Using intentional mistuning in the design of turbomachinery rotors

The maximum blade forced response amplitudes for mistuned turbomachinery rotors are generally much greater than those of their tuned counterparts. However, it is known that the ratio of mistuned to tuned maximum vibration amplitudes, the amplitude magnification, is often largest at a relatively small level of mistuning. Increasing the level of mistuning beyond this critical value actually leads to a decrease in the amplitude magnification. This suggests that it might be beneficial to introduce some level of mistuning into the nominal design of the system intentionally. In this study, the effectiveness of this intentional mistuning strategy is investigated. Intentional mistuning is introduced into the rotor design by varying the nominal blade stiffnesses in harmonic and square-wave patterns. In addition, the unavoidable, random mistuning of the blades is included in the model as usual. The statistics of the forced response are examined for lumped parameter models, as well as for a finite element based reduced-order model of an industrial rotor. For the cases considered, it is found that intentional mistuning can greatly reduce a rotor's sensitivity to random mistuning.