Recent Theory and Applications on Numerical Algorithms and Special FunctionsView this Special Issue
Research Article | Open Access
A Meshfree Method for Numerical Solution of Nonhomogeneous Time-Dependent Problems
We propose a new numerical meshfree scheme to solve time-dependent problems with variable coefficient governed by telegraph and wave equations which are more suitable than ordinary diffusion equations in modelling reaction diffusion for such branches of sciences. Finite difference method is adopted to deal with time variable and its derivative, and radial basis functions method is developed for spatial discretization. The results of numerical experiments are presented and are compared with analytical solutions to confirm the accuracy of our scheme.
This paper is devoted to the numerical computation of the nonhomogeneous time-dependent problem with the following form: where , , are constants, and are given analytic functions, and . The cases and correspond to the telegraph problem and wave problem, respectively.
Telegraph equations describe various phenomena in many applied fields, such as a planar random motion of a particle in fluid flow, transmission of electrical impulses in the axons of nerve and muscle cells, propagation of electromagnetic waves in superconducting media, and propagation of pressure waves occurring in pulsatile blood flow in arteries. The wave equation is also an important second-order linear partial differential equation for the description of waves, such as sound waves, light waves, and water waves. It arises in fields like acoustics, electromagnetics, and fluid dynamics.
Over the past several decades, many numerical methods have been developed to solve boundary-value problems involving ordinary and partial differential equations, such as the finite difference, finite elements, and multigrid methods. As one kind of mesh method, finite difference methods are adopted to solve this partial differential equations [1, 2]. Although these methods are effective for solving various kinds of partial differential equations, conditional stability of explicit finite difference procedures and the need to use large amount of CPU time in implicit finite difference schemes limit the applicability of these methods. Furthermore, numerical solution can be provided only on mesh points from these methods , and the accuracy of these well-known techniques is reduced in nonsmooth and nonregular domains. Some authors used Legendre-Gauss-Lobatto collocation method and Chebyshev-tau method to solve the space-fractional advection diffusion equation and got high accuracy results [4, 5].
Recently, meshfree techniques have attracted attention of researchers in order to avoid the mesh generation. Some meshfree schemes are the element free Galerkin method, the reproducing kernel particle, the point interpolation, and so forth. For more description see  and the references therein. Dehghan and Shokri  have solved the second order telegraph equations with constant coefficients using meshfree method. In this paper, we extend this problem considered in  to one kind of partial differential equations with variable coefficients.
Radical basis functions (RBFs) method is known as a powerful approximating tool for scattered data interpolation problem. As a meshfree method, the usage of RBFs to solve numerical solution of partial differential equations is based on the collocation scheme. The major advantage of numerical procedures by using RBFs is meshfree compared with the traditional techniques. RBFs are used actively for solving PDEs, and the examples can be found in [10–14].
In the last decade, the development of the RBFs method as a truly meshfree approach for approximating the solutions of partial derivative equations has drawn the attention of many researchers in science and engineering. Meshfree method has become an important numerical computation method, and there are many academic monographs published [15–21].
In this paper, we present an effective numerical scheme to solve time-dependent problems governed by telegraph and wave equations using the meshfree method with RBFs. The results of numerical experiments are presented and are compared with analytical solutions to confirm the good accuracy of the presented scheme.
The layout of the paper is as follows. In Section 2, the overview about RBFs and the numerical scheme of our method on the time-dependent problems are introduced. The results of numerical experiments are presented in Section 3. Section 4 is dedicated to a brief conclusion.
2. The Meshfree Method
2.1. Radial Basis Function Approximation
The approximation of a distribution , using RBFs, may be written as a linear combination of radial basis functions, and usually it takes the following form: where is the number of data points, , is the dimension of the problem, the ’s are coefficients to be determined, and is the radial basis function. Equation (2) can be written without the additional polynomial . In that case, must be a positive definite function to guarantee the solvability of the resulting system. However, is usually required when is conditionally positive definite, that is, when has a polynomial growth towards infinity. We will use RBFs, which defined as where is the Euclidean norm. Since given by (3) is continuous, we can use it directly.
