# Cubit Mesh

## Geometry

We construct the geometry by taking a horizontal cross-section of a 3D block.
Alternatively, we could have constructed the geometry by building it up from points and curves like we did with Gmsh.
{numref}`fig:example:strikeslip:2d:geometry:cubit` shows the geometry and variables names of the vertices and curves.

:::{figure-md} fig:example:strikeslip:2d:geometry:cubit
<img src="figs/geometry-cubit.*" alt="Geometry created in Cubit for generating the mesh." scale="75%"/>

Geometry created in Cubit for generating the finite-element mesh.
The names of the vertices and curves match the ones we use in the Cubit journal files.
:::

## Meshing using Python Script

:::{note}
New in v4.1.0.

We use a Python script to generate the mesh rather than Journal files.
We also use the skeleton sizing function for the surface.
:::

We use the Python script `generate_cubit.py` to generate the mesh.
The Python script is setup so that it can be run from within Cubit or as a standalone Python script without the Cubit GUI interface.
In this example, we will run the script from within Cubit using the Journal editor.

Open the Python script `generate_cubit.py` in the Cubit journal editor.
Play the selected script or play the lines, making sure you play the first line so that Cubit uses the Python interpreter when running the script.
We specify the parameters controlling the geometry, mesh size, and cell shape near the top of the script.

:::{important}
We reference objects by their centroid so the resulting script should be independent of the version of Cubit that is being used.
:::

:::{note}
We use the skeleton sizing function to gradually increase the cell size with distance from the fault.
This is much simpler than applying the bias sizing function to the curves and surfaces.
We set the minimim size equal to the cell size on the fault and the maximum gradient equal to the "bias".
:::

After running the Python script in Cubit, you will have a corresponding Exodus-II file (`mesh_tri.exo` or `mesh_quad.exo`).

:::{figure-md} fig:example:strikeslip:2d:cubit:tri
<img src="figs/cubit-tri.*" alt="Finite-element mesh with triangular cells generated by Cubit." width="400px"/>

Finite-element mesh with triangular cells generated by Cubit.
:::

:::{figure-md} fig:example:strikeslip:2d:cubit:quad
<img src="figs/cubit-quad.*" alt="Finite-element mesh with quadrilateral cells generated by Cubit." width="400px"/>

Finite-element mesh with quadrilateral cells generated by Cubit.
:::