# Pressure Coefficients

As stated before, the pressure data is relevant for structural engineers to understand the **wind induced stress**.

However, it comes in handy to adimensionalize the pressure data into **coefficients**.
Therefore, the analysis is **independent of the scale or unit system**.
It is a way to generalize the pressure data.

## Available Coefficients

There are several different coefficients, that are commonly used in the wind industry, such as:

* [Pressure Coefficient](./pressure_coefficient.md): It is a fundamental adimensionalization of the pressure data. It is obtained by dividing a pressure difference by the **dynamic pressure**.
* [Shape Coefficient](./shape_coefficient.md): It is equivalent to a resulting pressure coefficient over an **area of interest**. It is used to combine the pressure effects over the area, in a way to **sum the exerted force in each triangle** inside this area. Some peaks in each triangle **may cancel each other** in when calculating the shape coefficient.
* [Force Coefficient](./force_coefficient.md): It is a general adimensionalization of the resulting **wind induced force** over a body. It is calculated by summing the resulting force of each triangle, and dividing it by a **representative area**.
* [Moment Coefficient](./moment_coefficient.md): It is a general adimensionalization of the resulting **wind induced momentum** over a body. It is calculated by summing the resulting momentum of each triangle, and dividing it by a **representative volume**. The anchor point to define the momentum lever is an **arbitrary point for the whole body**.

## Geometry Artifact

Each coefficient has its own artifacts dependencies.
However, a common artifact shared between all of them, is a **description of the structure geometry**.

The pipeline accepts geometry in any of the following formats and dispatches by suffix via {func}`cfdmod.io.load_mesh`:

* `.lnas` -- the AeroSim native LNAS file with authored surfaces.
* `.stl` -- a triangle mesh; treated as a single `"all"` surface.
* `.h5` -- an XDMF+H5 with embedded `/Triangles + /Geometry`; single `"all"` surface.
* `.xdmf` -- redirects to its sibling `.h5`.

The mesh is attached to a Cp time series by the `mesh_attach` op, which pulls per-triangle areas, normals and centroids from the `.lnas` (or `.h5`) geometry. The Cf / Cm / Ce templates all start with a `mesh_attach` step; the Cp template itself does not need a mesh.

When the same building is simulated at several wind directions, each solver run produces a body H5 in its own wind-aligned ("spun") coordinate frame. Point every template's `mesh_attach` at a single fixed-frame mesh and all coefficients are expressed in that shared frame. Triangle ordering must match the body H5 -- only vertex coordinates may differ.

:::{note}
For LNAS-specific details, see the documentation inside the [LNAS repository](https://github.com/AeroSim-CFD/stl2lnas).
:::

## Filtering between coefficients

A coefficient time series is carried by a {class}`cfdmod.DataSource`, so
signal-processing steps are just more ops in the pipeline. To smooth a
series, insert a `moving_average` step; the window is expressed in the
input time units and the result is another field on the same data source,
ready to feed the next step.

```yaml
- id: cp_smoothed
  kind: moving_average
  source: cp_t
  field: cp
  window: 3.0
  out: cp
```

Placing smoothing in the pipeline (rather than inside a statistics block)
keeps the lineage explicit: every step is recorded in the template that
produced the output.

## Worked example

An end-to-end example lives at `examples/container_pack/process_container_pack.ipynb`
in the repo. It builds a Cp / Cf / Cm / Ce pipeline as YAML templates,
runs them with `run_template` over the on-disk XDMF+H5 storage, and
streams the results back onto a coarse mesh. The per-coefficient
tutorials below run the same shipped templates
(`fixtures/tests/pressure/templates/`) step by step.

```{toctree}
:maxdepth: 1
:caption: Pressure use Cases
:hidden:

Pressure Coefficient <./pressure_coefficient.md>
Shape Coefficient <./shape_coefficient.md>
Force Coefficient <./force_coefficient.md>
Moment Coefficient <./moment_coefficient.md>
Time Normalization <./time_normalization.md>
```