The nonlinear Landweber method applied to an inverse scattering problem for sound-soft obstacles in 3D

The inverse scattering problem for sound-soft obstacles is considered for both smooth and piecewise smooth surfaces in 3D. The nonlinear and ill-posed integral equation of the first kind is solved by the nonlinear Landweber method. It is an iterative regularization scheme to obtain approximations for the unknown boundary of the obstacle. It is stable with respect to noise and essentially no extra work is required to incorporate several incident waves. So far, it has only been applied to the two dimensional case. Two different integral equations are presented to obtain far-field data. Furthermore, the domain derivative and its adjoint are characterized. The integral equations of the second kind are approximated by a boundary element collocation method. The two-grid method is used to solve the large and dense linear systems. Numerical examples are illustrated to show that both smooth and piecewise smooth obstacles can be reconstructed with this method, where the latter case has not yet been reported.     

求解二维随机多区域声波散射问题的快速多极边界元方法

快速多极算法(FMM)是求解大尺度边界元问题的一种很有效的快速算法.应用快速多极算法求解二维随机多区域声散射问题的边界积分方程.首先给出了求解该问题的边界积分方程,进而给出快速多极算法求解的算法实现过程以及积分算子的相应多极展开、局部展开和相应系数的转化关系式.最后通过对数值例子的计算表明快速多极算法在求解随机多区域声散射问题时的可行性及高效性,其求解存储量和计算量都是O(N).     

Adaptive multilevel BEM for acoustic scattering

We study the h- and p-versions of the Galerkin boundary element method for integral equations of the first kind in 2D and 3D which result from the scattering of time harmonic acoustic waves at hard or soft scatterers. We derive an abstract a-posteriori error estimate for indefinite problems which is based on stable multilevel decompositions of our test and trial spaces. The Galerkin error is estimated by easily computable local error indicators and an adaptive algorithm for h- or p-adaptivity is formulated. The theoretical results are illustrated by numerical examples for hard and soft scatterers in 2D and 3D.     

A coupled formulation of finite and boundary element methods in two-dimensional elasticity

A symmetric stiffness formulation based on a boundary element method is studied for the structural analysis of a shear wall, with or without cutouts. To satisfy compatibility requirements with finite beam elements and to avoid problems due to the eventual discontinuities of the traction vector, different interpolation schemes are adopted to approximate the boundary displacements and tractions. A set of boundary integral equations is obtained with the collocation points on the boundary, which are selected by the error minimization technique proposed in this paper, and the stiffness matrix is formulated with those equations and symmetric coupling techniques of finite and boundary element methods. The newly developed plane stress element can have the openings in its interior domain and can be easily linked with the finite beam/column elements.     

Element integral approach for water waves

The problems described by the inviscid linearized theory for water waves are faced mainly by two different kind of approaches, that is, the integral and the finite element approaches. Although various authors have studied in the past several procedures able to match them together to get the maximum advantage, these two techniques have always been felt as completely different and far-away from each other. In this paper we try to analyse with care the nature of their differences, pointing out especially the common features. In particular, it is shown how the integral method can be adapted to a finite element scheme. Thus, three new different kinds of integral approaches are detected and described, while their connection with the variational formulations of the original problem is also illustrated. While the first two methods can be applied only in some cases, the third one is in principle more general. Their numerical features have been investigated and suitable comparisons with other existing methods carried out with reference to the particular problem of the wave-induced oscillations in a harbour.     

Meshfree Particle Methods in the Framework of Boundary Element Methods for the Helmholtz Equation

In this paper, we study electromagnetic wave scattering from periodic structures and eigenvalue analysis of the Helmholtz equation. Boundary element method (BEM) is an effective tool to deal with Helmholtz problems on bounded as well as unbounded domains. Recently, Oh et al. (Comput. Mech. 48:27–45, 2011) developed reproducing polynomial boundary particle methods (RPBPM) that can handle effectively boundary integral equations in the framework of the collocation BEM. The reproducing polynomial particle (RPP) shape functions used in RPBPM have compact support and are not periodic. Thus it is not ideal to use these RPP shape functions as approximation functions along the boundary of a circular domain. In order to get periodic approximation functions, we consider the limit of the RPP shape function as its support is getting infinitely large. We show that the basic approximation function obtained by the limit of the RPP shape function yields accurate solutions of Helmholtz problems on circular, or annular domains as well as on the infinite domains.     

A symmetric variational finite element-boundary integral equation coupling method

This paper is concerned with the development of a mixed variational principle for coupling finite element and boundary integral methods in interface problems, using the generalized Poisson's equation as a prototype situation. One of its primary objectives is to compare the performance of fully variational procedures with methods that use collocation for the treatment of boundary integral equations. A distinctive feature of the new variational principle is that the discretized algebraic equations for the coupled problem are automatically symmetric since they are all derived from a single functional. In addition, the condition that the flux remain continuous across interfaces is satisfied naturally. In discretizing the problem within inhomogeneous or loaded regions, domain finite elements are used to approximate the field variable. On the other hand, only boundary elements are used for regions where the medium is homogeneous and free of external agents. The corresponding integral equations are discretized both by fully variational and by collocation techniques. Results of numerical experiments indicate that the accuracy of the fully variational procedure is significantly greater than that of collocation for the complete interface problem, especially for complex disturbances, at little additional computational cost. This suggests that fully variational procedures may be preferable to collocation, not only in dealing with interface problems, but even for solving integral equations by themselves.     

