A new finite element method for analysing symmetrically loaded thin shells of revolution

An essential feature of finite element methods of analysis for symmetrically loaded shells of revolution is the setting up of equations representing the response of a short ‘ring’ element to edge loading. In this paper the axisymmetric behaviour of a short elastic cylindrical shell element under edge loading is described in a new way by means of a matrix which is a combination of stiffness, flexibility and ‘neutral’ sub-matrices. The coefficients of the matrix are derived direct from the equations of the problem, which involves a trivial amount of work in comparison with conventional methods. The corresponding matrix for a short section of an arbitrary shell of revolution is set up with little additional effort, and its use is described for calculation of edge response coefficients for portions of spherical shells. Finally, the method is used to study by iteration the behaviour of a thin spherical shell of viscous material containing a rigid boss which is loaded radially inwards: changes in meridional profile are followed as deformation proceeds. Results are presented for both linear and non-linear viscous material.     

An algorithm for exact eigenvalue calculations for rotationally periodic structures

An existing algorithm ensures that no eigenvalues are missed when using the stiffness matrix method of structural analysis, where the eigenvalues are the natural frequencies of undamped free vibration analyses or the critical load factors of buckling problems. The algorithm permits efficient multi-level substructuring and gives ‘exact’ results when the member equations used are those obtained by solving appropriate differential equations. The present paper extends this algorithm to cover rotationally periodic (i.e. cyclically symmetric) three-dimensional structures which are analysed by using complex arithmetic to obtain a stiffness matrix which involves only one of the rotationally repeating portions of the structure. Nodes and members are allowed to coincide with the axis of rotational periodicity and the resulting modes are classified. Rigid body freedoms are accounted for empirically, and the ‘exact’ member equations and efficient multi-level substructuring of the earlier algorithm can be used when assembling the stiffness matrix of the repeating portion.     

Velocity potential computation by finite differences for compressible flow in ducts

Preliminary studies of computation with velocity potential are made with a view to the analysis of complex three-dimensional flows. The methods used are applicable more generally to quasilinear elliptic problems with derivative boundary conditions on irregular domains. Second order finite difference approximations are constructed in simple form for plane ducts of general shape by using an irregular net. Derivative boundary conditions are handled quite easily. An iterative method is described which corresponds to freezing the coefficients in the quasilinear differential equation for velocity potential. The discretization is such that this is a ‘generalized Newton’ method for the non-linear algebraic equations. Good convergence has been found in practice even when there are small supersonic zones. The discretization accuracy is tested by comparisons with the exact solution for incompressible flow between confocal hyperbolas.     

A two-dimensional finite element algorithm for the simultaneous solution of the semiconductor device equations with automatic convergence

The proposed algorithm solves equations governing the behaviour of semiconductor devices using a finite element technique. Electrostatic potential and the hole and electron quasi-Fermi potentials are chosen as the solution variables. The equation set is written in a steady-state form using these three variables and this gives rise to a system of three nonlinear partial differential equations. The equations, which are intimately coupled, are solved simultaneously using a weighted residual formulation. Convergence of the nonlinear solution procedure using any initial guess is guaranteed by employing ‘incremental loading’ coupled to a test for divergence that is applied at each iterative step. The triangular elements used in the program are automatically generated from a mesh of eight-node isoparametric elements that is itself an automatically generated subdivision of a small number of eight-node (super) elements. A novel method of generating an initialisation state using the boundary element method is also described.     

A solution method for linear and geometrically nonlinear MDOF systems with random properties subject to random excitation

A method for computing the lower-order moments of response of randomly excited multi-degree-of-freedom (MDOF) systems with random structural properties is proposed. The method is grounded in the techniques of stochastic calculus, utilizing a Markov diffusion process to model the structural system with random structural properties. The resulting state-space formulation is a system of ordinary stochastic differential equations with random coefficients and deterministic initial conditions which are subsequently transformed into ordinary stochastic differential equations with deterministic coefficients and random initial conditions. This transformation facilitates the derivation of differential equations which govern the evolution of the unconditional statistical moments of response. Primary consideration is given to linear systems and systems with odd polynomial nonlinearities, for in these cases there is a significant reduction in the number of equations to be solved. The method is illustrated for a five-story shear-frame structure with nonlinear interstory restoring forces and random damping and stiffness properties. The results of the proposed method are compared to those estimated by extensive Monte-Carlo simulation.     

A FINITE POINT METHOD IN COMPUTATIONAL MECHANICS. APPLICATIONS TO CONVECTIVE TRANSPORT AND FLUID FLOW

The paper presents a fully meshless procedure fo solving partial differential equations. The approach termed generically the ‘finite point method’ is based on a weighted least square interpolation of point data and point collocation for evaluating the approximation integrals. Some examples showing the accuracy of the method for solution of adjoint and non-self adjoint equations typical of convective-diffusive transport and also to the analysis of compressible fluid mechanics problem are presented.     

