An h-hierarchical adaptive scaled boundary finite element method for elastodynamics

A posteriori h-hierarchical adaptive scaled boundary finite element method (ASBFEM) for transient elastodynamic problems is developed. In a time step, the fields of displacement, stress, velocity and acceleration are all semi-analytical and the kinetic energy, strain energy and energy error are all semi-analytically integrated in subdomains. This makes mesh mapping very simple but accurate. Adaptive mesh refinement is also very simple because only subdomain boundaries are discretised. Two 2D examples with stress wave propagation were modelled. It is found that the degrees of freedom needed by the ASBFEM are only 5%–15% as needed by adaptive FEM for the examples.     

On the CFL condition for the finite element immersed boundary method

The finite element version of the immersed boundary method proved to be a robust alternative to the original one which was based on finite differences. In this paper we highlight the advantages of the new method and discuss a stability analysis for its space–time discretization. Numerical experiments confirm the theoretical results.     

A mesh-refinement scheme for finite element computations

The paper considers the use of unorthodox grids where rapid transition from refined zones to coarser zones is effected, thus introducing exposed nodal freedoms at the zone interfaces. A technique for automated mesh enrichment of finite element discretizations is devised. Suitable convergence criteria are designed to delineate those subregions of a domain where refinement is necessary. Enriched and unaltered regions are separated by a fringe of semirefined elements. A valid finite element theory is provided for the fringe elements. A pilot numerical study illustrates the practical implementation of the automated refinement strategy. The principal features of the program are described.     

Scaling and the pre-asymptotic behavior of the condition of high order finite element stiffness matrices

Scaling is a simple transformation designed to reduce the condition of the global stiffness matrix K formed by the finite element method. The effectiveness of scaling depends on the problem; that is, on the order of differentiation in the nodal values, the physical dimensions of the elements, and the order of the boundary value problem. High order elements exhibit an optimistic pre-asymptotic behavior.     

A high order isoparametric finite element method for the computation of waveguide modes

We have studied the approximation of optical waveguide eigenvalues by a high order isoparametric vector finite element method. Isoparametric mappings are used for the approximation of domains with curved boundaries or curved material interfaces. Eigenvalue convergence for curved elements is investigated. Numerical results verify the predicted order of convergence and show the remarkable accuracy of the method.     

A generalization of the vertex-centered finite volume scheme to arbitrary high order

A higher order finite volume method for elliptic problems is proposed for arbitrary order ${p \in \mathbb{N}}$ . Piecewise polynomial basis functions are used as trial functions while the control volumes are constructed by a vertex-centered technique. The discretization is tested on numerical examples utilizing triangles and quadrilaterals in 2D. In these tests the optimal error is achieved in the H ¹-norm. The error in the L ²-norm is one order below optimal for even polynomial degrees and optimal for odd degrees.     

A mixed finite element for shell model with free edge boundary conditions Part 2. The numerical scheme

From the mixed variational formulation which was suggested in Part 1, we construct a finite element scheme. Then, a mathematical justification concerning the error estimate is developed. The most important point is to justify two compatibility conditions between the approximation of the kinematics and the stresses. Because of the structure of the shell operator they are not obvious at all and several restrictions on the mesh are necessary. Therefore, the classical result known in fluid mechanics for Stokes equations cannot be applied directly.     

A high-order absorbing boundary condition for 2D time-harmonic elastodynamic scattering problems

In this paper, we are concerned with the construction of a new high-order Absorbing Boundary Condition (ABC) for 2D-elastic scattering problems. It is defined by an approximate local Dirichlet-to-Neumann (DtN) map. First, we explain the derivation of this approximation. Next, a detailed analytical study in terms of Hankel functions in the circular case is addressed. The new ABC is compared with the standard low-order Lysmer–Kuhlemeyer ABC. Finally, its accuracy and efficiency are investigated for various numerical examples, particularly at high frequencies.     

