How to manipulate superelements with DMIGMOD in OptiStruct

viniciusgomes

Table of Contents

• Models
• Use Case
• Format and Definition
• Example and Validation
  • Superelement Generation
  • Residual Run with DMIGMOD
  • Validation with Full Mesh model
• Useful links

Models

 

The models used in this article can be found here: Models

Use Case

Superelements are a very useful technique to simplify complex models. There are several benefits to using them such as reducing computational cost and runtime, improved model management, and security.

To maximize the advantages of superelements, OptiStruct has the DMIGMOD feature which allows you to modify, manipulate, and move an existing H3D superelement within your residual model. A few application examples are the layout of battery packs, tyre-and-wheel assembly in a car, seats in a plane, and agricultural machinery sub-systems.

This article explores the use cases and applications of DMIGMOD.

 

Format and Definition

DMIGMOD is represented within OptiStruct by the following format:

Since it has multiple parameters and possible inputs, in this article we will highlight the most important ones. For a detailed description of each one of them, please refer to our documentation: https://help.altair.com/hwsolvers/os/topics/solvers/os/dmigmod_bulk_r.htm

 

- MTXNAME: Matrix name defined on ASSIGN,H3DDMIG

- SHF_____: All fields in the first line starting with the SHF prefix represent an ID shift for a particular entity of the superelement (grids, SPOINTs, elements, coordinate systems, …). This is useful if you need to simply renumber the superelement due to an ID conflict or if you plan to use the superelement several times in the residual model.

 

- GIDMAP + GIDn, GIDnA: Define mapped grid ID pairs. GIDMAP is to renumber the interface grid IDs such that they match the IDs used in the residual structure.

 

- RELOC + PAn, PBn: Use to reposition the superelement in the residual run, pointing grid ID pairs in the residual structure (PA1, PA2, PA3) and superelement (PB1, PB2, PB3). MIRROR flag can be used for reflecting the superelement.

- GRDTOL: Defines the tolerance settings between matching residual and superelement grids. The default tolerance setting is 1E-15, OptiStruct will error out if this value is violated.

 

Example and Validation

As an example, we will use the following model, which can be found at the beginning of this article:

The model consists of a modal analysis of a structure with four similar components attached to it that we will call instances (green, blue, purple, and gray components). These instances will be replaced by superelements with the DMIGMOD feature.

The process will be divided into 3 steps: running the full model as a reference, creating the superelement of only one instance (INST1), and running the reduced model with the superelement. To facilitate our job later, the interface nodes between the instances and the structure were renumbered as below:

 

Superelement Generation

In this step, we will create a superelement only for the green component (Instance 1).  For that, there are no extra or special inputs necessary, as it is done in a traditional reduction. ASETs will be defined at interface nodes and CBN method will be used with CMSMETH load collector. MODEL card is created to recover results later in the residual run. Refer to 02_Superelement_INST1\superelement_inst1.fem in the attached zip for more details.

 

Residual Run with DMIGMOD

 

For the residual run, we will import the 01_Full_Mesh\baseline_full_mesh.fem and delete all four instances of the model:

 

The next step is to import the superelement generated of Instance 1 four times since we will replace all four instances with the same superelement. This can be done through the ASSIGN, H3DDMIG entry as follows:

Note that INST1, INST2, INST3, and INST4 are the matrix names that we will use to reference in DMIGMOD.

Now, INST1 superelement is the original one and does not require any modification. Using the DMIGMOD we will modify and reposition the other three superelements as follows:

 

For INST2:

The original nodes from the INST1 superelement need to be mapped with GIDMAP and then relocated with RELOC to INST2 position.

 

DMIGMOD	MTXNAME INST2	SHFGID 2000001	SHFSPID 2000001	SHFSPID_F	SHFCID	SHFEID 2000001	SHFRID 2000001
+	GIDMAP	GID1 101	GID1A 201	GID2 102	GID2A 202	GID3 103	GID3A 203
+	GIDMAP	GID4 104	GID4A 204
+	RELOC	PA1 201	PA2 202	PA3 203	PB1 101	PB2 102	PB3 103
+	GRDTOL	ERREXT WARN	TOLEXT 1E-1

 

For INST3 and INST4:

For the other two instances, the process is quite similar, adding only the MIRROR flag in RELOC line since we need to reflect them to the other side of the model.

 

DMIGMOD	MTXNAME INST3	SHFGID 3000001	SHFSPID 3000001	SHFSPID_F	SHFCID	SHFEID 3000001	SHFRID 3000001
+	GIDMAP	GID1 101	GID1A 301	GID2 102	GID2A 302	GID3 103	GID3A 303
+	GIDMAP	GID4 104	GID4A 304
+	RELOC	PA1 301	PA2 302	PA3 303	PB1 101	PB2 102	PB3 103	MIRROR
+	GRDTOL	ERREXT WARN	TOLEXT 1E-1

 

DMIGMOD	MTXNAME INST4	SHFGID 4000001	SHFSPID 4000001	SHFSPID_F	SHFCID	SHFEID 4000001	SHFRID 4000001
+	GIDMAP	GID1 101	GID1A 401	GID2 102	GID2A 402	GID3 103	GID3A 403
+	GIDMAP	GID4 104	GID4A 404
+	RELOC	PA1 301	PA2 302	PA3 303	PB1 101	PB2 102	PB3 103	MIRROR
+	GRDTOL	ERREXT WARN	TOLEXT 1E-1

 

In the deck, this will look like:

 

The solved model can be found in 03_Residual_DMIGMOD\residual_DMIGMOD.fem. In the .out file, it is possible to check if all superelements were successfully processed:

 

Validation with Full Mesh model

 

To validate the methodology and results, we can compare baseline_full_mesh.h3d with residual_DMIGMOD.h3d results.

 

Mode	Baseline Model - Full Mesh	Residual Model with DMIGMOD	Error (%)
1	16.48	15.91	3.5%
2	57.11	55.36	3.1%
3	70.40	69.70	1.0%
4	78.73	77.31	1.8%
5	80.82	80.44	0.5%
6	124.56	123.53	0.8%
7	192.78	192.00	0.4%
8	232.72	232.24	0.2%
9	410.06	409.85	0.1%
10	413.66	413.31	0.1%

 

 

Useful links

 

DMIGMOD

Direct Matrix Input (Superelements)

Superelement tutorial