8 research outputs found
Short note on the mass matrix for Gauss-Lobatto grid points
The mass matrix for Gauss-Lobatto grid points is usually approximated by
Gauss-Lobatto quadrature because this leads to a diagonal matrix that is easy
to invert. The exact mass matrix and its inverse are full. We show that the
exact mass matrix \emph{and} its inverse differ from the approximate diagonal
ones by a simple rank-1 update (outer product). They can thus be applied to an
arbitrary vector in operations instead of
Efficient Explicit Time Stepping of High Order Discontinuous Galerkin Schemes for Waves
This work presents algorithms for the efficient implementation of
discontinuous Galerkin methods with explicit time stepping for acoustic wave
propagation on unstructured meshes of quadrilaterals or hexahedra. A crucial
step towards efficiency is to evaluate operators in a matrix-free way with
sum-factorization kernels. The method allows for general curved geometries and
variable coefficients. Temporal discretization is carried out by low-storage
explicit Runge-Kutta schemes and the arbitrary derivative (ADER) method. For
ADER, we propose a flexible basis change approach that combines cheap face
integrals with cell evaluation using collocated nodes and quadrature points.
Additionally, a degree reduction for the optimized cell evaluation is presented
to decrease the computational cost when evaluating higher order spatial
derivatives as required in ADER time stepping. We analyze and compare the
performance of state-of-the-art Runge-Kutta schemes and ADER time stepping with
the proposed optimizations. ADER involves fewer operations and additionally
reaches higher throughput by higher arithmetic intensities and hence decreases
the required computational time significantly. Comparison of Runge-Kutta and
ADER at their respective CFL stability limit renders ADER especially beneficial
for higher orders when the Butcher barrier implies an overproportional amount
of stages. Moreover, vector updates in explicit Runge--Kutta schemes are shown
to take a substantial amount of the computational time due to their memory
intensity
Unified discontinuous Galerkin scheme for a large class of elliptic equations
We present a discontinuous Galerkin internal-penalty scheme that is applicable to a large class of linear and nonlinear elliptic partial differential equations. The unified scheme can accommodate all second-order elliptic equations that can be formulated in first-order flux form, encompassing problems in linear elasticity, general relativity, and hydrodynamics, including problems formulated on a curved manifold. It allows for a wide range of linear and nonlinear boundary conditions, and accommodates curved and nonconforming meshes. Our generalized internal-penalty numerical flux and our Schur-complement strategy of eliminating auxiliary degrees of freedom make the scheme compact without requiring equation-specific modifications. We demonstrate the accuracy of the scheme for a suite of numerical test problems. The scheme is implemented in the open-source SpECTRE numerical relativity code
Simulating magnetized neutron stars with discontinuous Galerkin methods
Discontinuous Galerkin methods are popular because they can achieve high order where the solution is smooth, because they can capture shocks while needing only nearest-neighbor communication, and because they are relatively easy to formulate on complex meshes. We perform a detailed comparison of various limiting strategies presented in the literature applied to the equations of general relativistic magnetohydrodynamics. We compare the standard minmod/ limiter, the hierarchical limiter of Krivodonova, the simple WENO limiter, the HWENO limiter, and a discontinuous Galerkin-finite-difference hybrid method. The ultimate goal is to understand what limiting strategies are able to robustly simulate magnetized TOV stars without any fine-tuning of parameters. Among the limiters explored here, the only limiting strategy we can endorse is a discontinuous Galerkin-finite-difference hybrid method
SpECTRE: A Task-based Discontinuous Galerkin Code for Relativistic Astrophysics
We introduce a new relativistic astrophysics code, SpECTRE, that combines a
discontinuous Galerkin method with a task-based parallelism model. SpECTRE's
goal is to achieve more accurate solutions for challenging relativistic
astrophysics problems such as core-collapse supernovae and binary neutron star
mergers. The robustness of the discontinuous Galerkin method allows for the use
of high-resolution shock capturing methods in regions where (relativistic)
shocks are found, while exploiting high-order accuracy in smooth regions. A
task-based parallelism model allows efficient use of the largest supercomputers
for problems with a heterogeneous workload over disparate spatial and temporal
scales. We argue that the locality and algorithmic structure of discontinuous
Galerkin methods will exhibit good scalability within a task-based parallelism
framework. We demonstrate the code on a wide variety of challenging benchmark
problems in (non)-relativistic (magneto)-hydrodynamics. We demonstrate the
code's scalability including its strong scaling on the NCSA Blue Waters
supercomputer up to the machine's full capacity of 22,380 nodes using 671,400
threads.Comment: 41 pages, 13 figures, and 7 tables. Ancillary data contains
simulation input file
Discontinuous galerkin methods for general relativistic hydrodynamics
Diese Arbeit dokumentiert die Implementierung und Überprüfung einer neuen numerischen Methodik zur allgemeinrelativistischen Simulation von Neutronensternen. Hierbei wurden die GHG-Gleichungen mit Hilfe einer pseudospektralen Penalty-Methode diskretisiert, um die Metrik-Variablen zu evolvieren. Zur Behandlung der hydrodynamischen Variablen wurde sowohl eine diskontinuierliche Galerkin-Methode, als auch ein Finite-Volumen-Verfahren auf krummlinigen Gittern implementiert. Zusätzlich wurden dem Code Techniken zur Schockerkennung und -behandlung hinzugefügt. Diese Bestandteile wurden in umfangreichen Tests validiert. Hierbei wurden numerische Fehler in der Massenerhaltung, in der Einhaltung von physikalischen Zwangsbedingungen und im Vergleich mit analytischen Lösungen betrachtet. Die zugehörigen Konvergenzordnungen aller Methoden wurden untersucht. Dabei wurde eine Konvergenzordnung ~2 bei aktiver Schockbehandlung und mögliche Konvergenzordnungen >2 für die reine diskontinuierliche Galerkin-Methode festgestellt. Weiterhin wurde bestätigt, dass die numerische Einhaltung der metrischen Identitäten essentiell für die Massenerhaltung in Fluiddynamik-Simulationen ist. Eine aktive Schockbehandlung erlaubte die stabile und numerisch konvergente Simulation von statischen, rotierenden oder oszillierenden Neutronensternen. Dank der vorgestellten Modifikationen des bamps Codes ist dieser nun in der Lage, die frontale Kollision zweier Neutronensterne zu simulieren. Neben der Extraktion des zugehörigen Gravitationswellensignals wurde die Simulation genutzt, um das nahezu ideale Skalierungsverhalten der Rechenzeit von bamps aufzuzeigen. Abschließend wurde der neuartige Duale-Blätterung-Formalismus angewandt, um die Wellengleichung auf einem numerischen Gitter zu lösen, welches die lichtartige Zukunftsunendlichkeit beinhaltet. Wir zeigen erste erfolgreiche Tests dieses Verfahrens