基于边界节点法的MATLAB 工具箱开发

0引言 Helmholtz方程作为简化的波动方程,长期以来备受关注.求解Helmholtz方程的数值方法主要有有限差分法、有限元法和边界元法等.这些方法主要基于网格近似或者需要背景网格积分,在处理高波数或高频率Helmholtz方程及高维问题时存在网格划分困难和内存消耗大等缺点.基于点的近似思想,无网格方法在一定程度上减少对网格的依赖,甚至不需要网格。     

二维Helmholtz方程外问题的数值解法

利用位势理论把Helmholtz方程外问题转化为第二类积分方程的求解问题．在处理积分算子核时,采用了一种新的裂解方式,再利用Nystrom方法求解数值结果．最后针对该方法给出数值实例,以表明此方法的有效性．     

Helmholtz方程内问题的数值方法

应用位势理论把Helmholtz方程内问题转化为含有Cauchy奇性的第二类积分方程的求解问题.并利用Nystom方法求得数值结果,试验结果表明了此方法的简单与有效性.     

多重网格方法求解两类Helmholtz方程

详细给出了多重网格方法的实现过程,借助正定Helmholtz方程及不定Helmholtz方程的求解来探讨多重网格方法的特性。对多重网格V环、W环以及F环三种不同迭代格式的收敛效果进行了对比。通过正定Helmholtz方程的求解,发现多重网格的确有很高的计算效率。对于不定Helmholtz方程,随着波数的增加,利用多重网格方法得到结果不收敛,原因出在细网格光滑和粗网格矫正过程。如何针对此问题对多重网格进行有效改进还有待进一步研究。     

用CCD法离散求解二维Helmholtz方程的数值方法

用联合紧致差分格式(CCD)离散Helmholtz方程,具有6阶精度.然而对于得到的线性方程组,我们仍需一种高效求解方法.本文针对二维的Helmholtz方程CCD离散所得的线性方程组给出高效的数值方法.数值例子表明所提出的方法是有效的.     

复波数Helmholtz方程和时谐Maxwell方程组的平面波间断Petrov-Galerkin方法

平面波方法己证明是实波数Helmholtz方程和时谐Maxwell方程组的高效离散化方法,但是还鲜有工作研究复波数情形的平面波离散化方法.本文基于平面波间断Galerkin方法的思想,导出了离散复波数Helmholtz方程和时谐Maxwell方程组的平面波间断Petrov-Galerkin方法.数值结果表明由该方法得到的数值解具有高的精度.     

GRAPES模式中Helmhothz方程两种求解方法的对比研究

GRAPES是中国气象局自主研发的一个全球/区域分析预报系统。其模式计算方程组经过离散化之后,积分求解过程最终归结为对一个椭圆方程或Helmholtz(赫姆霍兹)方程的求解,这个求解是整个动力框架计算的核心。在目前GRAPES全球模式的准业务计算中,对于分辨率为0.5o的系统,Helmholtz方程的求解时间占到了整个模式计算时间的三分之一强。而且随着未来高分辨率模式的进一步加细,以及模式计算精度的提高,方程求解计算总量更是呈指数式增长。为此,本文分析了GRAPES模式中求解Helmholtz方程所采用的广义共轭余差法(GCR),并对比给出了利用PETSC函数库中提供的GMRES方法求解Helmholtz方程的一些初步测试结果。结果表明,采用高精度的GMRES方法可以减少模式预报偏差,改善模式预报准确度,在大规模并行计算时具有更好的可扩展性能。     

一种针对大波数Helmholtz方程的高性能并行预条件迭代求解算法

针对传统串行迭代法求解大波数Helmholtz方程存在效率低下且受限于单机内存的问题,提出了一种基于消息传递接口(Message Passing Interface,MPI) 的并行预条件迭代法。该算法利用复移位拉普拉斯算子对Helmholtz方程进行预条件处理,联合稳定双共轭梯度法和基于矩阵的多重网格法来求解预条件方程离散后的大规模线性系统,在Linux集群系统上基于 MPI环境实现了求解算法的并行计算,重点解决了多重网格的并行划分、信息传递和多重网格组件的构建问题。数值实验表明,对于大波数问题,提出的算法具有良好的并行加速比,相较于串行算法极大地提高了计算效率。     

