Some Insights into the Importance of Rotational Degrees-of-freedom and Residual Terms in Coupled Structure Analysis

There are several sources of error in predicting a coupled system’s dynamic behaviour. This paper is concerned with two of these many so”rces: exclusion of rotational degrees-of-freedom (RDOF) from coupled analysis and inadequate compensation of residual terms. It is shown that even 1 the rotational coordinates are included, but with inadequate compensation for the high-frequency outof-range modes, an erroneous prediction of the response is still obtained. The RDOFs are normally neglected from experimental modal analysis due to the difficulty in measuring them. Therefore, the results presented here wil l be related with a finite element (FE) model of the structure used, where all six DOFs could be easily obtained. This paper is divided into three main parts: first, the problems related to the RDOFs’ high-frequency residual terms are presented. Second, the problems faced in coupled structure analysis when the RDOFs are not included at all (or are wrongly included) in the analysis are shown. Finally, the FE based results will be compared with experimental results. Problems faced during the tests will be pointed out.