A comparison study of the LMAPS method and the LDQ method for time-dependent problems

This paper compares numerical solutions of spatial-temporal partial differential equations based on two RBF-based meshless methods: the local method of approximate particular solutions (LMAPS) and the local RBFs-based DQ method (LDQ). To avoid the ill-conditioned problems of the global version, the weighting coefficients at the supporting points are determined by solving low-order linear systems instead of large dense linear systems. The Runge–Kutta method is adopted for time stepping schemes. The numerical experiments have shown that the LMPAS method and the LDQ method are capable of solving the initial boundary value problem for spatial-temporal partial differential equations with high accuracy and efficiency.     

Group method solutions of the generalized forms of Burgers, Burgers�CKdV and KdV equations with time-dependent variable coefficients

Solutions for the generalized forms of Burgers, Burgers?CKdV, and KdV equations with time-dependent variable coefficients and with initial and boundary conditions are constructed. The analysis rests mainly on the standard group method. Similarity solutions are found which reduce the nonlinear system of partial differential equations to systems of ordinary differential equations to obtain some exact solutions and others as numerical solutions.     

The nonlinear vibration of an initially stressed laminated plate

Nonlinear partial differential equations of motion for a laminated plate in a general state of non-uniform initial stress are presented in various plate theories. This study uses Lo’s displacement field to derive the governing equations. The higher-order terms in Lo’s theory can be disregarded, to obtain the equations of simpler forms and even other theories for laminated plate. These nonlinear partial equations are transformed to ordinary nonlinear differential equations using the Galerkin method. The Runge–Kutta method is used to obtain the ratio of nonlinear frequency to linear frequency. The numerical solutions of an initially stressed laminate plate based on various plate theories obtained by the Galerkin and Runge–Kutta method are presented herein. Using these equations with various theories, the nonlinear vibration behavior of laminated plate is studied. The results show that apparent discrepancies exist among the various displacement fields, which indicates the transverse shear strain, normal strain and initial stress state have great effect on the vibration behavior of laminate plate under nonlinear vibration.     

低速碰撞下复合材料叠层板的微裂纹演化及其对弹性模量的影响

采用双线性特性破坏模型研究了复合材料叠层板各层内部开裂裂纹的演化;通过引入弹性模量的裂纹影响系数表示,推导出裂纹影响系数与应变及应变率之间的微分关系,并得到裂纹耗散功率与裂纹影响系数变化率之间的关系。通过计算不同初始碰撞速度下复合材料叠层板的应变、应变率响应以及裂纹影响系数的演化,得到整个冲击过程中各层内任意点附近裂纹开裂情形及其对弹性模量的影响;通过检查界面各点处的裂纹影响系数是否发生改变,预测了碰撞完成之后复合材料叠层板中各层内微裂纹的分布区域位置与大小;并将该预测结果与其他破坏准则计算结果进行了比较。计算结果表明,在碰撞过程中各层内任意点处的应力值超过其屈服强度后,该点附近的弹性模量开始发生衰减,衰减大小随铁球初始碰撞速度的增大而增大。在四边夹支的边界条件下,复合材料叠层板的裂纹分布区域同样最先出现在碰撞点及边界中点位置,区域面积随初始碰撞速度的增大不断扩大     

Postbuckling characteristics of nanobeams based on the surface elasticity theory

In the present paper, an attempt is made to numerically investigate the postbuckling response of nanobeams with the consideration of the surface stress effect. To accomplish this, the Gurtin–Murdoch elasticity theory is exploited to incorporate surface stress effect into the classical Euler–Bernoulli beam theory. The size-dependent governing differential equations are derived and discretized along with various end supports by employing the principle of virtual work and the generalized differential quadrature (GDQ) method. Newton’s method is applied to solve the discretized nonlinear equations with the aid of an auxiliary normalizing equation. After solving the governing equations linearly, to obtain each eigenpair in the nonlinear model, the linear response is used as the initial value in Newton’s method. Selected numerical results are given to show the surface stress effect on the postbuckling characteristics of nanobeams. It is found that by increasing the thickness of nanobeams, the postbuckling equilibrium path obtained by the developed non-classical beam model tends to the one predicted by the classical beam theory and this anticipation is the same for all selected boundary conditions.     

Finite calculus formulation for incompressible solids using linear triangles and tetrahedra

Many finite elements exhibit the so‐called ‘volumetric locking’ in the analysis of incompressible or quasi‐incompressible problems.In this paper, a new approach is taken to overcome this undesirable effect. The starting point is a new setting of the governing differential equations using a finite calculus (FIC) formulation. The basis of the FIC method is the satisfaction of the standard equations for balance of momentum (equilibrium of forces) and mass conservation in a domain of finite size and retaining higher order terms in the Taylor expansions used to express the different terms of the differential equations over the balance domain. The modified differential equations contain additional terms which introduce the necessary stability in the equations to overcome the volumetric locking problem. The FIC approach has been successfully used for deriving stabilized finite element and meshless methods for a wide range of advective–diffusive and fluid flow problems. The same ideas are applied in this paper to derive a stabilized formulation for static and dynamic finite element analysis of incompressible solids using linear triangles and tetrahedra. Examples of application of the new stabilized formulation to linear static problems as well as to the semi‐implicit and explicit 2D and 3D non‐linear transient dynamic analysis of an impact problem and a bulk forming process are presented. Copyright © 2004 John Wiley & Sons, Ltd.     

