Equidistribution Principle Research Articles

An Adaptive Moving Mesh Method for Forced Curve Shortening Flow.

We propose a novel adaptive moving mesh method for the numerical solution of a forced curve shortening geometric evolution equation. Control of the mesh quality is obtained using a tangential mesh velocity derived from a mesh equidistribution principle, where a positive adaptivity measure or monitor function is approximately equidistributed along the evolving curve. Central finite differences are used to discretize in space the governing evolution equation for the position vector, and a second-order implicit scheme is used for the temporal integration. Simulations are presented indicating the generation of meshes which resolve areas of high curvature and are of second-order accuracy. Furthermore, the new method delivers improved solution accuracy compared to the use of uniform arc-length meshes.

Adaptive moving knots meshless method for simulating time dependent partial differential equations

This paper presents an improvement of Huang’s moving knots method (Huang et al.) for solving the PDE on the contours of Equidistribution Principle (EP-contours). Firstly, a new moving knots strategy is generated by discovering a significant factor ignored (dropped) by Huang (Huang et al.). The proposed strategy ensures that arbitrary initial knots could asymptotically converge to the EP-contours. Then the moving knots strategy and PDE are simulated by using multi-quadric (MQ) quasi-interpolation step by Step (3.3, 3.4). Error estimates of the algorithm are given. At last, numerical experiments are provided to illustrate the validity of the algorithm. Both theoretical analysis and numerical results show that the proposed algorithm benefits the methods in Huang et al. and Wu.

A meshing strategy for a quadratic iso-parametric FEM in cavitation computation in nonlinear elasticity

The approximation properties of a quadratic iso-parametric finite element method for a typical cavitation problem in nonlinear elasticity in two dimensions are analyzed. More precisely, (1) the finite element interpolation errors are established in terms of the mesh parameters; (2) a mesh distribution strategy based on an error equi-distribution principle is given; (3) the convergence of finite element cavity solutions is proved. Numerical experiments show that, in fact, the optimal convergence rate can be achieved by the numerical cavity solutions.

A second order numerical method for a class of parameterized singular perturbation problems on adaptive grid

AbstractA nonlinear singularly perturbed boundary value problem depending on a parameter is considered. First, we solve the problem using the backward Euler finite difference scheme on an adaptive grid. The adaptive grid is a special nonuniform mesh generated through equidistribution principle by a positive monitor function depending on the solution. The behavior of the solution, the stability and the error estimates are discussed. Then, the Richardson extrapolation technique is applied to improve the accuracy of the computed solution associated to the backward Euler scheme. The proofs of the uniform convergence for the backward Euler scheme and the Richardson extrapolation are carried out. Numerical experiments validate the theoretical estimates and indicates that the estimates are sharp.

Parameter-Free Time Adaptivity Based on Energy Evolution for the Cahn-Hilliard Equation

AbstractIt is known that large time-stepping method are useful for simulating phase field models. In this work, an adaptive time-stepping strategy is proposed based on numerical energy stability and equi-distribution principle. The main idea is to use the energy variation as an indicator to update the time step, so that the resulting algorithm is free of user-defined parameters, which is different from several existing approaches. Some numerical experiments are presented to illustrate the effectiveness of the algorithms.

Discrete analysis of domain decomposition approaches for mesh generation via the equidistribution principle

Moving mesh methods based on the equidistribution principle are powerful techniques for the space-time adaptive solution of evolution problems. Solving the resulting coupled system of equations, namely the original PDE and the mesh PDE, however, is challenging in parallel. Recently several Schwarz domain decomposition algorithms were proposed for this task and analyzed at the continuous level. However, after discretization, the resulting problems may not even be well posed, so the discrete algorithms requires a different analysis, which is the subject of this paper. We prove that when the number of grid points is large enough, the classical parallel and alternating Schwarz methods converge to the unique monodomain solution. Thus, such methods can be used in place of Newton’s method, which can suffer from convergence difficulties for challenging problems. The analysis for the nonlinear domain decomposition algorithms is based on M–function theory and is valid for an arbitrary number of subdomains. An asymptotic convergence rate is provided and numerical experiments illustrate the results.

