TransientBase.C

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//* This file is part of the MOOSE framework
//* https://mooseframework.inl.gov
//*
//* All rights reserved, see COPYRIGHT for full restrictions
//* https://github.com/idaholab/moose/blob/master/COPYRIGHT
//*
//* Licensed under LGPL 2.1, please see LICENSE for details
//* https://www.gnu.org/licenses/lgpl-2.1.html
 
#include "TransientBase.h"
 
// MOOSE includes
#include "Factory.h"
#include "SubProblem.h"
#include "TimeStepper.h"
#include "MooseApp.h"
#include "Conversion.h"
#include "FEProblem.h"
#include "NonlinearSystem.h"
#include "Control.h"
#include "TimePeriod.h"
#include "MooseMesh.h"
#include "TimeIntegrator.h"
#include "Console.h"
#include "AuxiliarySystem.h"
#include "Convergence.h"
#include "ConvergenceIterationTypes.h"
 
#include "libmesh/implicit_system.h"
#include "libmesh/nonlinear_implicit_system.h"
#include "libmesh/transient_system.h"
#include "libmesh/numeric_vector.h"
 
// C++ Includes
#include <iomanip>
#include <iostream>
#include <fstream>
#include <sstream>
#include <iomanip>
 
InputParameters
TransientBase::defaultSteadyStateConvergenceParams()
{
  InputParameters params = emptyInputParameters();
 
  params.addParam<Real>("steady_state_tolerance",
                        1.0e-08,
                        "Whenever the relative residual changes by less "
                        "than this the solution will be considered to be "
                        "at steady state.");
  params.addParam<bool>("check_aux",
                        false,
                        "Whether to check the auxiliary system for convergence to steady-state. If "
                        "false, then the solution vector from the solver system is used.");
  params.addParam<bool>(
      "normalize_solution_diff_norm_by_dt",
      true,
      "Whether to divide the solution difference norm by dt. If taking 'small' "
      "time steps you probably want this to be true. If taking very 'large' timesteps in an "
      "attempt to *reach* a steady-state, you probably want this parameter to be false.");
 
  params.addParamNamesToGroup("steady_state_tolerance check_aux normalize_solution_diff_norm_by_dt",
                              "Steady State Detection");
 
  return params;
}
 
InputParameters
TransientBase::validParams()
{
  InputParameters params = Executioner::validParams();
  params += TransientBase::defaultSteadyStateConvergenceParams();
 
  params.addClassDescription("Executioner for time varying simulations.");
 
  std::vector<Real> sync_times(1);
  sync_times[0] = -std::numeric_limits<Real>::max();
 
  MooseEnum schemes("implicit-euler explicit-euler crank-nicolson bdf2 explicit-midpoint dirk "
                    "explicit-tvd-rk-2 newmark-beta",
                    "implicit-euler");
 
  params.addParam<Real>("start_time", 0.0, "The start time of the simulation");
  params.addParam<Real>("end_time", 1.0e30, "The end time of the simulation");
  params.addParam<Real>("dt", 1., "The timestep size between solves");
  params.addParam<Real>("dtmin", 1.0e-12, "The minimum timestep size in an adaptive run");
  params.addParam<Real>("dtmax", 1.0e30, "The maximum timestep size in an adaptive run");
  params.addParam<bool>(
      "reset_dt", false, "Use when restarting a calculation to force a change in dt.");
  params.addParam<unsigned int>("num_steps",
                                std::numeric_limits<unsigned int>::max(),
                                "The number of timesteps in a transient run");
  params.addParam<int>("n_startup_steps", 0, "The number of timesteps during startup");
 
  params.addParam<bool>(
      "steady_state_detection", false, "Whether or not to check for steady state conditions");
  params.addParam<ConvergenceName>(
      "steady_state_convergence",
      "Name of the Convergence object to use to assess whether the solution has reached a steady "
      "state. If not provided, a default Convergence will be constructed internally from the "
      "executioner parameters.");
  params.addParam<Real>(
      "steady_state_start_time",
      0.0,
      "Minimum amount of time to run before checking for steady state conditions.");
 
