Attractors of a rotating viscoelastic beam

We investigate the non-linear oscillations of a rotating viscoelastic beam with variable pitch angle. The governing equations of motion are two coupled partial differential equations for the longitudinal and transversal displacements. Using a perturbation technique and Galerkin's projection, we reduce the equations of motion to a non-autonomous ordinary differential equation. Our regular perturbation technique is based on the expansion of longitudinal displacement and the amplitude of first transversal mode in terms of a small parameter. We numerically generate the Poincaré maps of the reduced equations and reveal that the system exhibits regular and chaotic attractors. The regular attractors are stable limit-cycles that are relevant to stable, short-period oscillations of the beam. A bifurcation analysis has also been performed when the pitch angle is constant.     

Comparative analysis of chaos control methods: A mechanical system case study

Chaos may be exploited in order to design dynamical systems that may quickly react to some new situation, changing conditions and their response. In this regard, the idea that chaotic behavior may be controlled by small perturbations allows this kind of behavior to be desirable in different applications. This paper presents an overview of chaos control methods classified as follows: OGY methods - include discrete and semi-continuous approaches; multiparameter methods - also include discrete and semi-continuous approaches; and time-delayed feedback methods that are continuous approaches. These methods are employed in order to stabilize some desired UPOs establishing a comparative analysis of all methods. Essentially, a control rule is of concern and each controller needs to follow this rule. Noisy time series is treated establishing a robustness analysis of control methods. The main goal is to present a comparative analysis of the capability of each chaos control method to stabilize a desired UPO.     

Complex and chaotic response of a non-linear oscillator with an isothermal gas spring

In this paper we study the dynamics of a non-linear one-degree-of-freedom system subjected to an external harmonic excitation, representing a simplified model for the synchronous hydraulic oscillations that can occur in the draft tube of Francis turbines at partial loads. The application of different typical numerical techniques has shown the existence of multiple coexisting periodic solutions, and the non-periodic bounded solutions which exhibit deterministic chaotic behaviour. The relevant strange attractor has been defined and the loss of memory associated with an exponential divergence in time of close initial conditions resulting in chaotic dynamics have been found and measured. A partial classification of qualitatively different dynamical behaviours for the system has been outlined in the control parameter space.

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Sommario In questo articolo viene studiata la dinamica di un sistema non-lineare ad un singolo grado di liberta' soggetto ad una forzante armonica esterna, rappresentante un modello semplificato per le oscillazioni idrauliche sincrone che hanno luogo nei diffusori delle turbine tipo Francis a carico parziale. Applicando differenti tecniche numeriche, viene mostrata l'esistenza di soluzioni periodiche multiple, oltre che soluzioni non-periodiche limitate con tipico comportamento caotico deterministico. L'attrattore strano corrispondente e' stato definito e caratterizzato: la perdita di memoria associata alla divergenza esponenziale di orbite inizialmente vicine, tipica della dinamica caotica, e' stata individuata e calcolata numericamente. Una prima parziale classificazione dei vari comportamenti dinamici per il sistema viene evidenziata attraverso la rappresentazione nello spazio parametrico.

     

Phase space structure of spinning disks

Using a Hamiltonian formalism and a sequence of canonical transformations, we show that the ordinary differential equations associated with the forced oscillations of rotating circular disks admit the first integral of motion. This reduces the phase space dimension of the governing equations from five to three. The phase space flows of the reduced system are then visualized using Poincaré maps. Our results show that single mode oscillations of rotating disks are subject to chaotic behavior through the emergence of higher-order resonant islands that surround fundamental periodic cycles. We extend our new formalism to imperfect disks and construct adiabatic invariants near to and far from resonances. For low-speed imperfect disks, we find a new kind of bifurcations of the phase space flows as the system parameters vary. We study the effect of structural damping using Hamilton's principle for non-conservative systems and reveal the existence of asymptotically stable limit cycles for the damped system near the 1:1 resonance. We show that a low-speed disk is eventually flattened due to damping effect.     

