Rub-impact Rotor System Research Articles

Analysis of Reverse Whirl Characteristics of a Rub-Impact Rotor System

The rub-impact fault between the rotor and stator of the aeroengine can lead to a reverse whirl phenomenon, which will induce the instability of the rotor system. This paper uses the bending–torsion coupled vibration signal to analyze the reverse whirl characteristics of the single-rotor system under single-disc and multidisc rub-impact faults. In the full-spectrum analysis of the bending direction, the reverse whirl frequency, which is the nonpower frequency component caused by rubbing, is found. This whirl frequency leads to the period-doubling bifurcation and the reverse whirl phenomenon of the system. In the analysis of the torsional direction, it is found that the torsional signal can better reflect the different types of rub-impact faults. Further research shows that the unbalanced phase between the rubbing discs is a crucial parameter influencing the bending–torsion coupled vibration and the whirl characteristics of the rotor system. An experimental platform with nonuniform clearance is designed and manufactured to reproduce the reverse whirl phenomenon. The reverse whirl motion near the second-order superharmonic resonance speed is designed to verify the mechanism of the reverse whirl.

Improved frequency sweep modeling method based on model prediction output error for rub-impact rotor system

Improved frequency sweep modeling method based on model prediction output error for rub-impact rotor system

Transient state analysis of a rub-impact rotor system during maneuvering flight

Maneuvering flight substantially affects the dynamic behavior of rotors; particularly, such flight may cause rubbing between a rotor and stator, which is one of the most serious damages in aircraft engines. In this paper, a nonlinear dynamic model for describing the dynamic characteristics of a rub-impact rotor system during maneuvering flight is established based on the Lagrange equations. Subsequently, numerical simulations employing the Newmark method are performed, delving into the detailed discussion of the influence of parameters such as rotational speed and maneuvering flight on the transient and steady-state responses of the rotor system. The effect mechanism of maneuver load and its coupling with rub impact is revealed. The results show that the impact response induced by maneuvering flight is more obvious in the subcritical state than in the supercritical state. The additional stiffness and damping are also induced; in particular, the additional damping has a coupling effect. Moreover, the rub impact imposes an additional constraint on the rotor system, thereby weakening the influence of the maneuver load and becoming the major factor that determines the dynamic behavior of the rotor system at high speeds.

Research on a quantitative fault diagnosis method for rotor rub-impact

The rotor system during its operation is susceptible to various faults such as unbalance, rub-impact, crack, and misalignment, which usually induce the rotor system to exhibit nonlinear behavior. Some linear diagnosis methods are unable to extract nonlinear characteristics of the faulty rotor system. However, existing nonlinear fault diagnosis methods can describe the nonlinear characteristics but cannot quantitatively indicate the severity of rub-impact faults. To address this issue, this study combines the nonlinear output frequency response functions weighted contribution rate (WNOFRFs) and JS divergence to develop an improved fault diagnosis approach, WNOFRFs based on the JS divergence (WNOFRFs-JS). And a superior NOFRFs-associated index JSRm is developed to indicate the severity of faults. In addition, a sensitive factor is defined to evaluate the sensitivity of the index. The performance of this approach is verified by an established dynamic model and a rotor rub-impact experimental rig. The results prove the effectiveness and merits of this approach for the identification of rotor rub-impact. JSRm is especially sensitive to rub-impact and can also quantitatively detect the severity of faults. The present approach can accurately and quantitatively identify the rub-impact rotor system. These advantages enable the improved WNOFRFs to be applied in the fault diagnosis and condition monitoring of rotating machinery and even other nonlinear systems.

Dynamic modeling and response analysis of rub-impact rotor system with squeeze film damper under maneuvering load

With the development of the aero-engine rotor system towards accurate modeling, it is necessary to establish a general rotor system model considering many important strong nonlinear factors. In this paper, based on the finite element method, a 6-node rub-impact rotor system model including the newly-built blade-casing rub-impact force, climbing maneuvering load, nonlinear Hertz contact force of rolling bearing, rotor eccentric unbalance force, gravity field and squeeze film damper oil film force is established. The dynamic responses of the system under different rub-impact stiffness, different oil film clearance and different bearing clearance are analyzed in detail by means of bifurcation diagram, three-dimensional spectrum diagram and axis trajectory diagram. Through comparative analysis, it is found that the change of the oil film clearance has the greatest influence on the rub-impact response of the system, followed by the bearing clearance, and the change of the rub-impact stiffness has the least influence on the rub-impact response of the system. The study of this paper can provide a theoretical basis for the rub-impact response of the rub-impact rotor system with squeeze film damper under climbing maneuvering flight.

