subdomains_ex1.C

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// The libMesh Finite Element Library.
// Copyright (C) 2002-2026 Benjamin S. Kirk, John W. Peterson, Roy H. Stogner

// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// Lesser General Public License for more details.

// You should have received a copy of the GNU Lesser General Public
// License along with this library; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA



// <h1>Subdomains Example 1 - Solving on a Subdomain</h1>
// \author Tim Kroger
// \date 2010
//
// This example builds on the example 4 by showing what to do in
// order to solve an equation only on a subdomain.


// C++ include files that we need
#include <iostream>
#include <algorithm>
#include <math.h>

// Basic include file needed for the mesh functionality.
#include "libmesh/libmesh.h"
#include "libmesh/mesh.h"
#include "libmesh/mesh_generation.h"
#include "libmesh/exodusII_io.h"
#include "libmesh/gmv_io.h"
#include "libmesh/gnuplot_io.h"
#include "libmesh/linear_implicit_system.h"
#include "libmesh/equation_systems.h"

// Define the Finite Element object.
#include "libmesh/fe.h"

// Define Gauss quadrature rules.
#include "libmesh/quadrature_gauss.h"

// Define the DofMap, which handles degree of freedom
// indexing.
#include "libmesh/dof_map.h"

// Define useful datatypes for finite element
// matrix and vector components.
#include "libmesh/sparse_matrix.h"
#include "libmesh/numeric_vector.h"
#include "libmesh/dense_matrix.h"
#include "libmesh/dense_vector.h"

// Define the PerfLog, a performance logging utility.
// It is useful for timing events in a code and giving
// you an idea where bottlenecks lie.
#include "libmesh/perf_log.h"

// The definition of a geometric element
#include "libmesh/elem.h"

#include "libmesh/mesh_refinement.h"

// Classes needed for subdomain computation.
#include "libmesh/system_subset_by_subdomain.h"

#include "libmesh/string_to_enum.h"
#include "libmesh/getpot.h"
#include "libmesh/enum_solver_package.h"

// Bring in everything from the libMesh namespace
using namespace libMesh;



// Function prototype.  This is the function that will assemble
// the linear system for our Poisson problem.  Note that the
// function will take the EquationSystems object and the
// name of the system we are assembling as input.  From the
// EquationSystems object we have access to the Mesh and
// other objects we might need.
void assemble_poisson(EquationSystems & es,
                      const std::string & system_name);

// Exact solution function prototype.
Real exact_solution (const Real x,
                     const Real y = 0.,
                     const Real z = 0.);

// Begin the main program.
int main (int argc, char ** argv)
{
  // Initialize libMesh and any dependent libraries, like in example 2.
  LibMeshInit init (argc, argv);

  // Only our PETSc interface currently supports solves restricted to
  // subdomains
  libmesh_example_requires(libMesh::default_solver_package() == PETSC_SOLVERS, "--enable-petsc");

  // Skip adaptive examples on a non-adaptive libMesh build
#ifndef LIBMESH_ENABLE_AMR
  libmesh_example_requires(false, "--enable-amr");
#else

  // Declare a performance log for the main program
  // PerfLog perf_main("Main Program");

  // Create a GetPot object to parse the command line
  GetPot command_line (argc, argv);

  // Check for proper calling arguments.
  libmesh_error_msg_if(argc < 3, "Usage:\n" << "\t " << argv[0] << " -d 2(3)" << " -n 15");

  // 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;

  // Read problem dimension from command line.  Use int
  // instead of unsigned since the GetPot overload is ambiguous
  // otherwise.
  const int dim = libMesh::command_line_next("-d", 2);

  // Skip higher-dimensional examples on a lower-dimensional libMesh build
  libmesh_example_requires(dim <= LIBMESH_DIM, "2D/3D support");

  // Create a mesh with user-defined dimension.
  // Read number of elements from command line
  const int ps = libMesh::command_line_next("-n", 15);

  // Read FE order from command line
  std::string order = "FIRST";
  order = libMesh::command_line_next("-o", order);
  order = libMesh::command_line_next("-Order", order);

  // Read FE Family from command line
  std::string family = "LAGRANGE";
  family = libMesh::command_line_next("-f", family);
  family = libMesh::command_line_next("-FEFamily", family);

  // Cannot use discontinuous basis.
  libmesh_error_msg_if((family == "MONOMIAL") || (family == "XYZ"),
                       "This example requires a C^0 (or higher) FE basis.");

  // 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
  // grid on the square [-1,1]^D.  We instruct the mesh generator
  // to build a mesh of 8x8 Quad9 elements in 2D, or Hex27
  // elements in 3D.  Building these higher-order elements allows
  // us to use higher-order approximation, as in example 3.

