Transient

!syntax description /Executioner/Transient

Normal Usage

The Transient Executioner is the primary workhorse Executioner in MOOSE. Most simulations will use it.

At its most basic the Transient Executioner allows a simulation to step through multiple steps in _time_... doing one nonlinear solve per timestep. Most of the time this type of execution will utilize one or more TimeDerivative Kernels on the variables to solve for their time evolution.

Primary Parameters

The most important parameters for Transient (beyond what Steady already provides) are:

- dt: The initial timestep size - num_steps: Number of steps to do - end_time: Finish time for the simulation - scheme: The TimeIntegrator to use (see below) - defaults to Implicit/Backward Euler.

See down below for the full list of parameters for this class.

TimeIntegrators

It's important to note that transient simulations generally use a TimeIntegrator. As mentioned above, there is a scheme parameter that is shortcut syntax for selection of that TimeIntegrator. However, there is also a whole TimeIntegrator system for creating your own or specifying detailed parameters for time integration.

TimeSteppers

Similarly, the choice of how to move through time (the choice of timestep size) is important as well. The default TimeStepper is ConstantDT but many other choices can be made using the TimeStepper system.

Transient can also be used for simulations that don't necessarily need _time_. In this context a "transient" calculation can simply be thought of as a series of nonlinear solves. The time parameter will move forward - but what you do with it, or what it means is up to you.

One good example of this is doing "load steps" for a solid mechanics calculation. If the only thing that is desired is the final, steady state, solution, but getting to it is extremely difficult, then you might employ "load steps" to slowly ramp up a boundary condition so you can more easily solve from the initial state (the "initial condition") to the final configuration. In this case you would use "time" as a parameter to control how much of the force is applied (for instance, by using FunctionDirichletBC).

In this case you don't use any TimeDerivative Kernels. The "transient" behavior comes from changing a condition based on "time". What that "time" means is up to you to identify (generally, I like to just step through time = 1,2,3,4.. and define my functions so that at time = end_steps the full load is applied.

Quasi-Transient

Similarly to Load Steps, you can use Transient to do "Quasi-Transient" calculations. This is where some variables are evolving with time derivatives, while others are solved to steady state each step.

A classic example of this is doing coupled thermo-mechanics. It's very normal for the heat flow to move much more slowly than the solid mechanics. Therefore, classically, it is normal to have a time derivative for your heat conduction equation but none for the solid mechanics so that at each timestep the solid-mechanics is solved to a full steady state based on the current configuration of heat.

This idea works perfectly in MOOSE with Transient: just simply only apply TimeDerivative Kernels to the equations you want and leave them off for the others.

Another use-case is to use Transient to solve to a steady state. In this case there are a few built-in parameters to help detect steady state and stop the solve when it's reached. You can see them down below in the "Steady State Detection Parameters" section.

It is important to know that you must turn _on_ steady state detection using steady_state_detection = true before the other two parameters will do anything.

Input Parameters

• reset_dtFalseUse when restarting a calculation to force a change in dt.

Default:False

C++ Type:bool

Description:Use when restarting a calculation to force a change in dt.

• line_search_packagepetscThe solver package to use to conduct the line-search

Default:petsc

C++ Type:MooseEnum

Description:The solver package to use to conduct the line-search

• verboseFalsePrint detailed diagnostics on timestep calculation

Default:False

C++ Type:bool

Description:Print detailed diagnostics on timestep calculation

• update_xfem_at_timestep_beginFalseShould XFEM update the mesh at the beginning of the timestep

Default:False

C++ Type:bool

Description:Should XFEM update the mesh at the beginning of the timestep

• petsc_options_inameNames of PETSc name/value pairs

C++ Type:MultiMooseEnum

Description:Names of PETSc name/value pairs

• petsc_optionsSingleton PETSc options

C++ Type:MultiMooseEnum

Description:Singleton PETSc options

• max_xfem_update4294967295Maximum number of times to update XFEM crack topology in a step due to evolving cracks

Default:4294967295

C++ Type:unsigned int

Description:Maximum number of times to update XFEM crack topology in a step due to evolving cracks

• num_steps4294967295The number of timesteps in a transient run

Default:4294967295

C++ Type:unsigned int

Description:The number of timesteps in a transient run

• line_searchdefaultSpecifies the line search type (Note: none = basic)

Default:default

C++ Type:MooseEnum

Description:Specifies the line search type (Note: none = basic)

• splittingTop-level splitting defining a hierarchical decomposition into subsystems to help the solver.