Uncertainty quantization and reliability analysis for rotor/stator rub-impact using advanced Kriging surrogate model

This study provides new insights into the uncertainty quantification and reliability analysis of a flexible rotor system undergoing rub-impact. A probabilistic nonlinear formulation is proposed by considering the parameter uncertainties of different random distribution characteristics. The harmonic balance and alternating frequency/time (HB-AFT) scheme are employed to analyze the nonlinear behaviors of the system. To avoid the computationally intensive nonlinear calculations, the efficient Kriging surrogate strategy is adopted for the propagation of uncertainties in rotor dynamics, which enables rapid estimation of the statistics of nonlinear responses. Then, the probability density function (PDF) is evaluated for reliability analysis to determine the conditions for the occurrence of the rub-impact fault. Numerical results are compared with experimental results and those obtained by Monte Carlo simulation (MCS) to verify the effectiveness of the new modeling. Effects of stochastic parameters on vibrational behaviors and reliability of the rub-impact rotor system are finally performed, which demonstrates the proposed method is high-efficiency and high-precision for predicting the high-nonlinearity between output response and input variables. This contribution will further enrich the theory and application of probabilistic statistical analysis and reliability evaluation for complex rotating machinery.

Coupled Lateral and Torsional Vibration of Rub‐Impact Rotor during Hovering Flight

When aircraft make a maneuvering during flight, additional loads acting on the engine rotor system are generated, which may induce rub‐impact faults between the rotor and stator. To study the rub‐impact response characteristics of the rotor system during hovering flight, the dynamic model of a rub‐impact rotor system is established with lateral‐torsional vibration coupling effect under arbitrary maneuvering flight conditions using the finite element method and Lagrange equation. An implicit numerical integral method combining the Newmark‐β and Newton–Raphson methods is used to solve the vibration response. The results indicate that the dynamic characteristics of the rotor system will change during maneuvering flight, and the subharmonic vibrations are amplified in both lateral and torsional vibrations due to maneuvering overload. The form of the rub‐impact is different during level and hovering flight conditions: the rub‐impact may occur at an arbitrary phase of the whole cycle under the condition of level flight, while only local rub‐impact occurs during hovering flight. Under the both flight conditions, the rub‐impact has a large effect on the spectral characteristics, periodicity, and stability of the rotor system.

Nonlinear stochastic dynamics of a rub-impact rotor system with probabilistic uncertainties

This paper presents a stochastic model for performing the uncertainty and sensitivity analysis of a Jeffcott rotor system with fixed-point rub-impact and multiple uncertain parameters. A probabilistic nonlinear formulation is developed based on the combination of the harmonic balance method and an alternate frequency/time procedure (HB-AFT) in stochastic form. The non-intrusive generalized polynomial chaos expansion (gPCE) with unknown deterministic coefficients is employed to represent the propagation of uncertainties on rotor dynamics. In conjunction with the path continuation scheme and the Floquet theory, the developed model enables one to expediently evaluate the uncertainty bounds and probability density functions (PDFs) on periodic solution branches and associated stabilities. A global sensitivity analysis is then carried out by evaluating Sobol’s indices from gPCE to quantitatively ascertain the relative influence of different stochastic parameters on vibrational behaviors and conditions for the occurrence of rub-impact. The efficiency of the proposed algorithm for nonlinear stochastic dynamics of rub-impact rotors is validated with Monte Carlo simulation. Parametric studies are finally carried out to investigate the effects of multiple random parameters on the probabilistic variability in nonlinear responses of rub-impact rotors, which reveals the necessary to consider input uncertainties in analyses and designs to ensure the sustainable system performance.

Mitigation of nonlinear rub-impact of a rotor system with magnetorheological damper

In this work, a new method of using a magnetorheological damper to improve the self-adaptive ability of a rotor system to rub-impact is developed. To validate the feasibility of this method, a finite element model of the rub-impact rotor system with magnetorheological damper is investigated. A revised formula describing the relationship between the yielding shear stress and magnetic field intensity is proposed. Focusing on the mitigation effect of magnetorheological damper on the nonlinear dynamic response of the rotor system, numerical simulation is conducted. The results show that magnetorheological damper has a considerable effect on the vibration and stability of a rub-impact rotor system. When a suitable current is applied, magnetorheological damper can effectively mitigate the vibration of the rotor system to prevent rub-impact. If contact of the rotor/stator is inevitable, magnetorheological damper can further stabilize heavy rub-impact to slight rub-impact by adjusting the current to an appropriate value. This research reveals the influence mechanisms of magnetorheological damper on normal and rub-impact rotor systems and is helpful for vibration control and fault self-healing of rotating machinery.

