Dynamics of mistuned bladed disks: Modeling and experiment.

Abstract

The effect of random blade mistuning on the dynamics of bladed disks is investigated. A reduced-order model (ROM) formulation is presented for examining the forced response of tuned and mistuned bladed disks. The technique uses modal information obtained from highly detailed finite element models to create, in a systematic manner, much simpler and computationally inexpensive models of bladed disks. In order to verify the validity of the ROM formulation, and to provide benchmark experimental data on the localization phenomenon in bladed disks, the effects of random blade mistuning on the free and forced dynamic responses of geocentrically simple bladed disks are investigated experimentally. Two experimental specimens are considered: a nominally periodic twelve-bladed disk with equal blade lengths, and the corresponding mistuned bladed disk, which features slightly different blades of random length. The free response experiment concentrates on the transition from spatially extended mode shapes to localized mode shapes as the number of natural frequencies in a given frequency region increases. The forced response experiment quantifies the increase in blade response amplitudes due to mistuning. In particular, the effect of localized mode shapes, engine order excitation, and disk structural coupling on forced response amplitudes is reported. This work reports one of the first systematic experiments carried out to demonstrate the occurrence of vibration localization in bladed disks. After verifying the validity of the ROM formulation with finite element and experimental data, the effects of random blade mistuning on the dynamics of an advanced gas turbine rotor are reported. Both free and forced responses of the rotor are examined using the finite element method and the reduced-order modeling technique. The spatially extended free modes of vibration of the tuned rotor are found to undergo severe localization upon the introduction of blade mistuning. In turn, this results in dramatic displacement and stress amplitude increases in the forced response of individual blades. The mistuned forced response amplitude is found to vary considerably with mistuning strength and the degree of aerodynamic and disk structural coupling between the blades.