Defining the Adaptive Time-Step

In non-linear structural mechanics the required time-step size may change over the course of the simulation due to the non-linear nature of the problem. To solve non-linear problems efficiently an automatic adaptive time-step size control may be required.

Here, you set up the solid stress time-step provider to compute the time-step based on the perceived displacement change in the simulation.
To enable the adaptive time-step model:
  1. Right-click the Continua > Structure > Models node and select Select Models...
  2. In the Structure Model Selection Window select the Adaptive Time-Step model from the Optional Models group and click Close.
  3. Right-click the Structure > Models > Adaptive Time-Step > Time-Step Providers node and select New > Solid Stress.
  4. Rename the Solid Stress node to Target Displacement.
    By default this time-step provider monitors displacement change in the simulation. As the displacement change is a varying field a norm value is calculated which enable you to compare the field to the reference value specified. For this simulation the Norm Type is set to Max Norm, which means that the reference value is being compared to the maximum component of the displacement change field.
When monitoring displacement, the solid stress time-step provider requires a reference value by which to calibrate the time-step. For this simulation, this value is defined as half the thickness of the leaflet. If the monitored value is below the target value the time-step is increased and if the displacement change is above the target value the time-step is decreased. Linear scaling applies to all time-step changes, and time-steps are kept within the upper and lower limits that you define.
  1. Select the Target Displacement > Intervals > Interval [-inf, inf] node and set the following properties:
    Properties Value
    Control Method Target Control - Linear
    Target Value ${LeafletThicknessReport}/2
  2. Select the Solvers > Adaptive Time-Step node and set the following properties:
    Node Properties Value
    Adaptive Time-Step Initial Time-Step Option Maximum
    Minimum Time-Step ${PeriodReport}/100000
    Time-Step Change Factor Bound Option Enabled
    Time-Step Change Factor Bound Minimum Change Factor 0.25
    Maximum Change Factor 1.5

    To fully define the adaptive time-step, the solver needs the initial time-step, the minimum time-step, and the maximum and minimum factors by which the time-step is allowed to change between each successive step. By setting the Initial Time-Step Option to Maximum, the solver takes the time-step defined in the Implicit Unsteady solver as the initial value. The minimum time-step is defined as 100000 times smaller than the period of the simulation.

To specify the initial time-step:
  1. Select the Solvers > Implicit Unsteady node and set Time-Step to ${PeriodReport}/200.
  2. Save the simulation.