求解半线性抛物问题的两种二重网格算法

用线性方法对半线性抛物问题进行求解。方法依赖粗、细二重网格,针对粗解在细网格上的修正提出了两种算法,算法1是乘积倍的增长精度而算法2是平方倍的增长精度,而且重复算法1、2的最后几步可以任意阶地逼近细网格上的非线性解。数值算例验证了算法的可行性和有效性。     

Galerkin-petrov limit schemes for the convection-diffusion equation

In the present paper, we suggest a method for constructing grid schemes for the multidimensional convection-diffusion equation. The method is based on the approximation of the integral identity that is used in the definition of a weak solution of the differential problem. The use of spaces of smooth trial functions and spaces of functions with possible discontinuities in which the solution of the original problem is sought naturally leads to Galerkin-Petrov methods. The suggested method for the construction of grid schemes is based on a finite-element semidiscretization of the original space with respect to space variables, which constructs the space of trial functions on the basis of the direction of the convective transport near the boundaries of finite elements, the limit passage from a scheme with smooth trial functions to schemes with discontinuous trial functions, and the further discretization of the resulting equations with respect to the time variable. We prove the stability of the constructed difference schemes and present the results of computations for model problems.     

First-order accurate finite difference schemes for boundary vorticity approximations in curvilinear geometries

In this work we develop first-order accurate, forward finite difference schemes for the first derivative on both a uniform and a non-uniform grid. The schemes are applied to the calculation of vorticity on a solid wall of a curvilinear, two-dimensional channel. The von Mises coordinates are used to transform the governing equations, and map the irregular domain onto a rectangular computational domain. Vorticity on the solid boundary is expressed in terms of the first partial derivative of the square of the speed of the flow in the computational domain, and the derived finite difference schemes are used to calculate the vorticity at the computational boundary grid points using combinations of up to five computational domain grid points. This work extends previous work (Awartani et al., 2005) [3] in which higher-order schemes were obtained for the first derivative using up to four computational domain grid points. The aim here is to shed further light onto the use of first-order accurate non-uniform finite difference schemes that are essential when the von Mises transformation is used. Results show that the best schemes are those that use a natural sequence of non-uniform grid points. It is further shown that for non-uniform grid with clustering near the boundary, solution deteriorates with increasing number of grid points used. By contrast, when a uniform grid is used, solution improves with increasing number of grid points used.     

Grid approximation of singularly perturbed parabolic reaction-diffusion equations on large domains with respect to the space and time variables

In an unbounded (with respect to x and t) domain (and in domains that can be arbitrarily large), an initial-boundary value problem for singularly perturbed parabolic reaction-diffusion equations with the perturbation parameter ε² multiplying the higher order derivative is considered. The parameter ε takes arbitrary values in the half-open interval (0, 1]. To solve this problem, difference schemes on grids with an infinite number of nodes (formal difference schemes) are constructed that converge ε-uniformly in the entire unbounded domain. To construct these schemes, the classical grid approximations of the problem on the grids that are refined in the boundary layer are used. Schemes on grids with a finite number of nodes (constructive difference schemes) are also constructed for the problem under examination. These schemes converge for fixed values of ε in the prescribed bounded subdomains that can expand as the number of grid points increases. As ε → 0, the accuracy of the solution provided by such schemes generally deteriorates and the size of the subdomains decreases. Using the condensing grid method, constructive difference schemes that converge ε-uniformly are constructed. In these schemes, the approximation accuracy and the size of the prescribed subdomains (where the schemes are convergent) are independent of ε and the subdomains may expand as the number of nodes in the underlying grids increases.     

