## Introduction

Meshes generated with Gmsh can be converted to OGS meshes, in particular to VTU file format. Currently, the only supported Gmsh format is 2.1, which can be specified for example with the command line flag -format msh2 when executing gmsh.

## Usage

Gmsh writes all elements (unless specified otherwise) including the lower dimensional elements, usually model boundaries. Whereas for OGS we separate the so called “bulk” mesh and the boundary (or subdomain) meshes. GMSH2OGS options -e (--exclude-lines) and -b (--boundaries) control whether the line elements are removed or written in separate files.

## Example

A 2D-mesh with two physical groups (MaterialIDs) was generated with Gmsh. It also includes the outer boundary and inner boundary line elements.

### Input files and results

All files are stored in Tests/Data/MeshLib/:

### Running GMSH2OGS

GMSH2OGS creates different meshes from the same input depending on the -e and -b flags; the results are shown (z-translated) in the figure below.

GMSH2OGS -i A2-gmsh.msh -o A2.vtu
• Conversion without any additional flags yields a single VTU file with outer and inner boundaries (line elements) included, shown in the very bottom.
• Adding only a -e flag results in a single VTU file, now without the line elements.
• Adding only a -b flag gives a single VTU file as in the first case and additional eight files, each containing a line-element mesh corresponding to the physical groups; these are the white lines in the figure, shown again z-translated.
• Finally specifying both flags (-e and -b) produces a single VTU file without the line elements and additional eight boundary (subdomain) files.

This article was written by Dmitri Naumov. If you are missing something or you find an error please let us know.
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