Superelement Stress Recovery for Multipel Superelements

[Omkar Kunjir]

Hello everyone,

I hope you are all doing well.

My name is Omkar Kunjir, and I am currently working on my Master’s thesis at Volkswagen AG. I am happy to joing FeResPost Googel group. My research focuses on the Superelement concept for defining connections in structural simulations. 

I come across steh FeResPost documenttaion and also read about the supereleemnt section from the documentation. I am facing some issue which i would like to explain here. i will really appreciate your help.

Problem Description:

I am following two step to use the Spurenelement. 

1. Step 1: Reduction of Superelement

2. Step 2: Using the superelement in the FEM assembly to define connections (multiple superelements in series)

I have created a FEM model with fine meshing and have defined the A-set nodes at the corners. I then reduced this model into an external superelement in the .op2 file, where I store only the reduced stiffness matrix and OTM matrix for stress calculations.

In my assembly, I use the reduced superelement in series to connect two sheets. The A-set node of the superelement is connected to the residual structure via RBE3 elements, allowing it to be considered in the global stiffness matrix. When I run the simulation for the entire structure, I obtain stress results in the .op2 file. However, I am only able to visualize the stresses for the residual structure, but not for the superelement.  

This issue arises because the geometric information of the superelement is lost during the reduction process. Although the stresses for the elements from the superelement are available in the .pch file of the global simulation, I am unable to visualize them in the global .op2 file.  

My Goal:

I would like to recover the stresses for the superelements and visualize them in the main global .op2 file, along with the geometric information of the superelements. This would allow me to post-process and visualize the stresses for the superelements in the same manner as the residual structure.

Request:

Has anyone faced a similar issue or does anyone know how to recover and visualize the stresses for superelements, along with their geometric information, in the global .op2 file?

Any suggestions or examples would be greatly appreciated!

Please refer to the screenshot below for reference to better understand the problem.

 Looking forward to your guidance.  

Thanks & Reagrds;
Omkar Kunjir
M.Sc. CAME, RWTH Aachen University, Germany
+4915758300703

[FeResPost]

Hello Omkar,

As your superelement corresponds to a condensed model, Nastran calculates no stress on the initial detailed model. The only way I know to obtain the stress in the detailed model is to recover interface loads or displacements on SE condensation nodes, and reapply them as loads on the corresponding model. But I know no tool that allows to automate this process and visualize the results.

Regards,

Renaud

[Omkar Kunjir]

Dear Renaud,

Thank you so much for your response.

While condensing the superelement, I am defining specific elements for which I want to calculate the stresses. During the condensation process, the stiffness matrix and reduced OTM matrix are generated, which relate the unit displacement at the A-set nodes to the stresses of the elements I want to evaluate. Once I use the reduced superelement in the assembly and run the simulation, the stresses are evaluated for the elements defined in the OTM matrix, and the results appear in the .pch and .f06 files. However, I am unable to visualize these stresses in the assembly file for superelement, as the geometric information is not available in the condensed superelement. I was hoping the stresses would also be available in the .op2 file, but I am currently unable to read the binary data.

In the FeResPost Documentation, I found that it is possible to map the stresses from the .op2 file into the .bdf file and then save the .op2 file with all the mapped stresses. My idea is to define the connection with the superelement and include additional elements for visualization. These additional elements would not contribute any stiffness to the structure but would allow me to visualize the stresses in post-processing. In this approach, we would have a .bdf file with all the geometric information, and we would already have the stresses in the .op2 file. Therefore, we could map the stresses from the .op2 file to the .bdf file.

An additional challenge I am facing is that I have multiple superelements in series, each containing element IDs. I would need to map these elements along with specific SEID values. I would really appreciate your support in this case, or if you have any similar examples, I would be very grateful. I also wanted to ask you whether it is possible to use FeResPost online-cloud base instead of installation for trial purposes or is there another way to use it ?

Thank you once again for your help.

Mit freundlichen Grüßen, 
Omkar Kunjir
+4915758300703
Linkedin : Omkar Kunjir | LinkedIn

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[FeResPost]

Dear Omkar,

To be honest with you, I am no expert in Nastran superelements. For example, I do not know what an OTM matrix is. So I cannot help you in solving the problems of stress visualisation with condensed models. Maybe you will have better information from MSC Nastran or NX Nastran support.

The only thing that is supported by FeResPost is the partitioning of BDF files + correct (I hope) association of results read to the different partitions. But I am not sure it will help you.

It is not possible to use FeResPost online. On the other hand, the sources and binaries are downloadable for free. So you can install and run FeResPost on your computer.

Regards,

Renaud

Le 12-09-25 à 10:52, Omkar Kunjir a écrit :

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