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# Cahn-Hilliard equation

This example demonstrates the solution of the Cahn-Hilliard equation,
a nonlinear, time-dependent fourth-order PDE.

- A mixed finite element method
- The $\theta$-method for time-dependent equations
- Automatic linearisation
- Use of the class
  {py:class}`NonlinearProblem<dolfinx.fem.petsc.NonlinearProblem>`
- The built-in Newton solver
  ({py:class}`NewtonSolver<dolfinx.nls.petsc.NewtonSolver>`)
- Form compiler options
- Interpolation of functions
- Visualisation of a running simulation with
  [PyVista](https://pyvista.org/)

This demo is implemented in {download}`demo_cahn-hilliard.py`.

## Equation and problem definition

The Cahn-Hilliard equation is a parabolic equation and is typically
used to model phase separation in binary mixtures.  It involves
first-order time derivatives, and second- and fourth-order spatial
derivatives.  The equation reads:

$$
\begin{align}
\frac{\partial c}{\partial t} -
  \nabla \cdot M \left(\nabla\left(\frac{d f}{dc}
  - \lambda \nabla^{2}c\right)\right) &= 0 \quad {\rm in} \ \Omega, \\
M\left(\nabla\left(\frac{d f}{d c} -
  \lambda \nabla^{2}c\right)\right) \cdot n
  &= 0 \quad {\rm on} \ \partial\Omega, \\
M \lambda \nabla c \cdot n &= 0 \quad {\rm on} \ \partial\Omega.
\end{align}
$$

where $c$ is the unknown field, the function $f$ is usually non-convex
in $c$ (a fourth-order polynomial is commonly used), $n$ is the
outward directed boundary normal, and $M$ is a scalar parameter.

### Operator split form

The Cahn-Hilliard equation is a fourth-order equation, so casting it
in a weak form would result in the presence of second-order spatial
derivatives, and the problem could not be solved using a standard
Lagrange finite element basis.  A solution is to rephrase the problem
as two coupled second-order equations:

$$
\begin{align}
\frac{\partial c}{\partial t} - \nabla \cdot M \nabla\mu
    &= 0 \quad {\rm in} \ \Omega, \\
\mu -  \frac{d f}{d c} + \lambda \nabla^{2}c &= 0 \quad {\rm in} \ \Omega.
\end{align}
$$

The unknown fields are now $c$ and $\mu$. The weak (variational) form
of the problem reads: find $(c, \mu) \in V \times V$ such that

$$
\begin{align}
\int_{\Omega} \frac{\partial c}{\partial t} q \, {\rm d} x +
    \int_{\Omega} M \nabla\mu \cdot \nabla q \, {\rm d} x
    &= 0 \quad \forall \ q \in V,  \\
\int_{\Omega} \mu v \, {\rm d} x - \int_{\Omega} \frac{d f}{d c} v \, {\rm d} x
  - \int_{\Omega} \lambda \nabla c \cdot \nabla v \, {\rm d} x
   &= 0 \quad \forall \ v \in V.
\end{align}
$$

### Time discretisation

Before being able to solve this problem, the time derivative must be
dealt with. Apply the $\theta$-method to the mixed weak form of the
equation:

$$
\begin{align}
\int_{\Omega} \frac{c_{n+1} - c_{n}}{dt} q \, {\rm d} x
+ \int_{\Omega} M \nabla \mu_{n+\theta} \cdot \nabla q \, {\rm d} x
       &= 0 \quad \forall \ q \in V  \\
\int_{\Omega} \mu_{n+1} v  \, {\rm d} x - \int_{\Omega} \frac{d f_{n+1}}{d c} v  \, {\rm d} x
- \int_{\Omega} \lambda \nabla c_{n+1} \cdot \nabla v \, {\rm d} x
       &= 0 \quad \forall \ v \in V
\end{align}
$$

where $dt = t_{n+1} - t_{n}$ and $\mu_{n+\theta} = (1-\theta) \mu_{n} + \theta \mu_{n+1}$.
The task is: given $c_{n}$ and $\mu_{n}$, solve the above equation to
find $c_{n+1}$ and $\mu_{n+1}$.