Two‐grid mixed finite‐element methods for nonlinear Schrödinger equations

Two‐grid mixed finite element schemes are developed for solving both steady state and unsteady state nonlinear Schrödinger equations. The schemes use discretizations based on a mixed finite‐element method. The two‐grid approach yields iterative procedures for solving the nonlinear discrete equations. The idea is to relegate all of the Newton‐like iterations to grids much coarser than the final one, with no loss in order of accuracy. Numerical tests are performed. © 2010 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 28: 63‐73, 2012     

Practical finite‐analytic method for solving differential equations by compact numerical schemes

Methodology for development of compact numerical schemes by the practical finite‐analytic method (PFAM) is presented for spatial and/or temporal solution of differential equations. The advantage and accuracy of this approach over the conventional numerical methods are demonstrated. In contrast to the tedious discretization schemes resulting from the original finite‐analytic solution methods, such as based on the separation of variables and Laplace transformation, the practical finite‐analytical method is proven to yield simple and convenient discretization schemes. This is accomplished by a special universal determinant construction procedure using the general multi‐variate power series solutions obtained directly from differential equations. This method allows for direct incorporation of the boundary conditions into the numerical discretization scheme in a consistent manner without requiring the use of artificial fixing methods and fictitious points, and yields effective numerical schemes which are operationally similar to the finite‐difference schemes. Consequently, the methods developed for numerical solution of the algebraic equations resulting from the finite‐difference schemes can be readily facilitated. Several applications are presented demonstrating the effect of the computational molecule, grid spacing, and boundary condition treatment on the numerical accuracy. The quality of the numerical solutions generated by the PFAM is shown to approach to the exact analytical solution at optimum grid spacing. It is concluded that the PFAM offers great potential for development of robust numerical schemes. © 2008 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2009     

套网格有限差分方法求解——一阶双曲方程初边值问题的稳定性条件

本文讨论用套网格(Nesting grid)有限差分方法求解一阶双曲方程初边值问题,即在不同的区域做不同的网格剖分,选用相同或不同精度的差分格式.这种方法亦称杂交法(Hybrid Difference Method)或混合型差分方法(Mixed Difference Method),它广泛应用于数值天气预报和流体力学数值计算中,特别,对于局部区域上的解,其梯度变化激烈,而在其余区域上解的梯度变化平稳时,选用这种方法更有优越性。 [5,8,10]是套网格差分格式稳定性方面的工作,上述工作均以Kreiss定理为基础,针对两层显式耗散格式讨论,因而不便于应用,本文旨在利用GKS理论,寻求一般形式套网格差分格式稳定性的判别条件,§1针对模型问题建立套网格差分格式的一般形式,并介绍GKS理论的一种变形;§2建立套网格差分格式稳定性判别条件;§3是对一类差分格式和网格条件给出易于检验的稳定性判别准则;§4推广了Ciment匹配定理,并证明§3中的主要结果,最后§5是数值例子, 本文采用[1],[3]的符号。     

Burgers方程的一类交替分组方法

对于Burgers方程给出了一组新的Saul'yev型非对称差分格式，并用这些差分格式构造了求解非线性Burgers方程的交替分组四点方法．该算法把剖分节点分成若干组，在每组上构造能够独立求解的差分方程．因此算法具有并行本性，能直接在并行计算机上使用．章还证明了所给算法线性绝对稳定．数值试验表明，该方法使用简便，稳定性好，有很好的精度。     

Fourth-order finite difference methods for three-dimensional general linear elliptic problems with variable coefficients

In this article, we give a simple method for deriving finite difference schemes on a uniform cubic grid. We consider a general, three-dimensional, second-order, linear, elliptic partial differential equation with variable coefficients. We derive two simple fourth-order schemes. When the coefficients of the second-order mixed derivatives are equal to zero, the fourth-order scheme requires only 19 grid points. When the coefficients of the mixed derivatives are not equal to zero and the coefficients of U_xx, U_yy, and U_zz are equal, we require the 27 points of the cubic grid. Numerical examples are given to demonstrate the performance of the two schemes derived. There does not exist a fourth-order scheme involving 27 grid points for the general case.     