A Newton-like finite element scheme for compressible gas flows

Semi-implicit and Newton-like finite element methods are developed for the stationary compressible Euler equations. The Galerkin discretization of the inviscid fluxes is potentially oscillatory and unstable. To suppress numerical oscillations, the spatial discretization is performed by a high-resolution finite element scheme based on algebraic flux correction. A multidimensional limiter of TVD type is employed. An important goal is the efficient computation of stationary solutions in a wide range of Mach numbers, which is a challenging task due to oscillatory correction factors associated with TVD-type flux limiters. A semi-implicit scheme is derived by a time-lagged linearization of the nonlinear residual, and a Newton-like method is obtained in the limit of infinite CFL numbers. Special emphasis is laid on the numerical treatment of weakly imposed characteristic boundary conditions. Numerical evidence for unconditional stability is presented. It is shown that the proposed approach offers higher accuracy and better convergence behavior than algorithms in which the boundary conditions are implemented in a strong sense.     

Lagrangian finite element modelling of dam–fluid interaction: Accurate absorbing boundary conditions

The dynamic dam–fluid interaction is considered via a Lagrangian approach, based on a fluid finite element (FE) model under the assumption of small displacement and inviscid fluid. The fluid domain is discretized by enhanced displacement-based finite elements, which can be considered an evolution of those derived from the pioneering works of Bathe and Hahn [Bathe KJ, Hahn WF. On transient analysis of fluid–structure system. Comp Struct 1979;10:383–93] and of Wilson and Khalvati [Wilson EL, Khalvati M. Finite element for the dynamic analysis of fluid–solid system. Int J Numer Methods Eng 1983;19:1657–68]. The irrotational condition for inviscid fluids is imposed by the penalty method and consequentially leads to a type of micropolar media. The model is implemented using a FE code, and the numerical results of a rectangular bidimensional basin (subjected to horizontal sinusoidal acceleration) are compared with the analytical solution. It is demonstrated that the Lagrangian model is able to perform pressure and gravity wave propagation analysis, even if the gravity (or surface) waves are dispersive. The dispersion nature of surface waves indicates that the wave propagation velocity is dependent on the wave frequency.For the practical analysis of the coupled dam–fluid problem the analysed region of the basin must be reduced and the use of suitable asymptotic boundary conditions must be investigated. The classical Sommerfeld condition is implemented by means of a boundary layer of dampers and the analysis results are shown for the cases of sinusoidal forcing.The classical Sommerfeld condition is highly efficient for pressure-based FE modelling, but may not be considered fully adequate for the displacement-based FE approach. In the present paper a high-order boundary condition proposed by Higdom [Higdom RL. Radiation boundary condition for dispersive waves. SIAM J Numer Anal 1994;31:64–100] is considered. Its implementation requires the resolution of a multifreedom constraint problem, defined in terms of incremental displacements, in the ambit of dynamic time integration problems. The first- and second-order Higdon conditions are developed and implemented. The results are compared with the Sommerfeld condition results, and with the analytical unbounded problem results.Finally, a number of finite element results are presented and their related features are discussed and critically compared.     

A space-time least-square finite element scheme for advection-diffusion equations

A space-time least-square finite element scheme is presented for the advection-diffusion problems at moderate to high Peclet numbers. This scheme is designed to eliminate spurious oscillations and can be used to define the steady-state solution as the asymptotic transient solution for large time. Numerical results, using linear elements in a 1D space and bilinear elements in a 2D space, demonstrate the accuracy and the stability of the new scheme.     

Wave-splitting and absorbing boundary condition for Maxwell's equations on a curved surface

Wave-splitting of Maxwell's equations on a curved surface in three-dimension is derived from the conditions for out-going and in-coming waves. The condition for out-going wave is used to derive an absorbing boundary condition on a non-planar surface. A local approximation of the absorbing boundary condition is given. As a special case, a spherical absorbing boundary which truncates the computational domain is also considered in detail.     

