一种基于WENO重构的半离散中心迎风格式

通过三阶WENO重构和半离散中心迎风数值通量的结合,给出了一种求解双曲型守恒律方程的三阶半离散中心迎风格式,格式保持了中心差分格式方法简单的优点．数值计算的结果表明该方法具有较高的分辨率．     

高阶驻点上精度保持的六阶WENO有限差分格式

在五阶WENO有限差分格式的基础上,六阶WENO有限差分格式引入了额外的四点模板,减少了WENO格式的数值耗散.然而,该格式在驻点上无法达到理想收敛阶.为解决此问题,本文在非线性权重中引入整体模板的光滑性修正因子,使得驻点上非线性权重更快地收敛于理想权重,理论分析表明改进后的六阶格式能够在驻点上达到理想的六阶精度.驻点上的收敛阶测试和间断问题的数值实验表明,新提出的六阶WENO格式不仅在驻点上能够保持理想收敛精度,在间断问题上能保持本质无振荡的激波捕捉性质,同时在双曲守恒律解的光滑区域有效地求解细小尺度结构,还能够保持原有的六阶格式的计算效率.     

一维抛物型方程的一个新的高精度显式差分格式

工程技术中,常常需要求解抛物型方程．一维情形下的模型问题为 用差分方法解上述问题,隐格式常因计算量很大而不便使用,构造稳定性好精度高的显格式是非常必要的．文山构造了求解Ｐ维抛物型方程的分支绝对稳定的显式差分格式,但格式的精度不高,截断误差仅为 ．本文就 p＝ １情形构造了一个解问题（１）－（３）的新的显格式,精度较文［１］有较大的提高,截断误差可达． §１．差分格式的构造 设△ｔＬ为时间步长,△ｘ= Ｌ／Ｍ（Ｍ为正整数）为空间步长,网函数ｕ（ｊ△ｘ,ｎ△ｔ）记为ｕｊｎ．对方程（１）建立如下的差分格式：其…     

双曲型守恒律的一种三阶松弛格式

对一维双曲型守恒律,给出了一种形式更简单、计算量更小的三阶松弛格式.该格式以三阶WENO重构和三阶显隐式Runge-Kutta方法为基础.由于不用求解Riemann问题和计算非线性通量函数的雅可比矩阵,所以本文格式保持了松弛格式简单的优点.数值试验表明:该方法具有较高的分辨率.     

一维抛物型方程的样条子域精细积分(SSPI)隐格式

对一维抛物型方程初边值问题的求解,以往已经有一些数值解法,它们或者无条件稳定但精度不高,或者精度高但仅为条件稳定,且稳定性条件严格.另外,以往的差分格式在处理第二、第三类边界条件问题时,对带导数边界条件都是进行简单的差分逼近,影响了数值解的精度.因此构造一个无条件稳定且对各类边值问题都具有良好精度的数值方法具有重要意义.为此,基于子域精细积分思想,结合三次样条函数,提出了求解一维抛物型方程初边值问题含参数的样条子域精细积分格式.该格式为绝对稳定且精度很高.由于三次样条函数的采用,避免了通常有限差分法中处理带导数边界条件时产生的逼近误差,大大提高了求解第二、三类边界条件问题时的精度.     

二维带分数阶Laplacian算子的对流扩散方程的格子Boltzmann模型研究

带有分数阶Laplacian算子的对流扩散方程常被用来刻画自然界与社会系统中的反常扩散现象.本文提出了一种新的格子Boltzmann模型,用于求解二维带分数阶Laplacian算子的对流扩散方程.首先,基于分数阶Laplacian算子的Fourier变换和Gauss型求积公式,得到控制方程的近似方程.然后,将速度空间、时间和空间进行离散,并构造合适的平衡态分布函数和离散作用力,建立有效的格子Boltzmann-BGK模型.通过Chapman-Enskog分析,可由建立的格子Boltzmann-BGK模型恢复出宏观方程,从而证明了模型的有效性.最后,将模型应用于求解带有解析解的数值算例和Allen-Cahn方程,数值结果进一步验证了模型的正确性和有效性.     

图像平滑的2维小波插值方法

在应用扩散方程进行图像平滑时,常规的方法是对扩散方程差分化构造差分方程,利用初边值条件求解。这种方法误差传播快,精度不高。因此,构造了2维小波插值函数,利用它来求解扩散方程,并分析得到用小波插值函数求解Alvarez模型的方法。由于小波函数具有良好的局部性,求解扩散方程比用差分方法求解具有精度高,误差传播速度慢,对时间步长不敏感等优点。在数值实验中,给出了本文方法的有效性及相对于差分方法求解的优点。     

稳定交错差分求解线性SFS问题的新算法

针对采用偏心格式求解明暗恢复形状问题中的图像辐照方程时只能针对特定光源计算的特点,提出了一种新的稳定差分解法。首先使用泰勒展式线性化反射图函数,然后结合定解条件,讨论了一种新的交错差分格式用于求解图像辐照方程的具体算法,并给出了新差分格式稳定性和收敛性的具体证明。结合最佳松弛因子的选取方法,表面高度的最终值采用超松弛法迭代计算求得。经过多组图像计算可知,该新方法适用于任意的光照环境,且重构精度高于已有算法。     

