Background mesh is a basic (initial) mesh to be modified and/or refined. Entire surface model is placed inside the background mesh box. Background mesh is created using blockMesh utility. Background mesh is simple block a bit larger than bounding box of the surface model. Alternatively, background mesh can be used to create inlet or outlet of the final mesh (not this case).

Dictionary for blockMesh utility have to be placed in constant/polyMesh/ directory. For rounded object typically an impeller or turbine, it is recommended to create a cylindrical background mesh.

After the blockMeshDict file is set up the background mesh is created, running:

# blockMesh

Background mesh is very simple initial mesh used as an input for snappyHexMesh utility. You can view it in paraview:

# paraFoam