Setting Up Adaptive Time-Stepping

The Adaptive Time-Step model automates the determination of time-step sizes. You define time-step providers that specify the criteria to achieve suitable time-steps for your particular application, and set the appropriate limits, such as minimum and maximum time-step values. The Adaptive Time-Step model uses the specified criteria to determine the appropriate time-step sizes at each stage of the simulation.

To set up adaptive time-stepping:

  1. In the [physics continuum], select the Adaptive Time-Step model from the Optional Models group box.
    See Adaptive Time-Step Model Reference.

Select the time-step providers that you want to use for your application. Only the time-step providers that are relevant to the currently-activated models are available. You can use any combination of the available time-step providers. At any given point, the simulation uses the smallest time-step from the enabled time-step providers.

  1. Right-click the [physics continuum] > Models > Adaptive Time-Step > Time-Step Providers node and select New > [time-step provider].
    For descriptions of the available time-step providers, see Adaptive Time-Stepping.
    The corresponding node is added under the Time-Step Providers node.

Define the time-step providers that you want to use in your simulation. For each time-step provider, you can either enable the provider for the entire simulation, or enable it for a specified time range. You can disable any providers that you do not want to use in a particular simulation.

  1. Select the Time-Step Providers > [time-step provider] node and set Enable/Disable to either:

    • 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 a specified time range.

      In the Enabled Time Range child node, set the appropriate Start Time and Stop Time.

  2. Set the time-step provider properties that are relevant to your simulation.

    Each time-step provider has its own set of properties. For details of each specific time-step provider, and a description of its properties, follow the appropriate link in the list of time-step providers. See Adaptive Time-Stepping.

Specify the minimum time-step, the maximum time step, and the initial time-step. Optionally specify the change factor bounds (the limits to the ratio of successive time-step sizes) and any necessary checkpoint times. You set these values as solver properties.
  1. To set the maximum time-step, select the Solvers > Implicit Unsteady node or the Solvers > PISO Unsteady node (whichever is relevant) and specify the Time-Step.
  2. Select the Solvers > Adaptive Time-Step node and set the following properties:
    Property Description
    Initial Time-Step Option

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

    By default, the initial time-step is derived from the enabled time-step providers in each continuum.

    Alternatively, you can specify a particular time-step size, or use the specified minimum or maximum values.

    Minimum Time-Step

    The lower bound for the adaptive time-step size. The default value is 0.0 s.

    Time-Step Change Factor Bounds Option

    Optional

    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.

    Checkpoint Times

    Optional

    A list of physical times (in seconds) that are to be reached exactly by the adaptive time-stepping. The specified checkpoint times are reached gradually over the course of a few time-steps.

    For more information, see Adaptive Time-Step Solver Properties.

Adaptive time-stepping can predict small time-steps due to bad quality cells. These small time-steps can increase the runtime of the simulation unnecessarily.

  1. If you want to exclude specific cells from the time-step prediction:
    1. Select the [physics continuum] > Model > Adaptive Time-Step node and activate Enable Filtering.
    2. Select the Adaptive Time-Step > Filtering Profile node and specify a field function that identifies certain cells, for example, Bad Cell Indicator.
    All suitable time-step providers ignore contributions from the cells that are identified by the field function.
The Adaptive Time-Step solver provides actions to recalculate 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. With this option, you can evaluate the impact of parameter changes on the upcoming time-step proposals before the next time-step starts. It also updates the proposal field functions, ensuring consistency between post-processing and the time-step proposals of the upcoming time-step.
  1. (Optional) If you want to evaluate the impact of parameter changes on the time-step proposal:
    1. Right-click the Adaptive Time-Step solver node and select Refresh Time-Step Proposals.
    2. Open a scalar scene with the time-step proposal field functions or use the solver diagnostics to examine the updated time-step.