Accurate Dynamical Modelling For Dual-rotor Aero-engine System And Its Responses Induced By Rub Impact

In the field of aeronautic and aerospace engineering,increasing engine trust-weight ratio and reducing fuel consumption have been the main concerns for developing aeroengine performance.In general case,some measures such as reducing the allowable stress margin of the structural parts and improve air tightness are utilized to accomplish above purposes.However,those measures have led to the development of aero-engines toward more compact layout and complicated structure,and made the dynamical characteristics of aero-engine more sensitive to operating conditions,rub-impact as well as other faults is more likely to occur.Currently,the study of vibration characteristics for aero-engine system are mostly based on low dimensional rotor dynamical model with simple structure,such model can’t meet the requirement of actual aero-engine design,because of its oversimplification.Thus,carrying out research on high precision dynamic modelling for aero-engine rotor system,solving the nonlinear responses under typical rub-impact quantitatively,clarifying the response stability and its fault vibration mechanism,can provide important theoretical significance and practical engineering value to the structural design,fault detection and simulation of aero-engine rotor system.In this paper,taking the actual aero-engine dual-rotor system with complex structure and rub-impact faults as research object,the dynamic property is studied from threedimensional finite element modelling,model reduction,solving method of high-dimensional nonlinear dynamic system,response characteristic and its bifurcation mechanism under rub-impact condition,numerical simulation and experimental verification of dual-rotor system,respectively.The main contents and achievements derived in this dissertation are listed as follows:High accurate dynamical modelling and its dimensionality reduction are studied for the dual-rotor system of aero-engine with complex structure.Firstly,the applicable conditions of equivalent particle model and equivalent rigid ring model substituting for blade structure are given after comparing with the vibration characteristics of the blade-disc rotor model.And then,a solid FE model having high precision is proposed for the a real aero-engine dual-rotor system based on the combined modelling method developed in this paper,which refers to solid FE modeling strategy and Craig-Bampton modal synthesis.The accurate mass matrix,stiffness matrix and gyroscope matrix of the reduced dimensional model are obtained by utilizing programming language.Finally,the critical speed characteristics of the real aero-engine dual-rotor system are investigated through reduced dimensional model.The analyzing results show that the differences of the critical speed values between co-rotating and counter-rotating dual-rotor system are mainly caused by gyroscopic torque.The proposed method can be used to guarantee the calculation accuracy and can reduce the calculation time greatly,and improve the computational efficiency.The solution and analysis of the vibration responses is investigated for the dualrotor system induced by rub-impact.Due to the rub-impact model involves segmentation and fractional exponential nonlinear characteristics,the traditional approximate analytic methods are difficult to deal with this type of problems.The combined solving and analyzing method is presented using by multiple harmonic balance method-alternating frequency/time domain technique-Hsu algorithm for Floquet stability(MHB-AFT-Hsu).The nonlinear response characteristics and bifurcation property of the dual-rotor system with four-point support form and rub-impact are investigated.The results show that MHBAFT-Hsu method can be used to solve nonlinear rotor dynamical system with high dimension and complex structure quickly,the whole solution including unsteady solution can be obtained accordingly.It is found that there exists secondary Hopf bifurcation which leads to the transformation between periodic solutions and almost periodic solutions,and saddle-node bifurcation which causes jump phenomenon of the response amplitude in the vibration responses of this dual-rotor system induced by rub-impact.The quantitative solution and response analysis are presented for the real aero engine dual-rotor system with rub-impact.Considering the local nonlinear characteristics of rub impact faults,the reduced dual-rotor dynamical model with rub impact is established accurately by means of the concept of connecting substructure and modal synthesis technique.In order to obtain high precision responses under rub-impact condition quickly,a new quantitative calculation method based on approximate analytic method is proposed.The results show that the radial displacement of the steady-state response for the aero engine dual-rotor system with rub-impact has sine or cosine characteristics.The response trajectory is affected by rub-impact forces in two aspects: changing the radial amplitude of the vibration response and its symmetry axis,causing cutting amplitude and the emergence of harmonic frequencies.In some cases(for example,the vibration responses in the second order resonance region aroused by unbalance excitation of high pressure rotor in fourth chapter),there may be a phenomenon that rub-impact causes the response amplitude increase.The stress value and its distribution in the aero-engine dual-rotor system with rub-impact are periodically changed,its periodic frequency and stress level are significantly correlated with rotational speed ratio and unbalance mass.The numerical simulation and experiment verification of the casing-dual-rotor test rig are carried out to verify the validity of the combined modeling method in corresponding to solid FE modeling and Craig-Bampton modal synthesis,and the MHB-AFT solving algorithm embedded with arc-length continuation strategy.The accurate solid FE model of this test rig is established,and the reduced rotor dynamical model with 58 DOF is obtained accurately.The vibration characteristics of the dual-rotor test rig are investigated by numerical calculation and experiment test.The results show that theoretical simulation results agree well with experimental results,thus it is concluded that both the combined modeling method and MHB-AFT solving algorithm developed in this paper are accurate effective.