C++ Type:std::vector

Description:Top-level splitting defining a hierarchical decomposition into subsystems to help the solver.

• contact_line_search_ltolThe linear relative tolerance to be used while the contact state is changing between non-linear iterations. We recommend that this tolerance be looser than the standard linear tolerance

C++ Type:double

Description:The linear relative tolerance to be used while the contact state is changing between non-linear iterations. We recommend that this tolerance be looser than the standard linear tolerance

• petsc_options_valueValues of PETSc name/value pairs (must correspond with "petsc_options_iname"

C++ Type:std::vector

Description:Values of PETSc name/value pairs (must correspond with "petsc_options_iname"

• end_time1e+30The end time of the simulation

Default:1e+30

C++ Type:double

Description:The end time of the simulation

• solve_typePJFNK: Preconditioned Jacobian-Free Newton Krylov JFNK: Jacobian-Free Newton Krylov NEWTON: Full Newton Solve FD: Use finite differences to compute Jacobian LINEAR: Solving a linear problem

C++ Type:MooseEnum

Description:PJFNK: Preconditioned Jacobian-Free Newton Krylov JFNK: Jacobian-Free Newton Krylov NEWTON: Full Newton Solve FD: Use finite differences to compute Jacobian LINEAR: Solving a linear problem

• mffd_typewpSpecifies the finite differencing type for Jacobian-free solve types. Note that the default is wp (for Walker and Pernice).

Default:wp

C++ Type:MooseEnum

Description:Specifies the finite differencing type for Jacobian-free solve types. Note that the default is wp (for Walker and Pernice).

• dt1The timestep size between solves

Default:1

C++ Type:double

Description:The timestep size between solves

• schemeimplicit-eulerTime integration scheme used.

Default:implicit-euler

C++ Type:MooseEnum

Description:Time integration scheme used.

• contact_line_search_allowed_lambda_cuts2The number of times lambda is allowed to be cut in half in the contact line search. We recommend this number be roughly bounded by 0 <= allowed_lambda_cuts <= 3

Default:2

C++ Type:unsigned int

Description:The number of times lambda is allowed to be cut in half in the contact line search. We recommend this number be roughly bounded by 0 <= allowed_lambda_cuts <= 3

Optional Parameters

• use_multiapp_dtFalseIf 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

Default:False

C++ Type:bool

Description: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

• enableTrueSet the enabled status of the MooseObject.

Default:True

C++ Type:bool

Description:Set the enabled status of the MooseObject.

• abort_on_solve_failFalseabort if solve not converged rather than cut timestep

Default:False

C++ Type:bool

Description:abort if solve not converged rather than cut timestep

• timestep_tolerance2e-14the tolerance setting for final timestep size and sync times

Default:2e-14

C++ Type:double

Description:the tolerance setting for final timestep size and sync times

• control_tagsAdds user-defined labels for accessing object parameters via control logic.

C++ Type:std::vector

Description:Adds user-defined labels for accessing object parameters via control logic.

