一个非线性奇异振子的谐波平衡解

应用谐波平衡法计算了一个恢复力与因变量成反比的非线性振子的近似频率和近似周期解。与Mickens的方法不同,直接求解了非线性奇异二阶微分方程。一阶和二阶谐波解所对应的非线性恢复力的傅立叶级数展开式的系数容易由相应的积分得到。由二阶谐波平衡法得到的非线性代数方程组很容易用符号运算软件求出。得到的一近似频率与精确频率的百分比误差是12.8%,而二阶近似频率与精确频率的百分比误差小于1.28%。与数值方法给出的“精确”周期解比较,二阶近似解析解要比一阶近似解析解精确得多。高阶谐波平衡法一般需要求解复杂的非线性代数方程组,但是借助于Matlab和Mathematica等符号运算软件,这一困难可以得到一定程度的克服.     

达芬-谐波振子的改进解析逼近解

研究达芬-谐波振子的解析逼近解。所谓达芬-谐波振子是指当位移远小于1时，系统可化为三次非线性振子，而当位移远大于1时，该系统则化为线性谐波振子。通过将变形后的控制方程的线性化与谐波平衡法组合起来，我们建立了达芬-谐波振子频率及周期解的改进解板逼近。改进的解析逼近在振幅的全部取值范围内，包括振幅趋于零及无穷的情况，都有令人非常满意的精度。     

非线性振动方程多重解求解方法

非线性振动方程多重解的求解过程中,迭代初值难以有效确定,不稳定周期解收敛域很小,利用通常的微分方程解法无法直接求解。针对这个问题,引入同伦算法,使得初始值的选取无任何限制;同时利用预测-校正算法对外激励参数变化下的解曲线进行追踪,得到系统的多重解。该方法不但可以计算稳定的周期解,而且不稳定的周期解也可以求出。采用Duffing振子运动方程对该方法进行了计算验证,通过与理论近似解以及龙格-库塔法计算结果的对比,验证了该方法的有效性。     

退化环面上的非线性jerk方程近似周期解的同伦分析方法

用同伦分析法求解退化环面上的非线性Jerk方程的近似周期和近似解析周期解。所得结果表明文中得到的一阶近似周期和一阶近似解析周期解与Gottlieb用低阶谐波平衡法求解得到的结果一样。当参数和初速度较大时,一阶近似周期与精确周期的百分比误差是4.831 8%,而二阶近似周期与精确周期的百分比误差小于0.219 9%。与数值方法给出的"精确"周期解比较,二阶近似解析周期解比一阶近似解析周期解要精确的多。因此,同伦分析法是求解非线性Jerk方程的一种非常有效的方法。     

带三次恢复力项频率依赖于速度的非线性振子研究

带三次恢复力项频率依赖于速度（Velocity-Dependent-Frequency, VDF）的非线性振子 的周期及其性质目前没有文献讨论,且使用传统的摄动法或谐波平衡法求解这类振子一阶近似解时往往失效。特别的,其频率在有限的幅值范围内奇异。首先求得了该振子周期的积分表达式,基于积分表达式可积性条件采用谐波平衡法得到了该振子一类初始条件下的精确解;研究了该振子的周期性质,给出了由第一类完全椭圆积分表示的周期-振幅的近似解析表达式,分析了振子的方波现象及产生原因。研究表明,振子周期最终随着幅值的增大衰减到0;振子方波现象产生原因是由于系统参数 ,随着幅值的增大,方波现象更明显。最后提出使用一种Hermite插值法求解该振子的周期解,该方法将时间变量转换为新的谐振时间变量,其频率为振子频率的一半,对应的控制微分方程转变为适合于Hermite插值分析的形式,其解与数值解的对比证明了该方法的有效性。     

五次非线性自激系统的同宿解及其分岔

推广了双曲函数Lindstedt-Poincaré (L-P)法摄动步骤,定量求解一类派生系统含五次强非线性项的自激振子的同宿解及其分岔值。对极限环的同宿分岔参数进行摄动展开,给出同宿摄动解奇异项的定义,以消除同宿摄动解奇异项作为确定极限环同宿分岔点的条件,给出能够严格满足同宿条件的同宿轨道显式摄动解,推导出任意阶解和同宿分岔点判别的一般表达式。应用该法具体分析了一类推广的Liénard振子的同宿解和同宿分岔问题,指出方法的优点和存在的问题。算例表明,在相平面内该方法的结果与Runge-Kutta法数值周期轨道的逼近结果较吻合,相应的同宿分岔点判定值也具备较好的精度。该方法可以进一步研究推广应用于分析其它形式更一般的系统的同(异)宿解和同(异)宿分岔问题。     

