Bifurcation of limit cycles in fluid film bearings

Rotors supported by journal bearings may become unstable due to self-excited vibrations when a critical rotor speed is exceeded. Linearised analysis is usually used to determine the stability boundaries. Non-linear bifurcation theory or numerical integration is required to predict stable or unstable periodic oscillations close to the critical speed. In this paper, a dynamic model of a short journal bearing is used to analyse the bifurcation of the steady state equilibrium point of the journal centre. Numerical continuation is applied to determine stable or unstable limit cycles bifurcating from the equilibrium point at the critical speed. Under certain working conditions, limit cycles themselves are shown to disappear beyond a certain rotor speed and to exhibit a fold bifurcation giving birth to unstable limit cycles surrounding the stable supercritical limit cycles. Numerical integration of the system of equations is used to support the results obtained by numerical continuation. Numerical simulation permitted a partial validation of the analytical investigation.     

Bifurcation of limit cycles in a quintic system with ten parameters

In this paper, center conditions and bifurcation of limit cycles at the nilpotent critical point in a class of quintic polynomial differential system are investigated. With the help of the computer algebra system MATHEMATICA, the first 11 quasi-Lyapunov constants are deduced. As a result, sufficient and necessary conditions in order to have a center are obtained. The fact that there exist 11 small amplitude limit cycles created from the three-order nilpotent critical point is also proved. Henceforth, we give a lower bound of cyclicity of three-order nilpotent critical point for quintic Lyapunov systems. The results of Jiang et al. (Int. J. Bifurcation Chaos 19:2107–2113, 2009) are improved.     

Bifurcation of limit cycles,classification of centers and isochronicity for a class of non-analytic quintic systems

Inspired by Llibre and Vallls (in J. Math. Anal. Appl. 357:427–437, 2009), the conditions of center and isochronous center at the origin for a class of non-analytic quintic systems are studied in this paper. By a transformation, we first transform the systems into analytic systems, then sufficient and necessary conditions for the origin of the systems being a center are obtained. The fact that 11 limit circles could be bifurcated is proved. A complete classification of the sufficient and necessary conditions is given for the origin of the systems being an isochronous center.     

Stability of limit cycles in autonomous nonlinear systems

Periodical solutions or limit cycles (LC) comprise a significant family among the response types of nonlinear autonomous systems. Their identification and stability assessment is of a great importance during the analysis of an unknown system. A new analytical/iterative method of LC identification and portrait investigation was presented recently. The current study proposes a novel technique for their stability assessment. This strategy facilitates the distinction of stable and unstable LCs, thereby allowing the definition of attractive and repulsive response fields. A narrow toroidal domain is constructed around the LC, which is arithmetized by an orthogonal system that is positioned by tangential and normal vectors to the LC. The stability of the LC is investigated using the transformed differential system of the normal components of the response, which are functions of the coordinate along the LC trajectory. Exponential LC stability criteria are also proposed, which are based on the first degree of the perturbation procedure. Theoretical considerations are illustrated using single and two degree of freedom systems including demonstrations with specific systems. The strengths, future steps, and shortcomings of this method are evaluated.     

The stability of limit cycles in nonlinear systems

In this paper, a necessary condition is first presented for the existence of limit cycles in nonlinear systems, then four theorems are presented for the stability, instability, and semistabilities of limit cycles in second order nonlinear systems. Necessary and sufficient conditions are given in terms of the signs of first and second derivatives of a continuously differentiable positive function at the vicinity of the limit cycle. Two examples considering nonlinear systems with familiar limit cycles are presented to illustrate the theorems.     

Stability of flows with discontinuity surfaces

A study is made in the linear formulation of flows with homogeneous distribution of the parameters in expanding regions separated by boundaries that are either discontinuity surfaces of an arbitrary nature or surfaces with effective boundary conditions. Examples of such flows are the decay of an arbitrary discontinuity [1] and flow in a tube with a region of heat release [2].Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 10–18, September–October, 1982.We thank A. G. Kulikovskii for helpful discussions.     

