ComposeTimeStepperAction

Add the composition time stepper if multiple time steppers have been created.

This action specifies the CompositionDT object when multiple time steppers are provided in the [TimeSteppers] block. If there are more than one time steppers provided, it will add a CompositionDT in the time stepper system and make the CompositionDT as the final time stepper to use in the transient simulations. An optional parameter "lower_bound" is provided to set a lower bound for the computed time step size. It can be specified with any time stepper(s) in the [TimeSteppers] block.

An example of using a growing lower bound to limit the time step size as:

[Executioner]
  type = Transient
  end_time = 0.8
  solve_type = PJFNK
  petsc_options_iname = '-pc_type -pc_hypre_type'
  petsc_options_value = 'hypre boomeramg'

  [TimeSteppers]
    lower_bound = 'LogConstDT'
    [ConstDT1]
      type = ConstantDT
      dt = 0.2
    []

    [ConstDT2]
      type = ConstantDT
      dt = 0.1
    []

    [LogConstDT]
      type = LogConstantDT
      log_dt = 2
      first_dt = 0.01
    []
  []
[]

More information about Time Stepper System and CompositionDT time stepper may be found on the Time Stepper System syntax documentation.

(../moose/test/tests/time_steppers/time_stepper_system/lower_bound.i)

[Mesh]
  type = GeneratedMesh
  dim = 2
  nx = 10
  ny = 10
[]

[Variables]
  [u]
  []
[]

[Kernels]
  [diff]
    type = CoefDiffusion
    variable = u
    coef = 0.1
  []
  [time]
    type = TimeDerivative
    variable = u
  []
[]

[BCs]
  [left]
    type = DirichletBC
    variable = u
    boundary = left
    value = 0
  []
  [right]
    type = DirichletBC
    variable = u
    boundary = right
    value = 1
  []
[]

[Executioner]
  type = Transient
  end_time = 0.8
  solve_type = PJFNK
  petsc_options_iname = '-pc_type -pc_hypre_type'
  petsc_options_value = 'hypre boomeramg'

  [TimeSteppers]
    lower_bound = 'LogConstDT'
    [ConstDT1]
      type = ConstantDT
      dt = 0.2
    []

    [ConstDT2]
      type = ConstantDT
      dt = 0.1
    []

    [LogConstDT]
      type = LogConstantDT
      log_dt = 2
      first_dt = 0.01
    []
  []
[]

[Postprocessors]
  [timestep]
    type = TimePostprocessor
    execute_on = 'timestep_end'
  []
[]

[Outputs]
  csv = true
  file_base='lower_bound'
[]