An integration scheme for electromagnetic scattering using plane wave edge elements

Finite element techniques for the simulation of electromagnetic wave propagation are, like all conventional element based approaches for wave problems, limited by the ability of the polynomial basis to capture the sinusoidal nature of the solution. The Partition of Unity Method (PUM) has recently been applied successfully, in finite and boundary element algorithms, to wave propagation. In this paper, we apply the PUM approach to the edge finite elements in the solution of Maxwell’s equations. The electric field is expanded in a set of plane waves, the amplitudes of which become the unknowns, allowing each element to span a region containing multiple wavelengths. However, it is well known that, with PUM enrichment, the burden of computation shifts from the solver to the evaluation of oscillatory integrals during matrix assembly. A full electromagnetic scattering problem is not simulated or solved in this paper. This paper is an addition to the work of Ledger and concentrates on efficient methods of evaluating the oscillatory integrals that arise. A semi-analytical scheme of the Filon character is presented.     

NiHu: An open source C++ BEM library

This paper introduces NiHu, a C++ template library for boundary element methods (BEM). The library is capable of computing the coefficients of discretised boundary integral operators in a generic way with arbitrarily defined kernels and function spaces. NiHu’s template core defines the workflow of a general BEM algorithm independent of the specific application. The core provides expressive syntax, based on the operator notation of the BEM, reflecting the mathematics behind boundary elements in the C++ source code. The customisable Component library contains elements specific to particular applications such as different numerical integration techniques and regularisation methods. The library can be used for creating a standalone C++ application using external open source libraries, or compiling a Matlab toolbox through the MEX interface. By massively exploiting C++ template metaprogramming, NiHu generates optimised codes for specific applications, including heterogeneous problems. The paper introduces the main concepts of the novel development, demonstrates its versatility and flexibility and compares the implementation’s performance to that of other open source projects.     

A method for preconditioning matrices arising from linear integral equations for elliptic boundary value problems

The discretization of linear integral equations for elliptic boundary value problems by the boundary element method yields linear systems of simultaneous equations with filled matrices. The structure of these matrices allows Fourier methods to be used to determine preconditioning matrices such that fast iterative solution of the linear system of algebraic equations is possible. The preconditioning method is applicable to Fredholm integral equations of the first kind with non-smooth convolutional principal part as well as to Fredholm integral equations of the second kind. Numerical examples are presented.     

Efficient iterative algorithms for the stochastic finite element method with application to acoustic scattering

In this study, we describe the algebraic computations required to implement the stochastic finite element method for solving problems in which uncertainty is restricted to right-hand side data coming from forcing functions or boundary conditions. We show that the solution can be represented in a compact outer product form which leads to efficiencies in both work and storage, and we demonstrate that block iterative methods for algebraic systems with multiple right-hand sides can be used to advantage to compute this solution. We also show how to generate a variety of statistical quantities from the computed solution. Finally, we examine the behavior of these statistical quantities in one setting derived from a model of acoustic scattering.     

A symmetric Galerkin BEM variational framework for multi-domain interface problems

In this paper we discuss an energy-based variational framework for the solution of interior problems in multiply-connected domains comprising multiple piecewise homogeneous subdomains, using exclusively boundary integral equations. The primary goal is to provide a unified variational setting that lends itself naturally to symmetric Galerkin boundary element formulations in terms of Dirichlet-type variables only.The approach hinges on the explicit imposition of the normal derivative of the classical integral representation of the interior solution on each subdomain via Lagrange multipliers in the augmented Lagrangian of the system. We use Maue-type identities to resolve the hypersingular kernels, leading to a scheme that requires only standard single- and double-layer evaluations. In addition, the usual difficulty with multi-valued normals at subdomain corners is treated here within the same variational framework, by incorporating into the variational formulation the constraint equation between the limiting normal derivatives at either side of the corner. The resulting scheme remains fully symmetric.The numerical implementation avoids the explicit presence of Neumann-type unknowns on the boundaries, through condensation at the subdomain level. In all integral evaluations, three- or four-point Gauss quadrature rules are sufficient for accurate results. We describe the theory and present illustrative examples for thermal and acoustic problems governed by Laplace and Helmholtz equations, respectively. This technique, however, can be applied without essential modification to more general problems.     