Bifurcation analysis in the system with the existence of two stable limit cycles and a stable steady state

Van der Pol–Duffing oscillator, which can be used a model for many dynamical system, has been widely concerned. However, most of the systems by scholars are either stable steady states or limit cycles. Here, the self-sustained oscillator with the coexistence of steady state and limit cycles, which is famous for describing the flutter of airfoils with large span ratio in low-speed wind tunnels, is treated in this paper. Using the energy balance method, the deterministic bifurcation of the tristable system with time-delay feedback is investigated. The presence of time-delay feedback expands the bifurcation range of the parameters, making the bifurcation phenomenon more abundant. In addition, according to the stationary probability density function obtained by the stochastic averaging method, stochastic bifurcation of the system with time-delay feedback and noise is explored theoretically. The numerical results confirm the correctness of the theoretical analysis. Transition between the unimodal structure, the bimodal structure and the trimodal structure is found. Many rich bifurcations are available by adjusting the time-delay and noise intensity, which may be conductive to achieve the desired phenomenon in the real-world application.

     

Identification of limit cycles for piecewise nonlinear aeroelastic systems

This paper describes two methods for the analysis of aeroelastic systems with complex piecewise nonlinear structural stiffness. These methods are tested and compared for low speed incompressible and transonic flows. The first technique employed in this paper uses a new application of the analytical solution of linear algebraic systems, the second technique utilises logarithmic and tanh functions to both represent discrete nonlinearities and to act as a switch between different nonlinear areas. The transonic aerodynamic models used are generated using an eigenvalue realisation algorithm (ERA) which produces reduced order models (ROMs) from the pulse responses of time linearised Euler simulations. It is shown that such aerodynamic models are well suited to use with continuation methods. Flutter boundaries and limit cycle oscillations can then be rapidly identified with good accuracy.     

A general mechanism to generate three limit cycles in planar Filippov systems with two zones

Discontinuous piecewise linear systems with two zones are considered. A general canonical form that includes all the possible configurations in planar linear systems is introduced and exploited. It is shown that the existence of a focus in one zone is sufficient to get three nested limit cycles, independently on the dynamics of the another linear zone. Perturbing a situation with only one hyperbolic limit cycle, two additional limit cycles are obtained by using an adequate parametric sector of the unfolding of a codimension-two focus-fold singularity.     

Averaging in a system with several limit cycles

We consider the construction of the positive cone [7] by the method of averaging, which makes it possible to determine oscillatory modes in a many-frequency system with a polynomial nonlinearity and to construct curves dividing identical behavior of the trajectories. The starting point is the set of results given in [1–6].S. P. Timoshenko Institute of Mechanics, National Academy of Sciences of the Ukraine, Kiev. Translated from Prikladnaya Mekhanika, Vol. 30, No. 9, pp. 88–94, September, 1994.     

Stability of surfaces of strong discontinuity in gases

The existence of solutions with surfaces of strong discontinuity is one of the principal features of the continua whose motions are described by systems of differential equations of hyperbolic type. Shock waves in gas dynamics, magnetohydrodynamics and in solids, detonation waves and combustion fronts, contact discontinuities, etc. are well-known examples of these surfaces. The discontinuities are usually investigated in accordance with the following scheme: 1) derivation of the boundary conditions on the discontinuity from the input system of differential equations in integral form; 2) verification of the fulfilment of the evolution conditions; 3) solution of the problem of the discontinuity structure and, when the occasion requires, obtaining supplementary boundary conditions; 4) investigation of the stability of the discontinuity. Only after obtaining positive results in all fours stages can we assert that the existence of the discontinuity is theoretically justified and that it can be used for constructing the solutions of particular boundary value problems. In the present paper attention will be concentrated on the problem of the stability of discontinuities, all the material, with the exception of the general results of Sec.1, being concerned with gas media and relating to discontinuities on whose surface the normal mass flow is nonzero. Having no way of exploring all the aspects of the problem of the stability of discontinuities in the same detail within the limited context of this paper, the authors hope to demonstrate the most general ideas and approaches which could subsequently be used to investigate the stability of discontinuities in various particular models of continua.Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 2, pp. 3–22, March–April, 1996.     

