Efficient low-order refined preconditioners for high-order matrix-free continuous and discontinuous Galerkin methods

Pazner, Will

doi:10.48550/arxiv.1908.07071

Cited by 1 publication

(1 citation statement)

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“…There exist other techniques as well with the aim to overcome the complexity of matrix-based methods for high polynomial degrees. Preconditioners and multigrid methods applied to a low-order re-discretization of the operator on a mesh with vertices located on the nodes of the high-order discretization is a well-known technique originating from [79,80] and has been analyzed for example in [81,31,53,14,61,82]. Such an approach is not considered here.…”

Section: Multigrid For High-order Discretizations: State-of-the-artmentioning

confidence: 99%

Hybrid multigrid methods for high-order discontinuous Galerkin discretizations

Fehn,

Munch,

Wall

et al. 2019

Preprint

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The present work develops hybrid multigrid methods for high-order discontinuous Galerkin discretizations of elliptic problems, which are, for example, a key ingredient of incompressible flow solvers in the field of computational fluid dynamics. Fast matrix-free operator evaluation on tensor product elements is used to devise a computationally efficient PDE solver. The multigrid hierarchy exploits all possibilities of geometric, polynomial, and algebraic coarsening, targeting engineering applications on complex geometries. Additionally, a transfer from discontinuous to continuous function spaces is performed within the multigrid hierarchy. This does not only further reduce the problem size of the coarse-grid problem, but also leads to a discretization most suitable for state-of-the-art algebraic multigrid methods applied as coarse-grid solver. The relevant design choices regarding the selection of optimal multigrid coarsening strategies among the various possibilities are discussed with the metric of computational costs as the driving force for algorithmic selections. We find that a transfer to a continuous function space at highest polynomial degree (or on the finest mesh), followed by polynomial and geometric coarsening, shows the best overall performance. The success of this particular multigrid strategy is due to a significant reduction in iteration counts as compared to a transfer from discontinuous to continuous function spaces at lowest polynomial degree (or on the coarsest mesh). The coarsening strategy with transfer to a continuous function space on the finest level leads to a multigrid algorithm that is robust with respect to the penalty parameter of the symmetric interior penalty method. Detailed numerical investigations are conducted for a series of examples ranging from academic test cases to more complex, practically relevant geometries. Performance comparisons to state-of-the-art methods from the literature demonstrate the versatility and computational efficiency of the proposed multigrid algorithms.

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