Torsional oscillations in structures subject to ground motion

A multiple-scale technique has been employed to study nonlinear torsional oscillations in single-storey structures with cubic softening stiffness members subject to a single frequency ground excitation. The structures are studied under free oscillation conditions, primary resonance, and combination resonance. Time history analyses are employed to quantify the dynamic behaviour of a single-storey structure. The instabilities are related to the jump in response which exists in single degree-of-freedom softening oscillators subjected to harmonic excitation. It is shown that this jump can be quantified analytically, and appears as a cusp catastrophe in the bending mode. Time history analyses show that the bending mode can exhibit a limit cycle behaviour of constant amplitude.     

On sensitive elliptic singular perturbation problems and logarithmic oscillations: the case of shells with edges

This work deals with singular perturbation problems depending on small positive parameter ?. The limit problem as ? → 0 has no solution within the classical theory of PDEs, which uses distribution theory. A very particular and less‐known phenomenon appears: large oscillations. These problems exhibit some kind of instability; very small and smooth variations of the data imply large singular perturbations of the solution. That kind of problems appears in elasticity for highly compressible two‐dimensional bodies and thin shells with elliptic middle surface with a part of the boundary free. Here, we consider certain properties of that oscillations and extend the theory to shells with edges. Copyright © 2012 John Wiley & Sons, Ltd.     

Boundedness in nonlinear oscillations

In this paper, the boundedness of all solutions of the nonlinear differential equation (φ_p(x′))′ + αφ_p(x⁺) – βφ_p(x^–) + f(x) = e(t) is studied, where φ_p(u) = |u|^p–2 u, p ≥ 2, α, β are positive constants such that = 2w^–1 with w ∈ ?⁺\?, f is a bounded C⁵ function, e(t) ∈ C⁶ is 2π_p‐periodic, x⁺ = max{x, 0}, x^– = max{–x, 0}. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The global structure of periodic solutions to a suspension bridge mechanical model

We study two systems of nonlinearly coupled ordinary differentialequations that govern the vertical and torsional motions ofa cross-section of a suspension bridge. We observe numericallythat the structure of the set of periodic solutions changesconsiderably when we smooth the nonlinear terms. The smoothednonlinearities describe the force that we wish to model morerealistically and the resulting periodic solutions more accuratelyreplicate the phenomena observed at the Tacoma Narrows Bridgeon the day of its collapse. The main conclusion is that purelyvertical periodic forcing can result in subharmonic primarilytorsional motion.     

完全近似法的推广及其应用*

本文提出完全近似法的一种推广形式:引用渐近线性化的概念,通过对坐标作包含因变量的非线性泛函的变换,把原有的非线性问题线性化,从而以首项渐近解和相应的坐标变换给出原问题的较高阶的近似解析解.对模型方程和若干弱非线性振动和波动问题的分析表明,本文提出的方法是简捷而有效的.     

The analysis of the degenerate case of oscillations of a mechanical system

In this paper, the nonlinear model of the mechanism with two degrees of freedom will be studied. An approximate analytical solution of the differential equation of motion in the series showed the presence of features in the aspiration of the mass of one of the bodies to zero. It also gives an algorithm for finding the points of degeneracy of communication between small perturbations of the function of the problem and the derivatives of these functions at a time. Copyright © 2016 John Wiley & Sons, Ltd.     

Torsional vibrations and dispersion of torsional waves in orthotropic composite rods

The Barr’s refined theory of torsional vibrations of isotropic rods of noncircular cross section is generalized for an orthotropic material. An analysis of natural frequencies of torsional vibration of free-free orthotropic prismatic rods of rectangular cross section is carried out with the help of an exact solution of the frequency equation. For orthotropic CFRP and GFRP rods, the improved theory, which takes into account the normal stresses and inertia forces in the axial direction, in some cases, predicts a noticeable raise in the natural frequencies compared with those following from the Saint-Venant classical theory. A good agreement is obtained between the experimental and calculated values of natural frequencies of torsional vibrations of rectangular quartz and fiber glass rods. The dispersion of torsional waves in an orthotropic quasi-homogeneous rod is considered. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 44, No. 2, pp. 165–182, March–April, 2008.     

弹性圆柱壳扭转屈曲研究

本文给出两端固支的弹性圆柱壳扭转屈曲实验与理论计算结果.实验发现,对于较长的壳,其屈曲后的变形并不占据整个壳体的长度.另外在计算中仅考虑壳体的法向边界条件,而不考虑其周向和轴向边界条件,结果和Yamaki精确解以及本文实验结果相符较好,说明周向和轴向边界条件对圆柱壳的扭转屈曲影响较小.     

