1. Introduction
With the emergence of various engineering problems, an increasing number of numerical methods have been proposed in the past decades. To date, there are some well-known numerical methods, such as the finite difference method (FDM) [
1,
2], the finite element method (FEM) [
3] and the boundary element method (BEM) [
4]. These three methods are relatively mature, but they still have some obvious shortcomings and limitations. For example, with these methods, grid generation is difficult, especially for problems in complicated domains. They need to constantly remesh when calculating specific problems, which may result in high computational costs. On the contrary, there are some so-called meshless methods, which can solve problems without time-consuming mesh generation. From the present studies of meshless methods, it is evidently demonstrated that the meshless methods, proposed recently, are truly free from mesh and numerical quadrature, so they can be adopted to efficiently analyze physical problems and engineering applications with complex geometries.
In recent years, various meshless or meshfree numerical methods have been proposed to eliminate the time-consuming task of meshing. Meshless methods usually require node information only and do not need any orthogonal grid or unstructured mesh to acquire convergent, accurate and stable numerical solutions. These newly developed meshless methods have attracted the attention of many scholars [
5]. Among them, the Radial basis function (RBF)-related method is the most popular, which was proposed by Hardy [
6]. Duchon [
7] and Franke [
8] successively proposed different algorithms to select the optimal parameters. The RBF collocation method (RBFCM), developed by Kansa [
9,
10] in 1990, allows RBFs to solve partial differential equations (PDEs). The RBFCM is an approximate expression that uses the RBF as the basis function to construct the numerical solution. The RBFCM has been developed to form the local RBFCM (LRBFCM), which can yield a sparse matrix, so the LRBFCM is applied to solve large-scale engineering science problems [
11,
12,
13,
14,
15].
Another well-known meshless method is the method of fundamental solutions (MFS), which was first proposed by Kupradze and Aleksidze [
16]. The MFS takes the fundamental solution as the basis function and is a special case of the Trefftz method [
17]. The MFS require fictitious boundaries, which are called sources. The MFS only needs numerical calculations between boundary nodes and sources, and the homogeneous problems can be efficiently solved by discretization of boundary nodes only. However, in large-scale simulations, the conventional MFS cannot effectively and quickly solve problems in complex domains [
18,
19,
20] owing to the fully populated coefficients matrix. In order to extend the MFS to large-scale problems, the localized MFS was proposed in 2019 by combining the MFS and the concept of localization. The localized method of fundamental solutions (LMFS) approximate the numerical solution by implementing the MFS within each local subdomain and the sparse system of linear algebraic equations of the LMFS can be efficiently solved even for problems in complicated domains. Recently, some researchers successfully used the LMFS to analyze the three-dimensional interior acoustic field [
21]. Although the mathematical background of LMFS and the numerical implementation are simple, the determination of the fictitious boundary for sources is still a challenge in the LMFS [
22,
23].
In addition, the MFS in combination with RBF is sometimes applied for solving inhomogeneous problems [
24] or nonlinear problems (with the Picard iteration method or the homotopy analysis method) [
25]. Combining the advantages of RBF and MFS, the boundary knot method (BKM) was proposed by Chen [
26] in 2002 to avoid the problem of fictitious boundary and singularity in the arrangement of fundamental solutions in the MFS. The BKM uses an RBF, which satisfies the governing equation and is a nonsingular general solution, to replace the fundamental solution of the MFS, in order to avoid the singularity of the fundamental solution and retain the advantages of the MFS. As a boundary-type RBF methodology, the BKM has been widely and successfully applied to solve various types of PDEs [
26,
27,
28,
29,
30]. For many mathematical and physical problems, it is verified that the BKM can acquired highly accurate solutions, such as convection diffusion problems and Helmholtz [
31], heat conduction in nonlinear functionally graded material [
32], etc. However, the researchers also found that although the accuracy of the traditional BKM for two-dimensional and three-dimensional problems is relatively high, the interpolation matrix is a dense, ill-conditioned matrix, which may cause the instability of the numerical simulation.
In this study, we combined the BKM and the localization concept from the LMFS to form the localized BKM, which can yield a sparse system of linear algebraic equations instead of an ill-conditioned matrix. Moreover, the proposed localized BKM can efficiently and accurately analyze problems with complicated domains using limited computational time and computer memory. In this paper, the Laplace equation and the bi-harmonic equation were calculated by using localized BKM, and the effectiveness and accuracy of the proposed meshless method were verified by several numerical examples.
The organization of this paper is depicted as follows: The Laplace equation and the bi-harmonic equation are briefly introduced in
Section 2.
Section 3 presents the numerical procedures of the localized BKM, while in
Section 4, the numerical results of the localized BKM are compared with analytical solutions to verify the merits of the proposed meshless method. Finally, some conclusions and remarks are drawn in
Section 5.
2. Mathematical Formulation of Laplace and Bi-Harmonic Equations
In this study, the numerical procedures of the localized BKM was proposed to solve the two-dimensional boundary value problem, which are governed by Laplace and bi-harmonic equations, respectively. The governing equation of Laplace is demonstrated as follows:
where
is the two-dimensional Laplacian,
u(
x,
y) is the unknown variable in the computational domain Ω,
and
denote the boundary segments with Dirichlet boundary conditions and the Neumann boundary condition:
where
is the unit outward normal vector on the boundary and
and
are the given boundary conditions, respectively. The other boundary value problem is the bi-harmonic equation:
The Neumann boundary condition, Equation (2) and the Dirichlet boundary condition, Equation (3), should be simultaneously applied to all of the boundaries, since the bi-harmonic equation is a fourth-order PDE. In the next section, the numerical procedures of the proposed localized BKM will be clearly described.
5. Conclusions
In this paper, the localized BKM is proposed to solve two-dimensional Laplace and bi-harmonic equations accurately and efficiently. The proposed meshless method is the combination of the convectional BKM and the concept of localization from the LMFS and the LRBFCM. The proposed method is truly free from time-consuming mesh generation and numerical quadrature since only a set of randomly distributed nodes are required for the numerical simulation. Furthermore, the troublesome problems of determination of the fictitious boundary for sources can be simply avoided by using the nonsingular general solution. Since the resultant system of algebraic equations is sparse, it can be expected that the proposed localized BKM can be used to solve realistic engineering applications in complicated domains.
In this paper, six numerical examples are provided to demonstrate the merits of the proposed localized BKM. According to the numerical results and comparisons, the following conclusions can be drawn:
- (1)
In this paper, a novel meshless method, the localized BKM, is proposed to accurately and efficiently solve two-dimensional Laplace and bi-harmonic equations;
- (2)
As a result of the use of localization, the resultant system of algebraic equations is sparse, so it is evident that the proposed localized BKM is capable of efficiently solving large-scale problems;
- (3)
As compared with the MFS, the RBFCM and other meshless methods, the problems of ill-conditioned matrix and the determination of the fictitious boundary for sources are avoided in the proposed method;
- (4)
In the examples provided, it is evident that the proposed localized BKM can accurately solve problems in simply connected and multiply connected domains. Furthermore, both the Laplace equation with a non-harmonic condition and the bi-harmonic equation with a non-bi-harmonic conditions can be stably analyzed by the proposed meshless method.
In the future, the proposed localized BKM will be improved and extended to analyze various physical and mathematical problems, such as temporal transient problems, inverse problems, the acoustic problem, the moving-boundary problem, three-dimensional problems, etc.