An adaptive coupling method for exterior anisotropic elliptic problems

In this paper, we propose an adaptive coupling method for solving anisotropic elliptic PDEs in unbounded domains. Firstly, the existence and the uniqueness of the solution for the coupling method are proven, and the a priori error estimates in H1-norm and L2-norm that depend on the size of the FEM mesh, the location of the elliptic artificial boundary and the truncation of the infinite series in the artificial boundary integral condition are derived. Secondly, the a posteriori error estimates and the a posteriori error indicator of the coupling method are obtained. Finally, the adaptive coupling method refines the mesh distribution by the arc-length equidistribution principle and the a posteriori error indicator successively. Numerical examples confirm the advantage in accuracy and efficiency for the proposed method.

Adaptive reconnection-based arbitrary Lagrangian Eulerian method

eW present a new adaptive Arbitrary Lagrangian Eulerian (ALE) method. This method is based on the reconnection-based ALE (ReALE) methodology of Refs. [35,34,6]. The main elements in a standard ReALE method are: an explicit Lagrangian phase on an arbitrary polygonal (in 2D) mesh in which the solution and positions of grid nodes are updated; a rezoning phase in which a new grid is defined by changing the connectivity (using Voronoi tessellation) but not the number of cells; and a remapping phase in which the Lagrangian solution is transferred onto the new grid. In the standard ReALE method, the rezoned mesh is smoothed by using one or several steps toward centroidal Voronoi tessellation, but it is not adapted to the solution in any way.In the current paper we present a new adaptive ReALE method, A-ReALE, that is based on the following design principles. First, a monitor function (or error indicator) based on the Hessian of some flow parameter(s) is utilized. Second, an equi-distribution principle for the monitor function is used as a criterion for adapting the mesh. Third, a centroidal Voronoi tessellation is used to adapt the mesh. Fourth, we scale the monitor function to avoid very small and large cells and then smooth it to permit the use of theoretical results related to weighted centroidal Voronoi tessellation.In the A-ReALE method, both number of cells and their locations are allowed to change at the rezone stage on each time step. The number of generators at each time step is chosen to guarantee the required spatial resolution in regions where monitor function reaches its maximum value.We present all details required for implementation of new adaptive A-ReALE method and demonstrate its performance in comparison with standard ReALE method on series of numerical examples.

A moving mesh finite difference method for equilibrium radiation diffusion equations

An efficient moving mesh finite difference method is developed for the numerical solution of equilibrium radiation diffusion equations in two dimensions. The method is based on the moving mesh partial differential equation approach and moves the mesh continuously in time using a system of meshing partial differential equations. The mesh adaptation is controlled through a Hessian-based monitor function and the so-called equidistribution and alignment principles. Several challenging issues in the numerical solution are addressed. Particularly, the radiation diffusion coefficient depends on the energy density highly nonlinearly. This nonlinearity is treated using a predictor–corrector and lagged diffusion strategy. Moreover, the nonnegativity of the energy density is maintained using a cutoff method which has been known in literature to retain the accuracy and convergence order of finite difference approximation for parabolic equations. Numerical examples with multi-material, multiple spot concentration situations are presented. Numerical results show that the method works well for radiation diffusion equations and can produce numerical solutions of good accuracy. It is also shown that a two-level mesh movement strategy can significantly improve the efficiency of the computation.

