Piecewise Linear Basis Functions Research Articles

Robust space-time finite element methods for parabolic distributed optimal control problems with energy regularization

As in our previous work (SINUM 59(2):660–674, 2021) we consider space-time tracking optimal control problems for linear parabolic initial boundary value problems that are given in the space-time cylinder Q=Ω×(0,T)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Q = \\Omega \ imes (0,T)$$\\end{document}, and that are controlled by the right-hand side zϱ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$z_\\varrho $$\\end{document} from the Bochner space L2(0,T;H-1(Ω))\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$L^2(0,T;H^{-1}(\\Omega ))$$\\end{document}. So it is natural to replace the usual L2(Q)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$L^2(Q)$$\\end{document} norm regularization by the energy regularization in the L2(0,T;H-1(Ω))\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$L^2(0,T;H^{-1}(\\Omega ))$$\\end{document} norm. We derive new a priori estimates for the error ‖u~ϱh-u¯‖L2(Q)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Vert \\widetilde{u}_{\\varrho h} - \\overline{u}\\Vert _{L^2(Q)}$$\\end{document} between the computed state u~ϱh\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\widetilde{u}_{\\varrho h}$$\\end{document} and the desired state u¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\overline{u}$$\\end{document} in terms of the regularization parameter ϱ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\varrho $$\\end{document} and the space-time finite element mesh size h, and depending on the regularity of the desired state u¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\overline{u}$$\\end{document}. These new estimates lead to the optimal choice ϱ=h2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\varrho = h^2$$\\end{document}. The approximate state u~ϱh\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\widetilde{u}_{\\varrho h}$$\\end{document} is computed by means of a space-time finite element method using piecewise linear and continuous basis functions on completely unstructured simplicial meshes for Q. The theoretical results are quantitatively illustrated by a series of numerical examples in two and three space dimensions. We also provide performance studies for different solvers.

Verification of the modeling results of oscillations of an elastic-supported system with displacement limiters under moving load

ABSTRACT The article presents the results of experimental and numerical simulation of oscillations of the structural nonlinear dynamic system under moving load. Laboratory tests were carried out on an experimental facility designed and manufactured by the authors to research the features of the dynamic behaviour of elastic-supported beam systems with displacement limiters. The obtained experimental data converge well with the obtained results by the calculations performed in the MATLAB environment based on the numerical algorithm developed by the authors using piecewise linear basis functions. The article evaluated the possibility of using the modern computational complex LS-Dyna for the analysis of oscillations of load-bearing systems with limiting supports. The influence of the speed of moving load, stiffness of beam, and elastic support on the features of the oscillations of the dynamic system is investigated.

SOLVING LINEAR VOLTERRA INTEGRAL EQUATIONS WITH A PIECEWISE LINEAR MAXIMUM ENTROPY METHOD

Based on the principle of maximum entropy and piecewise linear basis functions, a numerical method for approximating a nonnegative solution of Volterra integral equations is proposed. Then a discrete form of the maximum entropy method is given to reduce the cost. The error analysis and the convergence rate are provided. Numerical experimental results are consistent with the theoretical analysis of errors and the convergence rate.

Closed-Form Expressions for Numerical Evaluation of Self-Impedance Terms Involved on Wire Antenna Analysis by the Method of Moments

This paper proposes new closed expressions of self-impedance using the Method of Moments with the Point Matching Procedure and piecewise constant and linear basis functions in different configurations, which allow saving computing time for the solution of wire antennas with complex geometries. The new expressions have complexity O(1) with well-defined theoretical bound errors. They were compared with an adaptive numerical integration. We obtain an accuracy between 7 and 16 digits depending on the chosen basis function and segmentation used. Besides, the computing time involved in the calculation of the self-impedance terms was evaluated and compared with the time required by the adaptative quadrature integration solution of the same problem. Expressions have a run-time bounded between 50 and 200 times faster than an adaptive numerical integration assuming full computation of all constant of the expressions.

A Fast Volume Integral Equation Solver With Linear Basis Functions for the Accurate Computation of EM Fields in MRI

A stable volume integral equation (VIE) solver based on polarization/magnetization currents is presented, for the accurate and efficient computation of the electromagnetic (EM) scattering from highly inhomogeneous and high contrast objects. We employ the Galerkin method of moments to discretize the formulation with discontinuous piecewise linear basis functions on uniform voxelized grids, allowing for the acceleration of the associated matrix-vector products in an iterative solver, with the help of FFT. Numerical results illustrate the superior accuracy and more stable convergence properties of the proposed framework, when compared against standard low-order (piecewise constant) discretization schemes and a more conventional VIE formulation based on electric flux densities. Finally, the developed solver is applied to analyze complex geometries, including realistic human body models, typically used in modeling the interactions between EM waves and biological tissue.