Bifurcation and chaos of a flag in an inviscid flow

A two-dimensional model is developed to study the flutter instability of a flag immersed in an inviscid flow. Two dimensionless parameters governing the system are the structure-to-fluid mass ratio M^⁎ and the dimensionless incoming flow velocity U^⁎. A transition from a static steady state to a chaotic state is investigated at a fixed M^⁎=1 with increasing U^⁎. Five single-frequency periodic flapping states are identified along the route, including four symmetrical oscillation states and one asymmetrical oscillation state. For the symmetrical states, the oscillation frequency increases with the increase of U^⁎, and the drag force on the flag changes linearly with the Strouhal number. Chaotic states are observed when U^⁎ is relatively large. Three chaotic windows are observed along the route. In addition, the system transitions from one periodic state to another through either period-doubling bifurcations or quasi-periodic bifurcations, and it transitions from a periodic state to a chaotic state through quasi-periodic bifurcations.     

Period-doubling route to chaos in a two-degree-of-freedom flexibly-mounted rigid plate placed in water

Flow-induced instabilities of a flexibly-mounted rigid flat plate placed in water were investigated experimentally, when the plate had either one degree of freedom in the torsional direction or two degrees of freedom in the torsional and transverse directions. Tests were conducted in a re-circulating water tunnel and bifurcation diagrams were used to summarize the system behavior. The 1DoF system became unstable by divergence at a critical flow velocity after which the plate buckled. At higher flow velocities, periodic oscillations were observed and the amplitude of oscillations increased with increasing flow velocity. No other instability was observed at higher flow velocities. In the 1DoF system, the variations in the response frequency were related to the added mass moment of inertia. For the 2DoF system, the plate׳s original stability was lost at a critical flow velocity by divergence followed by a dynamic instability resulting in periodic oscillations, which in turn became unstable giving rise to period-2, period-4 and eventually chaotic oscillations.     

Dynamical models and the onset of chaos in space debris

The increasing threat raised by space debris led to the development of different mathematical models and approaches to investigate the dynamics of small particles orbiting around the Earth. The choice of such models and methods strongly depend on the altitude of the objects above Earth's surface, since the strength of the different forces acting on an Earth orbiting object (geopotential, atmospheric drag, lunar and solar attractions, solar radiation pressure, etc.) varies with the altitude of the debris.In this review, our focus is on presenting different analytical and numerical approaches employed in modern studies of the space debris problem. We start by considering a model including the geopotential, solar and lunar gravitational forces and the solar radiation pressure. We summarize the equations of motion using different formalisms: Cartesian coordinates, Hamiltonian formulation using Delaunay and epicyclic variables, Milankovitch elements. Some of these methods lead in a straightforward way to the analysis of resonant motions. In particular, we review results found recently about the dynamics near tesseral, secular and semi-secular resonances.As an application of the above methods, we proceed to analyze a timely subject, namely the possible causes for the onset of chaos in space debris dynamics. Precisely, we discuss the phenomenon of overlapping of resonances, the effect of a large area-to-mass ratio, the influence of lunisolar secular resonances.We conclude with a short discussion about the effect of the dissipation due to the atmospheric drag and we provide a list of minor effects, which could influence the dynamics of space debris.     

Lyapunov functions for a non-linear model of the X-ray bursting of the microquasar GRS 1915+105

This paper introduces a biparametric family of Lyapunov functions for a non-linear mathematical model based on the FitzHugh-Nagumo equations able to reproduce some main features of the X-ray bursting behaviour exhibited by the microquasar GRS 1915+105. These functions are useful to investigate the properties of equilibrium points and allow us to demonstrate a theorem on the global stability. The transition between bursting and stable behaviour is also analyzed.     

Cross-well chaos and escape phenomena in driven oscillators

The paper is devoted to the study of common features in regular and strange behavior of the three classic dissipative softening type driven oscillators: (a) twin-well potential system, (b) single-well potential unsymmetric system and (c) single-well potential symmetric system.Computer simulations are followed by analytical approximations. It is shown that the mathematical techniques and physical concepts related to the theory of nonlinear oscillations are very useful in predicting bifurcations from regular, periodic responses to cross-well chaotic motions or to escape phenomena. The approximate analysis of periodic, resonant solutions and of period doubling or symmetry breaking instabilities in the Hill's type variational equation provides us with closed-form algebraic simple formulae; that is, the relationship between critical system parameter values, for which strange phenomena can be expected.     

