Perhaps the following will be useful:
There are two main rules which are crucial for any flow simulation:
- Your CAD needs to be watertight: There must not be any holes except inlets and outlets.
- Your CAD must only contain solid parts. Shells or Sheets must not appear.
Note: A sheet is a CAD geometry that is missing one or more faces.
In other words,
A few ways to do the CAD cleanup:
Also, try to import your model in a different file format. STL files are not recommended (unless for LBM or PWC simulations).
These documentation pages are useful for learning how to properly set up a CAD model :
and perhaps,
although it may be better to fix the issue using the same CAD tool used to generate the geometry.