View source for Cahn-Hilliard equation

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In this example we will show how to solve the [https://en.wikipedia.org/wiki/Cahn%E2%80%93Hilliard_equation Cahn-Hilliard] equation in Medusa. The solution provides an implementation of periodic boundary conditions and shows how to combine implicit and explicit operators. This example uses a custom biharmonic operator, so we recommend firstly reading the tutorial on [[Customization]].

== Cahn-Hilliard equation ==
The Cahn-Hilliard equation describes the separation of binary fluid into pure domains. We define concentration, a scalar field $c(x, y) \in [-1, 1]$ to describe the fluid, with values $c \pm 1$ representing pure phases. In this case the equation can be written as

\[
\frac{\partial c}{\partial t} = D \nabla^2 (c^3 - c - \gamma \nabla^2 c),
\]
where $D$ is the diffusion constant and $\sqrt{\gamma}$ is the characteristic transition length between domains.

We want to avoid any boundary effects and solve the equation for infinite fluid which can be approximated by using periodic boundary conditions.

== Periodic boundary conditions ==

<syntaxhighlight lang="cpp">
/// Create the square domain and fill it with nodes.
typedef mm::Vec<double, dim> vec_t;
mm::BoxShape<vec_t> box(-border, border);
mm::DomainDiscretization<vec_t> domain = box.discretizeBoundaryWithStep(dx);
mm::GeneralFill<vec_t> fill;
fill.seed(randomSeed);
domain.fill(fill, dx);
</syntaxhighlight>


== Solution ==
We need to handle two uncommon terms to solve the equation. Biharmonic ($\nabla^4$) operator can be implemented with custom operators as shown in [[Customization]] example. Nonlinear ($\nabla^2 c^3$) term proves to be more problematic, as it can not be solved implicitly. We either need to solve the entire equation with explicit time stepping or include the nonlinear term in the right hand side of our system of equations.

=== Explicit solution ===

<syntaxhighlight lang="cpp">

</syntaxhighlight>

=== Mixed solution ===

<syntaxhighlight lang="cpp">

</syntaxhighlight>


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