arXiv subjects

Weizhang Huang

Publications and source records attributed to Weizhang Huang.

At least 19 recordsLinked to original sources

arXiv2025

Parameter-free inexact block Schur complement preconditioning for linear poroelasticity under a hybrid Bernardi-Raugel and weak Galerkin finite element discretization

This work investigates inexact block Schur complement preconditioning for linear poroelasticity problems discretized using a hybrid approach: Bernardi-Raugel elements for solid displacement and lowest-order weak Galerkin elements for fluid pressure. When pure Dirichlet boundary conditions are applied to the displacement, the leading block of the resulting algebraic system becomes almost singular in the nearly incompressible (locking) regime, hindering efficient iterative solution. To overcome this, the system is reformulated as a three-field problem with an inherent regularization that maintains the original solution while ensuring nonsingularity. Analysis shows that both the minimal residual (MINRES) and generalized minimal residual (GMRES) methods, when preconditioned with inexact block diagonal and triangular Schur complement preconditioners, achieve convergence independent of mesh size and the locking parameter for the regularized system. Similar theoretical results are established for the situation with displacement subject to mixed boundary conditions, even without regularization. Numerical experiments in 2D and 3D confirm the benefits of regularization under pure Dirichlet conditions and the robustness of the preconditioners with respect to mesh size and the locking parameter in both boundary condition scenarios. Finally, a spinal cord simulation with discontinuous material parameters further illustrates the effectiveness and robustness of the proposed iterative solvers.

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arXiv2025

An inherent regularization approach to parameter-free preconditioning for nearly incompressible linear poroelasticity and elasticity

An inherent regularization strategy and block Schur complement preconditioning are studied for linear poroelasticity problems discretized using the lowest-order weak Galerkin FEM in space and the implicit Euler scheme in time. At each time step, the resulting saddle point system becomes nearly singular in the locking regime, where the solid is nearly incompressible. This near-singularity stems from the leading block, which corresponds to a linear elasticity system. To enable efficient iterative solution, this nearly singular system is first reformulated as a saddle point problem and then regularized by adding a term to the (2,2) block. This regularization preserves the solution while ensuring the non-singularity of the new system. As a result, block Schur complement preconditioning becomes effective. It is shown that the preconditioned MINRES and GMRES converge essentially independent of the mesh size and the locking parameter. Both two- and three-field formulations are considered for the iterative solution of the linear poroelasticity. The efficient solution of the two-field formulation builds upon the effective iterative solution of linear elasticity. For this case, MINRES and GMRES achieve parameter-free convergence when used with block Schur complement preconditioning, where the inverse of the leading block leverages efficient solvers for linear elasticity. The poroelasticity problem can also be reformulated as a three-field system by introducing a numerical pressure variable into the linear elasticity part. The inherent regularization strategy extends naturally to this formulation, and preconditioned MINRES and GMRES also show parameter-free convergence for the regularized system. Numerical experiments in both two and three dimensions confirm the effectiveness of the regularization strategy and the robustness of the block preconditioners.

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arXiv2025

Efficient parameter-robust preconditioners for linear poroelasticity and elasticity in the primal formulation

Poroelasticity problems play an important role in various engineering, geophysical, and biological applications. Their full discretization results in a large-scale saddle-point system at each time step that is becoming singular for locking cases and needs effective preconditioners for its fast iterative solution. Instead of constructing spectrally equivalent ones, we develop nonsingular preconditioners so that the eigenvalues of the preconditioned system consist of a cluster around $1$ and an outlier in the order of $1/λ$, where $λ$ is a Lamé constant that is large for locking cases. It is known that the convergence factor of GMRES is bounded by the radius of the cluster for this type of systems. Both two- and three-field block triangular Schur complement preconditioners are studied. Upper bounds of the radius of the eigenvalue cluster for those systems are obtained and shown to be related to the inf-sup condition but independent of mesh size, time step, and locking parameters, which reflects the robustness of the preconditioners with respect to parameter variations. Moreover, the developed preconditioners do not need to compute the Schur complement and neither require exact inversion of diagonal blocks except the leading one. A locking-free weak Galerkin finite element method and the implicit Euler scheme are used for the discretization of the governing equation. Both two- and three-dimensional numerical results are presented to confirm the effectiveness and parameter-robustness of the developed preconditioners.

