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This study investigates multiphysics block preconditioners that are critical in devising scalable Newton--Krylov iterative solvers for longer time-scale fully implicit fluid plasma models. The specific model of interest is the visco-resistive, low Mach number, compressible magnetohydro-dynamics (MHD) model. This model describes the dynamics of conducting fluids in the presence of electromagnetic fields and can be used to study aspects of astrophysical phenomena, important science and technology applications, and basic plasma physics. The specific application of interest that motivates this study is the macroscopic simulation of longer time-scale stability and disruptions of magnetic confinement fusion devices, specifically the ITER Tokamak. The computational solution of the governing balance equations for mass, momentum, heat transfer, and magnetic induction for resistive MHD systems can be extremely challenging. These difficulties arise from both the strong nonlinear, nonsymmetric coupling of fluid and electromagnetic phenomena as well as the significant range of time and length scales that the interactions of these physical mechanisms produce. To handle the range of time and spatial scales of interest, a fully implicit unstructured variational multiscale finite element formulation is employed. For the scalable solution of the Newton linearized systems, fully coupled block preconditioners are designed to leverage algebraic multigrid subsolves. Results are presented for the strong and weak scaling of the method as well as the robustness of these techniques for a large range of Lundquist numbers. [ABSTRACT FROM AUTHOR]

The one-fluid visco-resistive MHD model provides a description of the dynamics of a charged fluid under the influence of an electromagnetic field. This model is strongly coupled, highly nonlinear, and characterized by physical mechanisms that span a wide range of interacting time scales. Solutions of this system can include very fast component time scales to slowly varying dynamical time scales that are long relative to the normal modes of the model equations. Fully implicit time stepping is attractive for simulating this type of wide-ranging physical phenomena. However, it is essential that one has effective preconditioning strategies so that the overall fully implicit methodology is both efficient and scalable. In this paper, we propose and explore the performance of several candidate block preconditioners for this system. One of these preconditioners is based on an operator-split approximation. This method reduces the 3x3 system (momentum, continuity, and magnetics) into two 2x2 operators: a Navier-Stokes operator (momentum and continuity) and a magnetics-velocity operator (momentum and magnetics) which takes into account the critical Lorentz force coupling. Using previously developed preconditioners for Navier-Stokes, and an initial Schur-complement approximation for the magnetics-velocity system, we show that the split preconditioner is scalable and competitive with other preconditioners, including a fully coupled algebraic multigrid method. [ABSTRACT FROM AUTHOR]

This paper presents a numerical method to simulate an incompressible fluid through an immersed porous interface. The interface is modeled by a surface measure term in the Navier-Stokes equations and it is characterized by a resistance parameter. This approach can be used, for example, to model valves or to simulate blood flood through an immersed stent. Starting from a monolithic formulation proposed recently, a fractional step algorithm is derived. The difficult point is that this formulation is singular when the resistance vanishes, which can be a serious issue in some applications. We show that an appropriate Nitsche treatment of the interface condition fixes this problem and ensures uniform energy stability in time for any nonnegative value of the resistance. The theoretical stability and convergence results are illustrated with numerical experiments. [ABSTRACT FROM AUTHOR]