### Demo parameters

The following domains, functions and time stepping parameters are used
in this demo:

- $\Omega = (0, 1) \times (0, 1)$ (unit square)
- $f = 100 c^{2} (1-c)^{2}$
- $\lambda = 1 \times 10^{-2}$
- $M = 1$
- $dt = 5 \times 10^{-6}$
- $\theta = 0.5$

## Implementation

This demo is implemented in the {download}`demo_cahn-hilliard.py`
file.

```python
import os

from mpi4py import MPI
from petsc4py import PETSc

import numpy as np

import ufl
from basix.ufl import element, mixed_element
from dolfinx import default_real_type, log, plot
from dolfinx.fem import Function, functionspace
from dolfinx.fem.petsc import NonlinearProblem
from dolfinx.io import XDMFFile
from dolfinx.mesh import CellType, create_unit_square
from dolfinx.nls.petsc import NewtonSolver
from ufl import dx, grad, inner

try:
    import pyvista as pv
    import pyvistaqt as pvqt

    have_pyvista = True
    if pv.OFF_SCREEN:
        pv.start_xvfb(wait=0.5)
except ModuleNotFoundError:
    print("pyvista and pyvistaqt are required to visualise the solution")
    have_pyvista = False

# Save all logging to file
log.set_output_file("log.txt")
```

Next, various model parameters are defined:

```python
lmbda = 1.0e-02  # surface parameter
dt = 5.0e-06  # time step
theta = 0.5  # time stepping family, e.g. theta=1 -> backward Euler, theta=0.5 -> Crank-Nicholson
```

A unit square mesh with 96 cells edges in each direction is created,
and on this mesh a
{py:class}`FunctionSpace <dolfinx.fem.FunctionSpace>` `ME` is built
using a pair of linear Lagrange elements.

```python
msh = create_unit_square(MPI.COMM_WORLD, 96, 96, CellType.triangle)
P1 = element("Lagrange", msh.basix_cell(), 1)
ME = functionspace(msh, mixed_element([P1, P1]))
```

Trial and test functions of the space `ME` are now defined:

```python
q, v = ufl.TestFunctions(ME)
```

```{index} split functions
```

For the test functions, {py:func}`TestFunctions<function
ufl.argument.TestFunctions>` (note the 's' at the end) is used to
define the scalar test functions `q` and `v`. Some mixed objects of
the {py:class}`Function<dolfinx.fem.function.Function>` class on `ME`
are defined to represent $u = (c_{n+1}, \mu_{n+1})$ and $u0 = (c_{n},
\mu_{n})$, and these are then split into sub-functions:

```python
u = Function(ME)  # current solution
u0 = Function(ME)  # solution from previous converged step

# Split mixed functions
c, mu = ufl.split(u)
c0, mu0 = ufl.split(u0)
```

The line `c, mu = ufl.split(u)` permits direct access to the
components of a mixed function. Note that `c` and `mu` are references
for components of `u`, and not copies.

```{index} single: interpolating functions; (in Cahn-Hilliard demo)
```

The initial conditions are interpolated into a finite element space:

```python
# Zero u
u.x.array[:] = 0.0

# Interpolate initial condition
rng = np.random.default_rng(42)
u.sub(0).interpolate(lambda x: 0.63 + 0.02 * (0.5 - rng.random(x.shape[1])))
u.x.scatter_forward()
```

The first line creates an object of type `InitialConditions`.  The
following two lines make `u` and `u0` interpolants of `u_init` (since
`u` and `u0` are finite element functions, they may not be able to
represent a given function exactly, but the function can be
approximated by interpolating it in a finite element space).

```{index} automatic differentiation
```

The chemical potential $df/dc$ is computed using UFL automatic
differentiation:

```python
# Compute the chemical potential df/dc
c = ufl.variable(c)
f = 100 * c**2 * (1 - c) ** 2
dfdc = ufl.diff(f, c)
```

The first line declares that `c` is a variable that some function can
be differentiated with respect to. The next line is the function $f$
defined in the problem statement, and the third line performs the
differentiation of `f` with respect to the variable `c`.

