# 1. Geometry

After running the ABL case, it's very common to add a body of interest to that case. In
this step we will add the geometry of the body and make some considerations on its mesh.

## Adding the body

It's possible to either create a mesh from a primitive, as we did with the plane in the
ABL workshop, or to upload one from an STL. In this case we will add the STL for a small
building, which can be downloaded [here](/_static/stls/bodies/ar1_gable.stl).

The building resembles a house, and is {pv}`25.75 m` high at its roof ridge and
{pv}`20 m` at its eaves, over a {pv}`20 m` by {pv}`20 m` base.

```{eval-rst}
.. figure:: /_static/img/workshops/simulating-bodies-in-abl/01-ar1-gable.png
    :align: center

    The uploaded STL in the geometries tab, with its triangle metrics in the repair
    panel.
```

### Geometry mesh considerations

One very important aspect is the mesh. Note that the triangles are very small, even
though the same geometry could be represented with just two triangles per face.

This is due to how the AeroSim boundary condition works: it uses each triangle as a
reference to generate its IBM points, so a good quality triangulation matters,
especially for a body of interest. Three aspects to watch:

* **Shape** - triangles should be close to equilateral, which keeps the IBM points
  evenly distributed.
* **Size** - the same triangles carry the exported data, so their size should match the
  resolution we want in the results.
* **Normals** - they should point outwards, since the wall model relies on their
  orientation.

```{eval-rst}
.. figure:: /_static/img/workshops/simulating-bodies-in-abl/01-building-mesh.png
    :align: center

    The building triangulation, renamed to ``building``: mean triangle area of
    :math:`0.16\,\text{m}^2` and mean skewness of 0.83.
```

```{seealso}
* {doc}`Geometry Discretization <../../reference/geometry-discretization>` - the triangle
  refinement, the target areas and what makes a triangulation good enough.
* {doc}`Geometry Representation with IBM <../../reference/body-discretization>` - how the
  triangles become Lagrangian points and what the diffusive layer around them is.
```

## Adding it to the simulation

After that we can proceed to add the body to the simulation. We will duplicate the ABL
case, rename it to `Building case` and then proceed to configure it. The steps are the
same as in the ABL workshop.

For the boundary conditions, we will use the TBL (thin boundary layer) wall model, which
is well suited for bodies. It's very important that the body's geometry has good
quality, otherwise this BC may yield worse results than a simple no-slip.

```{eval-rst}
.. figure:: /_static/img/workshops/simulating-bodies-in-abl/01-building-domain.png
    :align: center

    The duplicated case with the building added, its wall BC set to the TBL wall model.
    The plane keeps the ABL wall model with :math:`z_0 = 0.05\,\text{m}`.
```

```{seealso}
{doc}`Boundary Conditions <../../reference/boundary-conditions>` covers the log-law and
the TBL wall models and how both couple with IBM.
```

## Next steps

Now that we have added the building it's time to
[adjust the mesh and the setup](02-adjustments.md) for it.