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arXiv2025

A general regularization strategy for singular Stokes problems and convergence analysis for corresponding discretization and iterative solution

A general regularization strategy is considered for the efficient iterative solution of the lowest-order weak Galerkin approximation of singular Stokes problems. The strategy adds a rank-one regularization term to the zero (2,2) block of the underlying singular saddle point system. This strategy includes the existing pressure pinning and mean-zero enforcement regularization as special examples. It is shown that the numerical error maintains the optimal-order convergence provided that the nonzero Dirichlet boundary datum is approximated numerically with sufficient accuracy. Inexact block diagonal and triangular Schur complement preconditioners are considered for the regularized system. The convergence analysis for MINRES and GMRES with corresponding block preconditioners is provided for different choices of the regularization term. Numerical experiments in two and three dimensions are presented to verify the theoretical findings and the effectiveness of the preconditioning for solving the regularized system.

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arXiv2025

New finite volume element schemes based on a two-layer dual strategy

A two-layer dual strategy is proposed in this work to construct a new family of high-order finite volume element (FVE-2L) schemes that can avoid main common drawbacks of the existing high-order finite volume element (FVE) schemes. The existing high-order FVE schemes are complicated to construct since the number of the dual elements in each primary element used in their construction increases with a rate $O((k+1)^2)$, where $k$ is the order of the scheme. Moreover, all $k$th-order FVE schemes require a higher regularity $H^{k+2}$ than the approximation theory for the $L^2$ theory. Furthermore, all FVE schemes lose local conservation properties over boundary dual elements when dealing with Dirichlet boundary conditions. The proposed FVE-2L schemes has a much simpler construction since they have a fixed number (four) of dual elements in each primary element. They also reduce the regularity requirement for the $L^2$ theory to $H^{k+1}$ and preserve the local conservation law on all dual elements of the second dual layer for both flux and equation forms. Their stability and $H^1$ and $L^2$ convergence are proved. Numerical results are presented to illustrate the convergence and conservation properties of the FVE-2L schemes. Moreover, the condition number of the stiffness matrix of the FVE-2L schemes for the Laplacian operator is shown to have the same growth rate as those for the existing FVE and finite element schemes.

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arXiv2025

Robust Adaptive Meshing, Mesh Density Functions, and Nonlocal Observations for Ensemble Based Data Assimilation

Adaptive spatial meshing has proven invaluable for the accurate, efficient computation of solutions of time dependent partial differential equations. In a DA context the use of adaptive spatial meshes addresses several factors that place increased demands on meshing; these include the location and relative importance of observations and the use of ensemble solutions. To increase the efficiency of adaptive meshes for data assimilation, robust look ahead meshes are developed that fix the same adaptive mesh for all ensemble members for the entire time interval of the forecasts and that incorporates the observations at the next analysis time. This allows for increased vectorization of the ensemble forecasts while minimizing interpolation of solutions between different meshes. The techniques to determine these robust meshes are based upon combining metric tensors or mesh density functions to define nonuniform meshes. We illustrate the robust ensemble look ahead meshes using traveling wave solutions of a bistable reaction-diffusion equation. Observation operators based on convolution type integrals and their associated metric tensors are derived. These further the goals of making efficient use of adaptive meshes in ensemble based DA techniques, developing and employing robust meshes that are effective for a range of similar behaviors in both the ensembles and the observations, and the integration with advanced numerical PDE techniques (a quasi-Lagrangian moving mesh DG technique employing embedded pairs for time stepping). Numerical experiments with different observation scenarios are presented for a 2D inviscid Burgers' equation, a multi-component system, a 2D Shallow Water model, and for a coupled system of two 1D Kuramoto-Sivashinsky equations.

