Adaptive Time-Step Model Reference

Table 1. Adaptive Time-Step Model Reference
Provided By	[physics continuum] > Models > Optional Models
Example Node Path	Continua > Physics 1 > Models > Adaptive Time-Step
Requires	Either of the following Time models: Implicit Unsteady or PISO Unsteady One of the following: Segregated Flow, Coupled Flow, or a Solid Energy model.
Properties	Enable Filtering See Adaptive Time-Step Properties. Each time-step provider has its own set of properties. See Adaptive Time-Step Provider Common Properties.
Specific Right-Click Actions	The following right-click action is available on the Time-Step Providers child node: New > [Time-Step Provider] Activates the selected time-step provider. Only the time-step providers that are relevant to the currently-activated models are available for selection.
Activates	Model Controls (child nodes)	Time-Step Providers For a full description of each specific time-step provider, follow the appropriate link in the list of time-step providers. See Adaptive Time-Stepping.
	Solvers	Adaptive Time-Step See Adaptive Time-Step Solver Properties
	Field Functions	Adaptive Time-Step Proposal (accumulated) See Field Functions.

Adaptive Time-Step Properties

Enable Filtering

When activated, provides the functionality to selectively disable suitable adaptive time-step providers for specific cells or a zone of cells.

Filtering Profile: Allows you to define a profile to exclude specific cells from the time-step adaptation considerations. A field function is defined for identifying the cells that are not intended for consideration by assigning a value greater than 0 to the Filtering Profile. As a result, the adaptive time-step provider is disabled for those cells, disregarding their influence on the time-step calculations.

The following image shows the CFL (Max.) Proposal distribution of a simulation before and after filtering. After the small cells (highlighted below) have been excluded from the time-step proposal calculation, the minimum time-step proposed is increased by an order of magnitude.

Before Profile Filtering	After Profile Filtering

Note	The filtering profile is not suitable for the Free Surface Explicit Multi-Step, the Solid Stress, the Max Particle Packing, and the In-cylinder time-step providers.

Adaptive Time-Step Provider Common Properties

The adaptive time-step providers are supplied by various models, and each time-step provider has a different set of properties. However, all time-step providers have the following property:

Enable/Disable

The available options are:

Enabled

The time-step provider is enabled for the entire simulation.
Disabled

The time-step provider is not used in the simulation.
Time Range

The time-step provider is enabled for the specified time range.

Adds the Enabled Time Range child node, with the following properties:
- Start Time
  
  The lower bound of the enabled time range. Once enabled, the time-step provider remains enabled until the specified stop time is reached. If the Enable Stop property is deactivated, the time-step provider remains enabled for the remainder of the simulation.
- Enable Stop
  
  Activates the Stop Time property.
- Stop Time
  
  The upper bound of the enabled time range.

Adaptive Time-Step Solver Properties

Initial Time-Step Option

Specifies the initial time-step size at $t = 0$ .

Auto

The enabled time-step providers in each continuum are queried for their proposals, and the smallest time-step is used.
Maximum

Use the time-step size that is specified in the Implicit Unsteady or PISO Unsteady solver.
Minimum

Use the time-step that is specified in the Minimum Time-Step property.
Specified

Use the value that is specific in the Initial Time-Step child node.

Minimum Time-Step

The lower limit for the time-step size. This value appears as $Δ t_{\min}$ in Eqn. (971). The default value is 0.0 s.

Verbosity

When activated, the Adaptive Time-Step solver sends additional information on its progress to the Output window. This setting is deactivated by default.

Time-Step Change Factor Bounds Option

If the enabled time-step providers have large variations in successive step size proposals, you can set this property to specify a lower bound and an upper bound for the change factor (that is, the ratio) of successive time-steps. A proposed time-step size is then restricted to the specified range.

When activated, the Time-Step Change Factor Bounds node is added, allowing you to specify the appropriate upper and lower bounds.

The properties are:

Minimum Change Factor

The factor bounding the time-step size decrease. The valid range is 0.0 – 1.0. The default value is 0.0.

A minimum change factor of 0.0 does not limit the allowed decrease of the time-step size. This setting may be required to support a sudden change in the magnitude of relevant time scales.

Maximum Change Factor

The factor bounding the time-step size increase. The valid range is 1.0 to infinity. The default value is 2.0.

The absolute lower bound and upper bound for the time-step proposal are the specified minimum (the Minimum Time-Step property) and maximum time-step (the Time-Step property set in the Implicit Unsteady or PISO Unsteady solver).

Checkpoint Times

A list of physical times (in seconds) that are to be reached exactly by the adaptive time-stepping. You might do this, for example, if you want to synchronize the simulation at a particular physical time for post-processing.

The specified checkpoint times are reached gradually over the course of a few time-steps.

Reaching a checkpoint time takes priority over any relative or absolute bounds. That is, you can get time-step sizes that are below the minimum time-step, or a step size change that is less than the minimum change factor bound. However, due to the gradual nature of approaching each checkpoint time, the deviations from these limits are small.

Adaptive Time-Step Solver Right-Click Actions

Refresh Time-Step Proposals: Refreshes the current time-step proposal without the need to step the simulation. The solver uses the current solution fields and properties of the Adaptive Time-Step model, its providers and solver to recalculate the time-step. If any of these properties are modified, this action allows the evaluation of property changes on upcoming time-step proposals without performing the simulation. If more than one time-step provider is used, the final calculated upcoming time-step is the minimum over all cells. You use the corresponding time-step proposal field functions or solver diagnostics to examine the recalculated time-step.

Field Functions

The following field function is available when the Adaptive Time-Step model is activated.

Adaptive Time-Step Proposal (accumulated): The accumulated time-step proposal from all active providers in the simulation. It represents the global minimum time-step proposal for each individual cell.