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PNNL-13501 - Pacific Northwest National Laboratory

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Invariant Discretization Methods for n-Dimensional Nonlinear Computational<br />

Reactive Transport Models<br />

Study Control Number: PN99034/1362<br />

Joseph S. Oliveira<br />

The objective of this research has been to investigate, develop, and apply a new set of scale invariant, discrete algebraic<br />

and geometric symmetry preserving algorithms to the problem of effectively defining and remapping multiscale,<br />

multidimensional hybrid computational meshes. This effort provides a critical part of the applied mathematical<br />

infrastructure needed to solve complex computational problems. The primary goal is to develop new combinatorial<br />

methods for generating and preserving computationally, symmetry-invariant, coordinate-free, three-dimensional and<br />

n-dimensional spatial discretizations, on mixed (structured and/or unstructured), hybrid, moving, and stochastic grids.<br />

Project Description<br />

We are building the mathematical tools to create invariant<br />

grid discretizations of adaptive moving hybrid grids that<br />

are coordinate free. We are using algebraic<br />

combinatorial, combinatorial geometric, and<br />

combinatorial topological invariants to remap, reconnect,<br />

and renormalize multiscale, multidimensional, adaptive,<br />

moving hybrid grids. In each of these cases, scalars,<br />

vectors, and tensors are remapped by sets of coordinatefree<br />

transformations between logical and real physical nspace.<br />

This emerging computational technology implies<br />

that the boundaries (external and internal) of the<br />

computational domain must be canonically based on the<br />

invariance of the solution(s) to the partial differential<br />

equations that model the physical problem domain. These<br />

may include, but are not limited to: material interfaces,<br />

liquid-gas interfaces, contact discontinuities, and shock<br />

fronts. Furthermore, the grid covering the computational<br />

domain (subdomains) has to be constructed using<br />

information about the position of the free moving<br />

boundary and the information about the invariance of the<br />

solution to the system as a whole. Lastly, finite difference<br />

schemes have to be reformulated in such a way that they<br />

do not depend on the components of vectors and tensors,<br />

but rather, the discrete analogs of coordinate-free<br />

differential forms. The coefficients of the finite<br />

difference equations should depend only on the<br />

coordinate-free characteristics of the grid, lengths of the<br />

edges, nearest neighbor point distributions, areas of faces,<br />

volumes of mesh cells, and angles between edges, and not<br />

specific vector coordinate representations.<br />

132 FY 2000 <strong>Laboratory</strong> Directed Research and Development Annual Report<br />

Approach<br />

The violation of physical symmetry in the discrete<br />

geometric representation of the partial differential<br />

equations is a principal cause of the numerical instabilities<br />

that lead to large numerical errors. If this type of problem<br />

is to be eliminated, we must generate computationally<br />

invariant hybrid discretization methods (threedimensionally,<br />

mixed [structured and unstructured] grids,<br />

and stable three-dimensional numerical schemes built on<br />

these grids) for obtaining stable, high-accuracy numerical<br />

solutions to the partial differential equations that preserve<br />

the inherent symmetry of the physical domain space. The<br />

overall goal is to generalize these methods to ndimensions,<br />

multiscale, hydro-transport physics models.<br />

The extension to n-dimensions will allow us to more<br />

accurately model physical parameter space by<br />

representing each algebraic parameter by a corresponding<br />

geometric dimension.<br />

The geometric invariance that is preserved by the grid<br />

discretization of the physical continuum is spatial<br />

symmetry, which is realized as algebraic invariant<br />

symmetry. For example, the preservation of physically<br />

based fluid flow symmetries—associated with multiscale,<br />

multiphase advective and reactive diffusive fluid flows in<br />

subsurface transport, magnetohydrodynamic flow,<br />

combustion systems, reactive atmospheric transport<br />

chemistry, and intra-inter cellular communications—must<br />

remain scale invariant. The divergence from spherical<br />

and rotational symmetries due to discretization errors can<br />

lead to large errors in the discretized systems of ordinary<br />

differential equations with large convergence ratios. The

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