Pseudo-arclength Continuation Research Articles

Nonlinear stability and bifurcations of an axially moving beam in thermal environment

The thermo-mechanical nonlinear dynamics of an axially moving beam with coupled longitudinal and transverse displacements subjected to a distributed harmonic external force is numerically investigated. This includes a case where the system is tuned to a three-to-one internal resonance between the first two transverse modes and a case where it is not is considered. Two coupled nonlinear partial differential equations for the longitudinal and transverse motions are obtained using Hamilton’s principle and constitutive relations, as well as taking into account the thermal effects. The Galerkin method is then used to discretize these equations into a set of coupled nonlinear ordinary differential equations. Two different techniques are employed to solve the resulting equations; the pseudo-arclength continuation method and direct time integration to investigate the periodic vibrations and the global dynamics of the system, respectively. The effect of different parameters on the dynamics of the system is investigated through the frequency-response curves of the system and the bifurcation diagrams of Poincaré maps. Furthermore, time histories, phase-plane portraits, and fast Fourier transforms are presented for a few different system parameter sets. It is illustrated that the system shows a broad variety of rich dynamics, depending on system parameters and the temperature rise.

In-plane and out-of-plane motion characteristics of microbeams with modal interactions

The three-dimensional nonlinear size-dependent motion characteristics of a microbeam are investigated numerically, with special consideration to one-to-one internal resonances between the in-plane and out-of-plane transverse modes. All of the in-plane and out-of-plane displacements and inertia are taken into account and Hamilton’s principle, in conjunction with the modified couple stress theory, is employed to obtain the nonlinear partial differential equations governing the motions of the system in the in-plane and out-of-plane directions. The discretization procedure is carried out by applying the Galerkin technique to the partial differential equations of motion, yielding a set of nonlinear ordinary differential equations. A linear analysis is performed upon this set of equations so as to obtain the size-dependent natural frequencies of the system. The nonlinear analysis of the discretized equations of motion is carried out by employing the pseudo-arclength continuation technique, resulting in the resonant responses of the system. It is shown that, due to the presence of one-to-one internal resonances between the in-plane and out-of-plane transverse modes, an in-plane excitation can give rise to an out-of-plane displacement; the internal resonances also cause the occurrence of extra solution branches and new bifurcation points.

Coupled longitudinal-transverse behaviour of a geometrically imperfect microbeam

Based on the modified couple stress theory, the coupled longitudinal-transverse nonlinear behaviour of an imperfect microbeam is investigated numerically. The equations governing the longitudinal and transverse motions are obtained using Hamilton’s principle for the system with an initial geometric imperfection. The Galerkin scheme is employed to discretize the two partial differential equations of motion, yielding a set of second-order nonlinear ordinary differential equations with coupled terms. This set is cast into new set of first-order nonlinear ordinary differential equations and solved by means of the pseudo-arclength continuation technique. The nonlinear resonant response of the system along with bifurcations are presented via frequency–response curves. Moreover, the effect of different system parameter on the frequency–response curves is highlighted.

Numerical Bifurcation Theory for High-Dimensional Neural Models.

Numerical bifurcation theory involves finding and then following certain types of solutions of differential equations as parameters are varied, and determining whether they undergo any bifurcations (qualitative changes in behaviour). The primary technique for doing this is numerical continuation, where the solution of interest satisfies a parametrised set of algebraic equations, and branches of solutions are followed as the parameter is varied. An effective way to do this is with pseudo-arclength continuation. We give an introduction to pseudo-arclength continuation and then demonstrate its use in investigating the behaviour of a number of models from the field of computational neuroscience. The models we consider are high dimensional, as they result from the discretisation of neural field models—nonlocal differential equations used to model macroscopic pattern formation in the cortex. We consider both stationary and moving patterns in one spatial dimension, and then translating patterns in two spatial dimensions. A variety of results from the literature are discussed, and a number of extensions of the technique are given.Electronic supplementary materialThe online version of this article (doi:10.1186/2190-8567-4-13) contains supplementary material, which is available to authorized users.