ON THE EXPLICIT COMPACT SCHEMES II: EXTENSION OF THE STCE/CE METHOD ON NONSTAGGERED GRIDS ON THE EXPLICIT COMPACT SCHEMES II: EXTENSION OF THE STCE/CE METHOD ON NONSTAGGERED GRIDS

1. IntroductionTills paper is interested in the genuinely nonlinear conserVation lawswith initial data u(0, x) = "o(x), x = (x', ...l x').It is well known that the above problem may not always have a smooth global solutinn even if the initial data no is adequately smooth[6]. Thus, we consider its weaksolutinn so that the Problem (1.1) Ililght have a global solution allowing discontinuities(e.g. shock wave.etc.). Moreover, the elltrOPy conditinn should be deposed inorder to single out a phyS…     

Arterial-based control of traffic flow in urban grid networks

Urban networks are typically composed of a grid of arterial streets. Optimal control of the traffic signals in the grid system is essential for the effective operation of the network. In this paper, we present mathematical programming models for the development of optimal arterial-based progression schemes. Such schemes are widely used for traffic signal control in arterial streets. Under such a scheme, a continuous green band is provided in each direction along the artery at the desired speed of travel to facilitate the movement of through traffic along the arterial. Traditional schemes consist of uniform-width progressions. New approaches generate variable bandwidth progressions in which each directional road section is allocated an individually weighted weighted that can be adapted to the prevailing traffic flows on that link. Mixed-integer linear programming is used for the optimization. Simulation results indicate that this method can produce considerable gains in performance when compared with traditional progression methods. By introducing efficient computational techniques, this method also lends itself to a natural extension for incorporation in a dynamic traffic management system.     

Uniformly Convergent Multigrid Methods for Convection–Diffusion Problems without Any Constraint on Coarse Grids

We construct a class of multigrid methods for convection–diffusion problems. The proposed algorithms use first order stable monotone schemes to precondition the second order standard Galerkin finite element discretization. To speed up the solution process of the lower order schemes, cross-wind-block reordering of the unknowns is applied. A V-cycle iteration, based on these algorithms, is then used as a preconditioner in GMRES. The numerical examples show that this method is convergent without imposing any constraint on the coarsest grid and the convergence of the preconditioned method is uniform.     

A Two-Grid Finite-Element Method for the Nonlinear Schrödinger Equation

In this paper, some two-grid finite element schemes are constructed for solving the nonlinear Schrödinger equation. With these schemes, the solution of the original problem is reduced to the solution of the same problem on a much coarser grid together with the solutions of two linear problems on a fine grid. We have shown, both theoretically and numerically, that our schemes are efficient and achieve asymptotically optimal accuracy.     

Accuracy,robustness and efficiency comparison in iterative computation of convection diffusion equation with boundary layers

Nine‐point fourth‐order compact finite difference scheme, central difference scheme, and upwind difference scheme are compared for solving the two‐dimensional convection diffusion equations with boundary layers. The domain is discretized with a stretched nonuniform grid. A grid transformation technique maps the nonuniform grid to a uniform one, on which the difference schemes are applied. A multigrid method and a multilevel preconditioning technique are used to solve the resulting sparse linear systems. We compare the accuracy of the computed solutions from different discretization schemes, and demonstrate the relative efficiency of each scheme. Comparisons of maximum absolute errors, iteration counts, CPU timings, and memory cost are made with respect to the two solution strategies. © 2000 John Wiley & Sons, Inc. Numer Methods Partial Differential Eq 16: 379–394, 2000     

Multidimensional upwind convection schemes for flow in porous media on structured and unstructured quadrilateral grids

