Mistuned Higher-Order Mode Forced Response of an Embedded Compressor Rotor: Part II — Mistuned Forced Response Prediction

Abstract

This paper is the second part of a two-part paper that presents a comprehensive study of the higher-order mode mistuned forced response of an embedded rotor blisk in a multi-stage axial research compressor. The resonant response of the second-stage rotor (R2) in its first chordwise bending (1CWB) mode due to the second harmonic of the periodic passing of its neighboring stators (S1 and S2) is investigated computationally and experimentally at three steady loading conditions in the Purdue Three-Stage Compressor Research Facility. A Non-Intrusive Stress Measurement System (NSMS, or blade tip-timing) is used to measure the blade vibration. Two reduced-order mistuning models of different levels of fidelity are used, namely the Fundamental Mistuning Model (FMM) and the Component Mode Mistuning (CMM), to predict the response. Although several modes in the 1CWB modal family appear in frequency veering and high modal density regions, they do not heavily participate in the response such that very similar results are produced by the FMM and the CMM models of different sizes. A significant response amplification factor of 1.5∼2.0 is both measured and predicted, which is on the same order of magnitude of what was commonly reported for low-frequency modes. This amplification is also a strong, non-monotonic function of the steady loading. Moreover, on average, the mistuned blades respond at an amplitude only approximately 40% that of the tuned, much lower than what was commonly reported (75∼80%). This is due to the very low level of structural coupling associated with the 1CWB family of the rotor blisk. In this study, a very good agreement between predictions and measurements is achieved for the deterministic analysis. This is complemented by a sensitivity analysis which shows that the mistuned system is highly sensitive to the discrepancies in the experimentally determined blade frequency mistuning.