  Real halfwidth = dim > 1 ? 1. : 0.;
  Real halfheight = dim > 2 ? 1. : 0.;

  if ((family == "LAGRANGE") && (order == "FIRST"))
    {
      // No reason to use high-order geometric elements if we are
      // solving with low-order finite elements.
      MeshTools::Generation::build_cube (mesh,
                                         ps,
                                         (dim>1) ? ps : 0,
                                         (dim>2) ? ps : 0,
                                         -1., 1.,
                                         -halfwidth, halfwidth,
                                         -halfheight, halfheight,
                                         (dim==1)    ? EDGE2 :
                                         ((dim == 2) ? QUAD4 : HEX8));
    }

  else
    {
      MeshTools::Generation::build_cube (mesh,
                                         ps,
                                         (dim>1) ? ps : 0,
                                         (dim>2) ? ps : 0,
                                         -1., 1.,
                                         -halfwidth, halfwidth,
                                         -halfheight, halfheight,
                                         (dim==1)    ? EDGE3 :
                                         ((dim == 2) ? QUAD9 : HEX27));
    }


  // To demonstrate solving on a subdomain, we will solve only on the
  // interior of a circle (ball in 3d) with radius 0.8.  So show that
  // this also works well on locally refined meshes, we refine once
  // all elements that are located on the boundary of this circle (or
  // ball).
  {
    // A MeshRefinement object is needed to refine meshes.
    MeshRefinement meshRefinement(mesh);

    // Loop over all elements.
    for (auto & elem : mesh.element_ptr_range())
      if (elem->active())
        {
          // Just check whether the current element has at least one
          // node inside and one node outside the circle.
          bool node_in = false;
          bool node_out = false;
          for (auto & n : elem->node_ref_range())
            {
              Real d = n.norm();
              if (d<0.8)
                node_in = true;
              else
                node_out = true;
            }
          if (node_in && node_out)
            elem->set_refinement_flag(Elem::REFINE);
          else
            elem->set_refinement_flag(Elem::DO_NOTHING);
        }
      else
        elem->set_refinement_flag(Elem::INACTIVE);

    // Now actually refine.
    meshRefinement.refine_elements();
  }

  // Print information about the mesh to the screen.
  mesh.print_info();

  // Now set the subdomain_id of all elements whose vertex average is inside
  // the circle to 1.
  for (auto elem : mesh.element_ptr_range())
    {
      Real d = elem->vertex_average().norm();
      if (d < 0.8)
        elem->subdomain_id() = 1;
    }

  // Make sure the mesh knows we added new subdomains.
  mesh.cache_elem_data();

  // Create an equation systems object.
  EquationSystems equation_systems (mesh);

  // Declare the system and its variables.
  // Create a system named "Poisson"
  LinearImplicitSystem & system =
    equation_systems.add_system<LinearImplicitSystem> ("Poisson");


  // Add the variable "u" to "Poisson".  "u"
  // will be approximated using second-order approximation.
  system.add_variable("u",
                      Utility::string_to_enum<Order>   (order),
                      Utility::string_to_enum<FEFamily>(family));

  // Give the system a pointer to the matrix assembly
  // function.
  system.attach_assemble_function (assemble_poisson);

  // Initialize the data structures for the equation system.
  equation_systems.init();

  // Print information about the system to the screen.
  equation_systems.print_info();
  mesh.print_info();

  // Restrict solves to those elements that have subdomain_id set to 1.
  std::set<subdomain_id_type> id_list;
  id_list.insert(1);
  SystemSubsetBySubdomain::SubdomainSelectionByList selection(id_list);
  SystemSubsetBySubdomain subset(system, selection);
  system.restrict_solve_to(&subset, SUBSET_ZERO);

  // Note that using SUBSET_ZERO will cause all dofs outside the
  // subdomain to be cleared.  This will, however, cause some hanging
  // nodes outside the subdomain to have inconsistent values.

  // Solve the system "Poisson", just like example 2.
  equation_systems.get_system("Poisson").solve();

  // After solving the system write the solution
  // to a GMV-formatted plot file.
  if (dim == 1)
    {
      GnuPlotIO plot(mesh, "Subdomains Example 1, 1D", GnuPlotIO::GRID_ON);
      plot.write_equation_systems("gnuplot_script", equation_systems);
    }
  else
    {
      GMVIO (mesh).write_equation_systems ((dim == 3) ?
                                           "out_3.gmv" : "out_2.gmv", equation_systems);
#ifdef LIBMESH_HAVE_EXODUS_API
      ExodusII_IO (mesh).write_equation_systems ((dim == 3) ?
                                                 "out_3.e" : "out_2.e", equation_systems);
#endif // #ifdef LIBMESH_HAVE_EXODUS_API
    }

#endif // #ifndef LIBMESH_ENABLE_AMR

  // All done.
  return 0;
}




// We now define the matrix assembly function for the
// Poisson system.  We need to first compute element
// matrices and right-hand sides, and then take into
// account the boundary conditions, which will be handled
// via a penalty method.
void assemble_poisson(EquationSystems & es,
                      const std::string & libmesh_dbg_var(system_name))
{
  // It is a good idea to make sure we are assembling
  // the proper system.
  libmesh_assert_equal_to (system_name, "Poisson");

