Discontinuous Finite Element Research Articles

Comparing non-hydrostatic extensions to a discontinuous finite element coastal ocean model

The unstructured mesh, discontinuous Galerkin finite element discretisation based coastal ocean model, Thetis, has been extended to include non-hydrostatic (buoyancy-driven and free surface) dynamics. Two alternative approaches to achieve this are described in this work. The first (a 3D based algorithm) makes use of prismatic element based meshes and uses a split-step pressure projection method for baroclinic and barotropic modes, while the second (a 2D based algorithm) adopts a novel multi-layer approach to convert a 3D problem into a combination of multiple 2D computations with only 2D triangle meshes required. Model development is carried out at high-level with the Firedrake library, using code generation techniques to automatically produce low-level code for the discretised model equations in an efficient and rapid manner. Through comparisons against several barotropic/baroclinic test cases where non-hydrostatic effects are important, the implemented approaches are verified and validated, and the proposed algorithms compared. Depending on whether the problems are dominated by dispersive, baroclinic or barotropic features, recommendation are given over the use of full 3D or multi-layer 2D based approaches to achieve optimal computational accuracy and efficiency. It is demonstrated that while in general the 2D approach is well-suited for barotropic problems and dispersive free surface waves, the 3D approach is more advantageous for simulating baroclinic buoyancy-driven flows due in part to the high vertical resolution typically required to represent the active tracer fields.

Calculated radiance errors induced by neglecting the polarization of the irradiation beam exposed to an atmosphere

The radiance calculation via the solution of the scalar approximation form of the rigorous vector radiative transfer equation (VRTE) leads to non-negligible errors in atmospheric radiative transfer problems. In this work, for the first time, a quantitative investigation of the calculated radiance errors induced by neglecting the polarization of incident irradiation beams exposed to a plane-parallel atmosphere is presented. The discontinuous finite element method, which is highly accurate in the spatial and angular spaces, was employed to solve both the VRTE and its scalar approximation. The calculated values of the radiance obtained via the scalar model are compared with the exact solutions calculated via the vector model and their relative errors are analyzed. The results show that neglecting the linearly polarized component of the incident irradiation leads to relative errors up to 94.00%. These values are much larger than the errors induced by neglecting the circularly polarized component.

A Mixed Discontinuous Galerkin Method for Linear Elasticity with Strongly Imposed Symmetry

In this paper, we study a mixed discontinuous Galerkin (MDG) method to solve linear elasticity problem with arbitrary order discontinuous finite element spaces in $d$-dimension ($d=2,3$). This method uses polynomials of degree $k+1$ for the stress and of degree $k$ for the displacement ($k\geq 0$). The mixed DG scheme is proved to be well-posed under proper norms. Specifically, we prove that, for any $k \geq 0$, the $H({\rm div})$-like error estimate for the stress and $L^2$ error estimate for the displacement are optimal. We further establish the optimal $L^2$ error estimate for the stress provided that the $\mathcal{P}_{k+2}-\mathcal{P}_{k+1}^{-1}$ Stokes pair is stable and $k \geq d$. We also provide numerical results of MDG showing that the orders of convergence are actually sharp.

Discontinuous finite volume element method for a coupled Navier-Stokes-Cahn-Hilliard phase field model

In this paper, we propose a discontinuous finite volume element method to solve a phase field model for two immiscible incompressible fluids. In this finite volume element scheme, discontinuous linear finite element basis functions are used to approximate the velocity, phase function, and chemical potential while piecewise constants are used to approximate the pressure. This numerical method is efficient, optimally convergent, conserving the mass, convenient to implement, flexible for mesh refinement, and easy to handle complex geometries with different types of boundary conditions. We rigorously prove the mass conservation property and the discrete energy dissipation for the proposed fully discrete discontinuous finite volume element scheme. Using numerical tests, we verify the accuracy, confirm the mass conservation and the energy law, test the influence of surface tension and small density variations, and simulate the driven cavity, the Rayleigh-Taylor instability.

