It is known that, under appropriate condition, amplitude of vibration of some blades in mistuned bladed disk system become large compared to what would be predicted by an analysis of the perfectly tuned system. In this paper, the effects of the mistuning pattern on the natural frequencies and the frequency response of mistuned bladed disk system are investigated. It is shown that, in the tuned system, the excitation with the wavenumber k excites only the vibration modes with the same wavenumber

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... and that, in the mistuned system, the excitation with the wavenumber k excites all vibration modes. The variations of the natural frequencies due to mistuning pattern are obtained by using sensitivity analysis. The relation between the mistuning patterns and the split of natural frequencies are discussed. It is noted that the mistuning pattern with the circumferential wavenumber 2k effectively splits the natural frequency of the vibration mode with the circumferential wavenumber k of the tuned system and that the amplitude of the responses of the system with the mistuning pattern with the circumferential wavenumber 2k subject to the excitation with the wavenumber k hardly become large. 1.