# ConstantDT

Timestepper that takes a constant time step size

## Description

The ConstantDT TimeStepper simply takes a constant time step size throughout the analysis, unless the solver fails to converge on an iteration.

ConstantDT begins the analysis taking the step specified by the user with the dt parameter. If the solver fails to obtain a converged solution for a given step, the executioner cuts back the step size and attempts to advance the time from the previous step using a smaller time step. The time step is cut back by multiplying the time step by 0.5.

If the solution with the cut-back time step is still unsuccessful, the time step size is repeatedly cut back until a successful solution is obtained. The user can specify a minimum time step through the dtmin parameter in the Executioner block. If the time step must be cut back below the minimum size without obtaining a solution, the problem exits with an error. If the time step is cut back using ConstantDT, that cut-back step size will be used for the remainder of the the analysis.

## Example Input Syntax

[Executioner]
type = Transient
petsc_options_iname = '-pc_type -pc_hypre_type'
petsc_options_value = 'hypre boomeramg'
line_search = none
nl_rel_tol = 1e-6
nl_max_its = 10
l_tol = 1e-8
l_max_its = 50
num_steps = 2 # 200
nl_abs_tol = 1e-10
nl_rel_step_tol = 1e-10
nl_abs_step_tol = 1e-10
[./TimeStepper]
type = ConstantDT
dt = 0.001
[../]
dtmin = .001
[]

(test/tests/misc/check_error/wrong_displacement_order.i)

## Input Parameters

• dtSize of the time step

C++ Type:double

Options:

Description:Size of the time step

### Required Parameters

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

Default:False

C++ Type:bool

Options:

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

• growth_factor2Maximum ratio of new to previous timestep sizes following a step that required the time step to be cut due to a failed solve.

Default:2

C++ Type:double

Options:

Description:Maximum ratio of new to previous timestep sizes following a step that required the time step to be cut due to a failed solve.

### Optional Parameters

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

C++ Type:std::vector

Options:

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

• enableTrueSet the enabled status of the MooseObject.

Default:True

C++ Type:bool

Options:

Description:Set the enabled status of the MooseObject.