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How to Find Faults in the CAD Model?


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.


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

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
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.


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

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