poisson-mfem.py

This is the raw source code of the poisson-mfem.py example. There is also a commented version of this example in the Examples section.

# SPDX-FileCopyrightText: Copyright © DUNE Project contributors, see file AUTHORS.md
# SPDX-License-Identifier: LicenseRef-GPL-2.0-only-with-DUNE-exception OR LGPL-3.0-or-later
 
#! [imports]
import numpy
import scipy.sparse.linalg
from scipy.sparse import lil_matrix
 
import dune.geometry
import dune.grid as grid
import dune.functions as functions
#! [imports]
 
# Compute the stiffness matrix for a single element
#! [getLocalMatrix]
def getLocalMatrix(localView):
 
    # Get the grid element from the local FE basis view
    element = localView.element()
 
    # Make dense element stiffness matrix
    n = len(localView)    # Number of local degrees of freedom (gradient + value)
    elementMatrix = numpy.zeros((n,n))
 
    # Get set of shape functions for this element
    fluxLocalFiniteElement     = localView.tree().child(0).finiteElement()
    pressureLocalFiniteElement = localView.tree().child(1).finiteElement()
 
    # The actual shape functions on the reference element
    localFluxBasis = fluxLocalFiniteElement.localBasis
    localPressureBasis = pressureLocalFiniteElement.localBasis
 
    nFlux = len(fluxLocalFiniteElement)
    nPressure = len(pressureLocalFiniteElement)
 
    # Get a quadrature rule
    fluxOrder     = element.mydimension * fluxLocalFiniteElement.localBasis.order
    pressureOrder = element.mydimension * pressureLocalFiniteElement.localBasis.order
    quadOrder     = numpy.max((2*fluxOrder, (fluxOrder-1)+pressureOrder))
 
    quadRule = dune.geometry.quadratureRule(element.type, quadOrder)
 
    # Loop over all quadrature points
    for quadPoint in quadRule:
 
        # Position of the current quadrature point in the reference element
        quadPos = quadPoint.position
 
        # The inverse Jacobian of the map from the reference element to the element
        geometryJacobianInverse = element.geometry.jacobianInverse(quadPos)
 
        # The multiplicative factor in the integral transformation formula
        integrationElement = element.geometry.integrationElement(quadPos)
 
        # --------------------------------------------------------------
        #  Shape functions - gradient variable
        # --------------------------------------------------------------
 
        # Values of the gradient variable shape functions on the current element
        fluxValues = localFluxBasis.evaluateFunction(quadPos)
 
        # Gradients of the gradient variable shape functions on the reference element
        fluxReferenceJacobians = localFluxBasis.evaluateJacobian(quadPos)
 
        fluxDivergence = numpy.zeros(nFlux)
 
        # Domain transformation of Jacobians and computation of div = trace(Jacobian)
        # TODO: Extend the Dune Python interface to allow to do this without casting to numpy.array
        for i in range(nFlux):
            fluxDivergence[i] = numpy.trace(numpy.array(fluxReferenceJacobians[i]) * geometryJacobianInverse)
 
        # --------------------------------------------------------------
        #  Shape functions - value variable
        # --------------------------------------------------------------
 
        # Values of the pressure shape functions
        pressureValues = localPressureBasis.evaluateFunction(quadPos)
 
        # --------------------------------------------------------------
        #  Gradient-variable--gradient-variable coupling
        # --------------------------------------------------------------
 
        for i in range(nFlux):
 
            row = localView.tree().child(0).localIndex(i)
 
            for j in range(nFlux):
 
                col = localView.tree().child(0).localIndex(j)
 
                # Compute the actual matrix entry contribution
                elementMatrix[row,col] += numpy.dot(fluxValues[i], fluxValues[j]) * quadPoint.weight * integrationElement
 
        # --------------------------------------------------------------
        #  Flux--pressure coupling
        # --------------------------------------------------------------
 
        for i in range(nFlux):
 
            fluxIndex = localView.tree().child(0).localIndex(i)
 
            for j in range(nPressure):
 
                pressureIndex = localView.tree().child(1).localIndex(j)
 
                # Matrix entry contribution
                tmp = (fluxDivergence[i] * pressureValues[j][0]) * quadPoint.weight * integrationElement
 
                # Add to both off-diagonal block matrices
                elementMatrix[fluxIndex, pressureIndex] += tmp
                elementMatrix[pressureIndex, fluxIndex] += tmp
 
    return elementMatrix
#! [getLocalMatrix]
 
 
# Compute the right-hand side for a single element
#! [getLocalVolumeTerm]
def getVolumeTerm(localView, localVolumeTerm):
 
