3,532 research outputs found
A multigroup diffusion solver using pseudo transient continuation for a radiation-hydrodynamic code with patch-based AMR
We present a scheme to solve the nonlinear multigroup radiation diffusion
(MGD) equations. The method is incorporated into a massively parallel,
multidimensional, Eulerian radiation-hydrodynamic code with adaptive mesh
refinement (AMR). The patch-based AMR algorithm refines in both space and time
creating a hierarchy of levels, coarsest to finest. The physics modules are
time-advanced using operator splitting. On each level, separate level-solve
packages advance the modules. Our multigroup level-solve adapts an implicit
procedure which leads to a two-step iterative scheme that alternates between
elliptic solves for each group with intra-cell group coupling. For robustness,
we introduce pseudo transient continuation (PTC). We analyze the magnitude of
the PTC parameter to ensure positivity of the resulting linear system, diagonal
dominance and convergence of the two-step scheme. For AMR, a level defines a
subdomain for refinement. For diffusive processes such as MGD, the refined
level uses Dirichet boundary data at the coarse-fine interface and the data is
derived from the coarse level solution. After advancing on the fine level, an
additional procedure, the sync-solve (SS), is required in order to enforce
conservation. The MGD SS reduces to an elliptic solve on a combined grid for a
system of G equations, where G is the number of groups. We adapt the partial
temperature scheme for the SS; hence, we reuse the infrastructure developed for
scalar equations. Results are presented. (Abridged)Comment: 46 pages, 14 figures, accepted to JC
Adaptive multiscale methods for 3D streamer discharges in air
We discuss spatially and temporally adaptive implicit-explicit (IMEX) methods
for parallel simulations of three-dimensional fluid streamer discharges in
atmospheric air. We examine strategies for advancing the fluid equations and
elliptic transport equations (e.g. Poisson) with different time steps,
synchronizing them on a global physical time scale which is taken to be
proportional to the dielectric relaxation time. The use of a longer time step
for the electric field leads to numerical errors that can be diagnosed, and we
quantify the conditions where this simplification is valid. Likewise, using a
three-term Helmholtz model for radiative transport, the same error diagnostics
show that the radiative transport equations do not need to be resolved on time
scales finer than the dielectric relaxation time. Elliptic equations are
bottlenecks for most streamer simulation codes, and the results presented here
potentially provide computational savings. Finally, a computational example of
3D branching streamers in a needle-plane geometry that uses up to 700 million
grid cells is presented.Comment: 17 pages, 5 figure
Algorithms and data structures for adaptive multigrid elliptic solvers
Adaptive refinement and the complicated data structures required to support it are discussed. These data structures must be carefully tuned, especially in three dimensions where the time and storage requirements of algorithms are crucial. Another major issue is grid generation. The options available seem to be curvilinear fitted grids, constructed on iterative graphics systems, and unfitted Cartesian grids, which can be constructed automatically. On several grounds, including storage requirements, the second option seems preferrable for the well behaved scalar elliptic problems considered here. A variety of techniques for treatment of boundary conditions on such grids are reviewed. A new approach, which may overcome some of the difficulties encountered with previous approaches, is also presented
Multilevel Sparse Grid Methods for Elliptic Partial Differential Equations with Random Coefficients
Stochastic sampling methods are arguably the most direct and least intrusive
means of incorporating parametric uncertainty into numerical simulations of
partial differential equations with random inputs. However, to achieve an
overall error that is within a desired tolerance, a large number of sample
simulations may be required (to control the sampling error), each of which may
need to be run at high levels of spatial fidelity (to control the spatial
error). Multilevel sampling methods aim to achieve the same accuracy as
traditional sampling methods, but at a reduced computational cost, through the
use of a hierarchy of spatial discretization models. Multilevel algorithms
coordinate the number of samples needed at each discretization level by
minimizing the computational cost, subject to a given error tolerance. They can
be applied to a variety of sampling schemes, exploit nesting when available,
can be implemented in parallel and can be used to inform adaptive spatial
refinement strategies. We extend the multilevel sampling algorithm to sparse
grid stochastic collocation methods, discuss its numerical implementation and
demonstrate its efficiency both theoretically and by means of numerical
examples
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