vector_fe_ex1.C File Reference

Go to the source code of this file.

Functions
void	assemble_poisson (EquationSystems &es, const std::string &system_name)
Real	exact_solution (const int component, const Real x, const Real y, const Real z=0.)
	This is the exact solution that we are trying to obtain.
int	main (int argc, char **argv)
void	assemble_poisson (EquationSystems &es, const std::string &libmesh_dbg_var(system_name))

Function Documentation

assemble_poisson() [1/2]

void assemble_poisson	(	EquationSystems &	es,
		const std::string &	libmesh_dbg_varsystem_name
	)

Definition at line 236 of file vector_fe_ex1.C.

{
 
  // It is a good idea to make sure we are assembling
  // the proper system.
  libmesh_assert_equal_to (system_name, "Poisson");
 
  // Get a constant reference to the mesh object.
  const MeshBase & mesh = es.get_mesh();
 
  // The dimension that we are running
  const unsigned int dim = mesh.mesh_dimension();
 
  // Get a reference to the LinearImplicitSystem we are solving
  LinearImplicitSystem & system = es.get_system<LinearImplicitSystem> ("Poisson");
 
  // A reference to the  DofMap object for this system.  The  DofMap
  // object handles the index translation from node and element numbers
  // to degree of freedom numbers.  We will talk more about the  DofMap
  // in future examples.
  const DofMap & dof_map = system.get_dof_map();
 
  // Get a constant reference to the Finite Element type
  // for the first (and only) variable in the system.
  FEType fe_type = dof_map.variable_type(0);
 
  // Build a Finite Element object of the specified type.
  // Note that FEVectorBase is a typedef for the templated FE
  // class.
  std::unique_ptr<FEVectorBase> fe (FEVectorBase::build(dim, fe_type));
 
  // A 2*p+1 order Gauss quadrature rule for numerical integration.
  QGauss qrule (dim, fe_type.default_quadrature_order());
 
  // Tell the finite element object to use our quadrature rule.
  fe->attach_quadrature_rule (&qrule);
 
  // Declare a special finite element object for
  // boundary integration.
  std::unique_ptr<FEVectorBase> fe_face (FEVectorBase::build(dim, fe_type));
 
  // Boundary integration requires one quadrature rule,
  // with dimensionality one less than the dimensionality
  // of the element.
  QGauss qface(dim-1, fe_type.default_quadrature_order());
 
  // Tell the finite element object to use our
  // quadrature rule.
  fe_face->attach_quadrature_rule (&qface);
 
  // Here we define some references to cell-specific data that
  // will be used to assemble the linear system.
  //
  // The element Jacobian * quadrature weight at each integration point.
  const std::vector<Real> & JxW = fe->get_JxW();
 
  // The physical XY locations of the quadrature points on the element.
  // These might be useful for evaluating spatially varying material
  // properties at the quadrature points.
  const std::vector<Point> & q_point = fe->get_xyz();
 
  // The element shape functions evaluated at the quadrature points.
  // Notice the shape functions are a vector rather than a scalar.
  const std::vector<std::vector<RealGradient>> & phi = fe->get_phi();
 
  // The element shape function gradients evaluated at the quadrature
  // points. Notice that the shape function gradients are a tensor.
  const std::vector<std::vector<RealTensor>> & dphi = fe->get_dphi();
 
  // Define data structures to contain the element matrix
  // and right-hand-side vector contribution.  Following
  // basic finite element terminology we will denote these
  // "Ke" and "Fe".  These datatypes are templated on
  //  Number, which allows the same code to work for real
  // or complex numbers.
  DenseMatrix<Number> Ke;
  DenseVector<Number> Fe;
 
  // This vector will hold the degree of freedom indices for
  // the element.  These define where in the global system
  // the element degrees of freedom get mapped.
  std::vector<dof_id_type> dof_indices;
 
  // The global system matrix
  SparseMatrix<Number> & matrix = system.get_system_matrix();
 