Extension of B-spline Material Point Method for unstructured triangular grids using Powell\u2013Sabin splines

The Material Point Method (MPM) is a numerical technique that combines a fixed Eulerian background grid and Lagrangian point masses to simulate materials which undergo large deformations. Within the original MPM, discontinuous gradients of the piecewise-linear basis functions lead to the so-called grid-crossing errors when particles cross element boundaries. Previous research has shown that B-spline MPM (BSMPM) is a viable alternative not only to MPM, but also to more advanced versions of the method that are designed to reduce the grid-crossing errors. In contrast to many other MPM-related methods, BSMPM has been used exclusively on structured rectangular domains, considerably limiting its range of applicability. In this paper, we present an extension of BSMPM to unstructured triangulations. The proposed approach combines MPM with C^1-continuous high-order Powell–Sabin spline basis functions. Numerical results demonstrate the potential of these basis functions within MPM in terms of grid-crossing-error elimination and higher-order convergence.

A Total-Lagrangian Material Point Method for solid mechanics problems involving large deformations

The material point method (MPM) has found successful applications in many engineering problems involving large displacement, large deformation and contacts. The standard MPM formulation, which adopts piece-wise linear basis functions, suffers from the so-called cell-crossing instability, low order of convergence and numerical fracture. Modifications have been made to this standard MPM to mitigate these issues: B-spline MPM (BSMPM), the generalized interpolation material point (GIMP) and convected particle domain interpolation (CPDI) all decrease cell-crossing instabilities and increase the order of convergence, but only CPDI effectively suppresses numerical fracture. However, these methods, CPDI in particular, significantly increase the method’s implementation and computational complexity. This paper presents a total Lagrangian MPM, dubbed TLMPM, that overcomes the issues of the conventional MPM while being more efficient and easier to implement than CPDI. The method is used for impact analyses of a cylinder bar made of steel and necking and fracture of cylinder alloy specimens. The numerical solutions are in satisfactory agreement with the experiment data. No numerical fracture occurred for simulations involving very large tensile deformation without special treatment such as done in the CPDI. Convergence analyses using the method of manufactured solutions show that the TLMPM is second-order accurate for problems of which boundary is axis-aligned. For the challenging generalized vortex problem, it also converges quadratically for relatively coarse meshes. Moreover, the model is able to simulate physically based fracture using continuum damage mechanics.

Cost-Efficient Numerical Schemes for the Boundary Integral Equation Solution in 2D Vortex Methods

The problem of the flow simulation around airfoils using Lagrangian vortex methods is considered. Numerical schemes of the second order of accuracy for free vortex sheet intensity distribution along the airfoil are developed for smooth and non-smooth airfoils. The schemes are based on the Galerkin approach with piecewise-constant and piecewise-linear basis functions. The finite element method ideas are used and the resulting piecewise-linear scheme has the same numerical complexity as the scheme with piecewise-constant numerical solution. The modification of a FEM-type scheme is developed for non-smooth airfoils which permits to take into account the discontinuity of the solution at the specified points; its computational cost increases insignificantly.

Octant-Based Stencil Selection for Meshless Finite Difference Methods in 3D

We study a simple meshless stencil selection algorithm in 3D for supporting the meshless finite difference method based on radial basis functions (RBF-FD) to solve the Dirichlet problem for the Poisson equation. Our numerical experiments show that the proposed method produces solutions that differ very little from the solutions by the finite element method with Courant’s piecewise linear basis functions, when the domain is discretized by the vertices of the respective tetrahedral triangulation.

Comparative analysis of models of limiting supports in the study of structurally nonlinear oscillations of elastically supported bar from mobile load

The method of modeling oscillations of an elastically supported strained bar with movement limiters at the ends under the action of a moving variable load is being improved and developed. Two models of limiting support are used. A computational algorithm was constructed. The model is formulated in terms of generalized functions, which makes it possible to effectively model pointwise in space and instantaneous in time forces. When solving partial differential equations, the boundary conditions are included in the original equations. To solve the differential equation, piecewise linear basis functions are used. They remain constant during the calculation. Numerical studies of the behavior of a dynamic system depending on the variation of a wide range of parameters using a computer program in Matlab have been performed. The developed method will allow analyzing the dynamic behavior of a number of structurally nonlinear technical systems.