Size-dependent vibrations of post-buckled functionally graded Mindlin rectangular microplates

In this paper, the free vibration behavior of post-buckled functionally graded (FG) Mindlin rectangular microplates are described based on the modified couple stress theory (MCST). This theory enables the consideration of the size-effect through introducing material length scale parameters. The FG microplates made of a mixture of metal and ceramic are considered whose volume fraction of components is expressed by a power law function. By means of Hamilton's principle, the nonlinear governing equations and associated boundary conditions are derived for FG micro-plates in the postbuckling domain. The governing equations and boundary conditions are then discretized by using the generalized differential quadrature (GDQ) method before solving numerically by the pseudo-arclength continuation technique. In the solution procedure, the postbuckling problem of microplates is investigated first. Afterwards, the free vibration of microplates around the buckled configuration is discussed. The effects of dimensionless length scale parameter, material gradient index and aspect ratio on the on the postbuckling path and frequency of FG microplates subject to arbitrary edge supports are thoroughly discussed.

Size-Dependent Thermal Buckling and Postbuckling of Functionally Graded Annular Microplates Based on the Modified Strain Gradient Theory

This article reports on the thermal instability of functionally graded (FG) annular microplates with different boundary conditions. The modified strain gradient elasticity theory is employed to capture size effects. The non-linear governing equations and boundary conditions are derived based on the first-order shear deformation theory (FSDT) and virtual displacements principle. The generalized differential quadrature technique is implemented so as to discretize. To obtain the critical buckling temperature, the set of linear discretized governing equations is solved as an eigenvalue problem. Also, the non-linear problem of thermal postbuckling is solved by the pseudo arc-length continuation method. The effects of boundary conditions, length scale parameter, and the variation of material through the thickness and geometrical properties on both critical buckling temperature and thermal postbuckling behavior are studied.

Nonlinear vibration analysis of Timoshenko nanobeams based on surface stress elasticity theory

Abstract In this article, the nonlinear free vibration behavior of Timoshenko nanobeams subject to different types of end conditions is investigated. The Gurtin–Murdoch continuum elasticity is incorporated into the Timoshenko beam theory in order to capture surface stress effects. The nonlinear governing equations and corresponding boundary conditions are derived using Hamilton's principle. A numerical approach is used to solve the problem in which the generalized differential quadrature method is applied to discretize the governing equations and boundary conditions. Then, a Galerkin-based method is numerically employed with the aim of reducing the set of partial differential governing equations into a set of time-dependent ordinary differential equations. Discretization on time domain is also done via periodic time differential operators that are defined on the basis of the derivatives of a periodic base function. The resulting nonlinear algebraic parameterized equations are finally solved by means of the pseudo arc-length continuation algorithm through treating the time period as a parameter. Numerical results are given to study the geometrical and surface properties on the nonlinear free vibration of nanobeams.

Vibration Analysis of Postbuckled Timoshenko Beams Using a Numerical Solution Methodology

In this article, a numerical solution methodology is presented to study the postbuckling configurations and free vibrations of Timoshenko beams undergoing postbuckling. The effect of geometrical imperfection is taken into account, and the analysis is carried out for different types of boundary conditions. Based on Hamilton's principle, the governing equations and corresponding boundary conditions are derived. After introducing a set of differential matrix operators that is used to discretize the governing equations and boundary conditions, the pseudo-arc length continuation method is applied to solve the postbuckling problem. Then, the problem of free vibration around the buckled configurations is solved as an eigenvalue problem using the solution obtained from the nonlinear problem in the previous step. This study shows that, when the axial load in the postbuckling domain increases, the vibration mode shape of buckled beam corresponding to the fundamental frequency may change. Another finding that can be of great technical interest is that, for all types of boundary conditions and in both prebuckling and postbuckling domains, the natural frequency of imperfect beam is higher than that of ideal beam. Also, it is observed that, by increasing the axial load, the natural frequency of both ideal and imperfect beams decreases in the prebuckling domain, while it increases in the postbuckling domain. The reduction of natural frequency in the transition area from the prebuckling domain to the postbuckling domain is due to the severe instability of the structure under the axial load.