Explorations and Expectations of Equidistribution Adaptations for Nonlinear Quenching Problems

Finite difference computations that involve spatial adaptation commonly employ an equidistribution principle. In these cases, a new mesh is constructed such that a given monitor function is equidistributed in some sense. Typical choices of the monitor function involve the solution or one of its many derivatives. This straightforward concept has proven to be extremely effective and practical. However, selections of core monitoring functions are often challenging and crucial to the computational success. This paper concerns six different designs of the monitoring function that targets a highly nonlinear partial differential equation that exhibits both quenching-type and degeneracy singularities. While the first four monitoring strategies are within the so-called primitive regime, the rest belong to a later category of the modified type, which requires the priori knowledge of certain important quenching solution characteristics. Simulated examples are given to illustrate our study and conclusions.

An Adaptive Grid Method for a Non-equilibrium PDE Model from Porous Media

An adaptive grid method is applied to a PDE model from geo-hydrology. Due to the higher mixed-order derivative, non-monotone waves can appear which could represent similar structures as observed in laboratory experiments [5, 16, 18]. The effectiveness of the adaptive grid, which is based on a smoothed equidistribution principle, is shown compared to uniformgrid simulations. On a uniformgrid (numerical) oscillating non-monotone wavesmay appearwhich are not present in the adaptive grid.

Error Analysis of a Dual-parametric Bi-quadratic FEM in Cavitation Computation in Elasticity

The orientation-preservation conditions and approximation errors of a dual-parametric bi-quadratic finite element method for the computation of both radially symmetric and general nonsymmetric cavity solutions in nonlinear elasticity are analyzed. The analytical results allow us to establish, based on an error equidistribution principle, an optimal meshing strategy for the method in cavitation computation. Numerical results are in good agreement with the analytical results.

A new hybrid adaptive mesh algorithm based on Voronoi tessellations and equi-distribution principle: Algorithms and numerical experiments

A new hybrid h-adaptive algorithm is design to optimize unstructured polygonal meshes in 2D based on Voronoi tessellations and its topological dual, Delaunay triangulations. The algorithm consists of several unique features: first, dynamic change of mesh topology (reconnection of nodes) is realized through refinement and coarsening iterations with specific feature (i.e. Voronoi diagram); second, two moving mesh strategies are included, one is the weighted global smoothing for mesh regulation which is based on centroidal Voronoi tessellations (CVTs), the other is a local smoothing for the purpose of speed-up of the adaptive iterations; third, a new marking strategy with double threshold values guarantee the convergence of adaptive iterations. The advantage of the hybrid algorithm is that the goal mesh resolution can converge to an arbitrarily small number; it asymptotically converges to a straight line in 2D (i.e., the volume equi-distribution error approaches zero). The h-adaptation is referred to as asynchronous process, if the processes of coarsening and refinement are connected through several global mesh regulation steps. Otherwise the hybrid adaptive process is synchronous. This will provide options to meet special requirement such as smooth transition of physical fields. The numerical results show that the numerical errors are indeed well distributed in an averaged sense in the domain, where the mesh sizes are optimal, and nodes of the mesh smoothly follow the underlying geometry or physics. The new hybrid algorithm demonstrates its potential for effectiveness, efficiency and robustness to handle generation and optimization of polygonal meshes for extreme scenarios with a wide spectrum of length scales.

Optimal error estimate using mesh equidistribution technique for singularly perturbed system of reaction–diffusion boundary-value problems

In this article, we study the problem of determining an appropriate grading of meshes for a system of coupled singularly perturbed reaction–diffusion problems having diffusion parameters with different magnitudes. The central difference scheme is used to discretize the problem on adaptively generated mesh where the mesh equation is derived using an equidistribution principle. An a priori monitor function is obtained from the error estimate. A suitable a posteriori analogue of this monitor function is also derived for the mesh construction which will lead to an optimal second-order parameter uniform convergence. We present the results of numerical experiments for linear and semilinear reaction–diffusion systems to support the effectiveness of our preferred monitor function obtained from theoretical analysis.

