Internal Superelements

Factorization of assembled matrices is computationally expensive for Implicit Finite Element analysis. The cost is even higher if the factorization has to be repeated multiple times such as for time domain or frequency domain dynamic analysis.

For more information about superelements, refer to Direct Matrix Input (Superelements) in the User Guide.

Standard Superelement versus Internal Superelement

With the Internal superelement process, both DMIG generation run and residual run are performed at the same time.

The main benefit of the Internal superelement process is to be able to retain the model hierarchy, similar to the full model analysis (which does not use any DMIG) and it is very easy to switch from the internal superelement process to full model analysis.

Input Data

Define Superelements in OptiStruct

Consider an example involving four superelements. Before the analysis, all the necessary superelements are created.

Figure 1. Superelement Definition

Each SUBCASE is created with a SUPER and a METHOD card. The SUPER card provides information about the individual superelement (SUPER refers to SESET which defines the interior points for a given superelement), while the dynamic modes for the reduction (Component mode synthesis) are specified using the METHOD card. SUBCASE specific parameter (PARAM,ORIGK4) may be used to replace all the material damping coefficients.

Figure 2. Subcase Definition of Superelements

With internal superelements, all the grids are by default, exterior points. The user selects the proper interior points by SESET for each superelement. For the given element, if the part of grids is chosen as interior points, the rest of grids remain as exterior points, as all the grids are exterior points by default (Figure 3).

Figure 3. Exterior and Interior Points Defined by CSUPEXT

Alternatively, you can explicitly pick certain grids as exterior points using CSUPEXT. This can be used to ensure that the chosen grids remain as the exterior points.

Review Superelements

Each Superelement is stored as a reduced matrix in the form of an .h3d file in the working directory (Figure 4).

Figure 4. Superelement Matrices Stored in the h3d Files

The interior and exterior grids for each Superelement can be reviewed from the <filename>.intsup file created in the working directory (Figure 5).

Figure 5. Interior and Exterior Points for each Superelement. from the .intsup file

The exterior points which are created automatically and the interior points generated by you can be visualized from the created sets in HyperMesh. This method can then be used as a verification for the points that have been created.

Figure 6. Exterior Points Defined by OptiStruct and the Corresponding Set

Figure 7. Interior Points User-Defined and the Corresponding Set

Recovery of Results

The results from internal superelements can be recovered similar to standard superelements (Figure 8 and Figure 9). However, in order to recover displacements, PLOTEL elements must be created inside the internal superelement. The PLOTEL elements would be automatically stored in the same .h3d file, where the reduced matrices are present. After the residual run, the displacement from the PLOTEL grid would be recovered, if displacement output is requested for the PLOTEL grids.

Figure 8.

Figure 9.

Multi-level Superelement Tree

The multi-level superelement tree is used to reduce the number of interface DoFs in subsequent residual runs. It is performed by aggregating a few lower-level components of the tree structure (Figure 10).

Figure 10. Multi-level Superelement Tree

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