External restarts

External restarts continue a nonlinear computation using existing results files, so you generate at least two solves:

1. Solve the solution to generate the results file that you want to reuse. This is the initial run. The initial run generates an .op2 file that contains the results for all subcases, and restart data for each static, dynamic, and preload subcase. You reference one of the static, dynamic, and preload restart subcases when you set up the restart run.

2. Solve a new solution, or, if the initial run failed to converge, solve a clone of the same solution. Either way, the solution uses the results of the initial run. This is the restart run. You can restart the solution from the end of a static, dynamic, or preload subcase, or from the last converged time step of one of those subcases. You cannot restart from an intermediate time step within a subcase. Note: An exception is SOL 401 preload subcases. A SOL 401 preload subcase can write output only at the final load. Thus, if a SOL 401 preload subcase does not converge, you cannot use it as a restart point in the restart run. The restart run generates an .op2 file that contains the combined results of the specified restart subcases from the initial run and the results of the restart run.

The options for the initial and restart run are on the Restart Management page of the Solution dialog box.

Requirements for the initial run

The initial run solution can contain any combination of subcases, but restart data is saved only for static, dynamic, and preload subcases. All subcases must be sequentially dependent on the previous subcase. You can set the sequential dependency in the Solution Step dialog box.

• If you plan to clone your initial run solution for the restart run, the initial run solution can contain all of the subcases you want for the final solution. However, when you use the results of the initial run solution for the restart solution, only the results for the preload, static, and dynamic subcases are used.

• If you plan to create a new solution for the restart run, the initial run solution can contain all of the subcases you need to generate the desired results.

The initial run generates an .op2 file with the results for all subcases and with restart data for preload, static, and dynamic subcases. Review these results in post-processing to determine the subcase that you want to use in the restart run. The Post Processing Navigator organizes the results according to subcase ID and time step.

To identify a solution as an initial run, set Generate Restart Data to Yes.

Requirements for the restart run

The requirements for the restart solution depend in part on the type of restart run:

• Restart run for a final solution

• Restart run for troubleshooting convergence

Note:

To use restarts with intermediate results files, you must select the 64-bit format for the .op2 file, which is the default. You can select the format in the Solution dialog box on the Parameters page.

All restart runs require that you specify the location of the results file (.op2) from the initial run, and that you identify which of those subcases you want to restart from.

Note:

For SOL 402, the .sdb, .adb, and .u18 files must be in the same location as the .op2 file.

The restart run generates an .op2 file that includes the initial run results and the restart run results.

Restart runs for final solutions

If you are applying pre-stress results to a new analysis, you typically want the restart run to be your final solution. The restart run for the final solution can be a clone of the original solution, or it can be a new solution.

An advantage to cloning the initial run solution is that subcase numbering is maintained. If you create a new solution, the software numbers the subcases starting with 1. In post-processing, however, the subcases are numbered consecutively starting with the restart subcases. For example, if you use subcases 1, 2, and 3 as your restart subcases, and your new solution has six subcases (subcases 1 through 6), in post-processing the subcases are numbered 1 through 9.

Requirements for restart runs for final solutions are as follows:

• If you clone the initial run solution, set Generate Restart Data to No.

• Identify the results from the initial run that you want to use to start the restart run. The subcase whose initial run results you want to use must be a nonlinear statics subcase, nonlinear dynamics subcase, or a preload subcase.To determine the subcase that you want to use in the restart run, open the results of the initial run in the Post Processing Navigator. The Post Processing Navigator organizes the results according to subcase ID and time step. To specify the results, in the Restart Subcase list, select the ID of the last subcase whose results you want to use. For example, if your initial run results file contains subcases 1 through 5, and you want to use the results from subcase 1 through subcase 3, select subcase 3. The solver uses the results that are available at the end of subcase 3, but not the results from subcases 4 and 5.

• (Optional) Identify the subcase that you want to use as the first subcase in the restart solution. This can be a nonlinear statics, nonlinear dynamics, or complex modes (SOL 402) subcase. To specify the subcase you want the restart solution to start from, use the Execute Subcase list. If you created a new restart solution, select First Subcase. This allows you to apply preload or pre-stress results from your initial run solution to a new analysis.Note: The restart time in the restart solution is the end time of the subcase or the end time of the last converged time step in the subcase that you specify. For example, if you specify subcases 1, 2, and 3 as the restart subcases, and subcase 3 ends at 10 seconds, the end time of the first subcase in the new solution must be greater than 10 seconds.If you cloned the initial solution for the restart solution, select Restart Subcase ID + 1 to start with the first subcase that follows the restart subcase.

Restart runs for troubleshooting convergence

When you troubleshoot convergence, instead of discrete initial and restart runs, you use your initial runs as restart runs. To do this, clone the initial solution to create the restart solution. Then, set the restart solution to also generate restart data. This way, the solver again saves all of the results up to the last converged time step (the restart point), and you can start from that last converged time step instead of solving from the beginning. Repeat this process—clone the solution and set it to generate restart data—until the solution converges.

Requirements for initial runs for troubleshooting convergence are as follows:

• To set the solution to generate restart data, set Generate Restart Data to Yes.

• Identify the subcase that last failed to converge and select it as the subcase whose results you want to use in the restart solution. This subcase must be a nonlinear statics subcase, nonlinear dynamics subcase, or a preload subcase.To specify the non-converged subcase, in the Restart Subcase list, select the subcase.

• Ensure that the restart solution will start with the same subcase that you select as the restart subcase. To do so, from the Execute Subcase list, select Restart Subcase ID, or leave the option blank.

Adjusting parameters for the restart run

Before you start the restart run, you might want to adjust boundary conditions and solution parameters to aid convergence, or you might want to add subcases to try different analyses with the restart results. The restart run performs a model validation check to ensure that the model you used for the initial run is compatible with the model you used for the restart run.

You can change the following boundary conditions and solution parameters:

• Loads

• Single-point and multi-point constraints

• Contact (activate and deactivate)

• Element Add/Remove simulation object (SOL 401)

• Subcase options, such as time steps.

However, the following changes are prohibited:

• Changing the number of degrees of freedom

• Moving nodes or creating nodes or elements in the mesh

• Modifying material properties

• Adding contact or glue conditions

• Changing the temperature of an Initial/Stress Free Temperature type of temperature set.

• Changing the loads in the last subcase, or, for a non-converged subcase, changing the load or its end time.

The solver compares the model in the initial run to ensure that it is the same as the model in the restart run except for these changes. You can override this model check when you need to make adjustments to aid convergence.

Adjusting bolt preloads for the restart run

For bolt preloads, the following restrictions apply:

• You cannot add new bolt preload bulk entries if none existed in the initial run.

• In the initial run, bolts (BOLT bulk entries) are ignored if they are not referenced by at least one bolt preload bulk entry (such as BOLTFOR or BOLTFRC). If a bolt in the initial run has preload values, then in the restart run, you can change the preload values, and you can add new preload values. (SOL 402)

How do I

Generate results to reuse with different analyses (external restart)

Troubleshoot convergence (external restart)

Restart a nonlinear solution (internal restart) (SOL 402)

Learn more

Restarting

Internal restarts (SOL 402)

Quick links

Command reference

Pre/Post video examples

Bulk Entry Descriptions

Simcenter 3D tutorials

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External restarts, Simcenter 3D 2021.1 Series

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