    # Get the grid element from the local FE basis view
    element = localView.element()
 
    # Construct an empty vector of the correct size
    localRhs = numpy.zeros(len(localView))
 
    # Get set of shape functions for this element
    # Only the pressure part has a non-zero right-hand side
    pressureLocalFiniteElement = localView.tree().child(1).finiteElement()
 
    # A quadrature rule
    quadOrder = 2*element.mydimension*pressureLocalFiniteElement.localBasis.order
    quadRule = dune.geometry.quadratureRule(element.type, quadOrder)
 
    nPressure = len(pressureLocalFiniteElement)
 
    # Loop over all quadrature points
    for quadPoint in quadRule:
 
        # Position of the current quadrature point in the reference element
        quadPos = quadPoint.position
 
        # The multiplicative factor in the integral transformation formula
        integrationElement = element.geometry.integrationElement(quadPos)
 
        # Evaluate the load density at the quadrature point
        functionValue = localVolumeTerm(quadPos)
 
        # Evaluate all shape function values at this point
        pressureValues = pressureLocalFiniteElement.localBasis.evaluateFunction(quadPos)
 
        # Actually compute the vector entries
        for j in range(nPressure):
            pressureIndex = localView.tree().child(1).localIndex(j)
            localRhs[pressureIndex] += - pressureValues[j][0] * functionValue * quadPoint.weight * integrationElement
 
    return localRhs
#! [getLocalVolumeTerm]
 
 
# Assemble the stiffness matrix for the mixed Poisson problem with the given basis
#! [assembleMixedPoissonMatrix]
def assembleMixedPoissonMatrix(basis):
 
    # Total number of DOFs (both FE spaces together)
    n = len(basis)
 
    # Make an empty stiffness matrix
    stiffnessMatrix = lil_matrix( (n,n) )
 
    # A view on the composite FE basis on a single element
    localView = basis.localView()
 
    # A loop over all elements of the grid
    for element in basis.gridView.elements:
 
        # Bind the local basis view to the current element
        localView.bind(element)
 
        # Assemble the element stiffness matrix
        elementMatrix = getLocalMatrix(localView)
 
        # Add element stiffness matrix onto the global stiffness matrix
        for i in range(len(localView)):
 
            # The global index of the i-th local degree of freedom of the element
            row = localView.index(i)[0]  # The index is an array of length 1
 
            for j in range(len(localView)):
 
                # The global index of the j-th local degree of freedom of the element
                col = localView.index(j)[0]
                stiffnessMatrix[row,col] += elementMatrix[i, j];
 
    # Transform the stiffness matrix to CSR format, and return it
    return stiffnessMatrix.tocsr()
#! [assembleMixedPoissonMatrix]
 
 
# Assemble the right-hand side vector of the mixed Poisson problem with the given basis
#! [assembleMixedPoissonRhs]
def assembleMixedPoissonRhs(basis, volumeTerm):
 
    # Get the basis for the pressure variable
    pressureBasis = functions.subspaceBasis(basis, 1)
 
    # Represent the volume term function as a FE function in the pressure space
    volumeTermCoeff = numpy.zeros(len(basis))
    pressureBasis.interpolate(volumeTermCoeff, volumeTerm)
    volumeTermGF = pressureBasis.asFunction(volumeTermCoeff)
 
    # A view on a single element
    localVolumeTerm = volumeTermGF.localFunction()
 
    # Create empty vector of correct size
    b = numpy.zeros(len(basis))
 
    # A view on the FE basis on a single element
    localView = basis.localView()
 
    # A loop over all elements of the grid
    for element in basis.gridView.elements:
 
        # Bind the local FE basis view to the current element
        localView.bind(element)
 
        # Get the local contribution to the right-hand side vector
        localVolumeTerm.bind(element)
        localRhs = getVolumeTerm(localView, localVolumeTerm)
 
        # Add the local vector to the global one
        for i in range(len(localRhs)):
            # The global index of the i-th degree of freedom of the element
            row = localView.index(i)[0]    # The index is an array of length 1.
            b[row] += localRhs[i]
 
    return b
#! [assembleMixedPoissonRhs]
 
 
# Mark all DOFs associated to entities for which # the boundary intersections center
# is marked by the given indicator functions.
#
# This method simply calls the corresponding C++ code.  A more Pythonic solution
# is planned to appear eventually...
#! [markBoundaryDOFsByIndicator]
def markBoundaryDOFsByIndicator(basis, vector, indicator):
    from io import StringIO
    code="""
    #include<utility>
    #include<functional>
    #include<dune/common/fvector.hh>
    #include<dune/functions/functionspacebases/boundarydofs.hh>
    template<class Basis, class Vector, class Indicator>
    void run(const Basis& basis, Vector& vector, const Indicator& indicator)
    {
      auto vectorBackend = vector.mutable_unchecked();
      Dune::Functions::forEachBoundaryDOF(basis, [&] (auto&& localIndex, const auto& localView, const auto& intersection) {
        if (indicator(intersection.geometry().center()).template cast<bool>())
          vectorBackend[localView.index(localIndex)] = true;
      });
    }
    """
    dune.generator.algorithm.run("run",StringIO(code), basis, vector, indicator)
#! [markBoundaryDOFsByIndicator]
 