On the predictive modeling of nonlinear frequency behaviors of an archetypal rub-impact rotor

Due to the existence of Duffing-type nonlinearity, Hertzian contact and bearing clearance, the dynamic characteristics of rub-impact rotor system are seriously affected. The paper presents a new kinematic equation in plural form for the typical rotor system by employing the Lagrange equation, which consists of a disk fixed on a shaft of Duffing-type nonlinearity, a stator of Hertz contact force and Coulomb friction. Furthermore, based on the complex harmonic balance method, the analytical frequencies of this nonlinear rotor system under unperturbed and perturbed rotating cases are comprehensive developed, respectively. Numerical simulations of the Runge–Kutta method were conducted to verify the analytical results. The comparison showed that out of the resonant range, the theoretical and numerical results were in good agreement.

Bifurcation and stability analysis of a nonlinear rotor system subjected to constant excitation and rub-impact

Abstract This paper focuses on the mechanism of a complex bifurcation behavior caused by flight maneuvers in an aircraft rub-impact rotor system with Duffing type nonlinearity. The maneuver load during flight maneuvers may induce a rub-impact phenomenon, accompanied by complex nonlinear behaviors such as periodic, sub-harmonic and quasi-periodic motions, but the bifurcation mechanism is not so clear. In this study, the harmonic balance method combined with an alternating frequency/time domain procedure (HB-AFT method) is formulated and used to derive the approximate periodic solutions of the system. In conjunction with the arc-length continuation, the solution branches for both periodic 1-T motion (synchronous oscillation) and periodic 2-T motion (sub-harmonic oscillation) are traced. Then with the aid of the Floquet theory, the stabilities of the obtained periodic solutions are examined. The changes in the number or the stability of the solutions lead to the identification of the bifurcation points, which can be classified qualitatively into three types, i.e. Neimark-Sacker bifurcation point (NSBP), quasi-periodic Hopf bifurcation point (QPHBP) and saddle-node bifurcation point (SNBP). In addition, the constant excitation significantly affects the instability as well as the bifurcation of the rotor system. In the case of a smaller constant excitation, the rotation region with respect to the quasi-periodic motions may disappear, accompanied with a PBP (pitchfork bifurcation point) instead of the two NSBPs and one QPHBP. The bifurcation analysis in this paper provides deep insight into the mechanism of the complex nonlinear phenomenon induced by the constant excitation. The results obtained will also contribute to a better understanding of the nonlinear dynamic behaviors of aircraft rotor systems during flight maneuvers.

Nonlinear Squeezing Time-Frequency Transform and Application in Rotor Rub-Impact Fault Diagnosis

Vibration signal analysis has been proved as an effective tool for condition monitoring and fault diagnosis for rotating machines in the manufacturing process. The presence of the rub-impact fault in rotor systems results in vibration signals with fast-oscillating periodic instantaneous frequency (IF). In this paper, a novel method for rotor rub-impact fault diagnosis based on nonlinear squeezing time-frequency (TF) transform (NSquTFT) is proposed. First, a dynamic model of rub-impact rotor system is investigated to quantitatively reveal the periodic oscillation behavior of the IF of vibration signals. Second, the theoretical analysis for the NSquTFT is conducted to prove that the NSquTFT is suitable for signals with fast-varying IF, and the method for rotor rub-impact fault diagnosis based on the NSquTFT is presented. Through a dynamic simulation signal, the effectiveness of the NSquTFT in extracting the fast-oscillating periodic IF is verified. The proposed method is then applied to analyze an experimental vibration signal collected from a test rig and a practical vibration signal collected from a dual-rotor turbofan engine for rotor rub-impact fault diagnosis. Comparisons are conducted throughout to evaluate the effectiveness of the proposed method by using Hilbert–Huang transform, wavelet-based synchrosqueezing transform (SST), and other methods. The application and comparison results show that the fast-oscillating periodic IF of the vibration signals caused by rotor rub-impact faults can be better extracted by the proposed method.

Nonlinear analysis of a rub-impact rotor with random stiffness under random excitation

Stochastic bifurcation and chaos of a rub-impact rotor system with random stiffness under random excitation are studied in this article. Due to the irrational and fractional expressions existing in the denominator of rub-impact force, the integral process is very complicated. Taylor series expansion is used to expand the irrational and fractional expressions into a series of polynomials. Chebyshev polynomial approximation method is applied to reduce the system equations with random parameter to its equivalent deterministic one, and the responses of stochastic system can be obtained by numerical methods. Numerical simulations show that random parameters have a significant effect on the rub-impact rotor system. It may promote the nonlinear response when the rotational speed is near the 1/2 first-order critical speed and may suppress the nonlinear response when the rotational speed is over the first-order critical speed.

Rationalize the irrational and fractional expressions in nonlinear analysis

Chebyshev polynomial approximation is an effective method to study the stochastic bifurcation and chaos. However, due to irrational and fractional expressions existing in the denominator of some mechanical systems, the integral process is very complicated. The Taylor series expansion is proposed to expand the irrational and fractional expressions into a series of polynomials. Smooth and discontinuous oscillator was taken as an example, and the results show that the Taylor series expansion method is acceptable. The rub-impact force was taken as another example. Numerical results indicate that the method is suitable for the rub-impact rotor system.

