Non-Linear Phenomena in Aero-Engine Rotor Vibration

Abstract

This paper describes a test facility which reproduces the essential features of a twin-rotor assembly for a medium-size jet engine. Its purpose is to investigate phenomena experienced in an actual engine, which relate to system non-linearities. These phenomena include subharmonics, combination oscillations and jumps in frequency response. All such phenomena manifested themselves in the test facility and explanations are given as to their cause and possible elimination.

The prediction of the non-linear dynamics of a squeeze-film damped aero-engine rotor housed in a flexible support structure

“…1, which respectively represent the low-pressure (LP) and high-pressure (HP) rotors of an aero-engine. The LP and HP assemblies can be interconnected through their bearing housings, as in reference [11]. If the rotors are then driven at different speeds, the unbalance on the two rotors produces dual-frequency external excitation of the non-linear system.…”

“…This solution is invariably stable but not necessarily periodic. Equations (11) were numerically integrated using a routine developed for numerically 'stiff ' differential equations [15].…”

A theoretical and experimental investigation of the dynamic response of a squeeze-film damped twin-shaft test rig

“…However, in the case of MFU excitation of a nonlinear system (as in the real aero-engine) the vibrations induced by the individual unbalances are effectively 'mixed', so that the resultant frequency spectrum of the nominal vibration response of a twin-spool engine, for example, will have 'combination frequencies' of the general form k 1 f 1 AE k 2 f 2 where k 1 , k 2 are integers and f 1 , f 2 are the rotor speeds. 8 Each of these frequencies is a potential source of resonance excitation. Such 'combination frequencies' are routinely observed in engine tests 8 and are missed out by SFU nonlinear analysis or MFU linear analysis.…”

“…8 Each of these frequencies is a potential source of resonance excitation. Such 'combination frequencies' are routinely observed in engine tests 8 and are missed out by SFU nonlinear analysis or MFU linear analysis. Moreover, the nominal combination frequency vibration is liable to bifurcation as in the SFU case.…”

“…It is revealed in this article that the choice of ' 2 is highly influential on the predicted response for certain speed ratios but markedly less so for others. With a few exceptions 2,4,8,9 the vast majority of research on nonlinear rotordynamics does not relate to the engine types depicted in Figure 1. Holmes and Dede 8 used the same twin-shaft SFD rig of this study (Figure 2), in which the two independently driven unbalanced rotors are nonlinearly coupled through the flexibly mounted housing of an SFD bearing without a retainer spring.…”