Standard reservoir simulation schemes employ first order upwind schemes for approximation of the convective fluxes when multiple phases or components are present. These convective flux schemes rely upon upwind information that is determined according to grid geometry. As a consequence directional diffusion is introduced into the solution that is grid dependent. The effect can be particularly important for cases where the flow is across grid coordinate lines and is known as cross-wind diffusion.Truly higher dimensional upwind schemes that minimize cross-wind diffusion are presented for convective flow approximation on quadrilateral unstructured grids. The schemes are locally conservative and yield improved results that are essentially free of spurious oscillations. The higher dimensional schemes are coupled with full tensor Darcy flux approximations.The benefits of the resulting schemes are demonstrated for classical test problems in reservoir simulation including cases with full tensor permeability fields. The test cases involve a range of structured and unstructured grids with variations in orientation and permeability that lead to flow fields that are poorly resolved by standard simulation methods. The higher dimensional formulations are shown to effectively reduce the numerical cross-wind diffusion effect, leading to improved resolution of concentration and saturation fronts.     

Stability of an operator-difference scheme for thermoelasticity problems

We study a linear three-layer operator-difference scheme with weights which generalizes a class of difference and projection-difference schemes for coupled thermoelasticity problems. Using the method of energy inequalities, we obtain stability estimates in grid energy norms under certain conditions on operator coefficients and parameters of the scheme.     

Three-point finite-difference schemes, Padé and the spectral Galerkin method. I. One-sided impedance approximation

A method for calculating special grid placement for three-point schemes which yields exponential superconvergence of the Neumann to Dirichlet map has been suggested earlier. Here we show that such a grid placement can yield impedance which is equivalent to that of a spectral Galerkin method, or more generally to that of a spectral Galerkin-Petrov method. In fact we show that for every stable Galerkin-Petrov method there is a three-point scheme which yields the same solution at the boundary. We discuss the application of this result to partial differential equations and give numerical examples. We also show equivalence at one corner of a two-dimensional optimal grid with a spectral Galerkin method.

     

On the convergence of solutions for a class of nonconformal FEM schemes for quasilinear elliptic equations

We consider versions of the nonconformal finite element method for the approximation to a second-order quasilinear elliptic equation in divergence form. For the construction of grid schemes, we use an approach used earlier for the nonstationary convection-diffusion equation and based on the Galerkin-Petrov limit approximation to the mixed statement of the original problem. The accuracy of solutions of nonconformal schemes with triangular linear finite elements is estimated in the absence of interior penalty terms, which are usually used in methods close to DG-methods for the stabilization of the scheme solution.     

Multisignal histogram‐based islanding detection using neuro‐fuzzy algorithm

Islanding is an important concern for grid‐connected distributed resources due to personnel and equipment safety issues. Several techniques based on passive and active detection schemes have been proposed previously. Although passive schemes have a large nondetection zone (NDZ), concerns have been raised about active methods because of their degrading effect on power quality. Reliably detecting this condition is regarded by many as an ongoing challenge because existing methods are not entirely satisfactory. This article proposes a new integrated histogram analysis method using a neuro‐fuzzy approach for islanding detection in grid‐connected wind turbines. The main objective of the proposed approach is to reduce the NDZ to as close as possible to zero and to maintain the output power quality unchanged. In addition, this technique can also overcome the problem of setting detection thresholds which is inherent in existing techniques. The method proposed in this study has a small NDZ and is capable of detecting islanding accurately within the minimum standard time. Moreover, for those regions which require better visualization, the proposed approach can serve as an efficient aid for better detecting grid‐power disconnection. © 2014 Wiley Periodicals, Inc. Complexity 21: 195–205, 2015     

Three-scale finite element eigenvalue discretizations

Some three-scale finite element discretization schemes are proposed and analyzed in this paper for a class of elliptic eigenvalue problems on tensor product domains. With these schemes, the solution of an eigenvalue problem on a fine grid may be reduced to the solutions of eigenvalue problems on a relatively coarse grid and some partially mesoscopic grids, together with the solutions of linear algebraic systems on a globally mesoscopic grid and several partially fine grids. It is shown theoretically and numerically that this type of discretization schemes not only significantly reduce the number of degrees of freedom but also produce very accurate approximations. AMS subject classification (2000)  65N15, 65N25, 65N30, 65N50