A mixed finite element method for boundary flux computation

Most finite element schemes for thermal problems estimate boundary heat flux directly from the derivative of the finite element solution. The boundary flux calculated by this approach is typically inaccurate and does not guarantee a global heat balance.In this paper we present a mixed finite element method for calculating the boundary flux and show the superiority of this method through numerical examples of both diffusion and advection-diffusion problems.     

A Dorodnitsyn finite element formulation for turbulent boundary layers

The Dorodnitsyn boundary layer formulation is combined with a modified Galerkin finite element formulation and an implicit, non-iterative marching scheme to generate a computational algorithm that is both accurate and very economical. For four representative pressure gradient cases taken from the 1968 Stanford Turbulent Boundary Layer Conference the Dorodnitsyn finite element formulation is compared with a Dorodnitsyn spectral formulation and a representative finite difference package. All methods produce solutions of high accuracy but the Dorodnitsyn finite element formulation is about ten times more economical than the other methods.     

A locking-free stabilized mixed finite element method for linear elasticity: the high order case

In this paper, we propose a locking-free stabilized mixed finite element method for the linear elasticity problem, which employs a jump penalty term for the displacement approximation. The continuous piecewise k-order polynomial space is used for the stress and the discontinuous piecewise \((k-1)\)-order polynomial space for the displacement, where we require that \(k\ge 3\) in the two dimensions and \(k\ge 4\) in the three dimensions. The method is proved to be stable and k-order convergent for the stress in \(H(\mathrm {div})\)-norm and for the displacement in \(L^2\)-norm. Further, the convergence does not deteriorate in the nearly incompressible or incompressible case. Finally, the numerical results are presented to illustrate the optimal convergence of the stabilized mixed method.     

A hybrid finite element-scaled boundary finite element method for crack propagation modelling

This study develops a novel hybrid method that combines the finite element method (FEM) and the scaled boundary finite element method (SBFEM) for crack propagation modelling in brittle and quasi-brittle materials. A very simple yet flexible local remeshing procedure, solely based on the FE mesh, is used to accommodate crack propagation. The crack-tip FE mesh is then replaced by a SBFEM rosette. This enables direct extraction of accurate stress intensity factors (SIFs) from the semi-analytical displacement or stress solutions of the SBFEM, which are then used to evaluate the crack propagation criterion. The fracture process zones are modelled using nonlinear cohesive interface elements that are automatically inserted into the FE mesh as the cracks propagate. Both the FEM’s flexibility in remeshing multiple cracks and the SBFEM’s high accuracy in calculating SIFs are exploited. The efficiency of the hybrid method in calculating SIFs is first demonstrated in two problems with stationary cracks. Nonlinear cohesive crack propagation in three notched concrete beams is then modelled. The results compare well with experimental and numerical results available in the literature.     

A performance study of plane wave finite element methods with a Padé-type artificial boundary condition in acoustic scattering

This paper proposes an original numerical method and studies its performance for solving high-frequency scattering problems involving elongated scatterers. The approach is based on coupling a high-order Padé-type non-reflecting boundary condition with plane wave finite element formulations. It is shown that for some numerical examples the approximate solution of suitable accuracy can be obtained using a small number of degrees of freedom.     

A nonconforming finite element method for a singularly perturbed boundary value problem

We analyze a new nonconforming Petrov-Galerkin finite element method for solving linear singularly perturbed two-point boundary value problems without turning points. The method is shown to be convergent, uniformly in the perturbation parameter, of orderh ^1/2 in a norm slightly stronger than the energy norm. Our proof uses a new abstract convergence theorem for Petrov-Galerkin finite element methods.     

带矢量ABC底面的共形完全匹配层

共形完全匹配层是一种有耗各向异性媒质组成的凸且光滑的壳体,其底面一般是PEC面或PMC面,但是PEC面或PMC面会对原散射场产生反射;为了减少底面反射,将CPML原有的PEC（或PMC）底面改为矢量ABC吸收边界,并给出了带矢量ABC底面的CPML泛函公式。通过数值算例证明,这种带矢量ABC底面的CPML边界不仅减少了底面反射,而且吸收效果好,计算精度高。