单摆大振幅振动的解析逼近解

构造了单摆大振幅振动的高精度解析逼近周期和周期解.首先,利用Maclaurin展开和Chebyshev多项式加速技术将单摆振动方程中的正弦型恢复力用三次多项式近似代替,得到一个Duffing型方程;然后,将牛顿法与谐波平衡法结合起来建立Duffing方程的解析逼近周期及周期解,从而给出单摆振动的解析逼近解.因此,在求解过程中避免了关于参数的非线性代数方程组的出现,只需解线性代数方程组就能建立单摆振动的解析逼近周期及周期解.几乎在振幅(初始摆角)的全部取值范围内,与数值方法给出"精确"周期及周期解比较,得到的解析逼近解都有很高的逼近精度.     

应用优化的同伦分析法求解非线性Jerk方程

应用优化的同伦分析法计算了具有三次非线性项的三阶微分方程(Jerk)的近似周期和近似解析周期解。文中给出一个算例说明由优化的同伦分析法可以容易得到精确的二阶近似周期解。当初速度 比较大时,一阶近似周期与精确周期的百分比误差为-0.415%,而二阶近似周期与精确周期的百分比误差为-0.0298%。与数值方法给出的“精确”周期解比较,一阶近似解析周期解和二阶近似周期解的精度很高。这个说明同伦分析法对求解非线性Jerk方程非常有效     

非线性耦合振子间的靶能量传递研究:保守系统中的完全能量传递

研究了两自由度非线性耦合振子间的靶能量传递现象。第一部分基于非线性耦合振子内在保守系统的慢变动力学方程,推导出了该结构的保守系统中两振子间完全能量传递所需要的条件。经数值仿真验证,本部分所推导的方程可准确计算两振子间完全能量传递所需的初始能量,并适用于强非线性系统。     

旋转几何非线性复合材料薄壁梁的自由振动分析

研究具有几何非线性的旋转复合材料薄壁梁的自由振动。梁的变形引入了Von Kármán几何非线性, 基于Hamilton原理和变分渐进法 (Variational-Asymptotical Method -VMA),导出旋转复合材料薄壁梁的非线性振动偏微分方程组。采用Galerkin法将振动方程离散化为常微分方程组。借助于谐波平衡法 (Harmonic Balance Method -HBM) 建立自由振动的振幅-非线性固有频率关系方程。将上述方程化为非线性特征值问题,采用迭代算法进行求解。将所建立的旋转复合材料薄壁梁非线性自由振动分析模型和计算方法,应用于周向均匀刚度配置(Circumferentially Uniform Stiffness –CUS) 构型复合材料薄壁梁,通过数值计算揭示了纤维铺层角、旋转速度对非线性振动固有频率-振幅关系的影响。     

时滞微分方程特征值的近似求解方法

通过将时滞微分方程变换为积分方程后并进行离散,得到相应的差分方程,从理论上建立了差分方程的系统矩阵的特征值与原线性时滞微分系统的特征值之间的关系,从而得到了一种求解时滞微分方程特征值的新方法。作为算例,计算了时滞的Logistic方程的前10阶特征值。误差分析表明,提出的近似方法不但简单,并且有很高的精度。     

Positive solutions for singular nonlocal boundary value problems

In this paper we present some existence results for singular nonlocal boundary value problems. Our nonlinearity may be singular in its dependent variable and it is allowed to change sign.     

分数阶单自由度间隙振子的受迫振动

研究了含有分数阶微分项的单自由度间隙振子的受迫振动,利用KBM渐近法获得了系统的近似解析解。分析了分段线性系统的主共振,得到了分数阶阶次在0～2时分数阶项的统一表达式;发现分数阶微分项在分段系统中以等效线性阻尼和等效线性刚度的形式影响着系统的动力学特性,而间隙以等效非线性刚度的形式影响着系统的动力学特性。获得了主共振幅频响应的表达式,并得到了系统的稳定性条件;比较了系统主共振幅频响应的近似解析解和数值解,发现两者符合程度较高,验证了近似解析解的正确性;详细分析了分数阶项和间隙对系统主共振幅频响应的影响。研究表明KBM渐近法是分析分数阶分段光滑系统动力学的有效方法。     

Analytical approximations to the solutions for a generalized oscillator with strong nonlinear terms

