Investigation Of The Dynamical Characteristics Of An Aircraft Engine High-pressure Rotor-hollow Shaft Crack System

The rotor and its support system are the core component of the aircraft engine.An aircraft engine high pressure rotor system is investigated in this paper. A hollowshaft rotor system is modeled, and the nonlinear dynamic characteristics of thesystem with a transverse crack is studied. The main contents are as follows:First, a linearized discrete model with the multi-disc and flexible shaft is builtaccording to structural characteristics of the aircraft high pressure rotor system. Andthe critical speed, modal shape, orbit of shaft centerline and the whirl frequency ofthe rotor system are calculated by the rotor dynamics finite element method for theRayleigh beam-shaft model and the Timoshenko beam-shaft model. Comparing withthe numerical simulation results for the commercial finite element software ANSYS,it is shown that the Timoshenko beam-shaft model system is more accurate andsuitable for the calculating of the dynamic characteristic for the aircraft highpressure rotor system.Then, aircraft high pressure rotor system is simplified based on the―momentconcentration‖and―reduce the inertia parameter‖principle for the complex discretemodel with the Timoshenko beam-shaft, and we obtained two simplified models:single-span with double-disc model and Jeffcott model. Using the finite elementmethod we calculate the dynamic characteristics for two simplified models, andcompared with the complex model, it indicates that the simplification is successfulin the error range, and we can select the proper simplified model for the nonlineardynamics qualitative analysis with the analytical method of the rotor system.And then, we built a single-span with double-disc finite element model for theaircraft high-pressure rotor system containing a transverse crack. Based on theprinciple that the neutral axis of the crack cross-sectional is time-varying,we solvedthe stiffness matrix of the rotor system, the object of study for solid Rayleighbeam-shaft open cracked rotor system is extended to the hollow Timoshenkobeam-shaft open cracked rotor system. Harmonic balance method(n=4) is used tosolve the differential equations of motion for the system. The critical speed of thesystem and other dynamic characteristics varied with the depth and the location ofthe crack section is analyzed and the results indicate that the exist of cracks willreduce the critical speed of the system, and for a asymmetric model, the small cracklocated near the bearings has small impact on the system because of the constraintsdominant, the deep crack located near the disc with a larger inertia parameters hasgreater impact on the system. Meanwhile, the nonlinear vibration response of thesystem is computed when rotational speed pass through the critical and sub-critical (c/n, n2,3,4) speed of the system, and we obtained the orbit of shaft centerline,three-dimensional amplitude-frequency curve and three-dimensional spectrogramwith non-dimensional crack depth of the system and the results indicate that thesystem nonlinear vibration response has high-frequency component at differentrotational speeds, and the3X harmonic component is always existed, but the2X and4X frequency may sometimes weaken or even disappear, while the orbit of shaftcenterline veering significantly when the speed is near the sub-critical speed. Inaddition, the impact for the orbit of shaft centerline is analyzed with the change ofthe crack initial position angle and the unbalance mass angle.Finally, we took the breathing crack model into consideration, also built asingle-span with double-disc finite element model for the aircraft high-pressurerotor system containing a transverse crack, and used the Harmonic balance method(n=4) to solve the equation too. By analyzing the three-dimensionalamplitude-frequency curve of the cracked rotor system and comparing with the openmodel, we find that at the speed of2b1/3there is also a resonance peak only forthe breathing model, the nonlinear vibration and the orbit of shaft centerline and soon have the same phenomenon for both open model and the breathing model.