• no_fe_reinitFalseSpecifies whether or not to reinitialize FEs

Default:False

C++ Type:bool

Description:Specifies whether or not to reinitialize FEs

• dtmax1e+30The maximum timestep size in an adaptive run

Default:1e+30

C++ Type:double

Description:The maximum timestep size in an adaptive run

• dtmin2e-14The minimum timestep size in an adaptive run

Default:2e-14

C++ Type:double

Description:The minimum timestep size in an adaptive run

• n_startup_steps0The number of timesteps during startup

Default:0

C++ Type:int

Description:The number of timesteps during startup

• start_time0The start time of the simulation

Default:0

C++ Type:double

Description:The start time of the simulation

• l_abs_step_tol-1Linear Absolute Step Tolerance

Default:-1

C++ Type:double

Description:Linear Absolute Step Tolerance

• nl_abs_tol1e-50Nonlinear Absolute Tolerance

Default:1e-50

C++ Type:double

Description:Nonlinear Absolute Tolerance

• nl_max_its50Max Nonlinear Iterations

Default:50

C++ Type:unsigned int

Description:Max Nonlinear Iterations

• l_max_its10000Max Linear Iterations

Default:10000

C++ Type:unsigned int

Description:Max Linear Iterations

• compute_initial_residual_before_preset_bcsFalseUse the residual norm computed *before* PresetBCs are imposed in relative convergence check

Default:False

C++ Type:bool

Description:Use the residual norm computed *before* PresetBCs are imposed in relative convergence check

• nl_rel_tol1e-08Nonlinear Relative Tolerance

Default:1e-08

C++ Type:double

Description:Nonlinear Relative Tolerance

• l_tol1e-05Linear Tolerance

Default:1e-05

C++ Type:double

Description:Linear Tolerance

• nl_max_funcs10000Max Nonlinear solver function evaluations

Default:10000

C++ Type:unsigned int

Description:Max Nonlinear solver function evaluations

• nl_rel_step_tol1e-50Nonlinear Relative step Tolerance

Default:1e-50

C++ Type:double

Description:Nonlinear Relative step Tolerance

• nl_abs_step_tol1e-50Nonlinear Absolute step Tolerance

Default:1e-50

C++ Type:double

Description:Nonlinear Absolute step Tolerance

Solver Parameters

• picard_abs_tol1e-50The absolute nonlinear residual to shoot for during Picard iterations. This check is performed based on the Master app's nonlinear residual.

Default:1e-50

C++ Type:double

Description:The absolute nonlinear residual to shoot for during Picard iterations. This check is performed based on the Master app's nonlinear residual.

• picard_rel_tol1e-08The relative nonlinear residual drop to shoot for during Picard iterations. This check is performed based on the Master app's nonlinear residual.

Default:1e-08

C++ Type:double

Description:The relative nonlinear residual drop to shoot for during Picard iterations. This check is performed based on the Master app's nonlinear residual.

• relaxed_variablesList of variables to relax during Picard Iteration

C++ Type:std::vector

Description:List of variables to relax during Picard Iteration

• picard_max_its1Number of times each timestep will be solved. Mainly used when wanting to do Picard iterations with MultiApps that are set to execute_on timestep_end or timestep_begin

Default:1

C++ Type:unsigned int

Description:Number of times each timestep will be solved. Mainly used when wanting to do Picard iterations with MultiApps that are set to execute_on timestep_end or timestep_begin

• relaxation_factor1Fraction of newly computed value to keep.Set between 0 and 2.

Default:1

C++ Type:double

Description:Fraction of newly computed value to keep.Set between 0 and 2.

Restart Parameters

• steady_state_start_time0Minimum amount of time to run before checking for steady state conditions.

Default:0

C++ Type:double

Description:Minimum amount of time to run before checking for steady state conditions.

• steady_state_tolerance1e-08Whenever the relative residual changes by less than this the solution will be considered to be at steady state.

Default:1e-08

C++ Type:double

Description:Whenever the relative residual changes by less than this the solution will be considered to be at steady state.

Default:False

C++ Type:bool

Description:Whether or not to check for steady state conditions

• time_period_startsThe start times of time periods

C++ Type:std::vector

Description:The start times of time periods

• time_period_endsThe end times of time periods

C++ Type:std::vector

Description:The end times of time periods

• time_periodsThe names of periods

C++ Type:std::vector

Description:The names of periods