How to retain watertight mesh after merging two stl using surfaceAdd?

Dear all,

I have an stl mesh which is watertight mesh (Checked with surfaceCheck utility of OpenFoam).

  1. I splitted this mesh to two stl’s using Paraview filters. (STL > GenerateNormals > Connectivity > Threshold > ExtractSurface).
  2. I saved these two stl files. Then I merged these two stls using surfaceAdd utility. (I merged again because I need separate patch in snappHexMesh).
  3. But, after merging, surfaceCheck shows the mesh is not closed. How do I retain this?

For reference, I have attached the zip file of stl.


Any leads will be appreciated.


Sunag R A.