Dear Alex,
Thank you for getting in touch. 
  1. In simple cases, you can first generate the linear mesh without boundary layers and then look at the end of the XML mesh file: 

  1. When you use more complex STEP files with hundreds of NURB surfaces, it might be a better idea to use the FieldConvert -m multiblock option (User Guide - 5.2.1.2) . You need to compile Nektar++ with VTK-ON.
    FieldConvert mesh.xml output.vtu:vtu:multiblock -v 
  2. Alternatively, you can extract the surfaces of interest and then FieldConvert it. 
    NekMesh mesh.xml mesh_extract.xml -m extract:surf=2,3,4 -v 
    FieldConvert mesh.xml output.vtu:vtu:vtuhighorder -v 

Last, if you are interested in complex geometries (>50 CAD surfaces), you might want to consider our third-party workflow, where you generate the linear mesh ( tet + prisms ) outside of NekMesh and we do all the high-order part in NekMesh, see ProjectCAD for the legacy version in the user guide. There you will be able to work on virtual geometries ( 1 composite corresponds to multiple CAD-NURBS). Then Option 1 should be the best.  

Kind Regards,
Kaloyan



From: nektar-users-bounces@imperial.ac.uk <nektar-users-bounces@imperial.ac.uk> on behalf of Alexander Schukmann <alexander.schukmann@protonmail.com>
Sent: Friday, March 29, 2024 7:12 PM
To: nektar-users <nektar-users@imperial.ac.uk>
Subject: [Nektar-users] BndLayerSurface IDs in step file for mesh generation with NekMesh
 

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Hello everyone,

to gain experience with NekMesh I first decided to generate a simple 3D boundary layer mesh after working my way through the very helpful tutorials. I started with creating a square duct in Gmsh, exporting the geometry as a STEP file (as recommended in the user guide) and finally feeding it to the mesh configuration file (MCF) and it worked very well. However, since in my case it is necessary to pass the parameter “BndLayerSurfaces” within the MCF, I tried to figure out where this information is stored inside the STEP file. Of course, to first set up the MCF I used the surface information written in the GEO file provided by Gmsh, but I wondered what would happen if I didn’t have a GEO but only a STEP file, e.g. of a more complex geometry created with a more sophisticated CAD tool? 
How would I know the necessary “BndLayerSurfaces” IDs in such a case?

Thanks in advance and all the best,
Alex