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Parallel algorithms for planar and spherical Delaunay construction with an application to centroidal Voronoi tessellations
D. W. Jacobsen,M. Gunzburger,T. Ringler,J. Burkardt
Geoscientific Model Development Discussions , 2013, DOI: 10.5194/gmdd-6-1427-2013
Abstract: A new algorithm, featuring overlapping domain decompositions, for the parallel construction of Delaunay and Voronoi tessellations is developed. Overlapping allows for the seamless stitching of the partial Delaunay tessellations constructed by individual processors. The algorithm is then modified, by the addition of stereographic projections, to handle the parallel construction of spherical Delaunay and Voronoi tessellations. The algorithms are then embedded into algorithms for the parallel construction of planar and spherical centroidal Voronoi tessellations that require multiple constructions of Delaunay tessellations. Computational tests are used to demonstrate the efficiency and scalability of the algorithms for spherical Delaunay and centroidal Voronoi tessellations. Compared to serial versions of the algorithm and to the STRIPACK-based approaches, the new parallel algorithm results in significant speedups for the construction of spherical centroidal Voronoi tessellations.
Manufactured solutions and the numerical verification of isothermal, nonlinear, three-dimensional Stokes ice-sheet models
W. Leng,L. Ju,M. Gunzburger,S. Price
The Cryosphere Discussions , 2012, DOI: 10.5194/tcd-6-2689-2012
Abstract: The technique of manufactured solutions is used for verification of computational models in many fields. In this paper we construct manufactured solutions for models of three-dimensional, isothermal, nonlinear Stokes flow in glaciers and ice sheets. The solution construction procedure starts with kinematic boundary conditions and is mainly based on the solution of a first-order partial differential equation for the ice velocity that satisfies the incompressibility condition. The manufactured solutions depend on the geometry of the ice sheet and other model parameters. Initial conditions are taken from the periodic geometry of a standard problem of the ISMIP-HOM benchmark tests and altered through the manufactured solution procedure to generate an analytic solution for the time-dependent flow problem. We then use this manufactured solution to verify a parallel, high-order accurate, finite element Stokes ice-sheet model. Results from the computational model show excellent agreement with the manufactured analytic solutions.
Manufactured solutions and the verification of three-dimensional Stokes ice-sheet models
W. Leng,L. Ju,M. Gunzburger,S. Price
The Cryosphere , 2013, DOI: 10.5194/tc-7-19-2013
Abstract: The manufactured solution technique is used for the verification of computational models in many fields. In this paper, we construct manufactured solutions for the three-dimensional, isothermal, nonlinear Stokes model for flows in glaciers and ice sheets. The solution construction procedure starts with kinematic boundary conditions and is mainly based on the solution of a first-order partial differential equation for the ice velocity that satisfies the incompressibility condition. The manufactured solutions depend on the geometry of the ice sheet, basal sliding parameters, and ice softness. Initial conditions are taken from the periodic geometry of a standard problem of the ISMIP-HOM benchmark tests. The upper surface is altered through the manufactured solution procedure to generate an analytic solution for the time-dependent flow problem. We then use this manufactured solution to verify a parallel, high-order accurate, finite element Stokes ice-sheet model. Simulation results from the computational model show good convergence to the manufactured analytic solution.
Goal-oriented A Posteriori Error Estimation for Finite Volume Methods
Qingshan Chen,Max Gunzburger
Mathematics , 2010,
Abstract: A general framework for goal-oriented a posteriori error estimation for finite volume methods is presented. The framework does not rely on recasting finite volume methods as special cases of finite element methods, but instead directly determines error estimators from the discretized finite volume equations. Thus, the framework can be ap- plied to arbitrary finite volume methods. It also provides the proper functional settings to address well-posedness issues for the primal and adjoint problems. Numerical results are presented to illustrate the validity and effectiveness of the a posteriori error estimates and their applicability to adaptive mesh refinement.
Peridynamics and Material Interfaces
Bacim Alali,Max Gunzburger
Mathematics , 2014,
Abstract: The convergence of a peridynamic model for solid mechanics inside heterogeneous media in the limit of vanishing nonlocality is analyzed. It is shown that the operator of linear peridynamics for an isotropic heterogeneous medium converges to the corresponding operator of linear elasticity when the material properties are sufficiently regular. On the other hand, when the material properties are discontinuous, i.e., when material interfaces are present, it is shown that the operator of linear peridynamics diverges, in the limit of vanishing nonlocality, at material interfaces. Nonlocal interface conditions, whose local limit implies the classical interface conditions of elasticity, are then developed and discussed. A peridynamics material interface model is introduced which generalizes the classical interface model of elasticity. The model consists of a new peridynamics operator along with nonlocal interface conditions. The new peridynamics interface model converges to the classical interface model of linear elasticity.