  // Now we will loop over all the elements in the mesh.
  // We will compute the element matrix and right-hand-side
  // contribution.
  //
  // Element iterators are a nice way to iterate through all the
  // elements, or all the elements that have some property.  The
  // iterator el will iterate from the first to the last element on
  // the local processor.  The iterator end_el tells us when to stop.
  // It is smart to make this one const so that we don't accidentally
  // mess it up!  In case users later modify this program to include
  // refinement, we will be safe and will only consider the active
  // elements; hence we use a variant of the active_elem_iterator.
  for (const auto & elem : mesh.active_local_element_ptr_range())
    {
      // Get the degree of freedom indices for the
      // current element.  These define where in the global
      // matrix and right-hand-side this element will
      // contribute to.
      dof_map.dof_indices (elem, dof_indices);
 
      // Compute the element-specific data for the current
      // element.  This involves computing the location of the
      // quadrature points (q_point) and the shape functions
      // (phi, dphi) for the current element.
      fe->reinit (elem);
 
      // Zero the element matrix and right-hand side before
      // summing them.  We use the resize member here because
      // the number of degrees of freedom might have changed from
      // the last element.  Note that this will be the case if the
      // element type is different (i.e. the last element was a
      // triangle, now we are on a quadrilateral).
 
      // The  DenseMatrix::resize() and the  DenseVector::resize()
      // members will automatically zero out the matrix  and vector.
      Ke.resize (dof_indices.size(),
                 dof_indices.size());
 
      Fe.resize (dof_indices.size());
 
      // We'll use an element-size-dependent h below, so the FDM error
      // doesn't easily dominate FEM error.
      const Real eps = 1.e-3 * elem->hmin();
 
      // Now loop over the quadrature points.  This handles
      // the numeric integration.
      for (unsigned int qp=0; qp<qrule.n_points(); qp++)
        {
          // Now we will build the element matrix.  This involves
          // a double loop to integrate the test functions (i) against
          // the trial functions (j).
          for (std::size_t i=0; i<phi.size(); i++)
            for (std::size_t j=0; j<phi.size(); j++)
              Ke(i,j) += JxW[qp] * dphi[i][qp].contract(dphi[j][qp]);
 
          // This is the end of the matrix summation loop
          // Now we build the element right-hand-side contribution.
          // This involves a single loop in which we integrate the
          // "forcing function" in the PDE against the test functions.
          {
            const Real x = q_point[qp](0);
            const Real y = q_point[qp](1);
 
            // "f" is the forcing function for the Poisson equation.
            // In this case we set f to be a finite difference
            // Laplacian approximation to the (known) exact solution.
            //
            // We will use the second-order accurate FD Laplacian
            // approximation, which in 2D is
            //
            // u_xx + u_yy = (u(i,j-1) + u(i,j+1) +
            //                u(i-1,j) + u(i+1,j) +
            //                -4*u(i,j))/h^2
 
            // Since the value of the forcing function depends only
            // on the location of the quadrature point (q_point[qp])
            // we will compute it here, outside of the i-loop
            const Real fx = -(exact_solution(0, x, y-eps) +
                              exact_solution(0, x, y+eps) +
                              exact_solution(0, x-eps, y) +
                              exact_solution(0, x+eps, y) -
                              4.*exact_solution(0, x, y))/eps/eps;
 
            const Real fy = -(exact_solution(1, x, y-eps) +
                              exact_solution(1, x, y+eps) +
                              exact_solution(1, x-eps, y) +
                              exact_solution(1, x+eps, y) -
                              4.*exact_solution(1, x, y))/eps/eps;
 
            const RealGradient f(fx, fy);
 
            for (std::size_t i=0; i<phi.size(); i++)
              Fe(i) += JxW[qp]*f*phi[i][qp];
          }
        }
 