Hysteresis modelling and chaos prediction in one- and two-DOF hysteretic models

In the present work hysteresis is simulated by means of internal variables. The analytical models of different types of hysteresis loops allow the reproduction of major and minor loops and provide a high degree of correspondence with experimental data. In models of this type adding an external periodic excitation or increasing the number of dimensions can lead to the occurrence of chaotic behaviour. Using an effective algorithm based on numerical analysis of the wandering trajectories [1–7], the evolution of the chaotic behaviour regions of oscillators with hysteresis is presented in various parametric planes. The substantial influence of a hysteretic dissipation value on the form and location of these regions, as well as the restraining and generating effects of hysteretic dissipation on the occurrence of chaos, are ascertained. Conditions for pinched hysteresis are defined. Furthermore, autonomous coupled hysteretic oscillators under sliding friction are investigated. Conditions for the occurrence of chaotic behaviour in a two-degree-of-freedom (two-DOF) hysteretic system are found in the plane of maximal static friction forces of both oscillators versus belt velocity.     

Testing a fast dynamical indicator: The MEGNO

To investigate non-linear dynamical systems, like for instance artificial satellites, Solar System, exoplanets or galactic models, it is necessary to have at hand several tools, such as a reliable dynamical indicator.The aim of the present work is to test a relatively new fast indicator, the Mean Exponential Growth factor of Nearby Orbits (MEGNO), since it is becoming a widespread technique for the study of Hamiltonian systems, particularly in the field of dynamical astronomy and astrodynamics, as well as molecular dynamics.In order to perform this test we make a detailed numerical and statistical study of a sample of orbits in a triaxial galactic system, whose dynamics was investigated by means of the computation of the Finite Time Lyapunov Characteristic Numbers (FT-LCNs) by other authors.     

Solitary waves and chaos in nonlinear visco-elastic rod

The dynamics behavior of a nonlinear visco-elastic rod subjected to axially periodic load is investigated theoretically and numerically. The weak longitudinal periodic load is distributed uniformly along the rod. Firstly, equation of motion of the rod is derived. Utilizing perturbation technique, we acquire Kdv type equation describing strain wave in the rod. By use traveling wave method, the elliptic cosine wave solution and the solitary wave solution in the rod are provided. Then, Melnikov method is applied to analyze the dynamic behaviour of the rod qualitatively. The explicit conditions for the onset of chaotic dynamics are yielded. With the help of the Poincare map method, phase trajectory and time-displacement history diagrams, the theoretical results obtained are checked.     

Bifurcation and chaos of a simple walking model driven by a rhythmic signal

The walk of animals is achieved by the interaction between the dynamics of their mechanical system and the central pattern generator (CPG). In this paper, we analyze dynamic properties of a simple walking model of a biped robot driven by a rhythmic signal from an oscillator. In particular, we examine the long-term global behavior and the bifurcation of the motion that leads to chaotic motion, depending on the model parameter values. The simple model consists of a hip and two legs connected at the hip through a rotational joint. The joint is driven by a rhythmic signal from an oscillator, which is an open loop. In order to analyze the bifurcation, we first obtained approximate solutions of the walking motion and then constructed discrete dynamics using the Poincaré map. As a result, we found that consecutive period-doubling bifurcations occur as the model parameter values change, and that the walking motion leads to chaotic motion over the critical value of the model parameters. Moreover, we approximately obtained the period-doubling solutions and the critical value by employing a Newton-Raphson method. Our analytical results were verified by the numerical simulations.     

On Local Analysis of Oscillations of a Non-ideal and Non-linear Mechanical Model

It is of major importance to consider non-ideal energy sources in engineering problems. They act on an oscillating system and at the same time experience a reciprocal action from the system. Here, a non-ideal system is studied. In this system, the interaction between source energy and motion is accomplished through a special kind of friction. Results about the stability and instability of the equilibrium point of this system are obtained. Moreover, its bifurcation curves are determined. Hopf bifurcations are found in the set of parameters of the oscillating system.     

Regular oscillations or chaos in a fractional order system with any effective dimension

This paper introduces a fractional order system which can generate regular oscillations or create chaos. It shows that this system is capable to create regular or nonregular oscillations under suitable conditions. These necessary conditions are achieved by violation of the no-chaos criteria. The effective dimension of the proposed system can be chosen any order less than three. Therefore, this system is a good example for limit cycle or chaos generation via fractional-order systems with low orders. Numerical simulations illustrate behavior of the proposed system in different situations.     

Averaging and periodic solutions in the plane and parametrically excited pendulum

We first approximate the solutions of the nonautonomous oscillating suspension point pendulum equation by the solutions of a second order autonomous differential equation. Using the strict monotonicity of the periodic solutions of the approximating equation, we prove the existence of a large number of subharmonic periodic solutions of the plane pendulum when its point of suspension is excited parametrically.