Simcenter 3D Solutions How to output residual forces in a SOL 401 analysis and visualize them with Simcenter 3D

Residual Forces can be useful for troubleshooting convergence issues in nonlinear analyses. For instance, in a contact analysis, plotting Residual Forces may help identify problematic contact regions with high values that do not decrease or only oscillate as the solution iterates. In such cases, softening the contact stiffness in these regions may improve convergence.

This article is divided into two sections: how to request residual forces and how to visualize them.

1. How to request residual forces

Unlike most results, which are are requested in the case control section of a Nastran deck, Residual Forces in a SOL 401 analysis are requested with the NRRFOUT parameter located in the NLCNTL card.

To modify this parameter in Simcenter 3D, right-click the solution in the Simulation Navigator and select Edit. Then, in the Solution dialog box, go to the Case Control tab and edit the "Nonlinear Control Parameters (NLCNTL)". This will open the Nonlinear Control Parameters dialog box. In this dialog box, navigate to the General Solution tab where you will find the NRRFOUT parameter. Refer to the images below for guidance.

To request the output of Residual Forces, adjust the NRRFOUT parameter value according to the Simcenter Nastran documentation: SOL 401 - Residual output. For example, setting NRRFOUT=-1 will output the residual forces for all iterations of the last computed time step.

After running the SOL 401 analysis, you will notice that the Residual Forces results are outputted as universal files (*.unv) rather than being included in the .op2 file. Therefore, additional steps are required for post-processing them in Simcenter 3D.

2. How to visualize the residual forces

In order to visualize the universal files, which contain the Residual Forces, you have to use the tool called "Add Companion Result Data". For that, in the Simulation Navigator, right-click the target results node and select "Add Companion Result Data". Then, in the dialog box, specify the location of the *.unv file that you wish to import. Refer to the following images for reference:




After adding the companion results, these will appear as a node under the target results node in the Simulation Navigator. When you open the results in the Post-Processing Navigator, the companion result will be listed under the solution node, allowing you to view the companion results alongside the standard solution results. See the following image for exemplification:

 

For further information on this command, you can refer to the Simcenter 3D documentation: Adding companion result data

Finally, it is important to mention that it is also possible to monitor the evolution of the Residual Forces using the Solution Monitor. However, to save the residual forces for post-processing, you have to change the default value of the NRRFOUT parameter (NRRFOUT = 0).



For more information about residual forces, the reader can be referred to the Simcenter Nastran Multi-Step Nonlinear User's Guide (SOL 401 and SOL 402)