Intentional mistuning effect in the forced response of rotors with aerodynamic damping

Intentional mistuning is a well known procedure to attenuate the negative effect of the existing random mistuning on the forced response of a turbomachinery rotor. Intentional mistuning can significantly decrease the vibration amplification due to the underlying random mistuning, and, what is perhaps more interesting, it can also reduce the rotor response high sensitivity to random mistuning. The idea is to introduce an intentional mistuning pattern that is small but still much larger than the existing random mistuning. This moves apart the frequency of adjacent blades, reducing the effective blade-to-blade coupling and thus the possibility of response amplification. The situation considered in this work corresponds to the case of forcing a blade dominated modal family, where all tuned modes have very similar vibration frequencies. The main source for the damping is assumed to be the effect of the aerodynamic forces; as it typically happens in an integrally bladed rotor with no damping coming from friction at the contact interfaces. This configuration is more complicated than the usual case of uniform material damping, because now the aerodynamic damping is clearly defined for the travelling wave vibration modes of the tuned rotor, but not for each single blade. The Asymptotic Mistuning Model methodology is applied to analyze this problem. It produces a very simplified model that gives information on the action mechanism of intentional mistuning and on its applicability; which is seen to be related to the relative magnitude of the elastic and aerodynamic coupling influence coefficients compared to the blade damping. The results from the reduced model are successfully compared with those from the finite element analysis of two rotor models with different intentional mistuning patterns and forcing configurations.