Computation of the time‐history response of dynamic problems using the boundary element method and modal techniques

The boundary element method in combination with modal techniques is used to calculate the response of transient excited structures in the time domain numerically. If the system matrices of a structure are evaluated with a fundamental solution in the frequency domain these matrices become functions of frequency which normally cannot be expressed analytically. The associated eigenvalue problem therefore is non‐linear and difficult to solve. For simplification a series expansion formula for the fundamental solution is used in different frequency ranges. Then the eigenvalue problem can be linearized and solved by direct or iterative methods. By using the orthogonal properties of the eigenfunctions, the normal modes of the dynamic problem can be uncoupled as is well known in vibration analysis. That way the transient response of a dynamic excited system in the time domain can be determined without difficulties. Displacements and stresses at different points of the structure are the result. Difficulties in the formulation of time‐dependent problems using the boundary element method can be avoided. There is no problem in considering modal damping factors, for general damping characteristics the associated fundamental solutions have to be found. Several examples are studied in the paper to illustrate how the new method can be applied. Copyright © 1999 John Wiley & Sons, Ltd.     

BOUNDARY ELEMENT–FINITE ELEMENT COUPLED EIGENANALYSIS OF FLUID–STRUCTURE SYSTEMS

The eigenanalysis of acoustical cavities with flexible structure boundaries, such as a fluid-filled container or an automobile cabin enclosure, is considered. An algebraic eigenvalue problem formulation for the fluid–structure problem is presented by combining the acoustic fluid boundary element eigenvalue analysis method and the structural finite elements. For many practical eigenproblems, use of finite elements to discretize the fluid domain leads to large stiffness and mass matrices. Since the acoustic boundary element discretization requires putting nodes only on the wetted surface of the structure, the size of the eigenproblem is reduced considerably, thus reducing the eigenvalue extraction effort. Futhermore, unlike in ordinary cases, the finite element discretization of pressure–displacement based fluid–structure problem gives rise to unsymmetric matrices. Therefore, the fact that the boundary element formulation produces unsymmetric matrices does not introduce additional difficulties here compared to the finite element case in the choice of an eigenvalue extraction procedure. Examples are included to demonstrate the fluid–structure eigenanalysis using boundary elements for the fluid domain and finite elements for the structure.     

超椭圆中厚板的自由振动

本文用一种新型的数值方法棗微分容积法求解任意边界条件下超椭圆形中厚板的自由振动问题。通过微分容积法将中厚板自由振动的控制微分方程和边界条件离散成为一组齐次的线性代数方程,这是一典型的特征值问题,用子空间迭代法可求出其特征值和特征向量。文中通过一些数值算例研究了该方法的收敛性和数值精度,展示了该方法的可行性和有效性。     

圆板轴对称自由振动的微分容积解法

本文用一种新型的数值方法——微分容积法求解任意边界条件下圆形中厚板的轴对称自由振动问题。通过微分容积法将中厚板轴对称自由振动的控制微分方程和边界条件离散成为一组关于各配点位移的齐次的线性代数方程，这是一典型的特征值问题，求解该特征值问题便可求得板的自振频率和振型。文中用一些数值算例研究了该方法的收敛性和数值精度，展示了该方法的可行性和有效性。     

Eigenvalue analysis for acoustic problem in 3D by boundary element method with the block Sakurai–Sugiura method

This paper presents accurate numerical solutions for nonlinear eigenvalue analysis of three-dimensional acoustic cavities by boundary element method (BEM). To solve the nonlinear eigenvalue problem (NEP) formulated by BEM, we employ a contour integral method, called block Sakurai–Sugiura (SS) method, by which the NEP is converted to a standard linear eigenvalue problem and the dimension of eigenspace is reduced. The block version adopted in present work can also extract eigenvalues whose multiplicity is larger than one, but for the complex connected region which includes a internal closed boundary, the methodology yields fictitious eigenvalues. The application of the technique is demonstrated through the eigenvalue calculation of sphere with unique homogenous boundary conditions, cube with mixed boundary conditions and a complex connected region formed by cubic boundary and spherical boundary, however, the fictitious eigenvalues can be identified by Burton–Miller's method. These numerical results are supported by appropriate convergence study and comparisons with close form.     