The IMQ radial basis function takes the form , . The accuracy of the numerical solution is severely influenced by the choice of parameter , since unsuitable parameter will produce the singular interpolation matrix. Moreover, the number of the chosen nodes can also affect the accuracy. Further learning about RBFs method can be got from [22, 23].
If denotes the space of -variate polynomial of order not exceeding more than and letting the polynomials be the basis of in , then the polynomial in (2) is usually written in the following form: where .
To get the coefficients and , the collocation method is used. However, in addition to the equations resulting from collocating (2) at the points, extra equations are required. This is ensured by the conditions Supposed that , is a linear partial differential operator, then can be approximated by
2.2. Nonhomogeneous Time-Dependent Problems
Let ue consider the following time-dependent problem: with initial conditions and Dirichlet boundary conditions where , , and are constant coefficients, , , , , and are given functions, and is the unknown function.
Equation (7) is discretized according to the following -weighted scheme: where , is the time step size, and is the Laplace operator. By using the notation with , we can get
Suppose that there are a total of interpolation points, and can be approximated by In order to determine the coefficients , the collocation method is used by applying (12) at every point , . Thus we obtain where . The additional conditions due to (5) can be written as Writing (13) together with (14) in a matrix form where and is given as follows:
Assuming that there are internal points and boundary points, then the matrix can be split into , where
Using the notation to designate the matrix of the same dimension as and containing the elements where , , then (11) together with the boundary conditions (9) can be written in matrix form as where The operator “*” means that the th component of vector is multiplied to all components of th row of matrix . Equation (19) is obtained by combining (11) applied to the domain points, and (9) applied to the boundary points meanwhile.
At , (19) has the following form: To approximate , the second initial condition can be used. For this purpose, the second initial condition is discretized as Writing (21) together with (22), we have where . Together with the initial condition (8) and (19), we can get all the ; thus we can get the numerical solutions.
Since the coefficient matrix is unchanged in each temporal step, we use the LU factorization to the coefficient matrix only once and use this factorization in our algorithm.
Remark. Although (19) is valid for any value of , we will use (the famous Crank-Nicolson scheme).
3. Numerical Example
In this section, we present some numerical results to test the efficiency of the new scheme for solving time-dependent problems governed by telegraph and wave equations.
3.1. Example 1
Let , , , and ; (1) becomes the telegraph equation where , with the boundary conditions and the initial conditions
The analytical solution of the equation is
We solve this problem by using the IMQ and TPS radial basis functions. These results are obtained for and . The , , and root-mean-square (RMS) errors are obtained in Table 1 for , and .
We also give the analysis of the parameter in IMQ for the results. In Table 2, the , , and RMS errors with different at time are presented.
The space-time graph of analytical and numerical results for with , , by using IMQ (with ) as the RBF is given in Figure 1. The results obtained show the very good accuracy and efficiency of the new approximate scheme. Note that we cannot distinguish the exact solution from the estimated solution in Figure 1.
3.2. Example 2
In this example, , , and (1) become the wave equation where , with the boundary conditions The initial conditions are given by and the analytical solution of the equation is given as
IMQ and TPS are used as the radial basis function in the discussed scheme, and these results are obtained for and .
Table 3 presents the and and RMS errors for , and .
Similar to Example 2, in Table 4, the , , and RMS errors with different at time are presented.
The analytical and numerical results for , and with , , are given in Figure 2 by using TPS (with ) as the RBF.
3.3. Example 3
In this example, we consider the following wave equation: where , with the boundary conditions and the initial conditions
The analytical solution of the equation is
We also use IMQ and TPS as the radial basis functions in the discussed scheme, and these results are obtained for and . The , , and RMS errors are obtained in Table 5 for and .
We also give the analysis of the parameter in IMQ for the results. In Table 6, the , , and RMS errors with different at time are presented.
The graph of analytical and numerical results for , and with , , and and using IMQ (with ) as the RBF are given in Figure 3.
3.4. Example 4
In this example, , , and ; then (1) becomes the telegraph equation where , with the initial conditions
The analytical solution of the equation is We extracted the boundary conditions from the exact solution.
The IMQ and TPS radial basis functions utilized in our scheme and these results are obtained for , and . The , , and RMS errors are obtained in Table 7 for and .