      // We have now reached the end of the RHS summation,
      // and the end of quadrature point loop, so
      // the interior element integration has
      // been completed.  However, we have not yet addressed
      // boundary conditions.  For this example we will only
      // consider simple Dirichlet boundary conditions.
      //
      // There are several ways Dirichlet boundary conditions
      // can be imposed.  A simple approach, which works for
      // interpolary bases like the standard Lagrange polynomials,
      // is to assign function values to the
      // degrees of freedom living on the domain boundary. This
      // works well for interpolary bases, but is more difficult
      // when non-interpolary (e.g Legendre or Hierarchic) bases
      // are used.
      //
      // Dirichlet boundary conditions can also be imposed with a
      // "penalty" method.  In this case essentially the L2 projection
      // of the boundary values are added to the matrix. The
      // projection is multiplied by some large factor so that, in
      // floating point arithmetic, the existing (smaller) entries
      // in the matrix and right-hand-side are effectively ignored.
      //
      // This amounts to adding a term of the form (in latex notation)
      //
      // \frac{1}{\epsilon} \int_{\delta \Omega} \phi_i \phi_j = \frac{1}{\epsilon} \int_{\delta \Omega} u \phi_i
      //
      // where
      //
      // \frac{1}{\epsilon} is the penalty parameter, defined such that \epsilon << 1
      {
        // The following loop is over the sides of the element.
        // If the element has no neighbor on a side then that
        // side MUST live on a boundary of the domain.
        for (auto side : elem->side_index_range())
          if (elem->neighbor_ptr(side) == nullptr)
            {
              // The value of the shape functions at the quadrature
              // points.
              const std::vector<std::vector<RealGradient>> & phi_face = fe_face->get_phi();
 
              // The Jacobian * Quadrature Weight at the quadrature
              // points on the face.
              const std::vector<Real> & JxW_face = fe_face->get_JxW();
 
              // The XYZ locations (in physical space) of the
              // quadrature points on the face.  This is where
              // we will interpolate the boundary value function.
              const std::vector<Point> & qface_point = fe_face->get_xyz();
 
              // Compute the shape function values on the element
              // face.
              fe_face->reinit(elem, side);
 
              // Loop over the face quadrature points for integration.
              for (unsigned int qp=0; qp<qface.n_points(); qp++)
                {
                  // The location on the boundary of the current
                  // face quadrature point.
                  const Real xf = qface_point[qp](0);
                  const Real yf = qface_point[qp](1);
 
                  // The penalty value.  \frac{1}{\epsilon}
                  // in the discussion above.
                  const Real penalty = 1.e10;
 
                  // The boundary values.
                  const RealGradient f(exact_solution(0, xf, yf),
                                       exact_solution(1, xf, yf));
 
                  // Matrix contribution of the L2 projection.
                  for (std::size_t i=0; i<phi_face.size(); i++)
                    for (std::size_t j=0; j<phi_face.size(); j++)
                      Ke(i,j) += JxW_face[qp]*penalty*phi_face[i][qp]*phi_face[j][qp];
 
                  // Right-hand-side contribution of the L2
                  // projection.
                  for (std::size_t i=0; i<phi_face.size(); i++)
                    Fe(i) += JxW_face[qp]*penalty*f*phi_face[i][qp];
                }
            }
      }
 
      // We have now finished the quadrature point loop,
      // and have therefore applied all the boundary conditions.
 
      // If this assembly program were to be used on an adaptive mesh,
      // we would have to apply any hanging node constraint equations
      //dof_map.constrain_element_matrix_and_vector (Ke, Fe, dof_indices);
 
      // The element matrix and right-hand-side are now built
      // for this element.  Add them to the global matrix and
      // right-hand-side vector.  The  SparseMatrix::add_matrix()
      // and  NumericVector::add_vector() members do this for us.
      matrix.add_matrix         (Ke, dof_indices);
      system.rhs->add_vector    (Fe, dof_indices);
    }
 
