Abstract
Efficient and robust iterative solvers for strongly anisotropic elliptic equations are very challenging. Indeed, the discretization of this class of problems gives rise to a linear system with a condition number increasing with anisotropic strength. This weakness is addressed clearly by adopting the asymptotic-preserving (AP) discretizations. In this paper a block preconditioning method is introduced to solve the linear algebraic systems of a class of micro–macro asymptotic-preserving (MMAP) scheme. The MMAP method was developed by Degond et al. in 2012 where its corresponding discrete matrix has a \(2\times 2\) block structure. Motivated by approximate Schur complements, a series of block preconditioners are constructed. We first analyze a natural approximate Schur complement that is the coefficient matrix of the original Non-AP discretization. However it tends to be singular for very small anisotropic parameters. We then improve it by using more suitable approximation for boundary rows of the exact Schur complement. With these block preconditioners, a preconditioned GMRES iterative method is developed to solve the discrete equations. Several numerical tests show that block preconditioning methods can be a practically useful strategy with respect to grid refinement and anisotropic strengths.



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Acknowledgements
The authors acknowledge fruitful discussions with Dr. Fabrice Deluzet from Institut de Mathématiques de Toulouse Université Paul Sabatier. The authors also would like to thank the anonymous reviewers for their valuable comments and suggestions to greatly improve the quality of the paper, and especially for the mention of the work by D. J. Mavriplis and I. Yavneh.
Funding
This work has been supported by the Natural Science Foundation of China under Grant (11901042, 12371432) and the Fundamental Research Funds for the Central Universities (2022FRFK060021). Chang Yang acknowledges support by a public grant from the “Laboratoire d’Excellence Centre International de Mathématiques et d’Informatique” (LabEx CIMI) overseen by the French National Research Agency (ANR) as part of the “Investissements d’Avenir” program (reference ANR-11-LABX-0040) in the frame of the PROMETEUS project (PRospect of nOvel nuMerical modEls for elecTric propulsion and low tEmperatUre plaSmas) and from the FrFCM (Fédération de recherche pour la Fusion par Confinement Magnétique) in the frame of the NEMESIA project (Numerical mEthods for Macroscopic models of magnEtized plaSmas and related anIsotropic equAtions).
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Appendix A: Finite Difference Discretizations
Appendix A: Finite Difference Discretizations
In this part, let us give the finite difference discretizations for the MMAP scheme introduced in Sect. 2.1.
1.1 A.1. Aligned Case
In the aligned case, we consider the magnetic field aligned with z-axis, i.e. \({\textbf{b}}=(0,1)^T\). For the convenience of matrix construction, we write the MMAP scheme of Sect. 2.1 as follows
and
The mesh is defined by
where \(\Delta x = \frac{1}{n_x-1}\) and \(\Delta z = \frac{1}{n_z-2}\). Furthermore, we take \(n_x=N\) and \(n_z=N+1\), so that \(h=\Delta x = \Delta z\). Then a second order finite difference discretization for (31) and (32) can be directly determined as
and
where \(\phi _{i,j}\) and \(q_{i,j}\) are numerical approximations of \(\phi (x_i,z_j)\) and \(q(x_i,z_j)\) respectively, and \(f_{i,j}= f(x_i,z_j)\). Now, to construct the linear system to (33)–(34), we define a lexicographic, that is to count first in z direction then in x direction. Denote iline the index of the linear system, we then define a function ’lexico’ to convert from the couple (i, j) to iline as
So the linear system corresponding to (33)–(34) is given by
At last, in order to get a more symmetrical matrix form, we make some algebraic operations, which yield in the first equation of (33) as
Thanks to these algebraic operations, the matrices \(A_{11}\) and \(A_{12}\) become symmetric.
1.2 A.2. Misaligned Case
Now let us concentrate on discretization for the general MMAP scheme (8)–(9). Again, this time we will consider a second order finite difference method.
Firstly, the mesh is defined different from the previous subsection, since we have to discretize cross derivative on all interior grids. For this, the mesh is defined by
where \(\Delta x = \frac{1}{n_x-1}\) and \(\Delta z = \frac{1}{n_z-1}\). Again, we take \(n_x=n_z=N\), so that \(h=\Delta x = \Delta z\).
Secondly, we notice that
its corresponding discretization on interior grids is
Using the usual central discretization, \(\Delta \phi \) can be approximated as
Then, let us denote \({\textbf{b}}\otimes {\textbf{b}}\) by
thus we can recast \(\Delta _\Vert \phi \) as
where \(G = b^{11}\partial _x\phi + b^{12}\partial _z\phi \), \(H = b^{21}\partial _x\phi + b^{22}\partial _z\phi \). Similarly, we get approximate \(\Delta _\Vert \phi \) as
where
At last, we obtain immediately approximated \(\Delta _\bot \phi \) by
Thanks to these discrete operators, the sub-blocks \(A_{22}\), \(A_{21}\), \(A_{11}\) and \(A_{12}\) from the linear system (10) correspond to \(-\Delta ^h_\Vert \phi \), \(\varepsilon \Delta ^h_\Vert q\), \(-\Delta ^h_\Vert q\) and \(-\Delta ^h_\bot \phi \) respectively on interior grids. Finally, let us consider discretization for boundary conditions. From (8)–(9), we see that it is necessary to approximate \({\textbf{n}}\cdot \nabla _\bot \phi \) and \({\textbf{n}}\cdot \nabla _\Vert q\) on \([0,1]\times \{0,1\}\). We only present the approximation on boundary \([0,1]\times \{0\}\), the one on boundary \([0,1]\times \{1\}\) is similar. Notice that the normal direction on boundary \([0,1]\times \{0\}\) is \({\textbf{n}}=(0,1)^T\), we thus has
Now applying center discretizations, we get a second approximation for above differential operations
At last, substituting \({\textbf{n}}\cdot \nabla _\bot \phi \) and \({\textbf{n}}\cdot \nabla _\Vert q\) in (8)–(9) by the above approximation, we obtain desired second order finite difference schemes.
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Li, L., Yang, C. Block Preconditioning Methods for Asymptotic Preserving Scheme Arising in Anisotropic Elliptic Problems. J Sci Comput 99, 63 (2024). https://doi.org/10.1007/s10915-024-02524-2
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DOI: https://doi.org/10.1007/s10915-024-02524-2
Keywords
- Block preconditioning methods
- Asymptotic preserving scheme
- Anisotropic elliptic problems
- Schur complement
- Finite difference method