A Stable Generalized/eXtended [formula omitted]-hierarchical FEM for three-dimensional linear elastic fracture mechanics

In this paper, the quadratic Stable Generalized Finite Element Method (SGFEM) proposed in Sanchez-Rivadeneira and Duarte (2019) is extended to 3-D fracture problems with non-planar crack surfaces and curved crack fronts. This SGFEM is based on p-hierarchical FEM enrichments and its approximation space is the same as in its GFEM counterpart. Singular enrichments are modified using a discontinuous finite element interpolant for conditioning control. This leads, with the proper choice of enrichments, to stiffness matrices with a scaled condition number of O(h−2) — the same order as in the standard FEM. The robustness of the method with respect to the position of a crack relative to the mesh is demonstrated. A scalar implementation of singular enrichments that can exactly reproduce the first term of the Mode I, II, and III of the asymptotic expansion of the elasticity solution is presented. Improved approximations of signed distance functions based on integration sub-elements are also presented. Convergence studies of a fully 3-D fracture problem with a non-planar crack surface and a curved crack front are presented. They show that the convergence rate of the proposed method on a sequence of uniform meshes is three times higher than quadratic quarter-point finite elements.

The weak Galerkin finite element method for the transport-reaction equation

We present and analyze a weak Galerkin finite element method for solving the transport-reaction equation in d space dimensions. This method is highly flexible by allowing the use of discontinuous finite element on general meshes consisting of arbitrary polygon/polyhedra. We derive the L2-error estimate of O(hk+12)-order for the discrete solution when the kth-order polynomials are used for k≥0. Moreover, for a special class of meshes, we also obtain the optimal error estimate of O(hk+1)-order in the L2-norm. A derivative recovery formula is presented to approximate the convection directional derivative and the corresponding superconvergence estimate is given. Numerical examples on compatible and non-compatible meshes are provided to show the effectiveness of this weak Galerkin method.

Uniform preconditioners for problems of positive order

Using the framework of operator or Caldéron preconditioning, uniform preconditioners are constructed for elliptic operators of order 2s∈[0,2] discretized with continuous finite (or boundary) elements. The cost of the preconditioner is the cost of the application an elliptic opposite order operator discretized with discontinuous or continuous finite elements on the same mesh, plus minor cost of linear complexity. Herewith the construction of a so-called dual mesh is avoided.

Benchmarking of a distributed-memory, high-order discontinuous finite element flow solver on a shared-memory parallel architecture

High-order numerical schemes implemented on high-performance parallel computers are of special interest for contemporary numerical simulations, especially in computational fluid dynamics. In this study, first, a high-order parallel flow solver is presented for some test cases of aerodynamic simulations. The flow solver is based on a discontinuous Galerkin finite element method on arbitrary grids with different orders of polynomial approximation for solving the compressible flow model. Second, the distributed-memory parallel implementation of the flow solver is benchmarked on a shared-memory multicore system. A distributed-memory parallel application can be executed on shared-memory architectures by assuming that each of the parallel processes assumes separate memory address space, although all are present in a common memory bank. This approach can offer an effective measure to address several issues related to limited resources, especially for uninterrupted electric supply. The scalability of the parallel application is analyzed by varying the problem workload per process for the test cases. For some test cases in the present study, over 90% parallel efficiency per process is also observed. The performance of the distributed-memory program on the shared-memory architecture establishes suitability and robustness of the approach for small to medium scale problems, at least.

An adaptive composite discontinuous Galerkin method for elliptic problems on complicated domains with discontinuous coefficients

In this paper, we introduce the multi-region discontinuous Galerkin composite finite element method (MRDGCFEM) with hp-adaptivity for the discretization of second-order elliptic partial differential equations with discontinuous coefficients. This method allows for the approximation of problems posed on computational domains where the jumps in the diffusion coefficient form a micro-structure. Standard numerical methods could be used for such problems but the computational effort may be extremely high. Small enough elements to represent the underlying pattern in the diffusion coefficient have to be used. In contrast, the dimension of the underlying MRDGCFE space is independent of the complexity of the diffusion coefficient pattern. The key idea is that the jumps in the diffusion coefficient are no longer resolved by the mesh where the problem is solved; instead, the finite element basis (or shape) functions are adapted to the diffusion pattern allowing for much coarser meshes. In this paper, we employ hp-adaptivity on a series of test cases highlighting the practical application of the proposed numerical scheme.

The Flux Reconstruction Method with Lax–Wendroff Type Temporal Discretization for Hyperbolic Conservation Laws

In this paper, we develop a Lax–Wendroff type time discretization method for high order Flux Reconstruction scheme to solve hyperbolic conservation laws. Through Cauchy–Kowalewski procedure, the resulting Lax–Wendroff Flux Reconstruction (LWFR) scheme is an alternative spatial–temporal coupling method to the popular Runge–Kutta Flux Reconstruction (RKFR) scheme. LWFR is a one-step explicit high order discontinuous finite element method and its discretization procedure is more compact and effective for certain problems than that of RKFR. Furthermore, aiming at accurate simulation of discontinuity, we propose a robust local artificial viscosity formulation of LWFR for the first time. A collection of successful numerical experiments show that LWFR can give essentially non-oscillatory and sharp solutions for discontinuity, and maintain designed order of accuracy for smooth regions, both in one-dimensional and two-dimensional Euler equations. In conclusion, LWFR scheme is cost effective and accuracy-preserving for certain problems and can be an attractive candidate to solve hyperbolic conservation laws.