Passage of a supersonic flow over intersecting surfaces

Using the linear formulation, the problem of passage of a supersonic flow over slightly curved intersecting surfaces whose tangent planes form small dihedral angles with the incident flow velocity at every point is considered. Conditions on the surfaces are referred to planes parallel to the incident flow forming angles 0≤γ≤2π at their intersection [1]. The problem reduces to finding the solution of the wave equation for the velocity potential with boundary conditions set on the surfaces flowed over and the leading characteristic surface. The Volterra method is used to find the solution [2]. This method has been applied to the problem of flow over a nonplanar wing [3] and flow around intersecting nonplanar wings forming an angle γ=π/n (n=1, 2, 3, ...) with consideration of the end effect on the wings forming the angle [4]. In [5] the end effect was considered for nonplanar wings with dihedral angle γ=m/nπ. In the general case of an arbitrary angle 0≤γ≤2π the problem of finding the velocity potential reduces to solution of Volterra type integrodifferential equations whose integrands contain singularities [1]. It was shown in [6] that the integrodifferential equations may be solved by the method of successive approximation, and approximate solutions were found differing slightly from the exact solution over the entire range of interaction with the surface and coinciding with the exact solution on the characteristic lines (the boundary of the interaction region, the edge of the dihedral angle). The solution of the problem of flow over intersecting plane wings (the conic case) for an arbitrary angle γ was obtained in terms of elementary functions in [7], which also considered the effect of boundary conditions set on a portion of the leading wave diffraction. In [8, 9] the nonstationary problem of wave diffraction at a plane angle π≤γ≤2π was considered. On the basis of the wave equation solution found in [8], this present study will derive a solution which permits solving the problem of supersonic flow over nonplanar wings forming an arbitrary angle π≤γ≤2π in quadratures. The solutions for flow over nonplanar intersecting surfaces for the cases 0≤γ≤π [6] and π≤γ≤2π, found in the present study, permit calculation of gasdynamic parameters near a wing with a prismatic appendage (fuselage or air intake). The study presents a method for construction of solutions in various zones of wing-air intake interaction.     

Kepler’s equation and limit cycles in a class of PWM feedback control systems

The aim of this paper is to point out some new results concerning the ripple instability in the closed-loop control system using pulse width modulators (PWM), with natural sampling, as power amplifier. The presented analysis, based on the dual-input describing function method and the theoretical framework of Kepler’s problem, shows an equivalence between the computation of switching instants of the PWM and the eccentric anomaly of the planet orbit around the sun, giving a simple stability criterion and a sufficient condition for the absence of solutions of the harmonic balance equation and, therefore, the probable absence of limit cycles of a period of a multiple of that characteristic of the modulator. The derived stability criterion, by using the describing function method, is successively compared with the local stability of the closed-loop PWM system for first- and second-order plants. In the first case it has been formally proved that the proposed criterion ensures the local stability of an equilibrium point, while in the second one a Monte Carlo simulation has confirmed that the selection of the modulator parameters, according to the proposed criterion, gives an effective method to avoid limit cycles and to ensure the local stability.     

Excitation of acoustic resonance in systems with double shear discontinuity

Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 3–9, May–June, 1991.     

Bifurcation of rotary motion in singly-linked systems with rocking

Translated from Prikladnaya Mekhanika, Vol. 32, No. 10, pp. 88–94, October, 1996.     

Investigation of surfaces of discontinuity in perfectly conducting magnetizing media

Relationships on discontinuities in magnetizing perfectly conducting media in a magnetic field are investigated. The magnetic permeabilities before and after the discontinuity are assumed to be constant, but unequal, quantities. It is shown that shocks of two kinds, fast and slow, are possible in the formulation under consideration in the hydrodynamics of magnetizing media, as in magnetic hydrodynamics: It is shown that the entropy decreases on the rarefaction shocks diminishing the magnetic permeability, but can grow on the rarefaction shocks increasing the magnetic permeability, but such waves are not evolutionary. The relationships on discontinuities in the mechanics of a continuous medium are written down in general form in [1] with the electromagnetic field, polarization, and magnetization effects taken into account. Relationships on discontinuities in the ferrohydrodynamic and elec trohydrodynamic approximations were written down in [2] and [3–5], respectively, for the cases when the magnetic permeability and dielectric permittivity of the medium ahead of and behind the discontinuity are arbitrary functions of their arguments and are identical. A system of relationships on discontinuities propagated into a magnetizing perfectly conducting medium is investigated in this paper. The method proposed in [6] is used in the investigation.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 104–110, January–February, 1976.We are grateful to A. A. Barmin for discussing the paper and for valuable remarks.     

On the (1, 3) distributions of limit cycles of plane quadratic systems

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Optimisation of chaotically perturbed acoustic limit cycles

Nonlinear Dynamics - In an acoustic cavity with a heat source, the thermal energy of the heat source can be converted into acoustic energy, which may generate a loud oscillation. If uncontrolled,...