VoxHenry: FFT-Accelerated Inductance Extraction for Voxelized Geometries

VoxHenry, a fast Fourier transform (FFT)-accelerated integral-equation-based simulator for extracting frequency-dependent inductances and resistances of structures discretized by voxels, is presented. VoxHenry shares many features with the popular inductance extractor, FastHenry. Just like FastHenry, VoxHenry solves a combination of the electric volume integral equation and the current continuity equation, but with three distinctions that make VoxHenry suitable and extremely efficient for analyzing voxelized geometries: 1) it leverages a carefully selected set of piecewise-constant and piecewise-linear basis functions; 2) it exploits FFTs to accelerate the matrix–vector multiplications during the iterative solution of system of equations; and 3) it employs a sparse preconditioner to ensure the rapid convergence of iterative solution. VoxHenry is capable of accurately computing frequency-dependent inductances and resistances of arbitrarily shaped and large-scale structures on a desktop computer. The accuracy, efficiency, and applicability of VoxHenry are demonstrated through inductance analysis of various structures, including square and circular coils as well as arrays of RF inductors (situated over ground planes).

Axial-Mode Performance Characteristics of a Thin-Wire Square Helical Antenna

This paper, using an adaptation of a method described by Kornhauser for a circular helix antenna in a 1951 publication, rigorously develops hitherto unavailable explicit analytical expressions for the far-zone electric fields radiated in free space, by a thin-wire helical antenna, of square cross-section. It is shown in the presentation that these radiation field expressions for the square helical antenna represent a general case, from which corresponding expressions, exactly as published elsewhere for the square-loop and linear geometries, are recoverable: to support the correctness of the formulation. And subsequently using the expressions as basis, the presentation undertakes a systematic investigation of the characteristic performance features of the square helical antenna, mounted on a ground plane of infinite extent; when operated in the axial mode, and for various values of square-loop perimeter and number of helix turns. For computational data, the antenna’s current distribution is determined via the method of moments with piece-wise linear basis and testing functions, after invoking the image theory. In addition to the radiated electric field, numerical results concerning axial ratio as well as the directive gain, for the square helix antenna, and for a range of antenna physical dimensions, are presented in graphical formats. These results very clearly indicate that the axial-mode performance of the square helical antenna is comparable with that of the conventional circular helical antenna.

Simulation of vibrations of a continuously elastic supported rod with varying boundary conditions under the action of a movable

To simulate the non-linear vibrations of a floating bridge of a continuous system on separate floating supports with additional limiting supports at the ends with a moving load solves the most complicated problem which is the problem of describing the behavior of a span structure. A technique for simulating the vibration of an elastically supported deformable rod with limiting supports at the ends, which is a design scheme of a span structure, under the action of a moving force is developed. A computational algorithm for solving partial differential equations with varying boundary conditions is proposed, which includes boundary conditions in the model equations and does not require the subordination of basis functions to the boundary conditions. During the calculation, the basis remains constant. Piecewise linear basis functions are used to solve the differential equation. The technique is tested using a computational program Matlab, which is implemented when performing numerical studies of the behavior of the dynamic system as a function of the parameter changes. The developed technique is universal for studying the dynamics of a number of constructively non-linear systems.

Versatile and Accurate Schemes of Discretization in the Scattering Analysis of 2-D Composite Objects With Penetrable or Perfectly Conducting Regions

The method-of-moment discretization of boundary integral equations in the scattering analysis of closed infinitely long (2-D) objects, perfectly conducting (PEC) or penetrable, is traditionally carried out with continuous piecewise linear basis functions, which embrace pairs of adjacent segments. This is numerically advantageous because the discretization of the transversal component of the scattered fields, electric (TE) or magnetic (TM), becomes free from hypersingular Kernel contributions. In the analysis of composite objects, though, the imposition of the continuity requirement around junction nodes, where the boundaries of several regions intersect, becomes especially awkward. In this paper, we present, for the scattering analysis of composite objects, a new combined discretization of the Poggio–Miller–Chang–Harrington–Wu–Tsai (PMCHWT) integral equation, for homogeneous dielectric regions, and the electric-field integral equation, for PEC regions, such that the basis functions are defined strictly on each segment, with no continuity constraint between adjacent segments. We show the improved observed accuracy with the proposed TE-PMCHWT implementation on several dielectric objects with sharp edges and corners and moderate or high contrasts. Furthermore, we illustrate the versatility of these schemes in the analysis of 2-D composite piecewise homogeneous objects without sacrificing accuracy with respect to the conventional implementations.

Advection on Cut-Cell Grids for an Idealized Mountain of Constant Slope

Abstract Cut cells use regular or nearly regular polygonal cells to describe fields. For a given orography, some cells may be completely under the mountain, some completely above the mountain, and some are partially filled with air. While there are reports indicating considerably improved simulations with cut cells, inaccuracies may arise with some approximations, producing noise in fields near the surface. This behavior may depend strongly on the approximations made for the advection terms near the surface. This paper investigates the accuracy of advection for numerical schemes for a nondivergent flow near a mountain surface. The schemes use C-grid staggering with densities located at cell centers or on the corners of cells. Also, a nonconserving scheme is considered, which was used in the past with real-data cut-cell simulations. Since the cut cells near the surface create an irregular resolution, the accuracy and order of some approximations may break down near the surface. The objective of this paper is to find schemes having the same accuracy for advection near the surface as in the interior of the domain. As a test problem, uniform advection by a nondivergent velocity field is used with a 45° slope mountain (represented as a straight line) on a rectangular grid. Along the surface a sequence of triangular and pentagonal cells of quite different sizes are generated. Some schemes being discussed for cut cells lead to inaccurate and noisy solutions for this perfectly smooth mountain. A scheme using piecewise linear basis functions in a C grid with density points at the cell corners avoids these inaccuracies.

