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Solving reaction-diffusion problems  with explicit Runge-Kutta exponential methods without order reduction 用不降阶的显式 Runge-Kutta 指数法解决反应扩散问题
Pub Date : 2024-02-08 DOI: 10.1051/m2an/2024011
Begoña Cano, María Jesús Moreta
Abstract. In this paper a technique is given to recover the classical order of the method when explicit exponential Runge-Kutta methods integrate reaction-diffusion problems. In the literature, methods of high enough stiff order for problems with vanishing boundary conditions have been constructed, but that implies restricting the coefficients and thus, the number of stages and the computational cost may significantly increase with respect to other methods without those restrictions. In contrast, the technique which is suggested here is cheaper because it just needs, for any method, to add some terms with information only on the boundaries. Moreover, time-dependent boundary conditions are directly tackled here.
摘要本文给出了一种在显式指数 Runge-Kutta 方法集成反应扩散问题时恢复方法经典阶数的技术。在文献中,人们已经为边界条件消失的问题构建了足够高的刚性阶次的方法,但这意味着对系数的限制,因此,与其他没有这些限制的方法相比,阶次数量和计算成本可能会显著增加。相比之下,本文提出的技术成本更低,因为对于任何方法而言,只需添加一些只包含边界信息的项即可。此外,这里还可以直接处理随时间变化的边界条件。
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引用次数: 0
Surface boundary layers   through  a scalar equation with  an eddy viscosity vanishing at the ground 通过标量方程计算地面涡流粘度消失的地表边界层
Pub Date : 2024-02-05 DOI: 10.1051/m2an/2024009
R. Lewandowski, François Legeais, L. Berselli
We introduce a scalar elliptic equation defined on a boundary layer given by $Pi_2 times [0, z_{top}]$, where $Pi_2$ is a two dimensional torus, with an eddy vertical eddy viscosity of order $z^alpha$, $alpha in [0, 1]$, an homogeneous boundary condition at $z=0$, and a Robin condition at $z=z_{top}$. We show the existence of weak solutions to this boundary problem, distinguishing the cases $0 le alpha <1$ and $alpha = 1$.  Then we carry out several numerical simulations, showing the ability of our model to accurately reproduce profiles close to those predicted by the Monin-Obukhov theory, by calculating stabilizing functions.
我们引入了一个定义在边界层上的标量椭圆方程,该边界层由 $Pi_2 times [0, z_{top}]$ 给出,其中 $Pi_2$ 是一个二维环,具有阶数为 $z^alpha$ 的垂直涡流粘度,$alpha in [0, 1]$,在 $z=0$ 处为均质边界条件,在 $z=z_{top}$ 处为罗宾条件。我们证明了该边界问题弱解的存在,并区分了 $0 le alpha <1$ 和 $alpha = 1$ 两种情况。 然后,我们进行了几次数值模拟,通过计算稳定函数,表明我们的模型能够准确地再现接近莫宁-奥布霍夫理论预测的剖面。
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引用次数: 0
Surface boundary layers   through  a scalar equation with  an eddy viscosity vanishing at the ground 通过标量方程计算地面涡流粘度消失的地表边界层
Pub Date : 2024-02-05 DOI: 10.1051/m2an/2024009
R. Lewandowski, François Legeais, L. Berselli
We introduce a scalar elliptic equation defined on a boundary layer given by $Pi_2 times [0, z_{top}]$, where $Pi_2$ is a two dimensional torus, with an eddy vertical eddy viscosity of order $z^alpha$, $alpha in [0, 1]$, an homogeneous boundary condition at $z=0$, and a Robin condition at $z=z_{top}$. We show the existence of weak solutions to this boundary problem, distinguishing the cases $0 le alpha <1$ and $alpha = 1$.  Then we carry out several numerical simulations, showing the ability of our model to accurately reproduce profiles close to those predicted by the Monin-Obukhov theory, by calculating stabilizing functions.
