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# Mixed formulation for the Poisson equation

+++

This demo illustrates how to solve Poisson equation using a mixed
(two-field) formulation. In particular, it illustrates how to

* Use mixed and non-continuous finite element spaces.
* Set essential boundary conditions for subspaces and $H(\mathrm{div})$ spaces.

{download}`Python script <./demo_mixed-poisson.py>`.\
{download}`Jupyter notebook <./demo_mixed-poisson.ipynb>`.

## Equation and problem definition

An alternative formulation of Poisson equation can be formulated by
introducing an additional (vector) variable, namely the (negative)
flux: $\sigma = \nabla u$. The partial differential equations
then read

$$
  \sigma - \nabla u &= 0 \quad {\rm in} \ \Omega, \\
  \nabla \cdot \sigma &= - f \quad {\rm in} \ \Omega,
$$
with boundary conditions

$$
  u = u_0 \quad {\rm on} \ \Gamma_{D},  \\
  \sigma \cdot n = g \quad {\rm on} \ \Gamma_{N}.
$$

The same equations arise in connection with flow in porous media, and are
also referred to as Darcy flow. Here $n$ denotes the outward pointing normal
vector on the boundary. Looking at the variational form, we see that the
boundary condition for the flux ($\sigma \cdot n = g$) is now an essential
boundary condition (which should be enforced in the function space), while
the other boundary condition ($u = u_0$) is a natural boundary condition
(which should be applied to the variational form). Inserting the boundary
conditions, this variational problem can be phrased in the general form: find
$(\sigma, u) \in \Sigma_g \times V$ such that

$$
   a((\sigma, u), (\tau, v)) = L((\tau, v))
   \quad \forall \ (\tau, v) \in \Sigma_0 \times V,
$$

where the variational forms $a$ and $L$ are defined as

$$
  a((\sigma, u), (\tau, v)) &=
    \int_{\Omega} \sigma \cdot \tau + \nabla \cdot \tau \ u
  + \nabla \cdot \sigma \ v \ {\rm d} x, \\
  L((\tau, v)) &= - \int_{\Omega} f v \ {\rm d} x
  + \int_{\Gamma_D} u_0 \tau \cdot n  \ {\rm d} s,
$$
and $\Sigma_g = \{ \tau \in H({\rm div})$ such that $\tau \cdot n|_{\Gamma_N}
= g \}$ and $V = L^2(\Omega)$.

To discretize the above formulation, two discrete function spaces $\Sigma_h
\subset \Sigma$ and $V_h \subset V$ are needed to form a mixed function space
$\Sigma_h \times V_h$. A stable choice of finite element spaces is to let
$\Sigma_h$ be the Brezzi-Douglas-Fortin-Marini elements of polynomial order
$k$ and let $V_h$ be discontinuous elements of polynomial order $k-1$.

We will use the same definitions of functions and boundaries as in the
demo for {doc}`the Poisson equation <demo_poisson>`. These are:

* $\Omega = [0,1] \times [0,1]$ (a unit square)
* $\Gamma_{D} = \{(0, y) \cup (1, y) \in \partial \Omega\}$
* $\Gamma_{N} = \{(x, 0) \cup (x, 1) \in \partial \Omega\}$
* $u_0 = 0$
* $g = \sin(5x)$   (flux)
* $f = 10\exp(-((x - 0.5)^2 + (y - 0.5)^2) / 0.02)$  (source term)

## Implementation

```python

import numpy as np

from basix.ufl import element, mixed_element
from dolfinx import fem, io, mesh
from dolfinx.fem.petsc import LinearProblem
from ufl import (Measure, SpatialCoordinate, TestFunctions, TrialFunctions,
                 div, exp, inner)

from mpi4py import MPI
from petsc4py import PETSc

domain = mesh.create_unit_square(MPI.COMM_WORLD, 32, 32, mesh.CellType.quadrilateral)

k = 1
Q_el = element("BDMCF", domain.basix_cell(), k)
P_el = element("DG", domain.basix_cell(), k - 1)
V_el = mixed_element([Q_el, P_el])
V = fem.functionspace(domain, V_el)

(sigma, u) = TrialFunctions(V)
(tau, v) = TestFunctions(V)

x = SpatialCoordinate(domain)
f = 10.0 * exp(-((x[0] - 0.5) * (x[0] - 0.5) + (x[1] - 0.5) * (x[1] - 0.5)) / 0.02)

dx = Measure("dx", domain)
a = inner(sigma, tau) * dx + inner(u, div(tau)) * dx + inner(div(sigma), v) * dx
L = -inner(f, v) * dx


fdim = domain.topology.dim - 1
facets_top = mesh.locate_entities_boundary(domain, fdim, lambda x: np.isclose(x[1], 1.0))
Q, _ = V.sub(0).collapse()
dofs_top = fem.locate_dofs_topological((V.sub(0), Q), fdim, facets_top)


def f1(x):
    values = np.zeros((2, x.shape[1]))
    values[1, :] = np.sin(5 * x[0])
    return values


f_h1 = fem.Function(Q)
f_h1.interpolate(f1)
bc_top = fem.dirichletbc(f_h1, dofs_top, V.sub(0))


facets_bottom = mesh.locate_entities_boundary(domain, fdim, lambda x: np.isclose(x[1], 0.0))
dofs_bottom = fem.locate_dofs_topological((V.sub(0), Q), fdim, facets_bottom)


def f2(x):
    values = np.zeros((2, x.shape[1]))
    values[1, :] = -np.sin(5 * x[0])
    return values


f_h2 = fem.Function(Q)
f_h2.interpolate(f2)
bc_bottom = fem.dirichletbc(f_h2, dofs_bottom, V.sub(0))


bcs = [bc_top, bc_bottom]

problem = LinearProblem(a, L, bcs=bcs, petsc_options={"ksp_type": "preonly", "pc_type": "lu",
                                                      "pc_factor_mat_solver_type": "mumps"})
try:
    w_h = problem.solve()
except PETSc.Error as e:  # type: ignore
    if e.ierr == 92:
        print("The required PETSc solver/preconditioner is not available. Exiting.")
        print(e)
        exit(0)
    else:
        raise e

sigma_h, u_h = w_h.split()

with io.XDMFFile(domain.comm, "out_mixed_poisson/u.xdmf", "w") as file:
    file.write_mesh(domain)
    file.write_function(u_h)
```