  params.addParam<std::vector<std::string>>("time_periods", "The names of periods");
  params.addParam<std::vector<Real>>("time_period_starts", "The start times of time periods");
  params.addParam<std::vector<Real>>("time_period_ends", "The end times of time periods");
  params.addParam<bool>(
      "abort_on_solve_fail", false, "abort if solve not converged rather than cut timestep");
  params.addParam<bool>(
      "error_on_dtmin",
      true,
      "Throw error when timestep is less than dtmin instead of just aborting solve.");
  params.addParam<MooseEnum>("scheme", schemes, "Time integration scheme used.");
  params.addParam<Real>("timestep_tolerance",
                        1.0e-12,
                        "the tolerance setting for final timestep size and sync times");
 
  params.addParam<bool>("use_multiapp_dt",
                        false,
                        "If true then the dt for the simulation will be "
                        "chosen by the MultiApps.  If false (the "
                        "default) then the minimum over the master dt "
                        "and the MultiApps is used");
 
  params.addParamNamesToGroup(
      "steady_state_detection steady_state_convergence steady_state_start_time",
      "Steady State Detection");
 
  params.addParamNamesToGroup("start_time dtmin dtmax n_startup_steps "
                              "abort_on_solve_fail timestep_tolerance use_multiapp_dt",
                              "Advanced");
 
  params.addParamNamesToGroup("time_periods time_period_starts time_period_ends", "Time Periods");
 
  // This Executioner supports --test-restep
  params.set<bool>("_supports_test_restep") = true;
 
  return params;
}
 
TransientBase::TransientBase(const InputParameters & parameters)
  : Executioner(parameters),
    _problem(_fe_problem),
    _aux(_fe_problem.getAuxiliarySystem()),
    _time_scheme(getParam<MooseEnum>("scheme").getEnum<Moose::TimeIntegratorType>()),
    _time_stepper(nullptr),
    _t_step(_problem.timeStep()),
    _time(_problem.time()),
    _time_old(_problem.timeOld()),
    _time_older(_problem.timeOlder()),
    _dt(_problem.dt()),
    _dt_old(_problem.dtOld()),
    _unconstrained_dt(declareRecoverableData<Real>("unconstrained_dt", -1)),
    _at_sync_point(declareRecoverableData<bool>("at_sync_point", false)),
    _last_solve_converged(declareRecoverableData<bool>("last_solve_converged", true)),
    _xfem_repeat_step(false),
    _end_time(getParam<Real>("end_time")),
    _dtmin(getParam<Real>("dtmin")),
    _dtmax(getParam<Real>("dtmax")),
    _num_steps(getParam<unsigned int>("num_steps")),
    _n_startup_steps(getParam<int>("n_startup_steps")),
    _steady_state_detection(getParam<bool>("steady_state_detection")),
    _steady_state_start_time(getParam<Real>("steady_state_start_time")),
    _sync_times(_app.getOutputWarehouse().getSyncTimes()),
    _abort(getParam<bool>("abort_on_solve_fail")),
    _error_on_dtmin(getParam<bool>("error_on_dtmin")),
    _time_interval(declareRecoverableData<bool>("time_interval", false)),
    _start_time(getParam<Real>("start_time")),
    _timestep_tolerance(getParam<Real>("timestep_tolerance")),
    _target_time(declareRecoverableData<Real>("target_time", -std::numeric_limits<Real>::max())),
    _use_multiapp_dt(getParam<bool>("use_multiapp_dt")),
    _testing_restep(false)
{
  _t_step = 0;
  _dt = 0;
  _next_interval_output_time = 0.0;
 
  // Either a start_time has been forced on us, or we want to tell the App about what our start time
  // is (in case anyone else is interested.
  if (_app.hasStartTime())
    _start_time = _app.getStartTime();
  else if (parameters.isParamSetByUser("start_time"))
    _app.setStartTime(_start_time);
 
  _time = _time_old = _time_older = _start_time;
  _problem.transient(true);
 
  setupTimeIntegrator();
 