我们引入了一个定义在边界层上的标量椭圆方程,该边界层由 $Pi_2 times [0, z_{top}]$ 给出,其中 $Pi_2$ 是一个二维环,具有阶数为 $z^alpha$ 的垂直涡流粘度,$alpha in [0, 1]$,在 $z=0$ 处为均质边界条件,在 $z=z_{top}$ 处为罗宾条件。我们证明了该边界问题弱解的存在,并区分了 $0 le alpha <1$ 和 $alpha = 1$ 两种情况。 然后,我们进行了几次数值模拟,通过计算稳定函数,表明我们的模型能够准确地再现接近莫宁-奥布霍夫理论预测的剖面。
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引用次数: 0
An efficient two-grid high-order compact difference scheme with variable-step BDF2 method for the semilinear parabolic equation 针对半线性抛物方程的高效双网格高阶紧凑差分方案与变步长 BDF2 方法
Pub Date : 2024-01-30 DOI: 10.1051/m2an/2024008
Bingyin Zhang, Hongfei Fu
Due to the lack of  corresponding analysis on appropriate mapping operator between two grids, high-order two-grid difference algorithms are rarely studied. In this paper, we firstly discuss the boundedness of a local bi-cubic Lagrange interpolation operator. And then, taking the semilinear parabolic equation as an example, we first construct a  variable-step high-order nonlinear difference algorithm using compact difference technique in space and  the second-order backward differentiation formula with variable temporal stepsize in time. With the help of discrete orthogonal convolution kernels, temporal-spatial error splitting idea  and a cut-off numerical technique, the unique solvability, maximum-norm stability and corresponding  error estimate of the high-order nonlinear difference scheme are established under assumption that the temporal stepsize ratio satisfies $ r_{k} := tau_{k}/tau_{k-1} < 4.8645 $. Then, an efficient two-grid high-order difference algorithm is developed by combining a small-scale variable-step high-order nonlinear difference algorithm on the coarse grid and a large-scale variable-step high-order linearized difference algorithm on the fine grid, in which the constructed piecewise bi-cubic Lagrange interpolation mapping operator is adopted to project the coarse-grid solution to the fine grid. Under the same temporal stepsize ratio restriction $ r_{k} < 4.8645 $  on the variable temporal stepsize,  unconditional and optimal fourth-order in space and second-order in time maximum-norm error estimates of the two-grid difference scheme is established. Finally, several numerical experiments are carried out to demonstrate the effectiveness and efficiency of the proposed scheme.
由于缺乏对两网格间适当映射算子的相应分析,高阶两网格差分算法很少被研究。本文首先讨论了局部双立方拉格朗日插值算子的有界性。然后,以半线性抛物线方程为例,首先利用空间上的紧凑差分技术和时间上可变时间步长的二阶反向微分公式,构建了一种可变步长的高阶非线性差分算法。借助离散正交卷积核、时空误差分割思想和截断数值技术,在时间步长比满足 $ r_{k} := tau_{k}/tau_{k-1} < 4.8645 $ 的假设条件下,建立了高阶非线性差分方案的唯一可解性、最大正态稳定性和相应的误差估计。然后,结合粗网格上的小尺度变步长高阶非线性差分算法和细网格上的大尺度变步长高阶线性化差分算法,建立了一种高效的双网格高阶差分算法,其中采用了构建的片断双立方拉格朗日插值映射算子将粗网格解投影到细网格。在相同的时间步长比限制下,$ r_{k}< 4.8645 $ 的限制下,建立了双网格差分方案的无条件最优空间四阶和时间二阶最大正则误差估计。最后,通过几个数值实验证明了所提方案的有效性和效率。
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引用次数: 0
Quasi-local and frequency robust preconditioners for the Helmholtz first-kind integral equations on the disk 圆盘上亥姆霍兹第一类积分方程的准局部和频率稳健预处理器
Pub Date : 2024-01-10 DOI: 10.1051/m2an/2023105
François Alouges, Martin Averseng
We propose preconditioners for the Helmholtz scattering problems by a planar, disk-shaped screen in $R^3$.  Those preconditioners are approximations of the square-roots of some partial differential operators acting on the screen. Their matrix-vector products involve only a few sparse system resolutions and can thus be evaluated cheaply in the context of iterative methods.      For the Laplace equation (i.e. for the wavenumber $k=0$) with Dirichlet condition on the disk and on regular meshes, we prove that the preconditioned linear system has a bounded condition number uniformly in the mesh size. We further provide numerical evidence indicating that the preconditioners also perform well for large values of $k$ and on locally refined meshes.
