Modal analysis and testing of rotating structures

This paper surveys the state of the art of modal testing or experimental mo dal analysis of rotating structures. When applied to ordinary, non-rotating structures, modal testing is considered to be well established. Rotating s tructures, on the other hand, impose special difficulties when one seeks to obtain the parameters of the dynamical model experimentally. This paper fo cuses on the necessary experimental techniques and their relationship to th e current state of the existing theory. Existing modal analysis methods, mo dels and techniques, and their advantages, limitations and relevance are ou tlined and compared. In addition, some new developments allowing us to circ umvent some of the above-mentioned difficulties are presented. Rotating machines appear in almost every aspect of our modern life: cars, a eroplanes, vacuum cleaners and steam-turbines all have many rotating struct ures whose dynamics need to be modelled, analysed and improved. The reliabi lity, stability and the response levels of these machines, predicted by ana lytical models, are generally not satisfactory until validated by experimen tally 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 dyn amic analysis of non-rotating and rotating structures are clarified. The im plications of the model structure, in the latter case, on the application o f modal testing are presented, as this is a point of great importance when experimental modal analysis is employed for rotating structures. Models wit h different degrees of complexity are being used for different types of rot ating machines. A classification of such models is outlined in this work an d the underlying assumptions and features are described in terms of a hiera rchical complexity. Several applications of modal testing are reported here and some experimental evidence to support the validity of the theory is pr esented. Desired future activities, which are required to advance the theor y and practice of this field, are summarized in conclusion.