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arXiv2025

A unifying moving mesh method for curves, surfaces, and domains based on mesh equidistribution and alignment

A unifying moving mesh method is developed for general $m$-dimensional geometric objects in $d$-dimensions ($d \ge 1$ and $1\le m \le d$) including curves, surfaces, and domains. The method is based on mesh equidistribution and alignment and does not require the availability of an analytical parametric representation of the underlying geometric object. Mathematical characterizations of shape and size of $m$-simplexes and properties of corresponding edge matrices and affine mappings are derived. The equidistribution and alignment conditions are presented in a unifying form for $m$-simplicial meshes. The equation for mesh movement is defined based on the moving mesh PDE approach, and suitable projection of the nodal mesh velocities is employed to ensure the mesh points stay on the underlying geometric object. The analytical expression for the mesh velocities is obtained in a compact matrix form. The nonsingularity of moving meshes is proved. Numerical results for curves ($m=1$) and surfaces ($m=2$) in two and three dimensions are presented to demonstrate the ability of the developed method to move mesh points without causing singularity and control their concentration.

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arXiv2024

Consistency enforcement for the iterative solution of weak Galerkin finite element approximation of Stokes flow

Finite element discretization of Stokes problems can result in singular, inconsistent saddle point linear algebraic systems. This inconsistency can cause many iterative methods to fail to converge. In this work, we consider the lowest-order weak Galerkin finite element method to discretize Stokes flow problems and study a consistency enforcement by modifying the right-hand side of the resulting linear system. It is shown that the modification of the scheme does not affect the optimal-order convergence of the numerical solution. Moreover, inexact block diagonal and triangular Schur complement preconditioners and the minimal residual method (MINRES) and the generalized minimal residual method (GMRES) are studied for the iterative solution of the modified scheme. Bounds for the eigenvalues and the residual of MINRES/GMRES are established. Those bounds show that the convergence of MINRES and GMRES is independent of the viscosity parameter and mesh size. The convergence of the modified scheme and effectiveness of the preconditioners are verified using numerical examples in two and three dimensions.

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arXiv2024

Convergence analysis of iterative solution with inexact block preconditioning for weak Galerkin finite element approximation of Stokes flow

This work is concerned with the convergence of the iterative solution for the Stokes flow, discretized with the weak Galerkin finite element method and preconditioned using inexact block Schur complement preconditioning. The resulting saddle point linear system is singular and the pressure solution is not unique. The system is regularized with a commonly used strategy by specifying the pressure value at a specific location. It is analytically shown that the regularized system is nonsingular but has an eigenvalue approaching zero as the fluid kinematic viscosity tends to zero. Inexact block diagonal and triangular Schur complement preconditioners are considered with the minimal residual method (MINRES) and the generalized minimal residual method (GMRES), respectively. For both cases, the bounds are obtained for the eigenvalues of the preconditioned systems and for the residual of MINRES/GMRES. These bounds show that the convergence factor of MINRES/GMRES is almost independent of the viscosity parameter and mesh size while the number of MINRES/GMRES iterations required to reach convergence depends on the parameters only logarithmically. The theoretical findings and effectiveness of the preconditioners are verified with two- and three-dimensional numerical examples.

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arXiv2024

Approximating and preconditioning the stiffness matrix in the GoFD approximation of the fractional Laplacian

In the finite difference approximation of the fractional Laplacian the stiffness matrix is typically dense and needs to be approximated numerically. The effect of the accuracy in approximating the stiffness matrix on the accuracy in the whole computation is analyzed and shown to be significant. Four such approximations are discussed. While they are shown to work well with the recently developed grid-over finite difference method (GoFD) for the numerical solution of boundary value problems of the fractional Laplacian, they differ in accuracy, economics to compute, performance of preconditioning, and asymptotic decay away from the diagonal line. In addition, two preconditioners based on sparse and circulant matrices are discussed for the iterative solution of linear systems associated with the stiffness matrix. Numerical results in two and three dimensions are presented.