我们针对亥姆霍兹散射问题提出了$R^3$中平面圆盘形屏幕的预处理。 这些先决条件器是作用于屏幕的某些偏微分算子的平方根近似值。它们的矩阵向量乘积只涉及几个稀疏的系统分辨率,因此可以在迭代法中便宜地进行评估。 对于在圆盘和规则网格上具有迪里夏特条件的拉普拉斯方程(即波长为 $k=0$),我们证明了预条件线性系统在网格大小上具有均匀的有界条件数。我们还进一步提供了数值证据,表明预条件器在 $k$ 的大值和局部细化网格上也表现良好。
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引用次数: 2
Low-order fictitious domain method with enhanced mass conservation for an interface Stokes problem 针对界面斯托克斯问题的增强质量守恒低阶虚构域法
Pub Date : 2023-12-19 DOI: 10.1051/m2an/2023103
Daniele Corti, Guillaume Delay, Miguel A. Fernández, Fabien Vergnet, Marina Vidrascu
One of the main difficulties that has to be faced with fictitious domain approximation of incompressible flows with immersed interfaces is related to the potential lack of mass conservation across the interface. In this paper, we propose and analyze a low order fictitious domain stabilized finite element method which mitigates this issue with the addition of a single velocity constraint. We provide a complete a priori numerical analysis of the method under minimal regularity assumptions. A comprehensive numerical study illustrates the capabilities of the proposed method, including comparisons with alternative fitted and unfitted mesh methods.
对具有浸入式界面的不可压缩流进行虚构域近似时,必须面对的主要困难之一与界面上可能缺乏质量守恒有关。在本文中,我们提出并分析了一种低阶虚构域稳定有限元方法,该方法通过添加单一速度约束来缓解这一问题。在最小正则性假设条件下,我们对该方法进行了完整的先验数值分析。全面的数值研究说明了所提方法的能力,包括与其他拟合和非拟合网格方法的比较。
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引用次数: 0
Discrete entropy inequalities via an optimization process 通过优化过程实现离散熵不等式
Pub Date : 2023-12-15 DOI: 10.1051/m2an/2023098
N. Aguillon, Emmanuel Audusse, Vivien Desveaux, Julien Salomon
The solutions of hyperbolic systems may contain discontinuities. These weak solutions verify not only the original PDEs, but also an entropy inequality that acts as a selection criterion determining whether a discontinuity is physical or not. Obtaining a discrete version of these entropy inequalities when approximating the solutions numerically is crucial to avoid convergence to unphysical solutions or even unstability. However such a task is difficult in general, if not impossible for schemes of order 2 or more. In this paper, we introduce an optimization framework that enables us to quantify a posteriori the decrease or increase of entropy of a given scheme, locally in space and time. We use it to obtain maps of numerical diffusion and to prove that some schemes do not have a discrete entropy inequality. A special attention is devoted to the widely used second order MUSCL scheme for which almost no theoretical results are known.
双曲系统的解可能包含不连续性。这些弱解不仅验证了原始的 PDE,还验证了熵不等式,该不等式是确定不连续性是否物理的选择标准。在数值近似求解时,获得这些熵不等式的离散版本对于避免收敛到非物理解甚至不稳定性至关重要。然而,对于阶数为 2 或更高的方案来说,这样的任务即使不是不可能,一般也很困难。在本文中,我们引入了一个优化框架,使我们能够对给定方案在空间和时间上的熵的减少或增加进行事后量化。我们用它来获得数值扩散图,并证明某些方案不存在离散熵不等式。我们特别关注了广泛使用的二阶 MUSCL 方案,目前几乎还没有关于该方案的理论结果。
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引用次数: 0
A PWDG method for the Maxwell system in anisotropic media with piecewise constant coefficient matrix 各向异性介质中麦克斯韦系统的 PWDG 方法,系数矩阵为片断常数
Pub Date : 2023-11-30 DOI: 10.1051/m2an/2023097
Long Yuan, Qiya Hu
In this paper we are concerned with plane wave discontinuous Galerkin (PWDG) methods for time-harmonic Maxwell equations in three-dimensional anisotropic media, for which the coefficients of the equations are piecewise constant symmetric matrices, where each constant symmetric matrix is defined on a medium (subdomain). By using suitable scaling transformations and coordinate (complex) transformations on every subdomain, the original Maxwell equation in anisotropic media is transformed into a Maxwell equation in isotropic media occupying a union domain of specific subdomains of complex Euclidean space. Based on these transformations, we define anisotropic plane wave basis functions and discretize the considered Maxwell equations by PWDG method with the proposed plane wave basis functions.  We derive error estimates of the resulting approximate solutions, and further introduce a practically feasible local $hp-$refinement algorithm, which substantially improves accuracies of the approximate solutions. Numerical results indicate that the approximate solutions generated by the proposed PWDG methods possess high accuracy for the case of strong discontinuity media.