  // All done!
}

References libMesh::SparseMatrix< T >::add_matrix(), libMesh::NumericVector< T >::add_vector(), libMesh::FEGenericBase< OutputType >::build(), libMesh::FEType::default_quadrature_order(), dim, exact_solution, libMesh::System::get_dof_map(), libMesh::EquationSystems::get_mesh(), libMesh::EquationSystems::get_system(), libMesh::ImplicitSystem::get_system_matrix(), mesh, libMesh::MeshBase::mesh_dimension(), libMesh::QBase::n_points(), libMesh::Real, libMesh::DenseVector< T >::resize(), libMesh::DenseMatrix< T >::resize(), and libMesh::ExplicitSystem::rhs.

assemble_poisson() [2/2]

void assemble_poisson	(	EquationSystems &	es,
		const std::string &	system_name
	)

Definition at line 261 of file miscellaneous_ex16.C.

{
  // Get a constant reference to the mesh object.
  const MeshBase & mesh = es.get_mesh();
 
  // The dimension that we are running
  const unsigned int dim = mesh.mesh_dimension();
 
  // Get a reference to the LinearImplicitSystem we are solving
  LinearImplicitSystem & system = es.get_system<LinearImplicitSystem>(system_name);
 
  // Get a pointer to the StaticCondensation class if it exists
  StaticCondensation * sc = nullptr;
  if (system.has_static_condensation())
    sc = &system.get_static_condensation();
 
  // A reference to the  DofMap object for this system.  The  DofMap
  // object handles the index translation from node and element numbers
  // to degree of freedom numbers.  We will talk more about the  DofMap
  // in future examples.
  const DofMap & dof_map = system.get_dof_map();
 
  // Get a constant reference to the Finite Element type
  // for the first (and only) variable in the system.
  FEType fe_type = dof_map.variable_type(0);
 
  // Build a Finite Element object of the specified type.  Since the
  // FEBase::build() member dynamically creates memory we will
  // store the object as a std::unique_ptr<FEBase>.  This can be thought
  // of as a pointer that will clean up after itself.  Introduction Example 4
  // describes some advantages of  std::unique_ptr's in the context of
  // quadrature rules.
  std::unique_ptr<FEBase> fe(FEBase::build(dim, fe_type));
 
  // A 5th order Gauss quadrature rule for numerical integration.
  QGauss qrule(dim, FIFTH);
 
  // Tell the finite element object to use our quadrature rule.
  fe->attach_quadrature_rule(&qrule);
 
  // Declare a special finite element object for
  // boundary integration.
  std::unique_ptr<FEBase> fe_face(FEBase::build(dim, fe_type));
 
  // Boundary integration requires one quadrature rule,
  // with dimensionality one less than the dimensionality
  // of the element.
  QGauss qface(dim - 1, FIFTH);
 
  // Tell the finite element object to use our
  // quadrature rule.
  fe_face->attach_quadrature_rule(&qface);
 
  // Here we define some references to cell-specific data that
  // will be used to assemble the linear system.
  //
  // The element Jacobian * quadrature weight at each integration point.
  const std::vector<Real> & JxW = fe->get_JxW();
 
  // The physical XY locations of the quadrature points on the element.
  // These might be useful for evaluating spatially varying material
  // properties at the quadrature points.
  const std::vector<Point> & q_point = fe->get_xyz();
 
  // The element shape functions evaluated at the quadrature points.
  const std::vector<std::vector<Real>> & phi = fe->get_phi();
 
  // The element shape function gradients evaluated at the quadrature
  // points.
  const std::vector<std::vector<RealGradient>> & dphi = fe->get_dphi();
 
  // Define data structures to contain the element matrix
  // and right-hand-side vector contribution.  Following
  // basic finite element terminology we will denote these
  // "Ke" and "Fe".  These datatypes are templated on
  //  Number, which allows the same code to work for real
  // or complex numbers.
  DenseMatrix<Number> Ke;
  DenseVector<Number> Fe;
 
