Transient Forced Response of a Mistuned Blisk Rotor in a Multistage Axial Compressor

Abstract Rotor forced response is a major aeromechanical problem causing blade high cycle fatigue, and mistuning can significantly increase the maximum blade vibrational amplitude due to mode localization. Most of the previous mistuned forced response experimental studies were conducted at slow rotational speed sweep rates through the resonance and analyzed utilizing the steady state assumption. However, in practice, engine sweep rate can be significantly high such that the classical steady state forced response analysis becomes invalid. To study the transient forced response in a realistic environment, a comprehensive experimental study was conducted in the Purdue three-stage axial research compressor by measuring rotor blade response at four different sweep rates in both acceleration and deceleration. An overall peak amplitude reduction and peak frequency shift were observed at higher sweep rates, along with a transient beating occurring at the highest sweep rate. Initially, an analytical transient analysis for a single degree of freedom (SDOF) system was conducted. While it explains the major features of the transient response well, it fails to accurately predict the amplitude change at different sweep rates. The transient effect differences measured between acceleration and deceleration are also completely missed by the SDOF analysis. Therefore, a transient mistuned rotor analysis was performed utilizing the Fundamental Mistuning Model (FMM). Through a mode superposition approach, the transient mistuned rotor analysis shows that the difference measured in the acceleration and deceleration cases was caused by the relative phase change of the dominant modes contributing to the total resonant response.