Merge meshes from two cylinders with different diameters
 

I would like to study the flow of gas in a cylindrical capillary followed by a larger cylindrical volume. I am using salome-platform to create the geometry and the mesh. I would like to know what is the best strategy to have such mesh.


My initial idea was to create a mesh for both cylinders and then to merge the meshes. I would simply generate two cylidners, select some mesh properties for the volumes and surfaces (inlet and outlet). Then I could export in .unv to OpenFOAM.

This idea comes from the fact I already tested an hexahedral mesh for the first capillary geometry and I would like to complete this geometry. The capillary is built as a divided cylinder that I mesh with 3D extrustion, quadrangle mapping for 2D and wire discretisation for 1D (see image 1). The inlet and outlet also have 2D and 1D parameters set to have a satisfying CFD simulation.


As a matter of fact, I already tried this, found something looking satisfying, with a fine mesh around the capillary (see image 2). Both meshes connect well and so, but my problem is that doing so I have 1000 pyramids, and unv format do not support pyramids. So maybe there is an alternative, I could do things differently.

https://i.postimg.cc/SYpQJsfx/mesh-example.png
https://i.postimg.cc/gwCmTJ5H/mesh-example2.png