  // This vector will hold the degree of freedom indices for
  // the element.  These define where in the global system
  // the element degrees of freedom get mapped.
  std::vector<dof_id_type> dof_indices;
 
  // The global system matrix
  SparseMatrix<Number> & matrix = system.get_system_matrix();
 
  // Now we will loop over all the elements in the mesh.
  // We will compute the element matrix and right-hand-side
  // contribution.
  //
  // Element ranges are a nice way to iterate through all the
  // elements, or all the elements that have some property.  The
  // range will iterate from the first to the last element on
  // the local processor.
  // It is smart to make this one const so that we don't accidentally
  // mess it up!  In case users later modify this program to include
  // refinement, we will be safe and will only consider the active
  // elements; hence we use a variant of the
  // active_local_element_ptr_range.
  for (const auto & elem : mesh.active_local_element_ptr_range())
  {
    // Get the degree of freedom indices for the
    // current element.  These define where in the global
    // matrix and right-hand-side this element will
    // contribute to.
    dof_map.dof_indices(elem, dof_indices);
 
    // Cache the number of degrees of freedom on this element, for
    // use as a loop bound later.  We use cast_int to explicitly
    // convert from size() (which may be 64-bit) to unsigned int
    // (which may be 32-bit but which is definitely enough to count
    // *local* degrees of freedom.
    const unsigned int n_dofs = cast_int<unsigned int>(dof_indices.size());
 
    // Compute the element-specific data for the current
    // element.  This involves computing the location of the
    // quadrature points (q_point) and the shape functions
    // (phi, dphi) for the current element.
    fe->reinit(elem);
 
    // With one variable, we should have the same number of degrees
    // of freedom as shape functions.
    libmesh_assert_equal_to(n_dofs, phi.size());
 
    // Zero the element matrix and right-hand side before
    // summing them.  We use the resize member here because
    // the number of degrees of freedom might have changed from
    // the last element.  Note that this will be the case if the
    // element type is different (i.e. the last element was a
    // triangle, now we are on a quadrilateral).
 
    // The  DenseMatrix::resize() and the  DenseVector::resize()
    // members will automatically zero out the matrix  and vector.
    Ke.resize(n_dofs, n_dofs);
 
    Fe.resize(n_dofs);
 
    // Now loop over the quadrature points.  This handles
    // the numeric integration.
    for (unsigned int qp = 0; qp < qrule.n_points(); qp++)
    {
 
      // Now we will build the element matrix.  This involves
      // a double loop to integrate the test functions (i) against
      // the trial functions (j).
      for (unsigned int i = 0; i != n_dofs; i++)
        for (unsigned int j = 0; j != n_dofs; j++)
        {
          Ke(i, j) += JxW[qp] * (dphi[i][qp] * dphi[j][qp]);
        }
 
      // This is the end of the matrix summation loop
      // Now we build the element right-hand-side contribution.
      // This involves a single loop in which we integrate the
      // "forcing function" in the PDE against the test functions.
      {
        const Real x = q_point[qp](0);
        const Real y = q_point[qp](1);
        const Real eps = 1.e-3;
 
        // "fxy" is the forcing function for the Poisson equation.
        // In this case we set fxy to be a finite difference
        // Laplacian approximation to the (known) exact solution.
        //
        // We will use the second-order accurate FD Laplacian
        // approximation, which in 2D is
        //
        // u_xx + u_yy = (u(i,j-1) + u(i,j+1) +
        //                u(i-1,j) + u(i+1,j) +
        //                -4*u(i,j))/h^2
        //
        // Since the value of the forcing function depends only
        // on the location of the quadrature point (q_point[qp])
        // we will compute it here, outside of the i-loop
        const Real fxy =
            -(exact_solution(x, y - eps) + exact_solution(x, y + eps) + exact_solution(x - eps, y) +
              exact_solution(x + eps, y) - 4. * exact_solution(x, y)) /
            eps / eps;
 
        for (unsigned int i = 0; i != n_dofs; i++)
          Fe(i) += JxW[qp] * fxy * phi[i][qp];
      }
    }
 