圆弧曲梁面内自由振动的微分容积解法

用微分容积法求解圆弧曲梁在面内的自由振动问题。通过微分容积法将曲梁自由振动的控制微分方程和边界约束方程离散成为一组线性齐次代数方程组，这是一典型的特征值问题，求解这一特征值问题可以求得其自由振动的圆频率。中采用了考虑轴向变形、剪切变形和转动效应的理论，并采用子空间迭代法求解频率方程。数值算例表明，本方法稳定收敛、精度较高，对圆弧曲梁问题简单、有效。     

Radial integration boundary element method for acoustic eigenvalue problems

In this paper, the radial integration boundary element method is developed to solve acoustic eigenvalue problems for the sake of eliminating the frequency dependency of the coefficient matrices in traditional boundary element method. The radial integration method is presented to transform domain integrals to boundary integrals. In this case, the unknown acoustic variable contained in domain integrals is approximated with the use of compactly supported radial basis functions and the combination of radial basis functions and global functions. As a domain integrals transformation method, the radial integration method is based on pure mathematical treatments and eliminates the dependence on particular solutions of the dual reciprocity method and the particular integral method. Eventually, the acoustic eigenvalue analysis procedure based on the radial integration method resorts to a generalized eigenvalue problem rather than an enhanced determinant search method or a standard eigenvalue analysis with matrices of large size, just like the multiple reciprocity method. Several numerical examples are presented to demonstrate the validity and accuracy of the proposed approach.     

Acoustic boundary element eigenproblem with sound absorption and its solution using Lanczos algorithm

A damped system eigenvalue analysis of acoustical cavities using the boundary element method is presented. The acoustic boundary element eigenproblem formulation found in the literature is extended to include sound absorption in acoustical cavities. A dissipative term is included in the eigenvalue matrix equation to account for boundary absorption. The resulting damped system eigenvalue problem is solved using a new Lanczos subspace algorithm for quadratic eigenproblems. Since the boundary element matrices are unsym-metric, the Lanczos algorithm presented is in its most general form for unsymmetric quadratic eigenprob-lems. Examples are presented to show the application of the method in computing the eigenfrequencies of acoustic cavities with sound absorption.     

A symmetric boundary integral approach to transient poroelastic analysis

 The problem of the transient quasi-static analysis of a poroelastic body subjected to a history of external actions is formulated in terms of four boundary integral equations, using time-dependent Green's functions of the “free” poroelastic space. Some of these Green's functions, not available in the literature are derived “ad hoc”. The boundary integral operator constructed is shown to be symmetric with respect to a time-convolutive bilinear form so that the boundary solution is characterized by a variational property and its approximation preserving symmetry can be achieved by a Galerkin boundary element procedure. Communicated by S. N. Atluri, 1 July 1996     

Structural design sensitivity analysis with general boundary conditions: Dynamic problem

In a previous paper¹, design sensitivity analysis of static response of structural systems when general boundary conditions are imposed during the analysis phase was developed and presented. This paper presents methods for design sensitivity analysis of dynamic response of structural systems when general boundary conditions are imposed during the analysis phase: Eigenvalue as well as transient response problems are discussed. Design sensitivity expression for eigenvalue constraints is derived. For the transient problem, point-wise as well as the integral-type constraint functions are treated. Advantages of these design sensitivity analysis procedures for the dynamic problem are the same as for the static problem. Namely, they are compatible with any existing finite element analysis computer code. Also, they can handle general boundary conditions that are design independent or dependent. Three simple examples are presented to show use of the procedures. Based on these applications, general-purpose numerical procedures can be developed and incorporated into existing computer codes.     

