# Boundary Conditions

The boundary conditions (BCs) must be configured in order to run a simulation. 
The computational domain is a parallelepiped as illustrated below:

```{figure} /_static/img/user_guide/02_config/computational_domain.png
:alt: Computational domain
:align: center

Computational domain
```

The BCs are defined for each plane in the `models.BC` field. 
The example below, shows a basic wind tunnel setup with a uniform inlet inflow:

```yaml
    models:
      BC:
        periodic_dims: [false, true, false]
        BC_map:
          - pos: W
            BC: UniformFlow
            wall_normal: W
            rho: 1.0
            ux: 0.05
            uy: 0
            uz: 0
            order: 2
          - pos: E
            BC: RegularizedNeumannOutlet
            rho: 1.0
            wall_normal: E
            order: 2
          - pos: F
            BC: Neumann
            wall_normal: F
            order: 1
          - pos: B
            BC: RegularizedHWBB
            wall_normal: B
            order: 1
```

If periodicity is set true for a certain direction, it does not require definition of a BC. 
The order determines which BC will overwrite the other at edges and corners of computational domain.

```{important}
It's recommended to execute the lateral boundaries first, then top/floor, and inlet/outlet. 
```

It's also possible to define a BC for edges as shown below:

```yaml
    models:
      BC:
        BC_map:
          - pos: NW
            BC: Neumann
            wall_normal: W
            order: 3
```

The `wall_normal` determines the offset direction to perform certain BCs and can only be defined for the domain face boundaries.

## Turbulent inflow

In many simulations it's desired to use a turbulent inflow.
To do this, the synthetic eddy method (SEM) is used.
The use of SEM is limited to the `W` boundary, and a simulation set to use SEM as inlet will have the following aspect:

```yaml
    models:
      BC:
        periodic_dims: [false, false, false]
        BC_map:
          - pos: E
            BC: RegularizedNeumannOutlet
            rho: 1.0
            wall_normal: E
            order: 2
          - pos: F
            BC: Neumann
            wall_normal: F
            order: 1
          - pos: B
            BC: RegularizedHWBB
            wall_normal: B
            order: 1
          - pos: N
            BC: Neumann
            wall_normal: N
            order: 0
          - pos: S
            BC: Neumann
            wall_normal: S
            order: 0
        SEM:
          eddies:
            lengthscale: {x: 14, y: 14, z: 14}
            eddies_vol_density: 10
            seed_rand: 0
            domain_limits_yz:
              start: [-14, -14]
              end: [174, 78]
          profile:
            csv_profile_data: "../SEM/vprofile.csv"
            z_offset: 3.6
            K: 1
            csv_y_height: "../SEM/y_heights.csv"
            length_mul: 1
            vel_mul: 1
```

The `.csv` file for `profile.csv_profile_data` describe the velocity profile and its stress tensor. 
It must contain the headers: `z,ux,Rxx,Ryy,Rzz,Rxz,Rxy,Ryz`

- `z`: Height for given values
- `ux`: Average velocity for the given height
- `Rxx,Ryy,Rzz,Rxz,Rxy,Ryz`: Reynolds stress tensor for given height

```{note}
The values in between are found using linear interpolation, or constant extrapolation for values higher or lower than `z` max, min.
```

Density of eddies is given by `eddies_vol_density`, which is the density of eddies by volume, so for 10 it means that every SEM domain node will have 10 eddies affecting it in average. 
The randomization of eddies generation can be controlled using the `seed_rand`, so when using the same `seed_rand` and other configurations the same series of eddies are generated (making it possible to reproduce the same inlet multiple times).

The height of velocity profile can be adjusted with `profile.z_offset`.
Also, if a variable height in y direction is desired $z(y)$, a csv file with the height profile can be provided in `profile.csv_y_height`.
The `.csv` for `csv_y_height` must contain the headers `y,z_height`.

```{note}
Examples can be found in `nassu/fixture/SEM/` folder.
```

```{important}
For more details check the [models documentation](file/models.md).
```