The fractional Laplacian operator on bounded domains as a special case of the nonlocal diffusion operator
Marta D'Elia,Max Gunzburger
Mathematics , 2013,
Abstract: We analyze a nonlocal diffusion operator having as special cases the fractional Laplacian and fractional differential operators that arise in several applications. In our analysis, a nonlocal vector calculus is exploited to define a weak formulation of the nonlocal problem. We demonstrate that, when sufficient conditions on certain kernel functions hold, the solution of the nonlocal equation converges to the solution of the fractional Laplacian equation on bounded domains as the nonlocal interactions become infinite. We also introduce a continuous Galerkin finite element discretization of the nonlocal weak formulation and we derive a priori error estimates. Through several numerical examples we illustrate the theoretical results and we show that by solving the nonlocal problem it is possible to obtain accurate approximations of the solutions of fractional differential equations circumventing the problem of treating infinite-volume constraints.
Optimal Point Sets for Total Degree Polynomial Interpolation in Moderate Dimensions
Max Gunzburger,Aretha L Teckentrup
Mathematics , 2014,
Abstract: This paper is concerned with Lagrange interpolation by total degree polynomials in moderate dimensions. In particular, we are interested in characterising the optimal choice of points for the interpolation problem, where we define the optimal interpolation points as those which minimise the Lebesgue constant. We give a novel algorithm for numerically computing the location of the optimal points, which is independent of the shape of the domain and does not require computations with Vandermonde matrices. We perform a numerical study of the growth of the minimal Lebesgue constant with respect to the degree of the polynomials and the dimension, and report the lowest values known as yet of the Lebesgue constant in the unit cube and the unit ball in up to 10 dimensions.
Identification of the diffusion parameter in nonlocal steady diffusion problems
Marta D'Elia,Max Gunzburger
Mathematics , 2013,
Abstract: The problem of identifying the diffusion parameter appearing in a nonlocal steady diffusion equation is considered. The identification problem is formulated as an optimal control problem having a matching functional as the objective of the control and the parameter function as the control variable. The analysis makes use of a nonlocal vector calculus that allows one to define a variational formulation of the nonlocal problem. In a manner analogous to the local partial differential equations counterpart, we demonstrate, for certain kernel functions, the existence of at least one optimal solution in the space of admissible parameters. We introduce a Galerkin finite element discretization of the optimal control problem and derive a priori error estimates for the approximate state and control variables. Using one-dimensional numerical experiments, we illustrate the theoretical results and show that by using nonlocal models it is possible to estimate non-smooth and discontinuous diffusion parameters.
A scale-invariant formulation of the anticipated potential vorticity method
Qingshan Chen,Max Gunzburger,Todd Ringler
Physics , 2010, DOI: 10.1175/MWR-D-10-05004.1
Abstract: The long-term success of climate models that operate on multi-resolution grids will depend on access to subgrid parameterizations that act appropriately across a wide range of spatial and temporal scales. As the first step in a series of efforts to obtain such scale-aware subgrid parametrizations, in this article we focus on the anticipated potential vorticity method (APVM) on quasi-uniform grids with varying resolutions. By a scale analysis technique and the phenomenological theories for two-dimensional turbulent flows, we derive a new formulation of the APVM, which depends on a single parameter that is, in principle, invariant with respect to the time step size, the grid resolution and the flow itself. Results of numerical experiments with this new formulation demonstrate that the optimal parameter of the APVM is invariant with respect to the time step size, insensitive to the flows, and is only weakly dependent on the grid resolution.
Partial and spectral-viscosity models for geophysical flows
Qingshan Chen,Max Gunzburger,Xiaoming Wang
Physics , 2010, DOI: 10.1007/s11401-010-0607-2
Abstract: Two models based on the hydrostatic primitive equa- tions are proposed. The first model is the primitive equations with partial viscosity only, and is oriented towards large-scale wave structures in the ocean and atmosphere. The second model is the viscous primitive equations with spectral eddy viscosity, and is oriented towards turbulent geophysical flows. For both models, the existence and uniqueness of global strong solutions is estab- lished. For the second model, the convergence of the solutions to the solutions of the classical primitive equations as eddy viscosity parameters tend to zero is also established.
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