  // Declare a performance log.  Give it a descriptive
  // string to identify what part of the code we are
  // logging, since there may be many PerfLogs in an
  // application.
  PerfLog perf_log ("Matrix Assembly");

  // 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.  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.
  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.
  // We begin with 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". More detail is in example 3.
  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.  See example 3 for a discussion of the
  // element iterators.
  for (const auto & elem : mesh.active_local_element_ptr_range())
    {
      // Elements with subdomain_id other than 1 are not in the active
      // subdomain.  We don't assemble anything for them.
      if (elem->subdomain_id()==1)
        {
          // Start logging the shape function initialization.
          // This is done through a simple function call with
          // the name of the event to log.
          perf_log.push("elem init");

          // 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).
          Ke.resize (dof_indices.size(),
                     dof_indices.size());

          Fe.resize (dof_indices.size());

          // Stop logging the shape function initialization.
          // If you forget to stop logging an event the PerfLog
          // object will probably catch the error and abort.
          perf_log.pop("elem init");

          // Now we will build the element matrix.  This involves
          // a double loop to integrate the test functions (i) against
          // the trial functions (j).
          //
          // We have split the numeric integration into two loops
          // so that we can log the matrix and right-hand-side
          // computation separately.
          //
          // Now start logging the element matrix computation
          perf_log.push ("Ke");

          for (unsigned int qp=0; qp<qrule.n_points(); qp++)
            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]*dphi[j][qp]);


          // Stop logging the matrix computation
          perf_log.pop ("Ke");

          // 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.
          //
          // Start logging the right-hand-side computation
          perf_log.push ("Fe");

          for (unsigned int qp=0; qp<qrule.n_points(); qp++)
            {
              // 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 on a structured grid is
              //
              // u_xx + u_yy = (u(i-1,j) + u(i+1,j) +
              //                u(i,j-1) + u(i,j+1) +
              //                -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 x = q_point[qp](0);
#if LIBMESH_DIM > 1
              const Real y = q_point[qp](1);
#else
              const Real y = 0.;
#endif
#if LIBMESH_DIM > 2
              const Real z = q_point[qp](2);
#else
              const Real z = 0.;
#endif
              const Real eps = 1.e-3;

              const Real uxx = (exact_solution(x-eps, y, z) +
                                exact_solution(x+eps, y, z) +
                                -2.*exact_solution(x, y, z))/eps/eps;

              const Real uyy = (exact_solution(x, y-eps, z) +
                                exact_solution(x, y+eps, z) +
                                -2.*exact_solution(x, y, z))/eps/eps;

              const Real uzz = (exact_solution(x, y, z-eps) +
                                exact_solution(x, y, z+eps) +
                                -2.*exact_solution(x, y, z))/eps/eps;

              Real fxy;
              if (dim==1)
                {
                  // In 1D, compute the rhs by differentiating the
                  // exact solution twice.
                  const Real pi = libMesh::pi;
                  fxy = (0.25*pi*pi)*sin(.5*pi*x);
                }
              else
                {
                  fxy = - (uxx + uyy + ((dim==2) ? 0. : uzz));
                }

              // Add the RHS contribution
              for (std::size_t i=0; i<phi.size(); i++)
                Fe(i) += JxW[qp]*fxy*phi[i][qp];
            }

          // Stop logging the right-hand-side computation
          perf_log.pop ("Fe");

          // At this point 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 imposed
          // via the penalty method. This is discussed at length in
          // example 3.
          {
            // Start logging the boundary condition computation.  We use a
            // macro to log everything in this scope.
            LOG_SCOPE_WITH("BCs", "", perf_log);

            // The following loops 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.  If there is a
            // neighbor, check that neighbor's subdomain_id; if that
            // is different from 1, the side is also located on the
            // boundary.
            for (auto side : elem->side_index_range())
              if ((elem->neighbor_ptr(side) == nullptr) ||
                  (elem->neighbor_ptr(side)->subdomain_id()!=1))
                {

                  // The penalty value.  \frac{1}{\epsilon}
                  // in the discussion above.
                  const Real penalty = 1.e10;

                  // 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);

                  // 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);
#if LIBMESH_DIM > 1
                      const Real yf = qface_point[qp](1);
#else
                      const Real yf = 0.;
#endif
#if LIBMESH_DIM > 2
                      const Real zf = qface_point[qp](2);
#else
                      const Real zf = 0.;
#endif


                      // The boundary value.
                      const Real value = exact_solution(xf, yf, zf);

                      // 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*value*phi_face[i][qp];
                    }
                }
          }

          // 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.
          // Start logging the insertion of the local (element)
          // matrix and vector into the global matrix and vector
          LOG_SCOPE_WITH("matrix insertion", "", perf_log);

          matrix.add_matrix         (Ke, dof_indices);
          system.rhs->add_vector    (Fe, dof_indices);
        }
    }

  // That's it.  We don't need to do anything else to the
  // PerfLog.  When it goes out of scope (at this function return)
  // it will print its log to the screen. Pretty easy, huh?
}