Impact of Propagating Cracks and Rub-Impact on Whirl Orbits and Post-Resonance Backward Whirl in Rotor Systems

Abstract

Fatigue cracks and rotor-stator rub-impact are some of the major faults that affect the proper functioning of rotor systems. Recurrent transition through resonance rotational speeds during runup, coast-down, and transient operation affects the development of faults in rotor systems. Moreover, the existence of such faults could excite the pre- and post-resonance backward whirling (Pr-BW and Po-BW), respectively. The Po-BW immediately appears after the faulty rotor system passes through its resonance rotational speed/speeds. As such, the existence of faults can be directly correlated with the excitation of backward whirling (BW), especially the Po-BW, which makes fault detection more reliable. Once the fault is accurately detected and identified, further damage can be potentially prevented, which saves human lives and equipment. The feasibility of Po-BW for crack detection was substantiated in a few recent research publications. However, the overall effect of multiple faults on BW still requires thorough analysis, which will be the focus of the present research. Robust numerical and experimental analysis is conducted to study the impact of multiple faults, such as crack and rotor-stator rub-impact, on Po-BW excitation. Accordingly, the findings indicate the existence of three types of BW phenomenon, which are categorized as pre-resonance BW, resonance BW, and Po-BW. These new findings provide additional information regarding the BW phenomenon when different kinds of faults are present in rotor systems. Additionally, the effects of unbalance forces on shaft’s whirling orbits in the presence of an open crack are carefully studied. It was established in previous literature that as the rotor system passed through 1/n (n=2, 3...) of the critical resonance speed, there is the presence of inner/outer loop patterns in the shaft’s whirl orbits. However, the present study provides new findings regarding the main factor attributing to the appearance of the inner loop pattern in the whirl orbit. The finite element model of the cracked rotor system along with an experimental analysis were conducted to verify the fundamental role of the unbalance force magnitude on the inner loop whirl orbit pattern and its associated pre-resonance and post-resonance rotational speeds. The findings of this research work potentially add significant insights to damage detection in rotordynamics field.

Date of Award	19 Jul 2024
Original language	American English
Supervisor	Mohammad Alshudeifat (Supervisor)