ChangeOverTimestepPostprocessor

Computes the change or relative change in a post-processor value over a timestep or the entire transient

The ChangeOverTimestepPostprocessor can be used to track convergence of additional quantities computed by postprocessors. For example, in conjugate heat transfer simulations, the total energy stored in the fluid should be tracked as an additional convergence metric, and not just the residual from the equations.

The 'INITIAL' execute_on flag should be included in the tracked postprocessor execute_on parameter if the ChangeOverTimestepPostprocessor is set to compute the difference with regards to the initial value.

Example input syntax

In this example, the ChangeOverTimestepPostprocessor is set to track the change of a FunctionValuePostprocessor, as a test of correctness. The FunctionValuePostprocessor's evolution is known as it is defined by a ParsedFunction.

[Postprocessors]
  [./my_postprocessor]
    type = FunctionValuePostprocessor
    function = my_function
    execute_on = 'initial timestep_end'
  [../]
  [./change_over_time]
    type = ChangeOverTimePostprocessor
    postprocessor = my_postprocessor
    change_with_respect_to_initial = false
    execute_on = 'initial timestep_end'
  [../]
[]

[Functions]
  [./my_function]
    type = ParsedFunction
    expression = '1 + t * t'
  [../]
[]

(../moose/test/tests/postprocessors/change_over_time/change_over_time.i)

# This test tests the ChangeOverTimePostprocessor, which computes the change
# in a postprocessor value with respect to the previous value or with respect to
# the initial value. This test creates a time-dependent function postprocessor
# and then computes its change over a timestep. The FE problem used here is a
# dummy problem and has no effect on the test.

[Mesh]
  type = GeneratedMesh
  dim = 1
  nx = 5
[]

[Variables]
  [./u]
  [../]
[]

[Kernels]
  [./time_derivative]
    type = TimeDerivative
    variable = u
  [../]
  [./diff]
    type = Diffusion
    variable = u
  [../]
[]

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

[Executioner]
  type = Transient

  start_time = 0.0
  dt = 1.0
  num_steps = 2
[]

[Functions]
  [./my_function]
    type = ParsedFunction
    expression = '1 + t * t'
  [../]
[]

[Postprocessors]
  [./my_postprocessor]
    type = FunctionValuePostprocessor
    function = my_function
    execute_on = 'initial timestep_end'
  [../]
  [./change_over_time]
    type = ChangeOverTimePostprocessor
    postprocessor = my_postprocessor
    change_with_respect_to_initial = false
    execute_on = 'initial timestep_end'
  [../]
[]

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