Using Monitors as Stopping Criteria

It is possible to create stopping criteria that are based on existing monitors. This function lets you use more meaningful criteria to judge convergence.

The node of a monitor-based stopping criterion has properties just like the automatically generated kind.

These monitor-based stopping criteria could include a reduction in a residual, or could be based upon some physical quantity that you are interested in obtaining from the solution. For example, if you are simulating the flow over an airfoil you can stop iterating when the drag and lift coefficients of the airfoil have reached steady values.

You can create stopping criteria that are based on any existing monitor. These criteria check the value of the associated monitor and return a satisfied condition when that value reaches some user-specified minimum, maximum, asymptotic limit, or standard deviation. You can also copy properties of these criteria.

Note	Using a multi-valued report monitor as an input extracts only the overall value of the report monitor.

Using Monitor-Based Stopping Criteria in Transient Simulations

If you are using the implicit unsteady model, these criteria apply automatically to the inner iterations. Make sure that the monitor Trigger property is set to Iteration when using the criterion to stop inner iterations. To apply the criteria to the outer iterations, so that they stop the entire simulation, deactivate the Stop Inner Iterations property of the corresponding criterion node.

In a transient analysis these criteria, when satisfied, allow only one inner iteration by default. To require more than one inner iteration:

Manually create the Minimum Inner Iterations criterion.
Set the properties of the Minimum Inner Iterations criterion:
- Set the Minimum Inner Iterations property to 3 or 4.
- Make sure that the Logical Rule property is set to And.
Set the Logical Rule property of the other monitor-based criteria to And.

Monitor-Based Properties

Monitor

The name of the monitor that is used to create this stopping criterion (read-only).

Stop Inner Iterations

When On, stops the inner iterations of the transient simulation when the criterion (as specified in Criterion Option) is satisfied. When Off, stops outer iterations of the transient simulation when the criterion (as specified in Criterion Option) is satisfied.

Criterion Option

The method to use for evaluating this criterion.

Minimum: Specifies that this criterion is satisfied when the monitor reaches a user-specified minimum value. A Minimum Limit node is added as a sub-node.
Maximum: Specifies that this criterion is satisfied when the monitor reaches a user-specified maximum value. A Maximum Limit node is added as a sub-node.
Asymptotic: Specifies that this criterion is satisfied when the monitor has stabilized to a particular range during a number of iterations. The range and iteration window are specified in the Asymptotic Limit sub-node.
Standard Deviation: Specifies that this criterion is satisfied when the monitor reaches a user-specified standard deviation. A Standard Deviation node is added as a sub-node.
Relative Change: Specifies that this criterion is satisfied when the ratio of current value to initial value (maximum of the first six relevant iterations) of the monitor reaches a user-specified relative change value. The initial value is determined at the start of each time-step for transient cases.

Enabled

When On, determines if this criterion is used to stop the solver.

Criterion Satisfied

Indicates if the stopping criterion has been satisfied (read-only).

Logical Rule

Defines how this criterion interacts with other stopping criteria that have been enabled.

AND: Requires that this criterion, along with one or more other criteria with the Logical Rule set to AND, all be satisfied before stopping the solver.
OR: Indicates that satisfying only this criterion is sufficient to stop the solver.

Creating a Monitor-Based Criterion

To create a stopping criterion from an existing monitor:

Right-click on the Stopping Criteria node and select Create New Criterion > Create from Monitor.
Select from the list of existing monitors.

A new stopping criterion node is added in the Stopping Criteria manager node. The new node is named after the selected monitor with Criterion appended to the name, and possibly a digit to make the new criterion unique among others with similar names. However, renaming the stopping criterion is possible. The new criterion is activated by default, with the Criterion Option set to Minimum, and the Logical Rule set to OR.

Min/Max Monitor

By default all criteria that are created from a monitor use a minimum limit to determine when to stop the simulation. The type of limit can be changed to a maximum, an asymptote or a standard deviation.

To change the type of limit, select the monitor stopping criterion node, then from the pull-down menu under the Criterion Option property in the Properties window, select the type. You can use a monitor to create more than one criterion. For example, a monitor could be used to stop the iteration when its value reaches a maximum or an asymptote.

If you select a Minimum or Maximum limit, you can set the limiting value by selecting the Minimum Limit or Maximum Limit node in the Stopping Criteria manager. In the properties window, you can then modify the Minimum Value or Maximum Value property.

Minimum Limit Properties

Minimum Value: The minimum monitor value that satisfies this criterion. The solver stops as soon as the monitor value is less than this specified value.

Maximum Limit Properties

Maximum Value: The maximum monitor value that satisfies this criterion. The solver stops as soon as the monitor value is greater than this specified value.

Asymptotic Monitor

It is also possible to set the criterion to stop once it has reached an asymptotic limit. It allows for definition of the range over which the value can fluctuate in a given number of samples.

This feature is useful when the exact limiting value is unknown but you do know the maximum change in the monitored value that you consider sufficient for convergence.

To select the asymptotic limit option, select the monitor stopping criterion node and select the Asymptotic option in the drop-down list under the Criterion Option property in the Properties window. The limit is set by selecting a maximum permissible change in the monitored value over a given number of successive iterations (samples). Expand the monitor stopping criterion node, select the Asymptotic Limit child node and edit the difference (|min – max|) in the monitored value and the number of samples in the Properties window.

Asymptotic Monitor Properties

Refresh Sampling: When On, this refreshes monitor sampling whenever an outer stopping criterion is satisfied.
|Max - Min|: The difference between the smallest and largest value that is collected during the previous Number of Samples. The criterion is satisfied once the difference lies within this limit.
Number of Samples: The number of samples that the monitor values check to see if the absolute change is exceeded.
Normalized: When On (the default setting), this normalizes the |Max - Min| values to the average value over Number of Samples. The default value for |Max - Min| is 0.1.

Standard Deviation Monitor

You can use the standard deviation for a given number of samples.

To select this option, select the monitor stopping criterion node and select the Standard Deviation option in the drop-down list under the Criterion Option property in the Properties window. Expand the monitor stopping criterion node, select the Standard Deviation child node and edit the standard deviation and the number of samples in the Properties window.

Standard Deviation Monitor Properties

Refresh Sampling: When On, this refreshes monitor sampling whenever an outer stopping criterion is satisfied.
Standard Deviation: The standard deviation of the samples, entered as text.
Number of Samples: The number of samples over which the standard deviation is determined and checked.

Relative Change Monitor

You can use relative change to stop the run when a selected value has changed by a given proportion.

To select this option, select the monitor stopping criterion node and select the Relative Change option in the drop-down list under the Criterion Option property in the Properties window. Expand the monitor stopping criterion node, select the Standard Deviation child node and edit the standard deviation and the number of samples in the Properties window.

Relative Change Monitor Properties

Refresh Sampling: When On, this refreshes monitor sampling whenever an outer stopping criterion is satisfied.
Relative Change: The ratio of the stopping value to the initial value.

Copying Properties of Monitor-Based Stopping Criteria

You can copy a property from one limiting value node to another.

If you have two (or more) monitor-based stopping criteria, and you want them to have the same limiting value, you can copy that property from one limiting value node to another. You can only copy from the same type of limiting value node (minimum, maximum, or asymptotic).

Renaming Monitor-Based Stopping Criteria

Similar to other objects, a monitor-based stopping criterion can be renamed within Simcenter STAR-CCM+.