A Linearized Finite Difference Scheme for the Richards Equation Under Variable-Flux Boundary Conditions

The paper presents a step-by-step construction of a finite-difference scheme for a heterogeneous biharmonic equation under zero boundary conditions superimposed on the desired function and its first-order partial derivatives. The finite-difference scheme is based on a square twenty-five-point pattern and has an implicit character. On analytical grid, the error of approximation of the biharmonic operator by the difference analog and the error of approximation of boundary conditions imposed on the first order partial derivatives are calculated by the expansion of the function in the Taylor series with the remainder term in the form of a Lagrange. The boundary conditions imposed on the sought function are satisfied precisely. A finite-difference scheme approximates a boundary value problem with a second order of accuracy over the mesh step. With the help of the Maple computer algebra system the solutions of the problem for different grid steps are obtained. The dependence of the minimum function and calculation time on the number of significant digits is revealed. The optimal number of significant digits is found. The convergence rate of the numerical scheme is analyzed. The dependence of the minimum value of the function and the calculation time on the value of the grid step is established. The optimal step value is found. A three-dimensional graph of the solution and its profiles in the middle sections are constructed. The advantages of the developed finite-difference scheme are indicated. Obtained results correspond to the physical meaning of the problem and are consistent with similar numerical and approximate analytical solutions.

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An Optimal Fourth-Order Finite Difference Scheme for the Helmholtz Equation Based on the Technique of Matched Interface Boundary

Discrete Dynamics in Nature and Society ◽

10.1155/2021/2539272 ◽

2021 ◽

Vol 2021 ◽

pp. 1-16

Author(s):

Dongsheng Cheng ◽

Jianjun Chen ◽

Guangqing Long

Keyword(s):

Finite Difference ◽

Boundary Conditions ◽

Difference Scheme ◽

Helmholtz Equation ◽

Fourth Order ◽

Finite Difference Scheme ◽

Weighted Average ◽

Numerical Dispersion ◽

Taylor Formula ◽

Interface Boundary

In this paper, a new optimal fourth-order 21-point finite difference scheme is proposed to solve the 2D Helmholtz equation numerically, with the technique of matched interface boundary (MIB) utilized to treat boundary problems. For the approximation of Laplacian, two sets of fourth-order difference schemes are derived firstly based on the Taylor formula, with a total of 21 grid points involved. Then, a weighted combination of the two schemes is employed in order to reduce the numerical dispersion, and the weights are determined by minimizing the dispersion. Similarly, for the discretization of the zeroth-order derivative term, a weighted average of all the 21 points is implemented to obtain the fourth-order accuracy. The new scheme is noncompact; hence, it encounters great difficulties in dealing with the boundary conditions, which is crucial to the order of convergence. To tackle this issue, the matched interface boundary (MIB) method is employed and developed, which is originally used to accommodate free edges in the discrete singular convolution analysis. Convergence analysis and dispersion analysis are performed. Numerical examples are given for various boundary conditions, which show that new scheme delivers a fourth order of accuracy and is efficient in reducing the numerical dispersion as well.

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