求解任意波数的三维Helmholtz方程

提出通过Adomian分解法求解任意波数的三维Helmholtz方程。通过Adomian分解法可以把三维Helmholtz微分方程转换成递归代数公式,并进一步把其边界条件转换成适用符号计算的简单代数公式。利用边界条件可以很容易得到方程的解析解表达式。Adomian分解法的主要特点在于计算简单快速,并且不需要进行线性化或离散化。最后通过数值计算以验证Adomian分解法求解任意波数下三维Helmholtz方程的有效性。数值计算结果表明:Adomian分解法的计算结果非常接近精确解,并且该方法在大波数情况下还具有良好的收敛性。     

全球非静力谱模式动力框架初步设计

基于我国目前已在业务运行的全球静力谱模式,参考ECMWF从静力谱模式到非静力谱模式的升级设计思想,从模式方程组的选取、模式方程组的线性化、水平球谐谱离散、时间积分方法、垂直有限差分离散、Helmholtz方程的求解等几个方面出发,针对浅薄近似的大气,采用Euler对流,初步设计了一个全球非静力谱模式干动力框架,并针对Helmholtz方程的求解,给出了一种将其转化为块三对角线性方程组的计算方法,该方法从计算效率上明显优于ECMWF目前所用计算方法。     

The boundary element-free method for 2D interior and exterior Helmholtz problems

The boundary element-free method (BEFM) is developed in this paper for numerical solutions of 2D interior and exterior Helmholtz problems with mixed boundary conditions of Dirichlet and Neumann types. A unified boundary integral equation is established for both interior and exterior problems. By using the improved interpolating moving least squares method to form meshless shape functions, mixed boundary conditions in the BEFM can be satisfied directly and easily. Detailed computational formulas are derived to compute weakly and strongly singular integrals over linear and higher order integration cells. Three numerical integration procedures are developed for the computation of strongly singular integrals. Numerical examples involving acoustic scattering and radiation problems are presented to show the accuracy and efficiency of the meshless method.     

Meshfree explicit local radial basis function collocation method for diffusion problems

This paper formulates a simple explicit local version of the classical meshless radial basis function collocation (Kansa) method. The formulation copes with the diffusion equation, applicable in the solution of a broad spectrum of scientific and engineering problems. The method is structured on multiquadrics radial basis functions. Instead of global, the collocation is made locally over a set of overlapping domains of influence and the time-stepping is performed in an explicit way. Only small systems of linear equations with the dimension of the number of nodes included in the domain of influence have to be solved for each node. The computational effort thus grows roughly linearly with the number of the nodes. The developed approach thus overcomes the principal large-scale problem bottleneck of the original Kansa method. Two test cases are elaborated. The first is the boundary value problem (NAFEMS test) associated with the steady temperature field with simultaneous involvement of the Dirichlet, Neumann and Robin boundary conditions on a rectangle. The second is the initial value problem, associated with the Dirichlet jump problem on a square. The accuracy of the method is assessed in terms of the average and maximum errors with respect to the density of nodes, number of nodes in the domain of influence, multiquadrics free parameter, and timestep length on uniform and nonuniform node arrangements. The developed meshless method outperforms the classical finite difference method in terms of accuracy in all situations except immediately after the Dirichlet jump where the approximation properties appear similar.     

Development of a meshless Galerkin boundary node method for viscous fluid flows

In this paper, a meshless Galerkin boundary node method is developed for boundary-only analysis of the interior and exterior incompressible viscous fluid flows, governed by the Stokes equations, in biharmonic stream function formulation. This method combines scattered points and boundary integral equations. Some of the novel features of this meshless scheme are boundary conditions can be enforced directly and easily despite the meshless shape functions lack the delta function property, and system matrices are symmetric and positive definite. The error analysis and convergence study of both velocity and pressure are presented in Sobolev spaces. The performance of this approach is illustrated and assessed through some numerical examples.     

A meshless Galerkin method for Stokes problems using boundary integral equations

A meshless Galerkin scheme for the simulation of two-dimensional incompressible viscous fluid flows in primitive variables is described in this paper. This method combines a boundary integral formulation for the Stokes equation with the moving least-squares (MLS) approximations for construction of trial and test functions for Galerkin approximations. Unlike the domain-type method, this scheme requires only a nodal structure on the bounding surface of a body for approximation of boundary unknowns, thus it is especially suitable for the exterior problems. Compared to other meshless methods such as the boundary node method and the element free Galerkin method, in which the MLS is also introduced, boundary conditions do not present any difficulty in using this meshless method. The convergence and error estimates of this approach are presented. Numerical examples are also given to show the efficiency of the method.     