  // Cut timesteps and end_time in half
  if (_app.testCheckpointHalfTransient())
  {
    mooseAssert(!_app.testReStep(), "Cannot use with restep");
    _end_time = (_start_time + _end_time) / 2.0;
    _num_steps /= 2.0;
 
    if (_num_steps == 0) // Always do one step in the first half
      _num_steps = 1;
  }
  // Retest a timestep (see options below for which timestep)
  else if (_app.testReStep())
  {
    if (_problem.shouldSolve())
    {
      // If num_steps is defined, we'll use that to determine restep timestep
      if (!parameters.isParamSetByAddParam("num_steps"))
        _test_restep_step = _t_step + (_num_steps + 1) / 2;
      // If end_time is defined, we'll use the half time to determine when to restep
      if (!parameters.isParamSetByAddParam("end_time"))
        _test_restep_time = (_start_time + _end_time) / 2;
      // If neither was set or we are doing pseudo-transient, pick the second timestep
      if ((!_test_restep_step && !_test_restep_time) || _steady_state_detection)
        _test_restep_step = 2;
 
      std::stringstream msg;
      if (_test_restep_step && _test_restep_time)
        msg << "Timestep " << *_test_restep_step << " or time " << *_test_restep_time
            << " (whichever happens first)";
      else if (_test_restep_step)
        msg << "Timestep " << *_test_restep_step;
      else if (_test_restep_time)
        msg << "Time " << *_test_restep_time;
      mooseInfo(msg.str(), " will be forcefully retried due to --test-restep.");
    }
    else
      mooseInfo(
          "A timestep is not being retried with --test-restep because Problem/solve=false.\n\nTo "
          "avoid this test being ran, you could set `restep = false` in the test specification.");
  }
 
  if (isParamValid("steady_state_convergence"))
    _problem.setSteadyStateConvergenceName(getParam<ConvergenceName>("steady_state_convergence"));
  else
    // Note that we create a steady-state Convergence object even if steady_state_detection ==
    // false. This could possibly be changed in the future, but TransientMultiApp would need to be
    // able to signal for the Convergence object to be created in case it uses steady-state
    // detection for sub-stepping.
    _problem.setNeedToAddDefaultSteadyStateConvergence();
}
 
void
TransientBase::init()
{
  _problem.execute(EXEC_PRE_MULTIAPP_SETUP);
  _problem.initialSetup();
  _fixed_point_solve->initialSetup();
 
  mooseAssert(getTimeStepper(), "No time stepper was set");
 
  _time_stepper->init();
 
  auto & conv = _problem.getConvergence(_problem.getSteadyStateConvergenceName());
  conv.checkIterationType(ConvergenceIterationTypes::STEADY_STATE);
 
  if (_app.isRecovering()) // Recover case
  {
    if (_t_step == 0)
      mooseError(
          "Internal error in TransientBase executioner: _t_step is equal to 0 while recovering "
          "in init().");
 
    _dt_old = _dt;
  }
}
 
void
TransientBase::preExecute()
{
  _time_stepper->preExecute();
 
  if (!_app.isRecovering())
  {
    _t_step = 0;
    _dt = 0;
    _next_interval_output_time = 0.0;
    if (!_app.isRestarting())
      _time = _time_old = _time_older = _start_time;
 
    _problem.outputStep(EXEC_INITIAL);
 
    computeDT();
    _dt = getDT();
    if (_dt == 0)
      mooseError("Time stepper computed zero time step size on initial which is not allowed.\n"
                 "1. If you are using an existing time stepper, double check the values in your "
                 "input file or report an error.\n"
                 "2. If you are developing a new time stepper, make sure that initial time step "
                 "size in your code is computed correctly.");
    const auto & tis = getTimeIntegrators();
    for (auto & ti : tis)
      ti->init();
  }
}
 
void
TransientBase::preStep()
{
  _time_stepper->preStep();
}
 
void
TransientBase::postStep()
{
  _time_stepper->postStep();
}
 
void
TransientBase::execute()
{
  preExecute();
 
  // Start time loop...
  while (keepGoing())
  {
    incrementStepOrReject();
    preStep();
    computeDT();
    takeStep();
    endStep();
    postStep();
  }
 
  if (lastSolveConverged())
  {
    _t_step++;
 