A coding error elimination procedure for verifying finite element programs with forced and natural boundary conditions

The general procedure developed by Shih for eliminating computer coding errors has been both simplified for finite element applications and extended for use in the verification of the coding of forced and natural boundary conditions. To accomplish these two objectives simultaneously, use is made of volume weighted residuals for all elements within the solution regime and of area weighted boundary residuals for all elements having natural boundary conditions on one or more of their borders. This procedure can thus be used to verify finite element codes with a combination of both forced (Dirichlet) and natural (Neumann and Robbins) boundary conditions. Depending on the equations being solved and the boundary conditions they are being subjected to, this modification can make possible the implementation of Shih's general procedure into existing finite element codes without having to carry out any additional discretization steps. The employment of this modified procedure is illustrated using two‐dimensional (2‐D) stress analysis and transient heat conduction problems. Copyright © 1999 John Wiley & Sons, Ltd.     

Normal mode solutions of linear dynamic field theories using Green's extended identity

Normal mode solutions of certain classes of linear, spatially time-invariant, self-adjoint and nonself-adjoint differential operators, with inhomogéneous boundary conditions in a finite region of arbitrary shape, are obtained by the use of Green's extended identity [1] in conjunction with the eigenvalue problems associated with the differential operators. Thus, continuum field theories belonging to these classes of operators, which encompass arbitrary (a) material and geometrical parameters, (b) spatial and time-dependent boundary conditions, (c) initial conditions can be solved by this technique. This is illustrated by the determination of the transient response of an axisymmetric, finite, thick transversely isotropic elastic hollow cylinder under inhomogeneous boundary conditions of all types (both pure and mixed). It is also shown that the displacement (separation-of-variables, integral transforms) and acceleration (Mindlin-Goodman, Williams) methods currently used for solutions of dynamic problems can both be derived from Green's extended identity.     

The dual reciprocity method and the numerical Green’s function for BEM fracture mechanic problems

The boundary element method has been applied with success to linear elastic fracture mechanicproblems, involving static and dynamic cases. In order to solve body force problems (e.g., gravitational forcesand transient problems with velocities and accelerations), Nardini and Brebbia presented, in 1982, the dualreciprocity formulation. Originally with the intention of solving transient problems using fundamental solutionsof the static formulation, the procedure was found to be very efficient in the solution of body force problemsas well. Also, a Green’s function corresponding to an embedded crack within the infinite medium can beintroduced into the boundary element formulation as the fundamental solution. This yields accurate means ofcalculating only the external boundary unknown displacements and tractions and, in a post-processing scheme,determining the crack opening displacements. This paper introduces an approach that involves the numericalGreen’s function procedure, of Telles and coworkers, and the dual reciprocity formulation. It compares beamsolutions with the simulated effect of the total weight applied as a concentrated boundary force, the actualself-weight as a body force and a frequency- and time-dependent transient Heaviside load applied to a platewith a central crack.     

Time domain boundary element method for dynamic stress intensity factor computations

This paper presents a procedure for transient dynamic stress intensity factor computations using traction singular quarter-point boundary elements in combination with the direct time domain formulation of the Boundary Element Method. The stress intensity factors are computed directly from the traction nodal values at the crack tip. Several examples of finite cracks in finite domains under mode-I and mixed mode dynamic loading conditions are presented. The computed stress intensity factors are represented versus time and compared with those obtained by other authors using different methods. The agreement is very good. The results are reliable and little mesh dependent. These facts allow for the analysis of dynamic crack problems with simple boundary discretizations. The versatile procedure presented can be easily applied to problems with complex geometry which include one or several cracks.     

