Comparison of the numerical accuracy of Superelements and FRF Assembly

As presented by Markus Herbst from BETA CAE Systems at the 9th BEFORE REALITY Conference.

Abstract:
Dynamic sub-structuring (DS) has always played a significant role in the field of structural dynamics. Performing the analysis of a structural system component wise instead of solving the entire system as a whole offers several advantages as it reduces the analysis time and facilitates additional root cause analysis types.

The DS techniques can be divided in two classes, the Component Mode Synthesis (CMS) methods and the FRF based sub-structuring (FBS) methods. In the first method components are represented by their modal contend and a well-known example is the Superelement (SE) technique implemented in many FE solvers. In the latter method the components are described by their Frequency Response Functions (FRF). The FRF-Assembly tool of META deploys an FBS method and it is used in the assembly process of NVH-Console, the NVH framework offered in ANSA.
In this presentation primarily the numerical accuracy of the Superelement technique (CMS) and the FRF-Assembly method of META (FBS) is investigated and compared for typical vibro-acoustic numerical FE analyses of automotive vehicles.  

One aspect of the investigation is the role of damping assumptions commonly used in FE calculations. It is demonstrated that the use of structural damping is an important enabler of achieving a good numerical accuracy for all investigated DS techniques.
It will also be shown that another important aspect is given by the type of boundary conditions used for the solution of the individual components. In the FRF assembly process implemented in META all components are unsupported. In contrast, most Superelement analyses are performed using the well-known Craig Bampton approach with fixed interface degrees of freedom. It will be presented that this is another crucial aspect for achieving a good agreement between the different DS techniques and the results obtained, if the entire system is solved in a one-shot run.


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