Time

The primary function of the time models in Simcenter STAR-CCM+ is to provide solvers that control the iteration and/or unsteady time-stepping.


Model Names	Steady
	Explicit Unsteady
	Implicit Unsteady
	PISO Unsteady
	Harmonic Balance
Provided By	[physics continuum] > Models > Time
Example Node Path	Continua > Physics 1 > Models > Steady
Properties	Key properties are: Continuum Iteration, Continuum Physical Time (unsteady models only) See Time Model Properties.
Activates	Solvers	Explicit Unsteady, Implicit Unsteady, PISO Unsteady, for the respective models. See Time Model Solvers.
	Field Functions	Iteration, Time, Time Level, Time Step. See Time Model Field Functions.

Steady

The Steady model is used for all steady-state calculations. When this model is activated, the concept of a physical time-step is meaningless. For those objects that offer a choice between Iteration or Time-Step in a trigger (such as Monitors or Scenes), choose only Iteration.

Explicit Unsteady

The Explicit Unsteady model is available only with the Coupled Energy model. It is only compatible with the Inviscid and Laminar viscous regime models. It uses the Explicit Unsteady solver. The explicit approach is the alternative to the Implicit Unsteady model. The choice between these two approaches is based on the time scales of the phenomena of interest. When this model is activated, objects that offer a choice between Iteration or Time-Step for a trigger (such as Monitors or Scenes), can be set to update at each time-step.

Implicit Unsteady

The Implicit Unsteady model is available with the Segregated Flow and Segregated Fluid Energy models, and with the Coupled Flow and Coupled Energy models. It uses the Implicit Unsteady solver. With the Coupled Flow and Coupled Energy models, the implicit unsteady approach is the alternative to the Explicit Unsteady one. The choice between these two approaches is based on the time scales of the phenomena of interest. When this model is activated, objects that offer a choice between Iteration or Time-Step for a trigger (such as Monitors or Scenes), can be set to update at each time-step.

PISO Unsteady

The PISO Unsteady time model is for use with the PISO algorithm, in transient cases. This option is appropriate in cases which have low coupling between the pressure, velocity, and volume fraction fields, or when the time-step is sufficiently small.

The PISO Unsteady time model applies the PISO algorithm when solving the discretized system of equations for a fluid continuum. For each time interval, PISO performs a predictor step followed by a number of corrector steps. During these steps, linear equation sets are solved iteratively for each main dependent variable. STAR-CCM+ decides the number of corrector steps based on a splitting error and inner residual levels. See PISO Algorithm.

Harmonic Balance

The Harmonic Balance time model is always used with the Harmonic Balance method which is designed for solving periodic unsteady problems, where the solution repeats itself with a known frequency. See Modeling with Harmonic Balance.

Time Model Properties

Continuum Iteration: The number of iterations for which this continuum has been active. (Read only.)
Continuum Physical Time: The duration of physical time for which this continuum has been active. Used only by Unsteady models. (Read only.)

Time Model Solvers

Explicit Unsteady, Implicit Unsteady, PISO Unsteady

Explicit Unsteady Solver

The Explicit Unsteady solver becomes active when the Explicit Unsteady model is active. Its primary function is to control the iterative update for the calculation.

Properties

Solver Frozen: When On, the solver does not update any quantity during an iteration. It is Off by default. This is a debugging option that can result in non-recoverable errors and wrong solutions due to missing storage. See Finite Volume Solvers Reference for details.

Implicit Unsteady Solver

The Implicit Unsteady solver becomes active when the Implicit Unsteady model is active. Its primary function is to control the update at each physical time for the calculation. It also controls the time-step size.

Note	The Implicit Unsteady solver does not have a Solver Frozen property. If you want to prevent any update of the flow field, activate the Freeze Flow property in the Segregated Flow solver, Coupled Implicit solver, or Viscous Flow Solver, whichever is appropriate.

Properties

Time-Step

The physical time-step. In addition to a constant, you can enter an expression directly. The default Time-Step is 0.001 s.

Temporal Discretization

Selects the temporal discretization scheme. For guidance on scheme selection, see the discussion on the transient term.

1st-order: Selects the first-order temporal scheme. When this scheme is selected together with a rigid body, morphing, or user-defined vertex motion model, the discretized grid flux is of first order.
2nd-order: Selects the second-order temporal scheme. When this scheme is selected together with a rigid body, morphing, or user-defined vertex motion model, the discretized grid flux is of second order.

Freeze Time

When On, fixes physical time at the current time-step. When Off, allows time to advance normally. The default is Off.

PISO Unsteady Solver

The PISO Unsteady solver controls the solution update for the Segregated Flow model or Volume of Fluid (VOF) model according to the PISO algorithm. The PISO Unsteady solver is available when the PISO Unsteady time model is active. Use the PISO algorithm in transient cases with small time-steps.

Properties

Time-Step

The physical time-step. In addition to a constant, you can enter an expression directly.

Verbosity

Specifies the amount of feedback (directed to the Output window) by the PISO solver.

None: Provides no feedback. The PISO solver runs silently.
Low: Provides the number of PISO iterations taken to converge .
High: Provides the same as Low, plus the residuals of the individual PISO iterations.
Diagnostics: Provides the highest degree of information. This option can affect performance, and therefore is not recommended for normal iteration.

Solver Frozen

When On, the solver does not update any quantity during an iteration. It is Off by default. This is a debugging option that can result in non-recoverable errors and wrong solutions due to missing storage. See Finite Volume Solvers Reference for details.

Stopping Criteria properties

Verbose: When the stopping criteria are satisfied, the details are printed to the Output panel.

Time Model Field Functions

Iteration: The current iteration of the simulation.
Time: The current physical time in the simulation. Used by all but the Steady time model.
Time Level: The total number of time-steps taken. Used by all Unsteady models.
Time Step: The current time-step of the simulation. Used by all Unsteady models.