An Immersed Finite Element Method for the Elasticity Problems with Displacement Jump

AbstractIn this paper, we propose a finite element method for the elasticity problems which have displacement discontinuity along the material interface using uniform grids. We modify the immersed finite element method introduced recently for the computation of interface problems having homogeneous jumps [20, 22]. Since the interface is allowed to cut through the element, we modify the standard Crouzeix-Raviart basis functions so that along the interface, the normal stress is continuous and the jump of the displacement vector is proportional to the normal stress. We construct the broken piecewise linear basis functions which are uniquely determined by these conditions. The unknowns are only associated with the edges of element, except the intersection points. Thus our scheme has fewer degrees of freedom than most of the XFEM type of methods in the existing literature [1,8,13]. Finally, we present numerical results which show optimal orders of convergence rates.

A New Approximate Closed Solution for Small Dipole Antenna with Method of Moments

In this paper, an approximate closed-form solution of the impedance coefficient for a small dipole using the method of moments is developed from the reduced or approximate Pocklington equation. As an application, the close-form self-impedance coefficient using the piecewise constant and linear basis functions are found, with well-defined rounding error functions. The authors found that the self-impedance using the piecewise constant basis function is inconsistent with the physics of the problem for segments close to wavelength tenths and arbitrary small wire diameters, independent of the reduced kernel approximation and the printed source model used.

Estimation of the Accuracy of Finite-Element Petrov–Galerkin Method in Integrating the One-Dimensional Stationary Convection-Diffusion-Reaction Equation

UDC 519.63; 004.75; 536.252 The accuracy and convergence of the numerical solutions of a stationary one-dimensional linear convection-diffusion-reaction equation (with Dirichlet boundary conditions) by the Petrov–Galerkin finiteelement method with piecewise-linear basis functions and piecewise-quadratic weighting functions are analyzed and the accuracy estimates of the method are obtained in certain norms depending on the choice of the collection of stabilization parameters of weight functions.

Scalable high-dimensional dynamic stochastic economic modeling

We present a highly parallelizable and flexible computational method to solve high-dimensional stochastic dynamic economic models. Solving such models often requires the use of iterative methods, like time iteration or dynamic programming. By exploiting the generic iterative structure of this broad class of economic problems, we propose a parallelization scheme that favors hybrid massively parallel computer architectures. Within a parallel nonlinear time iteration framework, we interpolate policy functions partially on GPUs using an adaptive sparse grid algorithm with piecewise linear hierarchical basis functions. GPUs accelerate this part of the computation one order of magnitude thus reducing overall computation time by 50%. The developments in this paper include the use of a fully adaptive sparse grid algorithm and the use of a mixed MPI-Intel TBB-CUDA/Thrust implementation to improve the interprocess communication strategy on massively parallel architectures. Numerical experiments on “Piz Daint” (Cray XC30) at the Swiss National Supercomputing Centre show that high-dimensional international real business cycle models can be efficiently solved in parallel. To the best of our knowledge, this performance on a massively parallel petascale architecture for such nonlinear high-dimensional economic models has not been possible prior to present work.

Uncertainty Quantification for Electromagnetic Systems Using ASGC and DGTD Method

In this paper, an adaptive hierarchical sparse grid collocation (ASGC) method combined with the discontinuous Galerkin time-domain method is leveraged to quantify the impacts of random parameters on the electromagnetics systems. The ASGC method approximates the stochastic observables of interest using interpolation functions over a set of collocation points determined by the Smolyak's algorithm integrated with an adaptive strategy. Instead of resorting to a full-tensor product sense, the Smolyak's algorithm constructs the collocation points in a hierarchical scheme with the interpolation level. Enhanced by an adaptive strategy, the Smolyak's algorithm will sample more points along important dimensions with sharp variations or discontinuities, resulting in a nonuniform sampling scheme. To flexibly handle different stochastic systems, either piecewise linear or Lagrange polynomial basis functions are applied. With these strategies, the number of collocation points is significantly reduced. The statistical knowledge of stochastic observables including the expected value, variance, probability density function, and cumulative distribution function are presented. The accuracy and robustness of the algorithm are demonstrated by various examples.