矩形板边界支撑优化设计

采用瑞利-里兹法分析、计算矩形板附加弹性铰(简)支撑的最小刚度和最优支撑位置,使板的第一阶固有频率达到原结构的第二阶频率。矩形板仅有一边固定(固支或简支),其他边自由,弹性支撑位于固定边相对的自由边界上。由振动系统能量泛函取极小值原理,构建特征频率方程,利用拉格朗日乘子施加最优支撑位置应满足的设计条件。算例结果表明,该文提出的方法是可靠的,能得到满意的结果。     

Buckling analysis of shear deformable plates by boundary element method

In this paper, the derivation and numerical implementation of boundary integral equations for the buckling analysis of shear deformable plates are presented. Plate buckling equations are derived as a standard eigenvalue problem. The formulation is formed by coupling boundary element formulations of shear deformable plate and two dimensional plane stress elasticity. The eigenvalue problem of plate buckling yields the critical load factor and buckling modes. The domain integrals which appear in this formulation are treated in two different ways: initially the integrals are evaluated using constant cells, and next, they are transformed into equivalent boundary integrals using the dual reciprocity method (DRM). Several examples with different geometry, loading and boundary conditions are presented to demonstrate the accuracy of the formulation. Copyright © 2004 John Wiley & Sons, Ltd.     

Localized wrinkling instabilities in radially stretched annular thin films

Summary We investigate a pre-stressed annular thin film subjected to a uniform displacement field along its inner boundary. This loading scenario leads to a variable stress distribution characterized by an orthoradial component that may change sign along a concentric circle within the annular domain. When the intensity of the applied field is strong enough, elastic buckling occurs circumferentially, leading to a localized wrinkling pattern near the inner edge. Using a linear non-homogeneous pre-bifurcation state, the eigenvalue problem describing this instability is cast as a singularly-perturbed fourth-order linear differential equation with variable coefficients. The dependence of the lowest eigenvalue on various non-dimensional quantities is numerically investigated using the compound matrix method. These results are complemented by a WKB analysis which suggests that the qualitative and quantitative features of the full model can be described by a simplified second-order eigenvalue problem which takes into account the finite stiffness of the system.     

基于谱方法分析有阻尼负载圆柱壳频散特性

以Chebyshev多项式系为基函数,采用谱方法离散弹性理论的波动方程,建立对应的广义特征值问题。依据壳体结构波运动、内部流体及外部阻尼材料在界面处的位移、应力连续条件,构造此复杂圆柱壳系统广义特征值方程。通过数值求解特征值获得对应频率下波数,进而获得圆柱壳结构的频散曲线。分别讨论充水与否、有阻尼负载圆柱壳的频散曲线,获得有价值结论。     

Null-field integral equation approach for free vibration analysis of circular plates with multiple circular holes

In this paper, a semi-analytical approach for the eigenproblem of circular plates with multiple circular holes is presented. Natural frequencies and modes are determined by employing the null-field integral formulation in conjunction with degenerate kernels, tensor rotation and Fourier series. In the proposed approach, all kernel functions are expanded into degenerate (separable) forms and all boundary densities are represented by using Fourier series. By uniformly collocating points on the real boundary and taking finite terms of Fourier series, a linear algebraic system can be constructed. The direct searching approach is adopted to determine the natural frequency through the singular value decomposition (SVD). After determining the unknown Fourier coefficients, the corresponding mode shape is obtained by using the boundary integral equations for domain points. The result of the annular plate, as a special case, is compared with the analytical solution to verify the validity of the present method. For the cases of circular plates with an eccentric hole or multiple circular holes, eigensolutions obtained by the present method are compared well with those of the existing approximate analytical method or finite element method (ABAQUS). Besides, the effect of eccentricity of the hole on the natural frequency and mode is also considered. Moreover, the inherent problem of spurious eigenvalue using the integral formulation is investigated and the SVD updating technique is adopted to suppress the occurrence of spurious eigenvalues. Excellent accuracy, fast rate of convergence and high computational efficiency are the main features of the present method thanks to the semi-analytical procedure.