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

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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.

stl_mesh_files

Any leads will be appreciated.

Regards,

Sunag R A.