MultiRegion Meshing Example: This case demonstrates multiregion meshing with OpenFOAM. 
The computational domain consists of a tube reactor filled with process gases, with a suspended solid substrate inside it. 

The mesh is divided into a fluid and a solid region: 
'gas' region – representing the fluid/ process gases inside the tube reactor
'shape' region – representing the suspeneded solid substrate on which deposition occurs

The meshes generated by this workflow are very large (≈30 GB each) and are not included in this repository.
You can generate them locally with ./Allrun

Here are the steps involved:
Step 0: constant/triSurface contains the inletWall, inlet, pipeWall and outlet as stl files.
The rectangular substrate will be created during meshing with snappyHexMesh.

Step 1: Use blockMesh to create a mesh that encompasses the entire tube reactor.
The region created is specificed as 'gas'.
This is important for splitting the regions in step 5 
(otherwise, the split is not good and unnecessary/extra regions are created).

Step 2: Use surfaceFeatureExtract to extract feature edges from the stl files.

Step 3: Use snappyHexMesh to do the meshing.  Two regions are created, 'gas' and 'shape'.

Step 4: Check for errors with checkMesh

Step 5: Check visually with paraview

Step 6: Split the regions 'gas' and 'shape':
splitMeshRegions -cellZonesOnly -overwrite
Check again with paraview

Step 7: You can check the mesh of each region separately after meshing with
checkMesh -region gas
checkMesh -region shape

List of commands (in Allrun):
blockMesh
surfaceFeatureExtract
snappyHexMesh
checkMesh 
splitMeshRegions -cellZonesOnly -overwrite