Nonlinear vibrations and multiple resonances of fluid filled arbitrary laminated circular cylindrical shells

Nonlinear forced vibrations of water-filled, laminated circular cylindrical shells are studied by using the Amabili–Reddy nonlinear higher-order shear deformation theory and energy approach in the Lagrangian description. The fluid is modeled by potential flow. It is assumed that the shell is subjected to a steady harmonic concentrated force acting in the radial direction. Pseudo arc-length continuation and collocation technique is used to carry out bifurcation analysis and to obtain nonlinear frequency–amplitude responses. Direct time integration of the equations of motion has also been performed by using Gear’s backward differentiation formula (BDF) to obtain time histories, phase space diagrams and Poincaré maps. The effect of internal fluid and lamination angle on the frequency–amplitude response in the neighborhood of the resonance frequency, traveling wave solution and internal resonances of simply supported shells are investigated. It is shown that water-filled composite shells may exhibit complex nonlinear dynamics including a rare and intricate 1:1:1:1 internal resonance.

Nonlinear forced vibration response of bimodular laminated composite plates

The nonlinear forced vibration characteristics of bimodular material laminated cross-ply composite plates subjected to periodic excitation are investigated. The analysis is carried out using Bert’s constitutive model employing first order shear deformation theory based finite element method (FEM) including von Kármán geometric nonlinearity. The periodic response is obtained using shooting technique coupled with Newmark time marching, arc length and pseudo-arc length continuation algorithms. The second order differential equation of motion is solved directly without transforming to first order differential equations thereby preserving the banded nature of equations. The nonlinear periodic vibration characteristics such as steady state response history, phase plane plots and frequency spectra are presented. A detailed parametric study is carried out to analyse the influence of bimodularity, aspect ratio, thickness ratio, excitation amplitude, support conditions and lamination scheme on the forced vibration response. The significant difference in the positive/negative half cycle amplitudes due to bimodularity is predicted with/without geometric nonlinearity. The through the thickness variation of fiber direction normal strain is presented to show the extent of assignment of tensile/compressive properties and restoring force due to geometric nonlinearity. Further, the unstable regions of the frequency response curves between turning points could not be traced due to the perturbations stemming from the numerical truncation in finite element discretization/solution of equations and the bimodular action resulting in deviation of solution in shooting iterations leading to convergence failure.

Nonlinear forced vibrations of rotating anisotropic beams

This work deals with forced vibration of nonlinear rotating anisotropic beams with uniform cross sections. Coupling the Galerkin method with the balance harmonic method, the nonlinear intrinsic and geometrically exact equations of motion for anisotropic beams subjected to large displacements, are converted into a static formulation. This latter is treated with two continuation methods. The first one is the asymptotic-numerical method, where power series expansions and Pade approximants are used to represent the generalized vector of displacement and the frequency. The second one is the pseudo-arclength continuation method. Numerical tests dealing with isotropic and anisotropic beams are considered. The natural frequencies obtained for prismatic beams are compared with the literature. Response curves are obtained and the nonlinearity is investigated for various geometrical conditions, excitation amplitudes and kinematical conditions. The nonlinearity related to the angular speed for prismatic isotropic beam is thus identified. The stability of the solution branch is examined, in the frequency domain using the Floquet theory.

Nonlinear Modal Analysis of a Full-Scale Aircraft

Nonlinear normal modes, which are defined as a nonlinear extension of the concept of linear normal modes, are a rigorous tool for nonlinear modal analysis. The objective of this paper is to demonstrate that the computation of nonlinear normal modes and of their oscillation frequencies can now be achieved for complex, real-world aerospace structures. The application considered in this study is the airframe of the Morane–Saulnier Paris aircraft. Ground vibration tests of this aircraft exhibited softening nonlinearities in the connection between the external fuel tanks and the wing tips. The nonlinear normal modes of this aircraft are computed from a reduced-order nonlinear finite element model using a numerical algorithm combining shooting and pseudo-arclength continuation. Several nonlinear normal modes, involving, e.g., wing bending, wing torsion, and tail bending, are presented, which highlights that the aircraft can exhibit very interesting nonlinear phenomena. Specifically, it is shown that modes with distinct linear frequencies can interact and generate additional nonlinear modes with no linear counterpart.