This paper is focused on solving the generalized second-order strongly nonlinear differential equation ${\ddot{x}+\sum_i {c_i^2 }x \left| x \right|^{i-1}=0}$ which describes the motion of a conservative oscillator with restoring force of series type with integer and noninteger displacement functions. The approximate analytical solution procedures are modified versions of the simple solution approach, the energy balance method, and the frequency?Camplitude formulation including the Petrov?CGalerkin approach. For the case where the linear term is dominant in comparison with the other series terms of the restoring force, the perturbation method based on the solution of the linear differential equation is applied. If the dominant term is nonlinear and the additional terms in the restoring force are small, the perturbation method based on the approximate solution of the pure nonlinear differential equation is introduced. Using the aforementioned methods, the frequency?Camplitude relations in the first approximation are obtained. The suggested solution methods are compared and their advantages and disadvantages discussed. A numerical example is considered, where the restoring force of the oscillator contains a linear and also a noninteger order term (i?=?5/3). The analytically obtained results are compared with numerical results as well as with some approximate analytical results for special cases from the literature.     

Analytical solutions for vibrations of laminated and sandwich plates using mixed theory

A semi-analytical method has been presented in this paper to evaluate the natural frequencies as well as displacement and stress eigenvectors for simply supported, cross-ply laminated and sandwich plates by using higher order mixed theory. Models based on equivalent single layer as well as layerwise (LW) theories have been formulated. By assuming a non-linear variation of axial displacements through the plate thickness, the warping of the transverse cross-section has been considered. Hamilton’s principle has been employed to derive the equilibrium equations. The proposed LW model fulfills a priori the continuity of displacements as well as the transverse and the normal stress components at each interface between two adjacent layers. Results obtained by present higher order mixed theory have been found in good agreement with those obtained by three-dimensional elasticity solutions. After establishing the accuracy of present results for orthotropic plates, new results for thin and thick sandwich plates have been presented which can serve as benchmark solutions for future investigations.     

基于Melnikov方法的分数阶Duffing振子混沌阈值解析研究

研究了含有分数阶微分项的Duffing振子的分岔与混沌行为,利用等效刚度和等效阻尼的概念对分数阶微分项进行处理,将分数阶微分项等效成三角函数与指数函数的形式,用Melnikov方法分析了分数阶Duffing振子产生分岔与混沌的必要条件,得到了其解析结果.进行了解析解和数值解的比较,证明了解析结果的精确度,并通过仿真计算...     

Analytical solutions for flexure of clamped rectangular cross-ply plates using an accurate zig–zag type higher-order theory

Clamped cross-ply plates are analyzed here using a higher-order theory based on a realistic thickness-wise displacement field that accounts for a zig–zag variation of the in-plane displacements. Rigorous analytical solutions are obtained for this difficult problem using a methodology developed recently wherein simple Fourier series solutions of a corresponding simply-supported plate are superposed, each such component solution being summed without truncation using MATHEMATICA. Numerical results, for square homogeneous and three-layer plates under uniform and central point loads, are compared with those based on the classical and first-order theories.     

Analytical solutions of plane structures using Mathematica

A study investigating the possibilities of using the program Mathematica to obtain analytical solutions ofplane structures is presented. Two typical applications are developed: plane frames with only bending deformations and plane trusses. In an attempt to keep the number of unknowns as low as possible, the indirect stiffness method and the force method are used to state the first and the second problems, respectively. All the formulations are developed by making use of kernel and generalized inverse matrices, which are also used to characterize the solution of overdetermined systems of equations. Two programs (PlaneFrame and PlaneTruss) were written to solve the first and second problems and the listing of the codes are given in the paper. The results obtained show that problems of a certain complexity can be solved. Copyright © 2001 John Wiley & Sons, Ltd.     

Analytical solutions using a higher-order refined theory for the static analysis of antisymmetric angle-ply composite and sandwich plates

Analytical formulations and solutions to the static analysis of simply supported anti-symmetric angle-ply composite and sandwich plates hitherto not reported in the literature based on a higher-order refined theory already reported in the literature are presented. The theoretical model presented herein incorporates laminate deformations, which account for the effect of transverse shear deformation and a non-linear variation of in-plane displacements with respect to the thickness coordinate. The transverse displacement is assumed to be constant throughout the thickness. The equations of equilibrium are obtained using principle of minimum potential energy. Solutions are obtained in closed form using Navier's technique by solving the boundary value problem. Accuracy of the theoretical formulations and the solution method is first ascertained by comparing the results with that already reported in the literature. After establishing the accuracy of the solutions, numerical results with real properties are presented for the multilayer antisymmetric angle-ply composite and sandwich plates, which will serve as a benchmark for future investigations.