    // We have now reached the end of the RHS summation,
    // and the end of quadrature point loop, so
    // the interior element integration has
    // been completed.  However, we have not yet addressed
    // boundary conditions.  For this example we will only
    // consider simple Dirichlet boundary conditions.
    //
    // There are several ways Dirichlet boundary conditions
    // can be imposed.  A simple approach, which works for
    // interpolary bases like the standard Lagrange polynomials,
    // is to assign function values to the
    // degrees of freedom living on the domain boundary. This
    // works well for interpolary bases, but is more difficult
    // when non-interpolary (e.g Legendre or Hierarchic) bases
    // are used.
    //
    // Dirichlet boundary conditions can also be imposed with a
    // "penalty" method.  In this case essentially the L2 projection
    // of the boundary values are added to the matrix. The
    // projection is multiplied by some large factor so that, in
    // floating point arithmetic, the existing (smaller) entries
    // in the matrix and right-hand-side are effectively ignored.
    //
    // This amounts to adding a term of the form (in latex notation)
    //
    // \frac{1}{\epsilon} \int_{\delta \Omega} \phi_i \phi_j = \frac{1}{\epsilon} \int_{\delta
    // \Omega} u \phi_i
    //
    // where
    //
    // \frac{1}{\epsilon} is the penalty parameter, defined such that \epsilon << 1
    {
 
      // The following loop is over the sides of the element.
      // If the element has no neighbor on a side then that
      // side MUST live on a boundary of the domain.
      for (auto side : elem->side_index_range())
        if (elem->neighbor_ptr(side) == nullptr)
        {
          // The value of the shape functions at the quadrature
          // points.
          const std::vector<std::vector<Real>> & phi_face = fe_face->get_phi();
 
          // The Jacobian * Quadrature Weight at the quadrature
          // points on the face.
          const std::vector<Real> & JxW_face = fe_face->get_JxW();
 
          // The XYZ locations (in physical space) of the
          // quadrature points on the face.  This is where
          // we will interpolate the boundary value function.
          const std::vector<Point> & qface_point = fe_face->get_xyz();
 
          // Compute the shape function values on the element
          // face.
          fe_face->reinit(elem, side);
 
          // Some shape functions will be 0 on the face, but for
          // ease of indexing and generality of code we loop over
          // them anyway
          libmesh_assert_equal_to(n_dofs, phi_face.size());
 
          // Loop over the face quadrature points for integration.
          for (unsigned int qp = 0; qp < qface.n_points(); qp++)
          {
            // The location on the boundary of the current
            // face quadrature point.
            const Real xf = qface_point[qp](0);
            const Real yf = qface_point[qp](1);
 
            // The penalty value.  \frac{1}{\epsilon}
            // in the discussion above.
            const Real penalty = 1.e10;
 
            // The boundary value.
            const Real value = exact_solution(xf, yf);
 
            // Matrix contribution of the L2 projection.
            for (unsigned int i = 0; i != n_dofs; i++)
              for (unsigned int j = 0; j != n_dofs; j++)
                Ke(i, j) += JxW_face[qp] * penalty * phi_face[i][qp] * phi_face[j][qp];
 
            // Right-hand-side contribution of the L2
            // projection.
            for (unsigned int i = 0; i != n_dofs; i++)
              Fe(i) += JxW_face[qp] * penalty * value * phi_face[i][qp];
          }
        }
    }
 
    // We have now finished the quadrature point loop,
    // and have therefore applied all the boundary conditions.
 
