Oversampled Modal Approach of a Turbocharger Rotor from the Experimental Lateral Vibrations

Abstract

The elements of rotors have inherent characteristics as geometry and material composition, which causes natural vibrations at frequencies that, due to the rotor unbalance, may coincide with the harmonics of the shaft speed, increasing stress and the probability of fractures even in transient conditions. Therefore, in this work, a theoretical-experimental hybrid method for calculating the natural frequencies and the mode shapes, at rest and non-supported conditions, of a turbocharger rotor is proposed. Firstly, a discrete model of low number of degrees of freedom is considered, and from an oversampled modal approach (OSMA) based on the axial oversampling, sectioning and coupling of the rotor, it is possible to use the oversampled mode shapes to increase the degrees of freedom of the system without major complications in the model. This spatial oversampling criterion is based on the Nyquist-Shannon theorem, and it is used to reduce the error in the estimates of the natural frequencies and to get a first approximation of the mode shapes. The natural frequencies were estimated by the transfer matrix method (TMM) and finite element method (FEM) in order to compare the proposed model results with well-founded numerical methods.