    /*
     * Call the multi-app executioners endStep and
     * postStep methods when doing Picard or when not automatically advancing sub-applications for
     * some other reason. We do not perform these calls for loose-coupling/auto-advancement
     * problems because TransientBase::endStep and TransientBase::postStep get called from
     * TransientBaseMultiApp::solveStep in that case.
     */
    if (!_fixed_point_solve->autoAdvance())
    {
      _problem.finishMultiAppStep(EXEC_MULTIAPP_FIXED_POINT_BEGIN,
                                  /*recurse_through_multiapp_levels=*/true);
      _problem.finishMultiAppStep(EXEC_TIMESTEP_BEGIN, /*recurse_through_multiapp_levels=*/true);
      _problem.finishMultiAppStep(EXEC_TIMESTEP_END, /*recurse_through_multiapp_levels=*/true);
      _problem.finishMultiAppStep(EXEC_MULTIAPP_FIXED_POINT_END,
                                  /*recurse_through_multiapp_levels=*/true);
    }
  }
 
  if (!_app.testCheckpointHalfTransient())
  {
    TIME_SECTION("final", 1, "Executing Final Objects");
    _problem.execMultiApps(EXEC_FINAL);
    _problem.finalizeMultiApps();
    _problem.execute(EXEC_FINAL);
    _problem.outputStep(EXEC_FINAL);
  }
 
  // This method is to finalize anything else we want to do on the problem side.
  _problem.postExecute();
 
  // This method can be overridden for user defined activities in the Executioner.
  postExecute();
 
  if (_test_restep_step || _test_restep_time)
    mooseError((_test_restep_step ? "Timestep " : "Time "),
               (_test_restep_step ? *_test_restep_step : *_test_restep_time),
               " was never retried because the simulation did not get to this timestep.\n\nTo "
               "support restep testing, specify `num_steps` in the input.\nOtherwise, set "
               "`restep = false` in this test specification.");
}
 
void
TransientBase::computeDT()
{
  // We're repeating the solve of the previous timestep,
  // so we use the same dt
  if (_testing_restep)
  {
    mooseAssert(!_test_restep_step && !_test_restep_time, "Should not be set");
    return;
  }
 
  _time_stepper->computeStep(); // This is actually when DT gets computed
}
 
void
TransientBase::incrementStepOrReject()
{
  if (lastSolveConverged())
  {
    if (!_xfem_repeat_step)
    {
#ifdef LIBMESH_ENABLE_AMR
      if (_t_step != 0)
        _problem.adaptMesh();
#endif
 
      _time_older = _time_old;
      _time_old = _time;
      _t_step++;
 
      bool advance_problem_state = true;
      const auto & tis = getTimeIntegrators();
      for (auto & ti : tis)
        // We do not want to advance the problem state here if a time integrator is already doing so
        // itself
        if (ti->advancesProblemState())
        {
          advance_problem_state = false;
          break;
        }
 
      if (advance_problem_state)
        _problem.advanceState();
      else if (tis.size() > 1)
        mooseError("Either there must be a single time integrator which advances state or none of "
                   "the time integrators should advance state.");
 
      if (_t_step == 1)
        return;
 
      /*
       * Call the multi-app executioners endStep and
       * postStep methods when doing Picard or when not automatically advancing sub-applications for
       * some other reason. We do not perform these calls for loose-coupling/auto-advancement
       * problems because TransientBase::endStep and TransientBase::postStep get called from
       * TransientBaseMultiApp::solveStep in that case.
       */
      if (!_fixed_point_solve->autoAdvance())
      {
        _problem.finishMultiAppStep(EXEC_MULTIAPP_FIXED_POINT_BEGIN);
        _problem.finishMultiAppStep(EXEC_TIMESTEP_BEGIN);
        _problem.finishMultiAppStep(EXEC_TIMESTEP_END);
        _problem.finishMultiAppStep(EXEC_MULTIAPP_FIXED_POINT_END);
      }
 