    // If this assembly program were to be used on an adaptive mesh,
    // we would have to apply any hanging node constraint equations
    dof_map.constrain_element_matrix_and_vector(Ke, Fe, dof_indices);
 
    if (sc)
      sc->set_current_elem(*elem);
 
    // The element matrix and right-hand-side are now built
    // for this element.  Add them to the global matrix and
    // right-hand-side vector.  The  SparseMatrix::add_matrix()
    // and  NumericVector::add_vector() members do this for us.
    matrix.add_matrix(Ke, dof_indices);
    system.rhs->add_vector(Fe, dof_indices);
  }
 
  matrix.close();
}

Referenced by main().

exact_solution()

Real exact_solution	(	const int	component,
		const Real	x,
		const Real	y,
		const Real	z = 0.
	)

This is the exact solution that we are trying to obtain.

We will solve

• (u_xx + u_yy) = f

and take a finite difference approximation using this function to get f. This is the well-known "method of manufactured solutions".

Definition at line 39 of file exact_solution.C.

{
  static const Real pi = acos(-1.);
 
  switch (component)
    {
    case 0:
      return cos(.5*pi*x)*sin(.5*pi*y)*cos(.5*pi*z);
    case 1:
      return sin(.5*pi*x)*cos(.5*pi*y)*cos(.5*pi*z);
    case 2:
      return sin(.5*pi*x)*cos(.5*pi*y)*cos(.5*pi*z)*cos(.5*pi*x*y*z);
    default:
      libmesh_error_msg("Invalid component = " << component);
    }
 
  // dummy
  return 0.0;
}

main()

int main	(	int	argc,
		char **	argv
	)

Definition at line 87 of file vector_fe_ex1.C.

{
  // Initialize libraries.
  LibMeshInit init (argc, argv);
 
  // This example requires a linear solver package.
  libmesh_example_requires(libMesh::default_solver_package() != INVALID_SOLVER_PACKAGE,
                           "--enable-petsc, --enable-trilinos, or --enable-eigen");
 
  // Brief message to the user regarding the program name
  // and command line arguments.
  libMesh::out << "Running " << argv[0];
 
  for (int i=1; i<argc; i++)
    libMesh::out << " " << argv[i];
 
  libMesh::out << std::endl << std::endl;
 
  // Skip this 2D example if libMesh was compiled as 1D-only.
  libmesh_example_requires(2 <= LIBMESH_DIM, "2D support");
 
  // Get the mesh size from the command line.
  const int nx = libMesh::command_line_next("-nx", 15),
            ny = libMesh::command_line_next("-ny", 15);
 
  // Create a mesh, with dimension to be overridden later, on the
  // default MPI communicator.
  Mesh mesh(init.comm());
 
  // Use the MeshTools::Generation mesh generator to create a uniform
  // 2D grid on the square [-1,1]^2.  We instruct the mesh generator
  // to build a mesh of 15x15 QUAD9 elements.
  MeshTools::Generation::build_square (mesh,
                                       nx, ny,
                                       -1., 1.,
                                       -1., 1.,
                                       QUAD9);
 
  // Print information about the mesh to the screen.
  mesh.print_info();
 
  // Create an equation systems object.
  EquationSystems equation_systems (mesh);
 
  // Declare the Poisson system and its variables.
  // The Poisson system is another example of a steady system.
  LinearImplicitSystem & poisson = equation_systems.add_system<LinearImplicitSystem> ("Poisson");
 
  // Read FE order from command line
  std::string order_str = "SECOND";
  order_str = libMesh::command_line_next("-o", order_str);
  order_str = libMesh::command_line_next("-Order", order_str);
  const Order order = Utility::string_to_enum<Order>(order_str);
 
  // Read FE Family from command line
  std::string family_str = "LAGRANGE_VEC";
  family_str = libMesh::command_line_next("-f", family_str);
  family_str = libMesh::command_line_next("-FEFamily", family_str);
  const FEFamily family = Utility::string_to_enum<FEFamily>(family_str);
 
  libmesh_error_msg_if(FEInterface::field_type(family) != TYPE_VECTOR,
                       "FE family " + family_str + " isn't vector-valued");
 
