Managing propagation of site-wide disturbances using model predictive control with forecasting

Abstract

In order to obtain higher aerodynamic efficiency and lower noise, novel architectures are being explored in the aero engine field. These designs include shorter, stiffer shafts carrying longer and more flexible blades; the shaft is in turn supported by complex structures. These arrangements imply that the dynamic behaviour of shaft and blades is no longer uncoupled and a dynamic interaction between the modes of the subcomponents can appear. This dynamic interaction can lead to high level of vibration in unexpected places, which can cause a decrease in performance and, ultimately, fatigue and wear of components.

In a shaft-disk-blades flexible assembly supported by linear bearings, the blade 1ND (Nodal Diameter) modes are known to couple with the shaft lateral modes, whilst the 0ND "umbrella" modes couple with the shaft axial modes. The objective of this research is to investigate if other families of coupled modes can appear in the presence of oriented bearing supports, which couple shaft axial and lateral displacements.

This phenomenon is investigated via different FE models, which consist of a flexible shaft carrying a single flexible disc, two flexible discs and a flexible bladed disc, respectively. It is found that, if the support structure of the shaft is asymmetric, some lateral modes couple with the axial mode, giving rise to the so called "mixed modes", which share both axial and lateral dynamic behaviour. These modes then in turn excite the disc and blades 0ND and 1ND modes simultaneously, giving rise to new families of coupled modes. If the blades are staggered, an additional coupling with the shaft torsional dynamics can be predicted.

In the second part of this work, an asymmetric bearing support structure was designed and installed on an existing rotor-dynamic rig, in order to provide an experimental validation for the numerical models. The experimental activity proved the hypothesis that an asymmetric bearing support structure can lead to a coupling between shaft bending and axial vibration, which then excites the disc 0ND and 1ND modes. Additionally, it was found that a coupling between shaft bending/axial modes and higher disc NDs can occur if the disc is mistuned.