求解多介质流体界面不稳定性问题的高精度WENO数值模拟方法

本文针对多介质流体界面不稳定性问题的数值模拟,把基于波传算法的高精度WENO数值格式用于守恒和非守恒形式的流体力学方程组计算。根据不同介质界面附近压强和速度保持一致的特点,求解了γ-model和体积分数形式的耦合型方程组,并与NND和NT2的模拟结果进行比较分析,表明该方法具有高分辨率和较强的捕捉界面的能力．     

不等距网格上求解ODE特征值问题若干高精度格式的计算与分析

本文针对不等距网格,从Raylei曲商(Raylei曲quotient)角度出发,构造了若干求解ODE特征值问题的高阶格式,并进行误差分析.文中高阶格式的构造是基于线性有限元及其对应的差分格式进行的.单纯的线性有限元及其对应的差分格式求解PDE特征值问题都只有二阶精度,我们利用质量集中和加权组合的思想通过将二者结合得到四阶精度的算法.本文从理论和实验的角度构造高阶格式并进行了相应的误差分析.通过在五种网格上计算四阶精度格式的误差阶系数,将四阶格式加权组合的新格式甚至可以达到六阶精度.最后用数值实验验证了构造的高阶格式的误差阶.同时,本文构造的两种四阶格式相对于传统的线性有限元方法,在同等量级误差的要求下,需要的网格数有量级的减少.     

Hermite WENO schemes and their application as limiters for Runge-Kutta discontinuous Galerkin method II: Two dimensional case

A class of fifth-order weighted essentially non-oscillatory (WENO) schemes based on Hermite polynomials, termed HWENO (Hermite WENO) schemes, for solving one dimensional non-linear hyperbolic conservation law systems, was developed and applied as limiters for the Runge-Kutta discontinuous Galerkin (RKDG) methods in [J. Comput. Phys. 193 (2003) 115]. In this paper, we extend the method to solve two dimensional non-linear hyperbolic conservation law systems. The emphasis is again on the application of such HWENO finite volume methodology as limiters for RKDG methods to maintain compactness of RKDG methods. Numerical experiments for two dimensional Burgers’ equation and Euler equations of compressible gas dynamics are presented to show the effectiveness of these methods.     

Lattice BBGKY scheme for two-phase flows: One-dimensional case

A novel lattice Boltzmann model for two-phase fluids is presented. We begin with the two-body BBGKY equation, and perform a coordinate transformation to split it into a Boltzmann equation for the one-body distribution, coupled to a kinetic equation for the correlation function. The coupling is accomplished by a self-consistent force. The resulting lattice Boltzmann model for nonideal fluids is grounded in the physics of the two-body distribution function. The discrete velocity model is described in detail, and numerical results are given for phase separation in one dimension.     

Numerical Study of the Nonlinear Combined Sine-Cosine-Gordon Equation with the Lattice Boltzmann Method

In this paper, a?lattice Boltzmann model is developed for solving the combined sine-cosine-Gordon equation through selecting equilibrium distribution function properly. With the Chapman-Enskog expansion, the governing evolution equation is recovered correctly from the continuous Boltzmann equation. Some problems, which have exact solutions, are validated by the present model. From the simulations, we find that the numerical results agree well with the exact solutions or better than the numerical solutions reported in previous studies. The study indicates that the present method is very effective and accurate. The present model can be used to solve more other nonlinear wave problems.     

Lattice Boltzmann model for elastic thin plate with small deflection

In this paper, a lattice Boltzmann model for solving problems of elastic thin plate with small deflection is proposed. In order to recover the Sophie–Germain equation for elastic thin plate by lattice Boltzmann method, we transform the equation into a set of Poisson equations. Two sets of distribution functions are employed in the lattice Boltzmann equation to recover the Poisson equations. Based on this model, some problems on the rectangular elastic thin plate with small deflection are simulated. The comparisons between the numerical results and the analysis solutions are given in detail. The numerical examples show that the lattice Boltzmann model can be used to solve problems of the elastic thin plate with small deflection.     

On different flux splittings and flux functions in WENO schemes for balance laws

In this paper we focus our attention on obtaining well-balanced schemes for balance laws by using Marquina’s flux in combination with the finite difference and finite volume WENO schemes. We consider also the Rusanov flux splitting and the HLL approximate Riemann solver. In particular, for the presented numerical schemes we develop corresponding discretizations of the source term, based on the idea of balancing with the flux gradient. When applied to the open-channel flow and to the shallow water equations, we obtain the finite difference WENO scheme with Marquina’s flux splitting, which satisfies the approximate conservation property, and also the balanced finite volume WENO scheme with Marquina’s solver satisfying the exact conservation property. Finally, we also present an improvement of the balanced finite difference WENO scheme with the Rusanov (locally Lax–Friedrichs) flux splitting, we previously developed in [Vuković S, Sopta L. ENO and WENO schemes with the exact conservation property for one-dimensional shallow water equations. J Comput Phys 2002;179:593–621].     