  // Adds the variable "u" to "Poisson".  "u" will be approximated
  // using the requested order of approximation and vector element
  // type. Since the mesh is 2-D, "u" will have two components.
  poisson.add_variable("u", order, family);
 
  // Give the system a pointer to the matrix assembly
  // function.  This will be called when needed by the
  // library.
  poisson.attach_assemble_function (assemble_poisson);
 
  // Initialize the data structures for the equation system.
  equation_systems.init();
 
  // Prints information about the system to the screen.
  equation_systems.print_info();
 
  // If we're using Eigen, the default BiCGStab solver does not seem
  // to converge robustly for this system.  Let's try some other
  // settings for them.
  if (libMesh::default_solver_package() == EIGEN_SOLVERS)
    poisson.get_linear_solver()->set_solver_type(GMRES);
 
  // Solve the system "Poisson".  Note that calling this
  // member will assemble the linear system and invoke
  // the default numerical solver.  With PETSc the solver can be
  // controlled from the command line.  For example,
  // you can invoke conjugate gradient with:
  //
  // ./vector_fe_ex1 -ksp_type cg
  //
  // You can also get a nice X-window that monitors the solver
  // convergence with:
  //
  // ./vector_fe_ex1 -ksp_xmonitor
  //
  // if you linked against the appropriate X libraries when you
  // built PETSc.
  poisson.solve();
 
  const Real l2_norm =
    poisson.calculate_norm(*poisson.solution, 0, L2);
 
  libMesh::out << "L2 norm of solution = " << std::setprecision(17) <<
    l2_norm << std::endl;
 
  libmesh_error_msg_if (libmesh_isnan(l2_norm),
                        "Failed to calculate solution");
 
  const Real error_in_norm = std::abs(l2_norm - sqrt(Real(2)));
 
  libMesh::out << "error in L2 norm = " << std::setprecision(17) <<
    error_in_norm << std::endl;
 
  // The error in the norm converges faster than the norm of the
  // error, at least until it gets low enough that floating-point
  // roundoff (and the penalty method) kill us.
  const int n = std::min(nx, ny);
  const int p = static_cast<int>(order);
  const Real expected_error_bound = 2*std::pow(n, -p*2);
  libMesh::out << "error bound = " << std::setprecision(17) <<
    expected_error_bound << std::endl;
  libMesh::out << "error ratio = " << std::setprecision(17) <<
    error_in_norm / expected_error_bound << std::endl;
  libmesh_error_msg_if (error_in_norm > expected_error_bound,
                        "Error exceeds expected bound of " <<
                        expected_error_bound);
 
#ifdef LIBMESH_HAVE_EXODUS_API
  ExodusII_IO(mesh).write_equation_systems("out.e", equation_systems);
#endif
 
#ifdef LIBMESH_HAVE_GMV
  GMVIO(mesh).write_equation_systems("out.gmv", equation_systems);
#endif
 
  // All done.
  return 0;
}

References libMesh::EquationSystems::add_system(), libMesh::System::add_variable(), assemble_poisson(), libMesh::System::attach_assemble_function(), libMesh::MeshTools::Generation::build_square(), libMesh::System::calculate_norm(), libMesh::command_line_next(), libMesh::default_solver_package(), libMesh::EIGEN_SOLVERS, libMesh::FEInterface::field_type(), libMesh::LinearImplicitSystem::get_linear_solver(), libMesh::GMRES, libMesh::EquationSystems::init(), libMesh::INVALID_SOLVER_PACKAGE, libMesh::L2, libMesh::libmesh_isnan(), main(), mesh, libMesh::out, libMesh::EquationSystems::print_info(), libMesh::MeshBase::print_info(), libMesh::QUAD9, libMesh::Real, libMesh::LinearSolver< T >::set_solver_type(), libMesh::System::solution, libMesh::LinearImplicitSystem::solve(), libMesh::TYPE_VECTOR, libMesh::MeshOutput< MT >::write_equation_systems(), and libMesh::ExodusII_IO::write_equation_systems().