A complex variable boundary element method for a class of boundary value problems in anisotropic thermoelasticity

A boundary element method based on the Cauchy's integral formulae, called the complex variable boundary element method (CVBEM), is proposed for the numerical solution of boundary value problems governing plane thermoelastic deformations of anisotropic elastic bodies. The method is applicable for a wide class of problems which do not involve inertia or coupling effects and can be easily and efficiently implemented on the computer. It is applied to solve specific test problems.     

Schwarz iterations for the efficient solution of screen problems with boundary elements

This paper investigates two domain decomposition algorithms for the numerical solution of boundary integral equations of the first kind. The schemes are based on theh-type boundary element Galerkin method to which the multiplicative and the additive Schwarz methods are applied. As for twodimensional problems, the rates of convergence of both methods are shown to be independent of the number of unknowns. Numerical results for standard model problems arising from Laplaces' equation with Dirichlet or Neumann boundary conditions in both two and three dimensions are discussed. A multidomain decomposition strategy is indicated by means of a screen problem in three dimensions, so as to obtain satisfactory experimental convergence rates.     

A Complete Factorization Method for Scattering by Periodic Surfaces

The Factorization Method, a well established method in inverse scattering problems for bounded obstacles, is extended to the case of scattering by a periodic surface. The method is rigorously proved to provide accurate reconstructions for the cases of the total field satisfying a Dirichlet or an impedance boundary condition on the scattering surface. A number of computational examples are given with an emphasis on exploring the number of evanescent modes for which data has to be reliably measured to obtain satisfactory reconstructions.     

Boundary element methods for magnetostatic field problems: a critical view

For the solution of magnetostatic field problems we discuss and compare several boundary integral formulations with respect to their accuracy, their efficiency, and their robustness. We provide fast boundary element methods which are able to deal with multiple connected computational domains, with large magnetic permeabilities, and with complicated structures with small gaps. The numerical comparison is based on several examples, including a controllable reactor as a real-world problem.     

Direct numerical simulation of transitional flow at high Mach number coupled with a thermal wall model

In transitional and turbulent high speed boundary-layer flows the wall thermal boundary conditions play an important role and in many cases an assumption of a constant temperature or a specified heat flux may not be appropriate for numerical simulations. In this paper we extend a formulation for direct numerical simulation of compressible flows to include a thin plate that is thermally fully coupled to the flow. Even without such thermal coupling compressible flows with shock waves and turbulence represent a challenge for numerical methods. In this paper we review the scaling properties of algorithms, based on explicit high-order finite differencing combined with shock capturing, that are suitable for dealing with such flows. An application is then considered in which an isolated roughness element is of sufficient height to trigger transition in the presence of acoustic forcing. With the thermal wall model included it is observed that the plate heats up sufficiently during the simulation for the transition process to be halted and the flow consequently re-laminarises.     

Applications of a fast multipole Galerkin in boundary element method in linear elastostatics

Abstact Boundary element methods provide a powerful tool for solving boundary value problems of linear elastostatics, especially in complicated three–dimensional structures. In contrast to the standard Galerkin approach leading to dense stiffness matrices, in fast boundary element methods such as the fast multipole method the application of matrix–vector products can be realized with almost linear complexity. Since all boundary integral operators of linear elastostatics can be reduced to those of the Laplacian, the discretization of the corresponding single and double layer potentials of the Laplace operator has to be employed only. This technique results in a fast multipole method which is an efficient tool for the simulation of elastic stress fields in engineering and industrial applications. This work has been supported by the German Research Foundation DFG under the Grant SFB 404 Multifield Problems in Continuum Mechanics. Dedicated to George C. Hsiao on the occasion of his 70th birthday.     

A Galerkin collocation method for some integral equations of the first kind

Here we present a certain modified collocation method which is a fully discretized numerical method for the solution of Fredholm integral equations of the first kind with logarithmic kernel as principal part. The scheme combines high accuracy from Galerkin's method with the high speed of collocation methods. The corresponding asymptotic error analysis shows optimal order of convergence in the sense of finite element approximation. The whole method is an improved boundary integral method for a wide class of plane boundary value problems involving finite element approximations on the boundary curve. The numerical experiments reveal both, high speed and high accuracy.     

Green element computation of the Sturm-Liouville equations

A mathematical derivation of a new numerical procedure called the Green element method (GEM) is presented and applied to the solution of Sturm-Liouville problems. The GEM is a numerical technique which expands the scope of application of the boundary element method (BEM) by implementing the singular boundary integral theory in an element-by-element fashion; and like the finite element method (FEM) gives rise to a banded coefficient matrix which is easy to handle numerically. For this application, the location of both the field and the source nodes within the same element makes it possible for integrations to be carried out accurately, thereby enhancing the accuracy of discrete equations. The method is therefore easy to apply and, because of its domain based implementation, it maintains the flexibility of the FEM. We apply the GEM to the solution of boundary value differential equations which represent the form of Sturm-Liouville problems, and its capability is demonstrated by comparing the results with those of the finite element methods available in the literature. Satisfactory results and a second-order accuracy were found to be exhibited.