A dual mesh multigrid preconditioner for the efficient solution of hydraulically driven fracture problems

We present a novel multigrid (MG) procedure for the efficient solution of the large non‐symmetric system of algebraic equations used to model the evolution of a hydraulically driven fracture in a multi‐layered elastic medium. The governing equations involve a highly non‐linear coupled system of integro‐partial differential equations along with the fracture front free boundary problem. The conditioning of the algebraic equations typically degrades as O(N³). A number of characteristics of this problem present significant new challenges for designing an effective MG strategy. Large changes in the coefficients of the PDE are dealt with by taking the appropriate harmonic averages of the discrete coefficients. Coarse level Green's functions for multiple elastic layers are constructed using a single dual mesh and superposition. Coarse grids that are sub‐sets of the finest grid are used to treat mixed variable problems associated with ‘pinch points.’ Localized approximations to the Jacobian at each MG level are used to devise efficient Gauss–Seidel smoothers and preferential line iterations are used to eliminate grid anisotropy caused by large aspect ratio elements. The performance of the MG preconditioner is demonstrated in a number of numerical experiments. Copyright © 2005 John Wiley & Sons, Ltd.     

Least-squares differential quadrature time integration scheme in the dual reciprocity boundary element method solution of diffusive–convective problems

Least-squares differential quadrature method (DQM) is used for solving the ordinary differential equations in time, obtained from the application of dual reciprocity boundary element method (DRBEM) for the spatial partial derivatives in diffusive–convective type problems with variable coefficients. The DRBEM enables us to use the fundamental solution of Laplace equation, which is easy to implement computationally. The terms except the Laplacian are considered as the nonhomogeneity in the equation, which are approximated in terms of radial basis functions. The application of DQM for time derivative discretization when it is combined with the DRBEM gives an overdetermined system of linear equations since both boundary and initial conditions are imposed. The least squares approximation is used for solving the overdetermined system. Thus, the solution is obtained at any time level without using an iterative scheme. Numerical results are in good agreement with the theoretical solutions of the diffusive–convective problems considered.     

A time–space finite element discretization technique for the calculation of the electromagnetic field in ferromagnetic materials

The stability of time-stepping methods for parabolic differential equations is mostly a critical issue. Furthermore, solving such equations with a classical time-stepping approach can be very expensive because many small time-steps have to be taken if steep gradients occur in the solution, even if these occur only in a narrow part of the space domain. In this paper we present a discretization technique in which finite element approximations are used in time and space simultaneously for a relatively large time period called a ‘time-slab’. This technique may be repeatedly applied to obtain further parts of the solution in subsequent time-intervals. It will be shown that, with the proposed method, the solution can be computed cheaply even if it has steep gradients and that stability is automatically guaranteed. For the solution of the non-linear algebraic equations on each time-slab fast iterative methods can be used.     

分数阶脉冲微分方程初值问题解的存在性

利用Green函数可以将分数阶微分方程初值问题转化为等价的积分方程.近来此方法被应用于讨论非线性分数阶微分方程初值问题解的存在性.本文讨论菲线性分数阶脉冲微分方程初值问题,应用Green函数,将其转化为等价的积分方程,并设非线性项满足Carathéodory条件,利用非紧性测度的性质和M(o)nch,8不动点定理证明解的存在性.     

An introduction to BEM by integral transforms

The paper provides a new method for obtaining the fundamental integral representation used in BEM. The method is based on integral transforms and can be applied to all linear differential equations with constant coefficients. Some explicit formulae pertaining to potential problems, linear elasticity and low Reynolds number flows are derived.     

Method of particular solutions for solving PDEs of the second and fourth orders with variable coefficients

The paper presents a new meshless numerical method for solving partial differential equations of the second and fourth orders with variable coefficients. The key idea of the method is the use of modified particular solutions which satisfy the homogeneous boundary conditions of the problem. This allows us to seek an approximate solution in the form which satisfies the boundary conditions of the initial problem. As a result we separate the approximation of the boundary conditions and the PDE inside the solution domain. Numerical experiments are carried out for accuracy and convergence investigations. A comparison of the numerical results obtained in the paper with the exact solutions or other numerical methods indicates that the proposed method is accurate in dealing with PDEs with variable coefficients.     

On the implementation of iterated defect correction to finite difference methods for elliptic boundary-value problems

We present a method which makes it possible to apply the idea of iterated defect correction to finite difference methods for the numerical treatment of partial differential equations. The method yields numerical approximations of very high accuracy for the solution, while the corresponding algebraic systems of equations still have ‘reasonable’ size.     

Levi functions for linear elliptic systems with variable coefficients including shell equations

This paper gives an algorithm to construct Levi functions of arbitrary degree for elliptic systems of linear partial differential equations with variable (real-analytic) coefficients. Further, an indirect method is described to transform elliptic boundary value problems into a system of integral equations. This method is applied to the shell equations in the non-shallow case. (In the shallow case the shell equations have constant coefficients.) Some questions of discretization are discussed and numerical results are presented.