Stability and steady-state response analysis of a single rub-impact rotor system

Using a lumped mass model of a single rub-impact rotor system considering the gyroscopic effect, the stability and steady-state response of the rotor system are investigated in this paper. The contact between the rotor and the stator is described by the simple Coulomb friction and piecewise linear spring models. An algorithm combining harmonic balance method with pseudo arc-length continuation is adopted to calculate the steady-state vibration response of a nonlinear system. Meanwhile, Hill’s method is used to analyze the stability of the system. The nonlinear dynamic characteristics of the system are investigated when the gap size, stator stiffness and unbalance are regarded as the control parameters. The results show that the gap size determines the location of the rub-impact; besides, the smaller gap can improve the stability of the system. The unsteady motion can be found as the stator stiffness increases. Moreover, the unbalance directly affects vibration amplitude, which becomes greater with the increasing imbalance.

Vibration Characteristics Analysis of the Rub- Impact Rotor System with Mass Unbalance

In this paper, vibration characteristics of a rub-impact Jeffcott rotor system excited by mass unbalance including the eccentric mass and the initial permanent deflection are investigated. Through the numerical calculation, rotating speeds, mass eccentricities, initial permanent deflections and phase angles between the eccentric mass direction and the rotor initial permanent deflection direction are used as control parameters to investigate their effect on the rub-impact rotor system with the help of bifurcation diagrams, Poincare maps, frequency spectrums and orbit maps. Result shows that these two kinds of mass unbalance have great but different effect on the dynamic characteristic of the rubbing rotor system. Different motion characteristics appear with the varying of these control parameters, and complicated motions such as periodic and quasi-periodic vibrations are observed. Corresponding results can be used to diagnose the rub-impact fault and decrease the effect of rub impact in the rotor system with mass unbalance.

Nonlinear vibration phenomenon of an aircraft rub-impact rotor system due to hovering flight

This paper focuses on the nonlinear vibration phenomenon caused by aircraft hovering flight in a rub-impact rotor system supported by two general supports with cubic stiffness. The effect of aircraft hovering flight on the rotor system is considered as a maneuver load to formulate the equations of motion, which might result in periodic response instability to the rotor system even the eccentricity is small. The dynamic responses of the system under maneuver load are presented by bifurcation diagrams and the corresponding Lyapunov exponent spectrums. Numerical analyses are carried out to detect the periodic, sub-harmonic and quasi-periodic motions of the system, which are presented by orbit diagrams, phase trajectories, Poincare maps and amplitude power spectrums. The results obtained in this paper will contribute an understanding of the nonlinear dynamic behaviors of aircraft rotor systems in maneuvering flight.

Nonlinear dynamic analysis of a rub-impact rotor supported by oil film bearings

A general model of a rub-impact rotor system is set up and supported by oil film journal bearings. The Jacobian matrix of the system response is used to calculate the Floquet multipliers, and the stability of periodic response is determined via the Floquet theory. The nonlinear dynamic characteristics of the system are investigated when the rotating speed and damping ratio is used as control parameter. The analysis methods are inclusive of bifurcation diagrams, Poincare maps, phase plane portraits, power spectrums, and vibration responses of the rotor center and bearing center. The analysis reveals a complex dynamic behavior comprising periodic, multi-periodic, chaotic, and quasi-periodic response. The modeling results thus obtained by using the proposed method will contribute to understanding and controlling of the nonlinear dynamic behaviors of the rotor-bearing system.

Non-Linear Characters of Rotor System with Crack and Rub-Impact Coupling Faults

Considering unsteady oil-film force and nonlinear rub-impact force, a dynamic model of crack and rub-impact coupling faults rotor system is developed. Based on the model mentioned, some parameters such as crack depth, eccentricity, as well as a combination parameter which consists of lubricating oil viscosity, rotor weight, bearing clearance and wide-diameter ratio are taken to be considered. According to the bifurcation diagram, the fractal dimension of poincare map and also the trajectory of the shaft center, the complex nonlinear dynamic behaviors are obtained, which may bring up theoretical references for fault diagnosis.

Effect of the Initial Deflection on Vibration Characteristics of the Rub-Impact Rotor System

Based on an elastic rub-impact model, a dynamic model of a rubbing rotor system with an initial deflection was set up and motion equations were derived and solved numerically. Poincaré maps, rotor orbits and bifurcation diagrams were drawn to investigate the effects of the initial deflection distance and the initial deflection angle on the vibration features of the rub-impact rotor system. The paper concluded that the initial deflection had great influence on the motion forms of the rubbing rotor system.