# 1. Geometries

The first thing to do before setting up a simulation is to bring in the geometries we
will work with. Here we will create the builtin geometries needed to run an ABL
simulation.

## Adding the plane

Start by creating a plane: go to **Add Geometry -> Generate Primitive**, then select
**Plane** in the form.

The plane size we will use is {pv}`8000 m` in X and {pv}`2000 m` in Y (streamwise is X,
height is Z, spanwise is Y).

The plane can be moved before or after generation. We will translate it by
{pv}`2000 m`, because we usually want a longer length downstream than upstream.

After that we should have our first geometry, as in the screen below.

```{eval-rst}
.. figure:: /_static/img/workshops/first-abl-simulation/01-geom-plane.png
    :align: center

    The generated plane, positioned in the domain.
```

## Adding roughness elements

To generate the inflow profile of the ABL, AeroSim uses roughness elements along with a
wall model on the plane, which keeps the profile and the turbulence intensity as the
flow develops.

This is similar to what a wind tunnel does with the blocks placed upstream of the
model: the flow enters near-uniform, develops over the fetch of blocks, and reaches the
model with the profile and the turbulence we want.

```{eval-rst}
.. figure:: /_static/img/workshops/first-abl-simulation/01-wt-roughness.svg
    :align: center
    :alt: Side view of a wind tunnel test section. Near-uniform flow enters at the
          left, passes a fetch of roughness blocks on the floor, and reaches the model
          as a developed boundary layer with a logarithmic mean velocity profile.

    Roughness elements upstream of the model in a wind tunnel test section.
```

To add them, use **Add Geometry -> Create Roughness Elements** and then choose the
parameters. For this case we will use a CAT 2 profile, so the catalogued AeroSim
roughness elements for CAT 2 will be used.

The parameters are free to be tweaked for tests. The default values are the ones
AeroSim has used and validated internally for each category.

Both **Linear** and **Radial** roughness elements can be generated, the radial ones
being arranged as a circular table for 360 degree cases.

```{eval-rst}
.. figure:: /_static/img/workshops/first-abl-simulation/01-roughness-form.png
    :align: center

    The roughness elements form, with the CAT 2 parameters.
```

With all this done, our geometries are set up.

```{eval-rst}
.. figure:: /_static/img/workshops/first-abl-simulation/01-final-geoms.png
    :align: center

    Plane and roughness elements, ready for the simulation setup.
```

## Next steps

With the geometries generated, we can move on to
[creating the simulation](02-simulation.md).