      /*
       * Ensure that we increment the sub-application time steps so that
       * when dt selection is made in the master application, we are using
       * the correct time step information
       */
      _problem.incrementMultiAppTStep(EXEC_MULTIAPP_FIXED_POINT_BEGIN);
      _problem.incrementMultiAppTStep(EXEC_TIMESTEP_BEGIN);
      _problem.incrementMultiAppTStep(EXEC_TIMESTEP_END);
      _problem.incrementMultiAppTStep(EXEC_MULTIAPP_FIXED_POINT_END);
    }
  }
  else
  {
    _problem.restoreMultiApps(EXEC_MULTIAPP_FIXED_POINT_BEGIN, true);
    _problem.restoreMultiApps(EXEC_TIMESTEP_BEGIN, true);
    _problem.restoreMultiApps(EXEC_TIMESTEP_END, true);
    _problem.restoreMultiApps(EXEC_MULTIAPP_FIXED_POINT_END, true);
    _time_stepper->rejectStep();
    _time = _time_old;
  }
}
 
void
TransientBase::takeStep(Real input_dt)
{
  if (lastSolveConverged())
    _dt_old = _dt;
 
  // We're repeating the solve of the previous timestep,
  // so we use the same dt
  if (_testing_restep)
  {
    mooseAssert(!_test_restep_step && !_test_restep_time, "Should not be set");
    _testing_restep = false;
  }
  else if (input_dt == -1.0)
    _dt = computeConstrainedDT();
  else
    _dt = input_dt;
 
  _time_stepper->preSolve();
 
  // Increment time
  _time = _time_old + _dt;
 
  _problem.timestepSetup();
 
  _problem.onTimestepBegin();
 
  _time_stepper->step();
  _xfem_repeat_step = _fixed_point_solve->XFEMRepeatStep();
 
  _last_solve_converged = _time_stepper->converged();
 
  // We're running with --test-restep and we have just solved
  // the timestep we are to repeat for the first time
  if ((_test_restep_step && *_test_restep_step == _t_step) ||
      (_test_restep_time && _time >= *_test_restep_time))
  {
    mooseAssert(!_testing_restep, "Should not be set");
 
    mooseInfo("Aborting and retrying solve for timestep ", _t_step, " due to --test-restep");
    _last_solve_converged = false;
    _testing_restep = true;
    _test_restep_step.reset();
    _test_restep_time.reset();
 
    return;
  }
 
  if (!lastSolveConverged())
  {
    _console << "Aborting as solve did not converge" << std::endl;
    return;
  }
 
  if (!(_problem.haveXFEM() && _fixed_point_solve->XFEMRepeatStep()))
  {
    if (lastSolveConverged())
      _time_stepper->acceptStep();
    else
      _time_stepper->rejectStep();
  }
 
  _time = _time_old;
 
  _time_stepper->postSolve();
 
  return;
}
 
void
TransientBase::endStep(Real input_time)
{
  if (input_time == -1.0)
    _time = _time_old + _dt;
  else
    _time = input_time;
 
  if (lastSolveConverged())
  {
    if (_xfem_repeat_step)
      _time = _time_old;
    else
    {
      // TODO: add linear system support
      const auto & tis = getTimeIntegrators();
      for (auto & ti : tis)
        ti->postStep();
 
      // Compute the Error Indicators and Markers
      _problem.computeIndicators();
      _problem.computeMarkers();
 
      // Perform the output of the current time step
      _problem.outputStep(EXEC_TIMESTEP_END);
 
      // output
      if (_time_interval && (_time + _timestep_tolerance >= _next_interval_output_time))
        _next_interval_output_time += _time_interval_output_interval;
    }
  }
}
 
Real
TransientBase::computeConstrainedDT()
{
  //  // If start up steps are needed
  //  if (_t_step == 1 && _n_startup_steps > 1)
  //    _dt = _input_dt/(double)(_n_startup_steps);
  //  else if (_t_step == 1+_n_startup_steps && _n_startup_steps > 1)
  //    _dt = _input_dt;
 
  Real dt_cur = _dt;
  std::ostringstream diag;
 
  // After startup steps, compute new dt
  if (_t_step > _n_startup_steps)
    dt_cur = getDT();
 
  else
  {
    diag << "Timestep < n_startup_steps, using old dt: " << std::setw(9) << std::setprecision(6)
         << std::setfill('0') << std::showpoint << std::left << _dt << " tstep: " << _t_step
         << " n_startup_steps: " << _n_startup_steps << std::endl;
  }
  _unconstrained_dt = dt_cur;
 
  if (_verbose)
    _console << diag.str() << std::flush;
 
  diag.str("");
  diag.clear();
 
