Tech Briefs

Families of Frequencies in Cyclic Symmetry Analysis

The ADINA analysis capabilities using cyclic symmetry are very powerful and have been strengthened further in ADINA 8.5 (see also the Nov. 30 News for the use of mesh gluing in a model using cyclic symmetry in ADINA 8.5). The user can now also select to limit the frequency solution to only some frequencies in the "families of frequencies" in which frequencies are very close and have similar mode shapes and modal stresses. Such families of frequencies are typically encountered in cyclic symmetric structures, e.g., turbines.

Fig. 1: Mesh Used in Problem

The figure below shows graphs of the first 128 natural frequencies of the 32-blade fan model shown above. The frequency groupings are very clear from the figure. The movie above shows the modal stresses for two frequencies in the second frequency group showing very similar modal stress patterns.

Fig. 2: First 128 Frequencies of Fan

Using the new features, the analysis can now be restricted to, for example, only 2 frequencies per family (for 4 families) resulting in the 8 representative frequencies marked in the figure (together with their mode shapes and modal stresses).

Comparison of Solution Time and Size of Results File

	Full Analysis with 128 Frequencies	Restricted Analysis with 8 Frequencies
Solution Time	178 s	10 s
Size of Results File	2.8 GB	0.2 GB

The table above shows the savings in total solution time, and in the size of the results file for the restricted analysis of only the 8 representative frequencies. The savings in solution time and disk space are clearly very large, showing the effectiveness of the ADINA solution.