A Natural BEM with non-uniform grids on the elliptic boundary and its coupling method

In this paper, for solving Poisson equation in 2-D unbounded domains, a natural BEM with non-uniform grids on the elliptic boundary is investigated, the coupling of NBEM–FEM with non-uniform grids on the elliptic artificial boundary is discussed, and the corresponding convergence theorems of the NBEM and the coupling method are proved. The moving mesh method based on the arc-length equidistribution principle is introduced into the NBEM and the coupling method respectively to solve exterior problems. Numerical examples confirm the convergence and demonstrate the advantage in accuracy and efficiency for the proposed methods.

Explorations and Expectations of Equidistribution Adaptations for Nonlinear Quenching Problems

AbstractFinite difference computations that involve spatial adaptation commonly employ an equidistribution principle. In these cases, a new mesh is constructed such that a given monitor function is equidistributed in some sense. Typical choices of the monitor function involve the solution or one of its many derivatives. This straightforward concept has proven to be extremely effective and practical. However, selections of core monitoring functions are often challenging and crucial to the computational success. This paper concerns six different designs of the monitoring function that targets a highly nonlinear partial differential equation that exhibits both quenching-type and degeneracy singularities. While the first four monitoring strategies are within the so-calledprimitiveregime, the rest belong to a later category of themodifiedtype, which requires the priori knowledge of certain important quenching solution characteristics. Simulated examples are given to illustrate our study and conclusions.

A Moving Mesh Method for Singularly Perturbed Problems

A singularly perturbed time dependent convection diffusion problem is solved on a rectangular domain, using the moving mesh method which uses the equidistribution principle. The problem has a boundary at the steady state. It is shown that the numerical approximations generated by the moving mesh method converge uniformly with respect to the singular perturbation parameter. Theoretical results are obtained which are verified using numerical results.

Adapting nodes in three dimensional irregular regions for meshless-type methods

In this paper, a new adaptive nodes technique based on equi-distribution principles and dimension reduction is presented for irregular regions in three dimensional cases. The mesh generation is performed by first producing some adaptive nodes in a cube based on equi-distribution along the coordinate axes and then transforming the generated nodes to the physical domain followed by a refinement process. The mesh points produced are appropriate for meshless-type methods which need only some scattered points rather than a mesh with some smoothness properties. The effectiveness of the generated mesh points is examined by a collocation meshless method using a well known radial basis function, namely ϕ(r) = r5 which is sufficiently smooth for our purpose. Some experimental results will be presented to illustrate the effectiveness of the proposed method.

Domain Decomposition Approaches for Mesh Generation via the Equidistribution Principle

Moving mesh methods based on the equidistribution principle are powerful techniques for the space-time adaptive solution of evolution problems. Solving the resulting coupled system of equations, namely the original PDE and the mesh PDE, however, is challenging in parallel. We propose in this paper several Schwarz domain decomposition algorithms for this task. We then study in detail the convergence properties of these algorithms applied to the nonlinear mesh PDE in one spatial dimension. We prove convergence for classical transmission conditions, and optimal and optimized variants for the generation of steady equidistributing grids. A classical, parallel, Schwarz algorithm is presented and analyzed for the generation of time dependent (moving) equidistributing grids. We conclude our study with numerical experiments.

A two-dimensional adaptive nodes technique in irregular regions applied to meshless-type methods

We propose a two-dimensional (2D) adaptive nodes technique for irregular regions. The method is based on equi-distribution principle and dimension reduction. The mesh generation is carried out by first producing some adaptive nodes in a rectangle based on equi-distribution along the coordinate axes and then transforming the generated nodes to the physical domain. Since the produced mesh is applied to the meshless-type methods, the connectivity of the points is not used and only the grid points are important, though the grid lines are utilized in the adapting process. The performance of the adaptive points is examined by considering a collocation meshless method which is based on interpolation in terms of a set of radial basis functions. A generalized thin plate spline with sufficient smoothness is used as a basis function and the arc-length is employed as a monitor in the equi-distribution process. Some experimental results will be presented to illustrate the effectiveness of the proposed method.