Robust spectral preconditioning for high-Péclet number convection-diffusion
For computational scientists solving large-scale convection-diffusion problems, this method provides a scalable preconditioner that remains effective at high Péclet numbers, addressing a known bottleneck in domain decomposition methods.
The paper introduces a two-level hybrid RAS preconditioner for high-Péclet number convection-diffusion equations, achieving robust GMRES convergence with low-dimensional coarse spaces. Numerical experiments demonstrate exponential convergence and effectiveness up to 10^5 subdomains, even for indefinite problems.
We introduce a two-level hybrid restricted additive Schwarz (RAS) preconditioner for heterogeneous steady-state convection-diffusion equations at high Péclet numbers. Our construction builds on the multiscale spectral generalized finite element method (MS-GFEM), wherein the coarse space is spanned by locally optimal basis functions obtained from local generalized eigenproblems on operator-harmonic spaces. Extending the theory of Ma (2025) to convection-diffusion problems in conservation form, we establish exponential convergence of the MS-GFEM approximation with respect to the dimension of the local approximation space. Rewriting MS-GFEM as a RAS-type iteration, we show for coercive problems that this exponential convergence property is inherited by the RAS-type iterative method (at least in the continuous setting). Employed as a preconditioner within the generalized minimal residual method (GMRES), the resulting method requires only a few iterations for high accuracy even with low-dimensional coarse spaces. Through extensive numerical experiments on problems with high-contrast diffusion and non-divergence-free, rotating velocity fields, we demonstrate robustness with respect to the grid Péclet number and the number of subdomains (tested up to $10^5$ subdomains), while coarse-space dimensions remain small as grid Péclet numbers increase. By adapting the coarse space and oversampling size, we are able to achieve arbitrarily fast convergence of preconditioned GMRES. As an extension, for which we do not have theory yet, we show effectiveness of the method even for indefinite problems and in the vanishing-diffusion limit.