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Stephan Rave

Christoph Lohmann (TU Dortmund): Towards an efficient parallel-in-time Navier-Stokes solver based on discretely divergence-free finite elements in 3D

Wednesday, 19.11.2025 14:15 im Raum M5

Mathematik und Informatik

In this talk, we investigate efficient and robust solution strategies for the incompressible Navier-Stokes equations, which are specifically designed to predict transient flow behaviors on modern hardware architectures. The main idea of the framework is to block the individual linear systems of equations at each time step into a single all-at-once saddle-point problem. To address the challenges associated with the zero block, we employ a global Schur complement technique, implicitly eliminating all velocity unknowns and leading to a problem for all pressure unknowns only. This so called pressure Schur complement (PSC) equation is then solved using a space-time multigrid algorithm with tailor-made preconditioners for smoothing purposes. However, as the Reynolds number increases, the accuracy of the PSC preconditioner deteriorates, leading to convergence issues in the overall multigrid solver. To overcome this disadvantage, we enhance the solution strategy by using the Augmented Lagrangian methodology in a global-in-time fashion. This approach guarantees a rapid convergence by drastically increasing the accuracy of the adapted PSC preconditioner by means of a strongly consistent modification of the velocity system matrix. For sufficiently large stabilization parameters, even the global-in-time Schur complement iteration by itself converges fast and makes the coarse grid acceleration of the multigrid algorithm redundant. The Augmented Lagrangian technique comes at the cost that the involved auxiliary problem for the velocity field becomes ill conditioned for large stabilization parameters so that standard iterative solvers prove to be practically unusable. This calls for more sophisticated iterative solvers, like a specialized multigrid solver utilizing modified intergrid transfer operators and block diagonal preconditioners. In the limit of large stabilization parameters, the Augmented Lagrangian approach implies that all intermediate velocity solutions satisfy the discrete incompressibility condition. A straightforward consequence is therefore the use of discretely divergence-free finite elements, eliminating the pressure variable in an a priori manner. Therefore, no saddle-point structure exists and a wide range of efficient and accurate preconditioners becomes applicable. After illustrating the potential of this approach for two-dimensional global-in-time problems, we give some insight into the rarely investigated extension to three dimensions.



Angelegt am 17.09.2025 von Stephan Rave
Geändert am 10.11.2025 von Stephan Rave
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Oberseminar Numerik