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arXiv2024

Meshfree finite difference solution of homogeneous Dirichlet problems of the fractional Laplacian

A so-called grid-overlay finite difference method (GoFD) was proposed recently for the numerical solution of homogeneous Dirichlet boundary value problems of the fractional Laplacian on arbitrary bounded domains. It was shown to have advantages of both finite difference and finite element methods, including its efficient implementation through the fast Fourier transform and ability to work for complex domains and with mesh adaptation. The purpose of this work is to study GoFD in a meshfree setting, a key to which is to construct the data transfer matrix from a given point cloud to a uniform grid. Two approaches are proposed, one based on the moving least squares fitting and the other based on the Delaunay triangulation and piecewise linear interpolation. Numerical results obtained for examples with convex and concave domains and various types of point clouds are presented. They show that both approaches lead to comparable results. Moreover, the resulting meshfree GoFD converges at a similar order as GoFD with unstructured meshes and finite element approximation as the number of points in the cloud increases. Furthermore, numerical results show that the method is robust to random perturbations in the location of the points.

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arXiv2024

A moving mesh finite element method for Bernoulli free boundary problems

A moving mesh finite element method is studied for the numerical solution of Bernoulli free boundary problems. The method is based on the pseudo-transient continuation with which a moving boundary problem is constructed and its steady-state solution is taken as the solution of the underlying Bernoulli free boundary problem. The moving boundary problem is solved in a split manner at each time step: the moving boundary is updated with the Euler scheme, the interior mesh points are moved using a moving mesh method, and the corresponding initial-boundary value problem is solved using the linear finite element method. The method can take full advantages of both the pseudo-transient continuation and the moving mesh method. Particularly, it is able to move the mesh, free of tangling, to fit the varying domain for a variety of geometries no matter if they are convex or concave. Moreover, it is convergent towards steady state for a broad class of free boundary problems and initial guesses of the free boundary. Numerical examples for Bernoulli free boundary problems with constant and non-constant Bernoulli conditions and for nonlinear free boundary problems are presented to demonstrate the accuracy and robustness of the method and its ability to deal with various geometries and nonlinearities.

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arXiv2024

A grid-overlay finite difference method for the fractional Laplacian on arbitrary bounded domains

A grid-overlay finite difference method is proposed for the numerical approximation of the fractional Laplacian on arbitrary bounded domains. The method uses an unstructured simplicial mesh and an overlay uniform grid for the underlying domain and constructs the approximation based on a uniform-grid finite difference approximation and a data transfer from the unstructured mesh to the uniform grid. The method takes full advantages of both uniform-grid finite difference approximation in efficient matrix-vector multiplication via the fast Fourier transform and unstructured meshes for complex geometries and mesh adaptation. It is shown that its stiffness matrix is similar to a symmetric and positive definite matrix and thus invertible if the data transfer has full column rank and positive column sums. Piecewise linear interpolation is studied as a special example for the data transfer. It is proved that the full column rank and positive column sums of linear interpolation is guaranteed if the spacing of the uniform grid is smaller than or equal to a positive bound proportional to the minimum element height of the unstructured mesh. Moreover, a sparse preconditioner is proposed for the iterative solution of the resulting linear system for the homogeneous Dirichlet problem of the fractional Laplacian. Numerical examples demonstrate that the new method has similar convergence behavior as existing finite difference and finite element methods and that the sparse preconditioning is effective. Furthermore, the new method can readily be incorporated with existing mesh adaptation strategies. Numerical results obtained by combining with the so-called MMPDE moving mesh method are also presented.

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arXiv2022

A study on CFL conditions for the DG solution of conservation laws on adaptive moving meshes

The selection of time step plays a crucial role in improving stability and efficiency in the Discontinuous Galerkin (DG) solution of hyperbolic conservation laws on adaptive moving meshes that typically employs explicit stepping. A commonly used selection of time step is a direct extension based on Courant-Friedrichs-Levy (CFL) conditions established for fixed and uniform meshes. In this work, we provide a mathematical justification for those time step selection strategies used in practical adaptive DG computations. A stability analysis is presented for a moving mesh DG method for linear scalar conservation laws. Based on the analysis, a new selection strategy of the time step is proposed, which takes into consideration the coupling of the $α$-function (that is related to the eigenvalues of the Jacobian matrix of the flux and the mesh movement velocity) and the heights of the mesh elements. The analysis also suggests several stable combinations of the choices of the $α$-function in the numerical scheme and in the time step selection. Numerical results obtained with a moving mesh DG method for Burgers' and Euler equations are presented. For comparison purpose, numerical results obtained with an error-based time step-size selection strategy are also given.

