Set zero Dirichlet/Neumann boundary conditions for derivative operators acting on cell-centered quantities.

This method is used to set zero Dirichlet and/or zero Neumann boundary conditions for differential operators that act on cell-centered quantities. The user may apply the same boundary conditions to all boundaries, or define the boundary conditions of boundary face (x, y and z) separately. The user may also apply boundary conditions to the lower and upper boundary face separately.

Cell gradient boundary conditions are enforced when constructing the following properties:

By default, the mesh assumes a zero Neumann boundary condition on the entire boundary. To define robin boundary conditions, see `cell_gradient_weak_form_robin`.

Parameters
BC`str` or `list` [dim,]

Define the boundary conditions using the string ‘dirichlet’ for zero Dirichlet conditions and ‘neumann’ for zero Neumann conditions. See examples for several implementations.

Examples

Here we demonstrate how to apply zero Dirichlet and/or Neumann boundary conditions for cell-centers differential operators.

```>>> from discretize import TensorMesh
>>> mesh = TensorMesh([[(1, 20)], [(1, 20)], [(1, 20)]])
```

Define zero Neumann conditions for all boundaries

```>>> BC = 'neumann'
```>>> BC = ['neumann', 'dirichlet', 'neumann']
```>>> BC = [['neumann', 'dirichlet'], 'dirichlet', 'dirichlet']