  // Allow the time stepper to limit the time step
  _at_sync_point = _time_stepper->constrainStep(dt_cur);
 
  // Don't let time go beyond next time interval output if specified
  if ((_time_interval) && (_time + dt_cur + _timestep_tolerance >= _next_interval_output_time))
  {
    dt_cur = _next_interval_output_time - _time;
    _at_sync_point = true;
 
    diag << "Limiting dt for time interval output at time: " << std::setw(9) << std::setprecision(6)
         << std::setfill('0') << std::showpoint << std::left << _next_interval_output_time
         << " dt: " << std::setw(9) << std::setprecision(6) << std::setfill('0') << std::showpoint
         << std::left << dt_cur << std::endl;
  }
 
  // If a target time is set and the current dt would exceed it, limit dt to match the target
  if (_target_time > -std::numeric_limits<Real>::max() + _timestep_tolerance &&
      _time + dt_cur + _timestep_tolerance >= _target_time)
  {
    dt_cur = _target_time - _time;
    _at_sync_point = true;
 
    diag << "Limiting dt for target time: " << std::setw(9) << std::setprecision(6)
         << std::setfill('0') << std::showpoint << std::left << _next_interval_output_time
         << " dt: " << std::setw(9) << std::setprecision(6) << std::setfill('0') << std::showpoint
         << std::left << dt_cur << std::endl;
  }
 
  // Constrain by what the multi apps are doing
  constrainDTFromMultiApp(dt_cur, diag, EXEC_MULTIAPP_FIXED_POINT_BEGIN);
  constrainDTFromMultiApp(dt_cur, diag, EXEC_TIMESTEP_BEGIN);
  constrainDTFromMultiApp(dt_cur, diag, EXEC_TIMESTEP_END);
  constrainDTFromMultiApp(dt_cur, diag, EXEC_MULTIAPP_FIXED_POINT_END);
 
  if (_verbose)
    _console << diag.str() << std::flush;
 
  return dt_cur;
}
 
void
TransientBase::constrainDTFromMultiApp(Real & dt_cur,
                                       std::ostringstream & diag,
                                       const ExecFlagType & execute_on) const
{
  Real multi_app_dt = _problem.computeMultiAppsDT(execute_on);
  if (_use_multiapp_dt || multi_app_dt < dt_cur)
  {
    dt_cur = multi_app_dt;
    _at_sync_point = false;
    diag << "Limiting dt for MultiApps on " << execute_on.name() << ": " << std::setw(9)
         << std::setprecision(6) << std::setfill('0') << std::showpoint << std::left << dt_cur
         << std::endl;
  }
}
 
Real
TransientBase::getDT()
{
  return _time_stepper->getCurrentDT();
}
 
bool
TransientBase::keepGoing()
{
  bool keep_going = !_problem.isSolveTerminationRequested();
 
  // Check for stop condition based upon steady-state check flag:
  if (lastSolveConverged())
  {
    if (!_xfem_repeat_step)
    {
      if (_steady_state_detection == true && _time > _steady_state_start_time)
      {
        // Check solution difference relative norm against steady-state tolerance
        if (convergedToSteadyState())
        {
          _console << "Steady-State Solution Achieved at time: " << _time << std::endl;
          // Output last solve if not output previously by forcing it
          keep_going = false;
        }
      }
 
      // Check for stop condition based upon number of simulation steps and/or solution end time:
      if (static_cast<unsigned int>(_t_step) >= _num_steps)
        keep_going = false;
 
      if ((_time >= _end_time) || (fabs(_time - _end_time) <= _timestep_tolerance))
        keep_going = false;
    }
  }
  else if (_abort)
  {
    _console << "Aborting as solve did not converge and input selected to abort" << std::endl;
    keep_going = false;
  }
  else if (!_error_on_dtmin && _dt <= _dtmin)
  {
    _console << "Aborting as timestep already at or below dtmin" << std::endl;
    keep_going = false;
  }
 
  return keep_going;
}
 
void
TransientBase::estimateTimeError()
{
}
 
bool
TransientBase::lastSolveConverged() const
{
  return _last_solve_converged;
}
 
void
TransientBase::postExecute()
{
  _time_stepper->postExecute();
}
 
void
TransientBase::setTargetTime(Real target_time)
{
  _target_time = target_time;
}
 