More accurate results for two-dimensional heat equation with Neumann’s and non-classical boundary conditions

In this article, recently proposed spectral meshless radial point interpolation (SMRPI) method is applied to the two-dimensional diffusion equation with a mixed group of Dirichlet’s and Neumann’s and non-classical boundary conditions. The present method is based on meshless methods and benefits from spectral collocation ideas. The point interpolation method with the help of radial basis functions is proposed to construct shape functions which have Kronecker delta function property. Evaluation of high-order derivatives is possible by constructing and using operational matrices. The computational cost of the method is modest due to using strong form equation and collocation approach. A comparison study of the efficiency and accuracy of the present method and other meshless methods is given by applying on mentioned diffusion equation. Stability and convergence of this meshless approach are discussed and theoretically proven. Convergence studies in the numerical examples show that SMRPI method possesses excellent rates of convergence.     

A fast boundary element method for the two-dimensional Helmholtz Equation

In this paper, we present a fast method for solving boundary integral equations arising from the exterior Dirichlet problem for the two-dimensional Helmholtz equation. This method combines a quadrature method for discretizing the boundary integral equations with a preconditioned iterative method for solving the resulting dense, nonsymmetric linear systems. Using this method, a polynomial rate of convergence can be obtained by performing a finite number of iterations, which yields high computational efficiency. Various numerical examples are presented.     

On the optimal shape parameter for Gaussian radial basis function finite difference approximation of the Poisson equation

We investigate the influence of the shape parameter in the meshless Gaussian radial basis function finite difference (RBF-FD) method with irregular centres on the quality of the approximation of the Dirichlet problem for the Poisson equation with smooth solution. Numerical experiments show that the optimal shape parameter strongly depends on the problem, but insignificantly on the density of the centres. Therefore, we suggest a multilevel algorithm that effectively finds a near-optimal shape parameter, which helps to significantly reduce the error. Comparison to the finite element method and to the generalised finite differences obtained in the flat limits of the Gaussian RBF is provided.     

An improved pseudospectral meshless radial point interpolation (PSMRPI) method for 3D wave equation with variable coefficients

In this paper, a pseudospectral meshless radial point interpolation (PSMRPI) technique is applied to the three-dimensional wave equation with variable coefficients subject to given appropriate initial and Dirichlet boundary conditions. The present method is a kind of combination of meshless methods and spectral collocation techniques. The point interpolation method along with the radial basis functions is used to construct the shape functions as the basis functions in the frame of the spectral collocation methods. These basis functions will have Kronecker delta function property, as well as unitary possession. In the proposed method, operational matrices of higher order derivatives are constructed and then applied. The merit of this innovative method is that, it does not require any kind of integration locally or globally over sub-domains, as it is essential in meshless methods based on Galerkin weak forms, such as element-free Galerkin and meshless local Petrov–Galerkin methods. Therefore, computational cost of PSMRPI method is low. Further, it is proved that the procedure is stable with respect to the time variable over some conditions on the 3D wave model, and the convergence of the technique is revealed. These latest claims are also shown in the numerical examples, which demonstrate that PSMRPI provides excellent rate of convergence.

     

Approximate analytic solution of the Dirichlet problems for Laplace’s equation in planar domains by a perturbation method

In this paper, we propose a regular perturbation method to obtain approximate analytic solutions of exterior and interior Dirichlet problems for Laplace’s equation in planar domains. This method, starting from a geometrical perturbation of these planar domains, reduces our problems to a family of classical Dirichlet problems for Laplace’s equation in a circle. Numerical examples are given and comparisons are made with the solutions obtained by other approximation methods.     

Detection of a two-dimensional moving cavity

The work here provides the mathematical and numerical groundwork for a potential thermography technique which can be used to track the time-dependent development of crystalline deposits. We develop a meshless numerical method, the method of fundamental solutions for solving the two-dimensional time-dependent heat equation, to locate an internal moving boundary ? D(t), where D(t) is assumed to be a perfectly insulating cavity. The inverse problem is non-linear and therefore, a least-squares minimization routine within MATLAB, FMINCON is employed to reconstruct the cavity, using known Dirichlet and Neumann data on the outer fixed boundary. The problem presented is also ill posed and, therefore, a Tikhonov regularization technique is employed.