 
# This incorporates essential constraints into matrix # and rhs of a linear system.
# The mask vector isConstrained # indicates which DOFs should be constrained,
# x contains the desired values of these DOFs. Other entries of x # are ignored.
# Note that this implements the symmetrized approach to modify the matrix.
#! [incorporateEssentialConstraints]
def incorporateEssentialConstraints(A, b, isConstrained, x):
    b -= A*(x*isConstrained)
    rows, cols = A.nonzero()
    for i,j in zip(rows, cols):
        if isConstrained[i] or isConstrained[j]:
          A[i,j] = 0
    for i in range(len(b)):
        if isConstrained[i]:
            A[i,i] = 1
            b[i] = x[i]
#! [incorporateEssentialConstraints]
 
 
 
 
 
#! [createGrid]
upperRight = [1, 1]   # Upper right corner of the domain
elements = [50, 50]   # Number of grid elements per direction
 
# Create a grid of the unit square
gridView = grid.structuredGrid([0,0],upperRight,elements)
#! [createGrid]
 
# Construct a pair of finite element space bases
# Note: In contrast to the corresponding C++ example we are using a single matrix with scalar entries,
# and plain numbers to index it (no multi-digit multi-indices).
#! [createBasis]
basis = functions.defaultGlobalBasis(gridView,
                                     functions.Composite(functions.RaviartThomas(order=0),
                                                         functions.Lagrange(order=0)))
 
fluxBasis     = functions.subspaceBasis(basis, 0);
pressureBasis = functions.subspaceBasis(basis, 1);
#! [createBasis]
 
#! [assembly]
# Compute the stiffness matrix
stiffnessMatrix = assembleMixedPoissonMatrix(basis)
 
# Compute the load vector
volumeTerm = lambda x : 2
 
b = assembleMixedPoissonRhs(basis, volumeTerm)
#! [assembly]
 
#! [dirichletDOFs]
# This marks the top and bottom boundary of the domain
fluxDirichletIndicator = lambda x : 1.*((x[1] > upperRight[1] - 1e-8) or (x[1] < 1e-8))
 
isDirichlet = numpy.zeros(len(basis))
 
# Mark all DOFs located in a boundary intersection marked
# by the fluxDirichletIndicator function.
markBoundaryDOFsByIndicator(fluxBasis, isDirichlet, fluxDirichletIndicator);
#! [dirichletDOFs]
 
 
 
#! [dirichletValues]
normal = lambda x : numpy.array([0.,1.]) if numpy.abs(x[1]-1) < 1e-8 else numpy.array([0., -1.])
fluxDirichletValues = lambda x : numpy.sin(2.*numpy.pi*x[0]) * normal(x)
 
# TODO: This should be constrained to boundary DOFs
fluxDirichletCoefficients = numpy.zeros(len(basis))
fluxBasis.interpolate(fluxDirichletCoefficients, fluxDirichletValues);
#! [dirichletValues]
 
# //////////////////////////////////////////
#   Modify Dirichlet rows
# //////////////////////////////////////////
#! [dirichletIntegration]
incorporateEssentialConstraints(stiffnessMatrix, b, isDirichlet, fluxDirichletCoefficients)
#! [dirichletIntegration]
 
# //////////////////////////
#    Compute solution
# //////////////////////////
#! [solving]
x = scipy.sparse.linalg.spsolve(stiffnessMatrix, b)
#! [solving]
 
# ////////////////////////////////////////////////////////////////////////////////////////////
#   Write result to VTK file
# ////////////////////////////////////////////////////////////////////////////////////////////
 
#! [vtkWriting]
# TODO: Improve file writing.  Currently this simply projects everything
# onto a first-order Lagrange space
vtkWriter = gridView.vtkWriter(subsampling=2)
 
fluxFunction = fluxBasis.asFunction(x)
fluxFunction.addToVTKWriter("flux", vtkWriter, grid.DataType.PointVector)
 
pressureFunction = pressureBasis.asFunction(x)
pressureFunction.addToVTKWriter("pressure", vtkWriter, grid.DataType.PointData)
 
vtkWriter.write("poisson-mfem-result")
#! [vtkWriting]