Real
TransientBase::computeSolutionChangeNorm(bool check_aux, bool normalize_by_dt) const
{
  return relativeSolutionDifferenceNorm(check_aux) / (normalize_by_dt ? _dt : Real(1));
}
 
void
TransientBase::setupTimeIntegrator()
{
  if (_pars.isParamSetByUser("scheme") && _problem.hasTimeIntegrator())
    mooseError("You cannot specify time_scheme in the Executioner and independently add a "
               "TimeIntegrator to the system at the same time");
 
  if (!_problem.hasTimeIntegrator())
  {
    // backwards compatibility
    std::string ti_str;
    using namespace Moose;
 
    switch (_time_scheme)
    {
      case TI_IMPLICIT_EULER:
        ti_str = "ImplicitEuler";
        break;
      case TI_EXPLICIT_EULER:
        ti_str = "ExplicitEuler";
        break;
      case TI_CRANK_NICOLSON:
        ti_str = "CrankNicolson";
        break;
      case TI_BDF2:
        ti_str = "BDF2";
        break;
      case TI_EXPLICIT_MIDPOINT:
        ti_str = "ExplicitMidpoint";
        break;
      case TI_LSTABLE_DIRK2:
        ti_str = "LStableDirk2";
        break;
      case TI_EXPLICIT_TVD_RK_2:
        ti_str = "ExplicitTVDRK2";
        break;
      case TI_NEWMARK_BETA:
        ti_str = "NewmarkBeta";
        break;
      default:
        mooseError("Unknown scheme: ", _time_scheme);
        break;
    }
 
    InputParameters params = _app.getFactory().getValidParams(ti_str);
    _problem.addTimeIntegrator(ti_str, ti_str, params);
  }
}
 
std::string
TransientBase::getTimeStepperName() const
{
  if (_time_stepper)
  {
    TimeStepper & ts = *_time_stepper;
    return demangle(typeid(ts).name());
  }
  else
    return std::string();
}
 
std::vector<std::string>
TransientBase::getTimeIntegratorNames() const
{
  const auto & tis = getTimeIntegrators();
  if (tis.empty())
    mooseError("Time integrator has not been built yet so we can't retrieve its name");
 
  std::vector<std::string> ret;
  for (const auto & ti : tis)
  {
    const auto & sys = ti->getCheckedPointerParam<SystemBase *>("_sys")->system();
    const auto & uvars = ti->getParam<std::vector<VariableName>>("variables");
 
    std::vector<VariableName> vars;
    for (const auto & var : uvars)
      if (sys.has_variable(var))
        vars.push_back(var);
 
    if (!uvars.empty() && vars.empty())
      continue;
 
    if (tis.size() > 1)
    {
      const std::string sys_prefix = _problem.numSolverSystems() > 1 ? sys.name() : "";
      const std::string var_prefix = MooseUtils::join(vars, ", ");
      const bool both = !sys_prefix.empty() && !var_prefix.empty();
      ret.push_back("[" + sys_prefix + (both ? " (" : "") + var_prefix + (both ? ")" : "") + "]:");
    }
 
    ret.push_back(ti->type());
  }
  return ret;
}
 
void
TransientBase::setTimeStepper(TimeStepper & ts)
{
  mooseAssert(!_time_stepper, "Already set");
  _time_stepper = &ts;
}
 
bool
TransientBase::convergedToSteadyState() const
{
  auto & convergence = _problem.getConvergence(_problem.getSteadyStateConvergenceName());
  const auto status = convergence.checkConvergence(_t_step);
 
  if (status == Convergence::MooseConvergenceStatus::DIVERGED)
    mooseError(
        "The steady-state Convergence object (", convergence.name(), ") reported divergence.");
  else if (status == Convergence::MooseConvergenceStatus::CONVERGED)
    return true;
  else // status == Convergence::MooseConvergenceStatus::ITERATING
    return false;
}
 
void
TransientBase::parentOutputPositionChanged()
{
  _fe_problem.parentOutputPositionChanged();
}