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arXiv2022

A well-balanced moving mesh discontinuous Galerkin method for the Ripa model on triangular meshes

A well-balanced moving mesh discontinuous Galerkin (DG) method is proposed for the numerical solution of the Ripa model -- a generalization of the shallow water equations that accounts for effects of water temperature variations. Thermodynamic processes are important particularly in the upper layers of the ocean where the variations of sea surface temperature play a fundamental role in climate change. The well-balance property which requires numerical schemes to preserve the lake-at-rest steady state is crucial to the simulation of perturbation waves over that steady state such as waves on a lake or tsunami waves in the deep ocean. To ensure the well-balance, positivity-preserving, and high-order properties, a DG-interpolation scheme (with or without scaling positivity-preserving limiter) and special treatments pertaining to the Ripa model are employed in the transfer of both the flow variables and bottom topography from the old mesh to the new one and in the TVB limiting process. Mesh adaptivity is realized using an MMPDE moving mesh approach and a metric tensor based on an equilibrium variable and water depth. A motivation is to adapt the mesh according to both the perturbations of the lake-at-rest steady state and the water depth distribution (bottom structure). Numerical examples in one and two dimensions are presented to demonstrate the well-balance, high-order accuracy, and positivity-preserving properties of the method and its ability to capture small perturbations of the lake-at-rest steady state.

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arXiv2021

Mesh Sensitivity Analysis for Finite Element Solution of Linear Elliptic Partial Differential Equations

Mesh sensitivity of finite element solution for linear elliptic partial differential equations is analyzed. A bound for the change in the finite element solution is obtained in terms of the mesh deformation and its gradient. The bound shows how the finite element solution changes continuously with the mesh. The result holds in any dimension and for arbitrary unstructured simplicial meshes, general linear elliptic partial differential equations, and general finite element approximations.

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arXiv2021

A Metric Tensor Approach to Data Assimilation with Adaptive Moving Meshes

Adaptive moving spatial meshes are useful for solving physical models given by time-dependent partial differentialequations. However, special consideration must be given when combining adaptive meshing procedures with ensemble-based data assimilation (DA) techniques. In particular, we focus on the case where each ensemble member evolvesindependently upon its own mesh and is interpolated to a common mesh for the DA update. This paper outlines aframework to develop time-dependent reference meshes using locations of observations and the metric tensors (MTs)or monitor functions that define the spatial meshes of the ensemble members. We develop a time-dependent spatiallocalization scheme based on the metric tensor (MT localization). We also explore how adaptive moving mesh tech-niques can control and inform the placement of mesh points to concentrate near the location of observations, reducingthe error of observation interpolation. This is especially beneficial when we have observations in locations that wouldotherwise have a sparse spatial discretization. We illustrate the utility of our results using discontinuous Galerkin(DG) approximations of 1D and 2D inviscid Burgers equations. The numerical results show that the MT localizationscheme compares favorably with standard Gaspari-Cohn localization techniques. In problems where the observationsare sparse, the choice of common mesh has a direct impact on DA performance. The numerical results also demonstratethe advantage of DG-based interpolation over linear interpolation for the 2D inviscid Burgers equation.

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arXiv2021

A quasi-conservative DG-ALE method for multi-component flows using the non-oscillatory kinetic flux

A high-order quasi-conservative discontinuous Galerkin (DG) method is proposed for the numerical simulation of compressible multi-component flows. A distinct feature of the method is a predictor-corrector strategy to define the grid velocity. A Lagrangian mesh is first computed based on the flow velocity and then used as an initial mesh in a moving mesh method (the moving mesh partial differential equation or MMPDE method ) to improve its quality. The fluid dynamic equations are discretized in the direct arbitrary Lagrangian-Eulerian framework using DG elements and the non-oscillatory kinetic flux while the species equation is discretized using a quasi-conservative DG scheme to avoid numerical oscillations near material interfaces. A selection of one- and two-dimensional examples are presented to verify the convergence order and the constant-pressure-velocity preservation property of the method. They also demonstrate that the incorporation of the Lagrangian meshing with the MMPDE moving mesh method works well to concentrate mesh points in regions of shocks and material interfaces.

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