of an imported geometry. Computers aren’t perfect, so vertices and edges each have a tolerance that can be imagined as a bubble. Any two components in the same bubble will be merged, and appear as one. This ensures that all models are closed, even if the numbers aren’t exactly equal.

Although the model tolerance will help ensure closed 3D geometry, there can be problems that arise as well. A mesh can never be refined past the model tolerance, and a factor of safety of 10 is automatically included. If you try to refine a mesh beyond the model tolerance, this error will appear in the meshing log:

meshing log showing an error about the model tolerance being higher than the desired fineness
Figure 3: Meshing log error that appears once the desired fineness is lower than the model tolerance.

If you are seeing this problem in the meshing log, the only solution is to make changes to the geometry within your CAD package.

Note

If none of the above suggestions solved your problem, then please post the issue on our forum or contact us.

Last updated: September 3rd, 2020

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