ExplicitTimeIntegrator

consistent

The consistent option builds a full ("consistent") "mass matrix" and uses it in a linear solve to get the update. This is done by calling FEProblem::computeJacobianTag() and specifying the TIME tag which includes all of the TimeKernel derived Kernels and NodalBC derived BoundaryConditions to compute :

    _fe_problem.computeJacobianTag(

A residual computation is also completed to use as the RHS ():

  _fe_problem.computeResidual(*_nonlinear_implicit_system->current_local_solution,

Finally, the following linear system is solved to obtain the solution update using the default linear solver from libMesh (usually PETSc, including the application of any command-line parameters specified):

lumped

The lumped option creates a "lumped" mass matrix to use in the solve. A lumped mass matrix is a diagonal matrix where the diagonal is the sum of all elements on the row from the original matrix. Here, to achieve the lumping, a matrix-vector product is performed between the consistent mass matrix and a vector of all ones.

The inverse of a diagonal matrix is simply the reciprocal of each diagonal entry. Then the matrix-vector product of the "inverse" lumped diagonal matrix is applied by simply doing a pointwise multiplication with the RHS.

Thus the lumped option does not actually solve a system of linear equations, allowing it be much faster. However, the lumping of the mass matrix may lead to unacceptable phase errors.

lump_preconditioned

This option is the combination of the other two options: the consistent mass matrix is used in the linear system, but the linear solve is preconditioned using the lumped mass matrix. This compromise retains the accuracy of the consistent option while benefiting from some of the speedup offered by the lumped option.

The lumped mass matrix preconditioner is applied with the class, LumpedPreconditioner:

#pragma once

// MOOSE includes
#include "NonlinearSystem.h"
#include "FEProblem.h"
#include "PetscSupport.h"

// libMesh includes
#include "libmesh/sparse_matrix.h"
#include "libmesh/nonlinear_solver.h"
#include "libmesh/preconditioner.h"

// Forward declarations
class LumpedPreconditioner;

/**
 * Class to that applies the lumped mass matrix preconditioner
 * in the ExplicitTimeIntegrator
 */
class LumpedPreconditioner : public Preconditioner<Real>
{
public:
  LumpedPreconditioner(const NumericVector<Real> & diag_inverse)
    : Preconditioner(diag_inverse.comm()), _diag_inverse(diag_inverse)
  {
  }

  virtual void init() override
  {
    // No more initialization needed here
    _is_initialized = true;
  }

  virtual void apply(const NumericVector<Real> & x, NumericVector<Real> & y) override
  {
    y.pointwise_mult(_diag_inverse, x);
  }

protected:
  /// The inverse of the diagonal of the lumped matrix
  const NumericVector<Real> & _diag_inverse;
};

This object simply applies the inverse of the diagonal, lumped mass-matrix as the preconditioner for the linear solve, which is very efficient. Note that when this option is applied you shouldn't specify any other preconditioners using command-line syntax or they will override this option.

Update Form

Note that even though we're doing an explicit solve we are currently doing it in "update form" similar to a single step Newton solve. This gives us good parity with the rest of MOOSE. We may want to change this in the future to make better use of the fact that the mass-matrix can be constant for a wider class of problems if we remove dt from it.

_ones

To get the sum of each row of the mass matrix for "lumping" purposes a vector consisting of all 1s is used in a matrix-vector product:

      mass_matrix.vector_mult(_mass_matrix_diag, *_ones);

This is actually the very same way MatGetRowSum is implemented in PETSc; however, doing it manually cuts down on vector creation/destruction and a few other book-keeping operations.

In the future this could be changed to use MatGetRowSum if a specialization for MPI_Aij format is created.

Time

Time in an explicit solve must be handled carefully. When evaluating the weak form (the integral parts) of the residual, time needs to be set to be the "old" time (the time we are solving "from"):

  _fe_problem.time() = _fe_problem.timeOld();

However, DirichletBC derived boundary conditions need to use the final** time, since the strong constraints they represent use the final time and are not affected by the time integrator. To achieve this, time is reset to the _current_time after the weak form residual evaluation and before NodalBC boundary condition application, which makes postResidual() the correct place to reset time for this purpose:

  _fe_problem.time() = _current_time;

After postResidual() the NodalBC BCs are applied with the time at the final time for the step.

meshChanged()

When the mesh changes the linear solver needs to be destroyed and recreated. This is done by simply building a new one and setting it up again. This happens automatically just by "overwriting" the std::unique_ptr to the LinearSolver.

Relevant Executioner solver options

You can ignore this section if using solve_type = lumped. No Executioner parameters are relevant to you in that case. However, for consistent or lump_preconditioned solve types, the l_tol and l_max_its parameters are used in the solution process. Nonlinear executioner options are not relevant. When using PETSc as the default solver package, pc and ksp options from the petsc_options* parameters will be used while snes options will be ignored.