Modal analysis and testing of rotating structures

This paper surveys the state of the art of modal testing or experimental modal analysis of rotating structures. When applied to ordinary, non–rotating structures, modal testing is considered to be well established. Rotating structures, on the other hand, impose special difficulties when one seeks to obtain the parameters of the dynamical model experimentally. This paper focuses on the necessary experimental techniques and their relationship to the current state of the existing theory. Existing modal analysis methods, models and techniques, and their advantages, limitations and relevance are outlined and compared. In addition, some new developments allowing us to circumvent some of the above–mentioned difficulties are presented. Rotating machines appear in almost every aspect of our modern life: cars, aeroplanes, vacuum cleaners and steam–turbines all have many rotating structures whose dynamics need to be modelled, analysed and improved. The reliability, stability and the response levels of these machines, predicted by analytical models, are generally not satisfactory until validated by experimentally obtained data. For this purpose, modal testing has to be employed and further advance is essential in order to overcome the difficulties in this area. In this paper, the differences between the mathematical models used for dynamic analysis of non–rotating and rotating structures are clarified. The implications of the model structure, in the latter case, on the application of modal testing are presented, as this is a point of great importance when experimental modal analysis is employed for rotating structures. Models with different degrees of complexity are being used for different types of rotating machines. A classification of such models is outlined in this work and the underlying assumptions and features are described in terms of a hierarchical complexity. Several applications of modal testing are reported here and some experimental evidence to support the validity of the theory is presented. Desired future activities, which are required to advance the theory and practice of this field, are summarized in conclusion.

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