Hi Syavash, Gmsh do recognise 1D entities, i.e. lines and curves. You just need to select curve in physical group and select the domain boundaries (lines or curves) and assign name to them. The fluid domain then becomes a surface and after dealing with the boundaries you need also to assign a pyhiscal group to the surface(s). Cheers, Mohsen From: nektar-users-bounces@imperial.ac.uk <nektar-users-bounces@imperial.ac.uk> on behalf of Ehsan Asgari <eh.asgari@gmail.com> Date: Tuesday, 9 May 2023 at 10:23 To: nektar-users <nektar-users@imperial.ac.uk> Subject: [Nektar-users] Modifying 2-D mesh in Gmsh ⚠ External sender. Take care when opening links or attachments. Do not provide your login details. This email from eh.asgari@gmail.com originates from outside Imperial. Do not click on links and attachments unless you recognise the sender. If you trust the sender, add them to your safe senders list<https://spam.ic.ac.uk/SpamConsole/Senders.aspx> to disable email stamping for this address. Dear All When I import a 3-D mesh in Gmsh, it can recognise 2-D surfaces and I label them as physical groups for Nektar to be used as boundary conditions. However, in case of a 2-D mesh, Gmsh cannot recognise 1-D line elements so I cannot label them to use as boundary conditions in Nektar. How can I get these 1-D elements recognised in Gmsh? Kind regards syavash