It is convenient to introduce an expression for $\mu_{n+\theta}$:

```python
# mu_(n+theta)
mu_mid = (1.0 - theta) * mu0 + theta * mu
```

which is then used in the definition of the variational forms:

```python
# Weak statement of the equations
F0 = inner(c, q) * dx - inner(c0, q) * dx + dt * inner(grad(mu_mid), grad(q)) * dx
F1 = inner(mu, v) * dx - inner(dfdc, v) * dx - lmbda * inner(grad(c), grad(v)) * dx
F = F0 + F1
```

This is a statement of the time-discrete equations presented as part
of the problem statement, using UFL syntax.

```{index} single: Newton solver; (in Cahn-Hilliard demo)
```

The DOLFINx Newton solver requires a
{py:class}`NonlinearProblem<dolfinx.fem.NonlinearProblem>` object to
solve a system of nonlinear equations

```python
# Create nonlinear problem and Newton solver
problem = NonlinearProblem(F, u)
solver = NewtonSolver(MPI.COMM_WORLD, problem)
solver.convergence_criterion = "incremental"
solver.rtol = np.sqrt(np.finfo(default_real_type).eps) * 1e-2

# We can customize the linear solver used inside the NewtonSolver by
# modifying the PETSc options
ksp = solver.krylov_solver
opts = PETSc.Options()  # type: ignore
option_prefix = ksp.getOptionsPrefix()
opts[f"{option_prefix}ksp_type"] = "preonly"
opts[f"{option_prefix}pc_type"] = "lu"
sys = PETSc.Sys()  # type: ignore
# For factorisation prefer MUMPS, then superlu_dist, then default.
if sys.hasExternalPackage("mumps"):
    opts[f"{option_prefix}pc_factor_mat_solver_type"] = "mumps"
elif sys.hasExternalPackage("superlu_dist"):
    opts[f"{option_prefix}pc_factor_mat_solver_type"] = "superlu_dist"
ksp.setFromOptions()
```

The setting of `convergence_criterion` to `"incremental"` specifies
that the Newton solver should compute a norm of the solution increment
to check for convergence (the other possibility is to use
`"residual"`, or to provide a user-defined check). The tolerance for
convergence is specified by `rtol`.

To run the solver and save the output to a VTK file for later
visualization, the solver is advanced in time from $t_{n}$ to
$t_{n+1}$ until a terminal time $T$ is reached:

```python
# Output file
file = XDMFFile(MPI.COMM_WORLD, "demo_ch/output.xdmf", "w")
file.write_mesh(msh)

# Step in time
t = 0.0

#  Reduce run time if on test (CI) server
if "CI" in os.environ.keys() or "GITHUB_ACTIONS" in os.environ.keys():
    T = 3 * dt
else:
    T = 50 * dt

# Get the sub-space for c and the corresponding dofs in the mixed space
# vector
V0, dofs = ME.sub(0).collapse()

# Prepare viewer for plotting the solution during the computation
if have_pyvista:
    # Create a VTK 'mesh' with 'nodes' at the function dofs
    topology, cell_types, x = plot.vtk_mesh(V0)
    grid = pv.UnstructuredGrid(topology, cell_types, x)

    # Set output data
    grid.point_data["c"] = u.x.array[dofs].real
    grid.set_active_scalars("c")

    p = pvqt.BackgroundPlotter(title="concentration", auto_update=True)
    p.add_mesh(grid, clim=[0, 1])
    p.view_xy(True)
    p.add_text(f"time: {t}", font_size=12, name="timelabel")

c = u.sub(0)
u0.x.array[:] = u.x.array
while t < T:
    t += dt
    r = solver.solve(u)
    print(f"Step {int(t / dt)}: num iterations: {r[0]}")
    u0.x.array[:] = u.x.array
    file.write_function(c, t)

    # Update the plot window
    if have_pyvista:
        p.add_text(f"time: {t:.2e}", font_size=12, name="timelabel")
        grid.point_data["c"] = u.x.array[dofs].real
        p.app.processEvents()

file.close()

# Update ghost entries and plot
if have_pyvista:
    u.x.scatter_forward()
    grid.point_data["c"] = u.x.array[dofs].real
    screenshot = None
    if pv.OFF_SCREEN:
        screenshot = "c.png"
    pv.plot(grid, show_edges=True, screenshot=screenshot)
```
