Reduced Order Modelling of Mistuned Integrally Bladed Rotors

Jhansi Reddy Dodda, Purdue University

Abstract

This work aims to study the mode localization behavior of the mistuned rotor, which is the root cause of the unexpected premature fatigue failure. The unsteady loading, flow separations, tip leakage flows, vortex shedding, and acoustic instabilities induce nonlinear blade vibrations and responses. An accurate finite element model can help predict the maximum dynamic response and shed light on the dynamics of the mistuned system, which can lay out guidelines for design and manufacturing processes. Cyclic symmetry structures are generally simplified as Finite Element (FE) models of single sectors for analysis purposes to reduce computational costs. However, inherent blade mistuning breaks the cyclic symmetry, and often the full blisk must be modeled, which has millions of degrees of freedom (DOF), making it computationally too expensive. These simulations are often coupled with Monte Carlo simulations (MCS) and Latin hypercube for the probabilistic analysis of random mistuning, which requires a large sample set, further increasing the computational costs. Previous research and aeromechanical analysis used lumped mass and beam frame assembly models, which were very robust but had a low order of accuracy. This paved the way for developing FE-based Reduced Order Models (ROM). These high-fidelity complex models can capture the simplified nonlinearities in reduced-order models. The CMM (Component Mode Mistuning) and FMM (Fundamental Mode Mistuning) models were studied on the embedded stage of the Purdue 3-Stage axial compressor to understand the accuracy and usability of these methods for regions of interest. A brief comparison between the ROM models is made in this study. Although the FMM model is a simple, accurate model for determining the impact of mistuning on forced response when we have an isolated family of blade modes, the accuracy decreases considerably in cases with strong modal participation from other families. The more complex CMM model is required to study mistuned responses in veering regions, regions with high modal density, and instances of disk-dominated modes. The FMM model estimates the amplification well for mistuning cases with low deviations and high nodal diameters. The CMM model captures the intricate details of the response well and converges rapidly with the increasing number of tuned system modes. The modal participation in the veering regions was also captured reasonably well by CMM. The forced response for cases with small standard deviation was predicted well by both the reduced order models. The effect of the arrangement of the deviations was also explored, which showed significant amplification reduction. This study will guide the future to predict forced response incorporating frequency mistuning and aerodynamic coupling, which would be validated with the experimental data.

Degree

M.E.

Advisors

Key, Purdue University.

Subject Area

Design|Fluid mechanics|Industrial engineering|Mathematics|Mechanics

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