传动系统扭振的重频条件

借助于传递矩阵法研究了两轴系扭振的重频条件。必要条件为:存在节点啮合对(JEC);充要条件为:四个边界传递系数(f_B)等于零的个数不少于3个。给出了两轴系存在重频时的振型选择。多级轴系重频的必要条件与两轴系的必要条件相同,而如果所有的相邻JEC之间的传递系数(f_I)等于零时,频率重数等于独立的f_B为零的个数减1.若存在f_I≠0,可将它所对应的连接部分删除,从而把整个轴系的JEC分成内部不含f_I≠0的几组,该阶频率的总重数等于各组独立解个数之和。     

基于扭转模态的角振动压电俘能器研究

以线性压电理论为基础,分析了压电陶瓷圆柱壳的扭转振动问题.由于压电陶瓷柱壳的扭转振动使得覆盖于柱壳两端的电极之间出现电势差,通过导线连接两电极与外负载,则圆柱壳在振动过程中所捕获的能量能有效地供给微电子器件工作.得到了输出电压、电流、能量、效率以及输出功率密度的解析表达式,并数值分析了这类压电俘能器的基本性能.结果表明,这种结构能够用作压电俘能器,将旋转机械能转化成为电能.     

A characterisation of oscillations in the discrete two-dimensional convection-diffusion equation

It is well known that discrete solutions to the convection-diffusion equation contain nonphysical oscillations when boundary layers are present but not resolved by the discretisation. However, except for one-dimensional problems, there is little analysis of this phenomenon. In this paper, we present an analysis of the two-dimensional problem with constant flow aligned with the grid, based on a Fourier decomposition of the discrete solution. For Galerkin bilinear finite element discretisations, we derive closed form expressions for the Fourier coefficients, showing them to be weighted sums of certain functions which are oscillatory when the mesh Péclet number is large. The oscillatory functions are determined as solutions to a set of three-term recurrences, and the weights are determined by the boundary conditions. These expressions are then used to characterise the oscillations of the discrete solution in terms of the mesh Péclet number and boundary conditions of the problem.

     

Specific heat oscillations in quasi-periodic structures

Most models on quasi-crystals, as well as the related experimental results, exhibit fractal energy spectra. We discuss the relevant thermodynamic implications of this feature by performing both analytical and numerical calculations of the specific heats associated with successive hierarchical approximations to fractal energy spectra. Instructive anomalies are displayed, namely the specific heat exhibits an infinite number of small-amplitude oscillations symmetrically disposed around a characteristic dimension of the spectrum. We also analyse a multifractal spectrum generated dynamically by the logistic map and conjecture a possible connection with Tsallis' generalized thermostatics.     

WFQEM‐based perturbation approach and its applications in analyzing nonlinear periodic structures

Based on the weak form quadrature element method, a perturbation approach is developed. Waves propagating in periodic beams on a nonlinear elastic foundation are studied by using the new proposed method. The feasibility and accuracy of the proposed method are verified by comparing the present results with those available in literatures in linear cases. Detailed modal analysis of the linear cases is conducted in order to obtain the dispersion relations of the nonlinear cases. The theoretical results show that the dispersion relations of the nonlinear cases are amplitude dependent. Furthermore, the effects of geometric parameters and degree of nonlinearity on the amplitude‐dependent dispersion relations are discussed in detail. This work provides a new method for analyzing the dispersion relations of nonlinear periodic structures and gives some useful guidelines for designing periodic beams or pipelines with nonlinear structure–foundation interaction. Copyright © 2016 John Wiley & Sons, Ltd.     

分析结构非线性问题的杂交可变基Galerkin方法

基于渐近摄动理论和Galer-kin方法,本文提出分析结构非线性问题的杂交可变基Galer-kin方法。本文方法首次引入可变基函数的概念,可大幅度降低计算量,而且在有限元法等数值方法中易于推广应用,在解决非线性问题领域有广泛应用前景。最后本文分析圆板大挠度问题和扁球壳大挠度问题,以验证本文方法的有效性。     

Dynamic visual cryptography based on chaotic oscillations

Dynamic visual cryptography scheme based on chaotic oscillations is proposed in this paper. Special computational algorithms are required for hiding the secret image in the cover moiré grating, but the decryption of the secret is completely visual. The secret image is leaked in the form of time-averaged geometric moiré fringes when the cover image is oscillated by a chaotic law. The relationship among the standard deviation of the stochastic time variable, the pitch of the moiré grating and the pixel size ensuring visual decryption of the secret is derived. The parameters of these chaotic oscillations must be carefully preselected before the secret image is leaked from the cover image. Several computational experiments are used to illustrate the functionality and the applicability of the proposed image hiding technique.     

Market oscillations induced by the competition between value-based and trend-based investment strategies

We consider financial market using mathematical models which incorporate an excess demand function that depends not only upon the price but on the price derivative. The classical (value-based) motivation for purchasing the equity is augmented with a trend-based strategy of buying due to rising prices. An analysis (based on money flow and the finiteness of assets) of the supply, demand and price as a function of time leads to a system of ordinary differential equations which is mathematically complete. The numerical study of our equations exhibits overshooting, abrupt reversals and oscillations in prices. We examine our models within the context of real markets and economic laboratory experiments by comparing its predictions with a set of Porter and Smith experiments and with all US stock market “crashes” since 1929.