(sec-user-physics-boundary-conditions)=
# Boundary Conditions

## Assigning Boundary Conditions

There are three basic steps in assigning a specific boundary condition to a portion of the domain.

1. Create sets of vertices in the mesh generation process for each boundary condition.
2. Set the parameters for each boundary condition group using `cfg` files or command line arguments.
3. Specify the spatial variation in parameters for the boundary condition using a spatial database.

## Creating Sets of Vertices

The procedure for creating sets of vertices differs depending on the mesh generator.
For meshes specified using the PyLith mesh ASCII format, the sets of vertices are specified using groups (see {ref}`sec-user-file-formats-meshio-ascii`).
In CUBIT the groups of vertices are created using nodesets.
Note that we chose to associate boundary conditions with groups of vertices because nearly every mesh generation package supports associating a string or integer with groups of vertices.
Note also that we currently associate boundary conditions with string identifiers.
Finally, note that the boundary conditions must be associated with a simply-connected surface;
that is, surfaces must be connected and not contain holes.

## Arrays of Boundary Condition Components

A dynamic array of boundary condition components associates a name (string) with each boundary condition. The default boundary condition for each component in the array is `DirichletTimeDependent`.
Other boundary conditions can be bound to the named items in the array by assining the component type to the named boundary condition.

```{code-block} cfg
---
caption: Array of boundary conditions in a `cfg` file
---
[pylithapp.problem]
# Array of four boundary conditions
bc = [x_neg, x_pos, y_pos, z_neg]
# Default boundary condition is DirichletBC
# Keep default value for x_neg and x_pos but assign new types to y_pos and z_neg
bc.y_pos = pylith.bc.AbsorbingDampers
bc.z_neg = pylith.bc.NeumannTimeDependent
```

## Diagnostic Information

The diagnostic information includes the outward normal direction (`normal_dir`) and the two tangential directions (`horizontal_tangential_dir` and `vertical_tangential_dir`).
The default basis order for discretizing these directions is 1, so these produce `vertex_fields` as opposed to `cell_fields` (basis order of 0).

## Boundary Condition Implementations

:::{toctree}
time-dependent.md
absorbing-dampers.md
:::

:::{seealso}
See {ref}`sec-user-governing-eqns` for the derivation of the finite-element formulation for each of the boundary conditions.
:::