SOLIDWORKS Simulation Nonlinear Analysis – Stepping Options

Article by Irfan Zardadkhan, PhD, CSWE updated April 24, 2014

Article

One of the most daunting tasks in creating nonlinear simulations is the proper settings of the time stepping options. This article will look at some recommendations for adjusting the time stepping options.

NLTimeStep

Nonlinear Time Step Options

The Nonlinear Autostepping Option

In general, the Automatic (autostepping) option is recommended. This is because the optimal time step size may vary throughout the solution process depending on local singularities, changes in loads, and other nonlinearities for instance. This option gives the solver the freedom to adjust the step as needed in order to smoothly get through any points in time where convergence difficulties are encountered.

If there are convergence issues in the first few steps, the following changes can help:

  • Decrease the initial time increment – for instance instead of the default value of 0.01, try 0.001.

If there are convergence issues in later steps, the following changes can help:

  • Decrease the Maximum time increment – for instance instead of the default 0.1, you might want to change it to 0.05 or even 0.01.

Please note that with the above change, you will most likely be significantly increasing the total number of steps and therefore the time it takes to solve as well as the size of the result data (.cwr file).

Best Practice Nonlinear Analysis Options

One way around this is to use the Save data for restarting the analysis option in conjunction with the Restart option. For example, if you know that in a study with an end time of 1, there will be convergence difficulty requiring smaller and tighter time steps at t = 0.3, you can run with the aforementioned reduced initial and maximum step sizes, making sure the Save data for restarting the analysis option is enabled. Once the solver has gotten past this time, you can cancel the analysis, go back to Study Properties, relax the time step settings somewhat, and enable the Restart checkbox before continuing the run. This should cause the solver to resume solving from the last time step, now using the newly changed stepping parameters.

The Minimum and Number of adjustments settings in the Automatic (autostepping) options are usually less helpful in resolving convergence issues. This is because by default, the minimum is set to 1e-8. Increasing it will almost never help with convergence. Conversely, making it even smaller is also unlikely to help since if such minuscule time steps are required, there are probably some issues causing the analysis to fail regardless of the time step.

The number of adjustments setting controls how many times in a given step the solver can retry the same step in a new iteration with a reduced time step. The actual “new” step chosen by the solver will depend on the range that is allowed by the minimum and maximum time step settings. By default, the number of adjustments is set to 5 which should help the solver get through most issues. If the solver needs to adjust the time step more than 5 times in a given step, there are most likely issues that cannot be resolved simply by adjusting time step parameters. Sometimes, the issue is also that the time steps were too large just prior to this localized problem, and no amount of reduction in the time step at this point will help the solver move further. In this case, the recommended approach is to rerun but with a decreased maximum time step. The approach of using the “Save data” and “Restart” options as mentioned previously can also help in such cases.

Using the “Fixed” option is generally not advised except in certain cases. One reason to use this option might be if results at very specific times are required. However, please note that if a time curve is applied for a load or restraint, any point in time specified in the curve will force the solver to put a step at that specific time. This approach can be an alternative to using the “Fixed” option. It is also possible to use the aforementioned “Save data” and “Restart” options along with fixed time steps to closely control the analysis. However, this approach requires careful supervision of solver progress and can ultimately be time consuming and inconvenient.

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Irfan Zardadkhan, PhD, CSWE

Irfan holds a PhD in Aerospace Engineering and is as Elite AE. He contributes regularly to the SIMULATION and COMPOSER tech blogs. He has won the TenLinks Top blogger award for SOLIDWORKS. He has presented at local user groups and at SOLIDWORKS World.