本文涉及三维各向异性介质中时谐麦克斯韦方程的平面波非连续伽勒金(PWDG)方法,方程的系数为片断常数对称矩阵,其中每个常数对称矩阵都定义在一个介质(子域)上。通过在每个子域上使用适当的缩放变换和坐标(复数)变换,各向异性介质中的原始麦克斯韦方程被转换为各向同性介质中的麦克斯韦方程,该方程占据复欧几里得空间特定子域的联合域。基于这些转换,我们定义了各向异性平面波基函数,并利用所提出的平面波基函数通过 PWDG 方法对所考虑的麦克斯韦方程进行离散化。 我们得出了所得到的近似解的误差估计值,并进一步引入了一种实际可行的局部$hp-$细化算法,该算法大大提高了近似解的精确度。数值结果表明,所提出的 PWDG 方法生成的近似解在强不连续介质情况下具有很高的精度。
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引用次数: 0
Stability analysis of a finite element approximation for the Navier-Stokes equation with free surface 带自由表面的纳维-斯托克斯方程有限元近似的稳定性分析
Pub Date : 2023-11-29 DOI: 10.1051/m2an/2023096
G. Barrenechea, E. Audusse, A. Decoene, Pierrick Quemar
In this work we study the numerical approximation of incompressible Navier-Stokes equations with free surface. The evolution of the free surface is driven by the kinematic boundary condition, and an Arbitrary Lagrangian Eulerian (ALE) approach is used to derive a (formal) weak formulation which involves three fields, namely, velocity, pressure, and the function describing the free surface. This formulation is discretised using finite elements in space and a time-advancing explicit finite difference scheme in time. In fact, the domain tracking algorithm is explicit: first, we solve the equation for the free surface, then move the mesh according to the sigma transform, and finally we compute the velocity and pressure in the updated domain. This explicit strategy is built in such a way that global conservation can be proven, which plays a pivotal role in the proof of stability of the discrete problem. The well-posedness and stability results are independent of the viscosity of the fluid, but while the proof of stability for the velocity is valid for all time steps, and all geometries, the stability for the free surface requires a CFL condition. The performance of the current approach is presented via numerical results and comparisons with the characteristics finite element method.
在这项工作中,我们研究了具有自由表面的不可压缩纳维-斯托克斯方程的数值近似。自由表面的演化由运动边界条件驱动,并采用任意拉格朗日欧拉(ALE)方法推导出一种(形式)弱公式,其中涉及三个场,即速度、压力和描述自由表面的函数。该公式在空间上使用有限元进行离散,在时间上使用时间递增显式有限差分方案。事实上,域跟踪算法是显式的:首先,我们求解自由表面方程,然后根据西格玛变换移动网格,最后计算更新域中的速度和压力。这种显式策略可以证明全局守恒,这对证明离散问题的稳定性起着关键作用。拟合度和稳定性结果与流体的粘度无关,但速度的稳定性证明适用于所有时间步长和所有几何形状,而自由表面的稳定性则需要 CFL 条件。通过数值结果以及与特征有限元法的比较,介绍了当前方法的性能。
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引用次数: 0
PDE-convergence in Euclidean norm of AMF-W methods for linear multidimensional parabolic problems 线性多维抛物线问题的 AMF-W 方法在欧氏规范下的 PDE 收敛性
Pub Date : 2023-11-16 DOI: 10.1051/m2an/2023094
S. González-Pinto, Ernst Hairer, D. Hernández-Abreu
This work considers space-discretised parabolic problems on a rectangular domain subject to Dirichlet boundary conditions. For the time integration s-stage AMF-W-methods, which are ADI (alternating direc- tion implicit) type integrators, are considered. They are particularly efficient when the space dimension m of the problem is large. Optimal results on PDE-convergence have recently been obtained in [J. Comput. Appl. Math., 417:114642, 2023] for the case m = 2. The aim of the present work is to extend these results to arbitrary space dimension m ≥ 3. It is explained which order statements carry over from the case m = 2 to m ≥ 3, and which do not.
本研究考虑了矩形域上的空间离散抛物线问题,该问题受德里希特边界条件的限制。对于时间积分,考虑了 s 级 AMF-W 方法,即 ADI(交替方向隐式)类型积分器。当问题的空间维数 m 较大时,这种方法尤其有效。最近,[J. Comput. Appl. Math., 417:114642, 2023]获得了 m = 2 情况下 PDE 收敛的最佳结果。本研究的目的是将这些结果扩展到任意空间维度 m ≥ 3。本文解释了哪些秩语句可以从 m = 2 的情况延续到 m ≥ 3,哪些不可以。
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引用次数: 0
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ESAIM: Mathematical Modelling and Numerical Analysis
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