A discontinuous Galerkin method for the Stolz–Adams approximate deconvolution model for turbulent flows

We consider the zeroth order model of the family of approximate deconvolution models of Stolz and Adams. We propose and analyze fully discrete schemes using discontinuous finite elements. Optimal error estimates are derived. The dependence of these estimates with respect to the Reynolds number Re is O(ReeRe), which is an improvement with respect to the classical continuous finite element method where the dependence is O(ReeRe3), Layton [1].

Discontinuous finite element solutions for coupled radiation-conduction heat transfer in irregular media

The discontinuous finite element method (DFEM) is used to investigate the coupled radiation-conduction heat transfer in an irregular medium, and the highly accurate solutions for several typical media are numerically obtained. Comparing with the traditional continuous finite element method, the computational domain in the DFEM application is discretized into unstructured meshes that are assumed to be separated from each other. The shape function construction, field variable approximation, and numerical solutions are obtained for every single element. The continuity of the computational domain is maintained by modeling a numerical flux with the up-winding scheme. Thus the DFEM has the salient feature of geometry flexibility and simultaneously supports locally conservative solutions. The DFEM discretization for the radiative transfer equation and the energy diffusion equation are first presented, and the accuracies of the DFEM for coupled radiation-conduction heat transfer problems are verified. Combined radiation-conduction heat transfer problems in several irregular media are afterward solved, and the highly accurate DFEM solutions are presented.

An a-priori error analysis for discontinuous Lagrangian finite elements applied to nonconforming dual-mixed formulations: Poisson and Stokes problems

In this paper, we discuss the well-posedness of a mixed discontinuous Galerkin (DG) scheme for the Poisson and Stokes problems in 2D, considering only piecewise Lagrangian finite elements. The complication here lies in the fact that the classical Babuška-Brezzi theory is difficult to verify for low-order finite elements, so we proceed in a non-standard way. First, we prove uniqueness, and then we apply a discrete version of Fredholm's alternative theorem to ensure existence. The a-priori error analysis is done by introducing suitable projections of the exact solution. As a result, we prove that the method is convergent, and, under standard additional regularity assumptions on the exact solution, the optimal rate of convergence of the method is guaranteed.

Simulation and Modeling of Tower Diesel Power Station in Samawah Using Finite Element Method with MATLAB and COMSOL Programs

In this paper we present work that explores governing equations for heat transfer and fluid dynamics, and we describe and discuss different forms for such equations, that can be applied to formulations of the discontinuous finite element. In general, heat transfer and fluid dynamics problems are categorized as boundary value problems. Thus, the desired initial and final boundary conditions for solving these equations are given in a simulation of a modeling tower for a diesel power plant in Samawah, Iraq, via the finite element method using MATLAB and COMSOL programs. As vertical expansion is limited by the frictional repression of the benthos, an effective vertical force is generated in the tower. If the effective vertical force overtakes the buckle initiation force, the tower will undergo Euler buckling to qualify the resulting high vertical forces in the tower wall. We present numerical results for a mesh in Multiphysics of the lower and upper parts for the tower; an analytical model was built of the upper portion of the tower.

A Robust Discontinuous Galerkin High-Order Finite Element Method for Elasticity Problems with Interfaces

A robust discontinuous Galerkin (DG) finite element method is proposed for elasticity problems with interfaces, where the continuity across the interfaces is weakly enforced by using Nitsche’s method. We employ a weighting for the interfacial consistency terms arising in the Nitsche variational form and present a detailed finite element formulation of this DG method. The stabilization parameter is evaluated by solving element level generalized eigenvalue problem for higher-order elements. Consequently, we give the choice of the weighting parameter that results in an estimate for the stabilization parameter such that the method remains well behaved in the pathological cases. The accuracy and robustness of the proposed method are then demonstrated through several numerical examples.