Multidomain WENO Finite Difference Method with Interpolation at Subdomain Interfaces

High order finite difference WENO methods have the advantage of simpler coding and smaller computational cost for multi-dimensional problems, compared with finite volume WENO methods of the same order of accuracy. However a main restriction is that conservative finite difference methods of third and higher order of accuracy can only be used on uniform rectangular or smooth curvilinear meshes. In order to overcome this difficulty, in this paper we develop a multidomain high order WENO finite difference method which uses an interpolation procedure at the subdomain interfaces. A simple Lagrange interpolation procedure is implemented and compared to a WENO interpolation procedure. Extensive numerical examples are shown to indicate the effectiveness of each procedure, including the measurement of conservation errors, orders of accuracy, essentially non-oscillatory properties at the domain interfaces, and robustness for problems containing strong shocks and complex geometry. Our numerical experiments have shown that the simple and efficient Lagrange interpolation suffices for the subdomain interface treatment in the multidomain WENO finite difference method, to retain essential conservation, full high order of accuracy, essentially non-oscillatory properties at the domain interfaces even for strong shocks, and robustness for problems containing strong shocks and complex geometry. The method developed in this paper can be used to solve problems in relatively complex geometry at a much smaller CPU cost than the finite volume version of the same method for the same accuracy. The method can also be used for high order finite difference ENO schemes and an example is given to demonstrate a similar result as that for the WENO schemes.     

A Speed-Up Strategy for Finite Volume WENO Schemes for Hyperbolic Conservation Laws

In this paper, a speed-up strategy for finite volume WENO schemes is developed for solving hyperbolic conservation laws. It adopts p-adaptive like reconstruction, which automatically adjusts from fifth order WENO reconstruction to first order constant reconstruction when nearly constant solutions are detected by the undivided differences. The corresponding order of accuracy for the solutions is shown to be the same as obtained by original WENO schemes. The strategy is implemented with both WENO and mapped WENO schemes. Numerical examples in different space dimensions show that the strategy can reduce the computational cost by 20–40%, especially for problems with large fraction of constant regions.     

Hermite WENO Schemes and Their Application as Limiters for Runge-Kutta Discontinuous Galerkin Method,III: Unstructured Meshes

In [J. Comput. Phys. 193:115–135, 2004] and [Comput. Fluids 34:642–663, 2005], Qiu and Shu developed a class of high order weighted essentially non-oscillatory (WENO) schemes based on Hermite polynomials, termed HWENO (Hermite WENO) schemes, for solving nonlinear hyperbolic conservation law systems, and applied them as limiters for the Runge-Kutta discontinuous Galerkin (RKDG) methods on structured meshes. In this continuation paper, we extend the method to solve two dimensional problems on unstructured meshes. The emphasis is again on the application of such HWENO finite volume methodology as limiters for RKDG methods to maintain compactness of RKDG methods. Numerical experiments for two dimensional Burgers’ equation and Euler equations of compressible gas dynamics are presented to show the effectiveness of these methods. The research was partially supported by the European project ADIGMA on the development of innovative solution algorithms for aerodynamic simulations, NSFC grant 10671091 and JSNSF BK2006511.     

A lattice Boltzmann model for the Korteweg-de Vries equation with two conservation laws

A lattice Boltzmann model for the Korteweg-de Vries (KdV) equation is presented by using the higher-order moment method. In contrast to the previous lattice Boltzmann model to the KdV equation, our method has higher-order accuracy. Two key steps in the development of this model are the addition of a momentum conservation condition, and the construction of a correlation between the first conservation law and the second conservation law. The numerical example shows the higher-order moment method can be used to raise the truncation error of the lattice Boltzmann scheme.     

Lax–Friedrichs Multigrid Fast Sweeping Methods for Steady State Problems for Hyperbolic Conservation Laws

Fast sweeping methods are efficient Gauss–Seidel iterative numerical schemes originally designed for solving static Hamilton–Jacobi equations. Recently, these methods have been applied to solve hyperbolic conservation laws with source terms. In this paper, we propose Lax–Friedrichs fast sweeping multigrid methods which allow even more efficient calculations of viscosity solutions of stationary hyperbolic problems. Due to the choice of Lax–Friedrichs numerical fluxes, general problems can be solved without difficult inversion. High order discretization, e.g., WENO finite difference method, can be incorporated to achieve high order accuracy. On the other hand, multigrid methods, which have been widely used to solve elliptic equations, can speed up the computation by smoothing errors of low frequencies on coarse meshes. We modify the classical multigrid method with regard to properties of viscous solutions to hyperbolic conservation equations by introducing WENO interpolation between levels of mesh grids. Extensive numerical examples in both scalar and system test problems in one and two dimensions demonstrate the efficiency, high order accuracy and the capability of resolving singularities of the viscosity solutions.