Nonlinear resonant behavior of microbeams over the buckled state

The present study investigates the nonlinear resonant behavior of a microbeam over its buckled (non-trivial) configuration. The system is assumed to be subjected to an axial load along with a distributed transverse harmonic load. The axial load is increased leading the system to lose the stability via a pitchfork bifurcation; the postbuckling configuration is obtained and the nonlinear resonant response of the system over the buckled state is examined. More specifically, the nonlinear equation of motion is obtained employing Hamilton’s principle along with the modified couple stress theory. The continuous system is truncated into a system with finite degrees of freedom; the Galerkin scheme is employed to discretize the nonlinear partial differential equation of motion into a set of ordinary differential equations. This set of equations is solved numerically employing the pseudo-arclength continuation technique; first a nonlinear static analysis is performed upon this set of equations so as to obtain the onset of buckling (supercritical pitchfork bifurcation) and the buckled configuration of the microbeam. The frequency-response and force-response curves of the system are then constructed over the buckled configurations. A comparison is made between the frequency-response curves obtained by means of the modified couple stress and the classical theories. The effect of different system parameters on the frequency-response and force-response curves is also examined.

Nonlinear dynamics of cantilevered extensible pipes conveying fluid

In this paper the nonlinear planar dynamics of a fluid-conveying cantilevered pipe is investigated. The centreline of the pipe is considered to be extensible; i.e., coupled longitudinal and transverse displacements are considered. The extended version of the Lagrange equations for systems containing non-material volumes is employed to derive the equations of motion, resulting directly in a set of coupled nonlinear ordinary differential equations. The pseudo-arclength continuation technique along with direct time integration are used to solve these equations. Bifurcation diagrams of the system are constructed as the flow velocity is increased; these diagrams are supplemented by time traces, phase-plane portraits, and fast Fourier transforms for some sets of system parameters. As opposed to the case of an inextensible pipe, an extensible pipe elongates in the axial direction as the flow velocity is increased from zero; depending on the system parameters, this static elongation can be considerable. At the critical flow velocity, the system loses stability via a supercritical Hopf bifurcation, emerging from the trivial solution for the transverse displacement and leading to a flutter.

Nonlinear behaviour of electrically actuated MEMS resonators

The present study investigates the nonlinear size-dependent behaviour of an electrically actuated MEMS resonator based on the modified couple stress theory; the microbeam is excited by an AC voltage which is superimposed on a DC voltage. A high-dimensional reduced order model of the continuous system is obtained by applying the Galerkin scheme to the nonlinear partial differential equation of motion. The pseudo-arclength continuation technique is employed to examine the nonlinear static and dynamic behaviour of the system. Specifically, the nonlinear static behaviour of the system is investigated when the microbeam is excited by the electrostatic excitation (DC voltage); this analysis yields the static deflected configuration of the system and the value of the DC voltage corresponding to the static pull-in instability. The size-dependent dynamic behaviour of the system is examined under primary and superharmonic excitations; the frequency- and force-response curves of the system as well as time histories and phase-plane portraits are constructed. Moreover, the effect of taking into account the length-scale parameter on the static and dynamic behaviour of the system is examined by comparing the results obtained by means of the classical and modified couple stress theories.

Nonlinear stability and bifurcations of an axially accelerating beam with an intermediate spring-support

The present work aims at investigating the nonlinear dynamics, bifurcations, and stability of an axially accelerating beam with an intermediate spring-support. The problem of a parametrically excited system is addressed for the gyroscopic system. A geometric nonlinearity due to mid-plane stretching is considered and Hamilton's principle is employed to derive the nonlinear equation of motion. The equation is then reduced into a set of nonlinear ordinary differential equations with coupled terms via Galerkin's method. For the system in the sub-critical speed regime, the pseudo-arclength continuation technique is employed to plot the frequency-response curves. The results are presented for the system with and without a three-to-one internal resonance between the first two transverse modes. Also, the global dynamics of the system is investigated using direct time integration of the discretized equations. The mean axial speed and the amplitude of speed variations are varied as the bifurcation parameters and the bifurcation diagrams of Poincare maps are constructed.