Discontinuous finite-element quadrature sets based on icosahedron for the discrete ordinates method

The discrete ordinates method (SN) is one of the major shielding calculation method, which is suitable for solving deep-penetration transport problems. Our objective is to explore the available quadrature sets and to improve the accuracy in shielding problems involving strong anisotropy. The linear discontinuous finite-element (LDFE) quadrature sets based on the icosahedron (in short, ICLDFE quadrature sets) are developed by defining projected points on the surfaces of the icosahedron. Weights are then introduced in the integration of the discontinuous finite-element basis functions in the relevant angular regions. The multivariate secant method is used to optimize the discrete directions and their corresponding weights. The numerical integration of polynomials in the direction cosines and the Kobayashi benchmark are used to analyze and verify the properties of these new quadrature sets. Results show that the ICLDFE quadrature sets can exactly integrate the zero-order and first-order of the spherical harmonic functions over one-twentieth of the spherical surface. As for the Kobayashi benchmark problem, the maximum relative error between the fifth-order ICLDFE quadrature sets and references is only −0.55%. The ICLDFE quadrature sets provide better integration precision of the spherical harmonic functions in local discrete angle domains and higher accuracy for simple shielding problems.

Discontinuous finite volume element method of two-dimensional unsaturated soil water movement problem

In this paper, a numerical approximation method for the two-dimensional unsaturated soil water movement problem is established by using the discontinuous finite volume method. We prove the optimal error estimate for the fully discrete format. Finally, the reliability of the method is verified by numerical experiments. This method is not only simple to calculate, but also stable and reliable.

The time discontinuous space-time finite element method for fractional diffusion-wave equation

In this paper a time discontinuous space-time finite element method for fractional diffusion-wave equation is studied. The existence and the uniqueness of the numerical solution are included. When we adopt the shape function as the piecewise r-order function, the convergence order is proved to be r+1 in the sense of L2 norm and r+1/2 in the sense of max norm, respectively. Numerical examples with comparisons the accuracy and effectiveness of proposed method are presented.

Fully meshless solution of the one-dimensional multigroup neutron transport equation with the radial basis function collocation method

In this paper a fully meshless method is proposed for the numerical solution of the one-dimensional multigroup neutron transport equation with anisotropic scattering. Both first-order and even-parity forms of the transport equation are studied. The radial basis function collocation method is chosen for the spatial treatment, and Legendre polynomials are used to approximate the angular variable. The selection of the Legendre polynomials instead of discrete ordinates approach resulted with a fully meshless algorithm in both independent variables. Multiquadric is utilized as the radial function. Seven problems are considered to evaluate the performance of the method. The results show that the method converges exponentially, and it is possible to obtain high levels of accuracies for the multiplication factor and neutron flux with a good stability in both spatial and angular domains. For the one-group isotropic benchmark problem, discrete ordinates solutions employing discontinuous linear finite elements for the spatial variable are also provided, and a comparison of the methods revealed that the fully meshless method produced more accurate results than the discrete ordinates-finite element scheme when the shape parameter is properly chosen.

A novel transport sweep architecture for efficient deterministic patient dose calculations in MRI-guided radiotherapy

Accurate and efficient patient dose calculations are essential for treatment planning in magnetic resonance imaging guided radiotherapy (MRIgRT). Achieving reasonable performance for a space-angle discontinuous finite element method (DFEM) grid based Boltzmann solver (GBBS) with magnetic fields for clinical MRIgRT applications largely depends on how the transport sweep is orchestrated. Compared to classical Discrete Ordinates, DFEM in angle introduces increased angular degrees of freedom and eliminates ray-effect artifacts. However, the inclusion of magnetic fields introduces additional serial dependencies such that parallelization of the space-angle transport sweeps becomes more challenging.Novel techniques for the transport sweep and right-hand source assembly are developed, predicated on limiting the number of bulk material densities modeled in the transport sweep scatter calculations. Specifically, k-means clustering is used to assign sub-intervals of mass-density for each spatial element to execute the scatter-dose calculations using batched multiplication by pre-inverted transport sweep matrices. This is shown to be two orders of magnitude more efficient than solving each elemental system individually at runtime.Even with discrete material densities used in the transport sweep scatter calculations, accuracy is maintained by optimizing the material density assignments using k-means clustering, and by performing the primary photon fluence calculations (ray-tracing) using the underlying continuous density of the computed tomography (CT) image. In the presence of 0.5 T parallel and 1.5 T perpendicular magnetic fields, this approach demonstrates high levels of accuracy with gamma 1%/1 mm passing rates exceeding 94% across a range of anatomical sites compared to GEANT4 Monte Carlo dose calculations which used continuous densities.This deterministic GBBS approach maintains unconditional stability, produces no ray-effect artifacts, and has the benefit of no statistical uncertainty. Runtime on a non-parallelized Matlab implementation averaged 10 min per beam averaging 80 000 spatial elements, paving way for future development based on this algorithmically efficient paradigm.