Three-dimensional nonlinear size-dependent behaviour of Timoshenko microbeams

The geometrically nonlinear size-dependent behaviour of a Timoshenko microbeam is examined numerically, taking into account the coupled longitudinal–transverse displacements as well as the rotation. The strain energy of a Timoshenko microbeam is obtained based on the modified couples stress theory. Hamilton’s principle is then employed to derive the nonlinear partial differential equations of motion for the longitudinal, transverse, and rotational motions. The Galerkin scheme is applied to these nonlinear partial differential equations, resulting in a set of nonlinear ordinary differential equations with coupled terms. The nonlinear resonant response of the system is examined by solving the discretized equations of motion via the pseudo-arclength continuation technique and constructing the frequency–response and force–response curves. In particular, the effect of the length scale parameter is investigated by comparing the results obtained using the modified couple and classical theories. The frequency–response curves of the present model are compared to those of the one in which the longitudinal displacement is neglected so as to highlight the importance of taking into account the longitudinal displacement. The effect of other system parameters on the frequency–response and force–response curves is also investigated.

Nonlinear vibrations of laminated and sandwich rectangular plates with free edges. Part 1: Theory and numerical simulations

Nonlinear vibrations of completely free laminated and sandwich rectangular plates are investigated for the first time. The equations of motion are obtained via multi-modal energy approach based on Lagrange equations and by using classical and higher-order shear deformation theories with von Kármán type nonlinearities. The numerical analysis is carried out in two steps. First, the plate displacements and rotations are expanded in terms of Chebyshev polynomials and a linear analysis is performed to obtain the natural frequencies and modes of vibration. Then, the energy functional is discretized by using the natural modes of vibration and a system of nonlinear ordinary differential equations with quadratic and cubic nonlinear terms is obtained. A code based on pseudo arc-length continuation and collocation scheme is used for bifurcation analysis and the near resonance nonlinear vibrations are studied. The frequency-response curves, time histories and phase space diagrams are presented.

Dynamics of a liquid film sheared by a co-flowing turbulent gas

Consider the dynamics of a thin laminar liquid film flowing over an inclined wall in the presence of a co-flowing turbulent gas. The solution to the full two-phase flow problem poses substantial technical difficulties. However, by making appropriate assumptions, the solution process can be simplified and can provide valuable insights. The assumptions allow us to solve the gas and liquid problems independently. Solving for the gas flow reduces to finding perturbations to pressure and tangential stresses at the interface, influencing the liquid problem through the boundary conditions. We analyze the effect of gas flow on the liquid problem by developing an integral-boundary-layer model, which is valid up to moderate liquid Reynolds numbers. We seek solitary-wave solutions of this model under the influence of gas flow via a pseudo-arclength continuation method. Our computations demonstrate that as a general trend, the wave speed increases with increasing the gas shear and the liquid flow rate. Further insight into the problem is provided via time-dependent computations of the integral-boundary-layer model.

Coupled nonlinear size-dependent behaviour of microbeams

The nonlinear resonant behaviour of a microbeam, subject to a distributed harmonic excitation force, is investigated numerically taking into account the longitudinal as well as the transverse displacement. Hamilton’s principle is employed to derive the coupled longitudinal-transverse nonlinear partial differential equations of motion based on the modified couple stress theory. The discretized form of the equations of motion is obtained by applying the Galerkin technique. The pseudo-arclength continuation technique is then employed to solve the discretized equations of motion numerically. Different types of bifurcations as well as the stability of solution branches are determined. The numerical results are presented in the form of frequency-response and force-response curves for different sets of parameters. The effect of taking into account the longitudinal displacement is highlighted.