A finite element based level set method for two-phase incompressible flows

We present a method that has been developed for the efficient numerical simulation of two-phase incompressible flows. For capturing the interface between the phases the level set technique is applied. The continuous model consists of the incompressible Navier–Stokes equations coupled with an advection equation for the level set function. The effect of surface tension is modeled by a localized force term at the interface (so-called continuum surface force approach). For spatial discretization of velocity, pressure and the level set function conforming finite elements on a hierarchy of nested tetrahedral grids are used. In the finite element setting we can apply a special technique to the localized force term, which is based on a partial integration rule for the Laplace–Beltrami operator. Due to this approach the second order derivatives coming from the curvature can be eliminated. For the time discretization we apply a variant of the fractional step θ-scheme. The discrete saddle point problems that occur in each time step are solved using an inexact Uzawa method combined with multigrid techniques. For reparametrization of the level set function a new variant of the fast marching method is introduced. A special feature of the solver is that it combines the level set method with finite element discretization, Laplace–Beltrami partial integration, multilevel local refinement and multigrid solution techniques. All these components of the solver are described. Results of numerical experiments are presented.     

不可压缩流两重稳定有限体积算法应用研究

通过将局部高斯积分稳定化方法和两重网格算法思想紧密结合,提出了粘性不可压缩流体的两重稳定有限体积算法。将该算法的三种迭代格式进行了效率的分析比较。理论分析和数值实验发现：当粗、细网格尺度比例选择适当时,两重算法与传统算法具有相同精度解的同时,效率大大提高;对不同格式的两重有限体积算法进行比较分析发现：Simple格式计算效率最高,Picard格式次之,Newton格式较低。     

Numerical instabilities in level set topology optimization with the extended finite element method

This paper studies level set topology optimization of structures predicting the structural response by the eXtended Finite Element Method (XFEM). In contrast to Ersatz material approaches, the XFEM represents the geometry in the mechanical model by crisp boundaries. The traditional XFEM approach augments the approximation of the state variable fields with a fixed set of enrichment functions. For complex material layouts with small geometric features, this strategy may result in interpolation errors and non-physical coupling between disconnected material domains. These defects can lead to numerical instabilities in the optimized material layout, similar to checker-board patterns found in density methods. In this paper, a generalized Heaviside enrichment strategy is presented that adapts the set of enrichment functions to the material layout and consistently interpolates the state variable fields, bypassing the limitations of the traditional approach. This XFEM formulation is embedded into a level set topology optimization framework and studied with “material-void” and “material-material” design problems, optimizing the compliance via a mathematical programming method. The numerical results suggest that the generalized formulation of the XFEM resolves numerical instabilities, but regularization techniques are still required to control the optimized geometry. It is observed that constraining the perimeter effectively eliminates the emergence of small geometric features. In contrast, smoothing the level set field does not provide a reliable geometry control but mainly improves the convergence rate of the optimization process.     

Viscous incompressible flow by a penalty function finite element method

A quantitative evaluation for the penalty function finite element method for two-dimensional viscous incompressible flow using primitive variables is made, using bilinear and biquadratic elements. We conclude that this procedure, more efficient than full velocity pressure formulation, can achieve excellent accuracy using rather coarse meshes, that biquadratic elements produce improved accuracy over the more commonly used bilinears, and that the penalty method effectively imposes the continuity constraint. We point out the dangers of modelling problems containing singularities, as is the case of the driven cavity flow, with finite element formulations using primitive variables and how to overcome these problems. The driven cavity flow for Reynolds number up to 400 is solved and the answers compared with the best available solutions, the Jeffery-Hamel flow in a convergent control channel is solved for Reynolds number up to 61 and the results compared with the analytical solution. Finally extensions of the method are indicated via an example of natural convection in a square cavity for Rayleigh number up to 10⁶, and the advantage of the method discussed.     

Higher order stabilized finite element method for hyperelastic finite deformation

This paper presents a higher order stabilized finite element formulation for hyperelastic large deformation problems involving incompressible or nearly incompressible materials. A Lagrangian finite element formulation is presented where mesh dependent terms are added element-wise to enhance the stability of the mixed finite element formulation. A reconstruction method based on local projections is used to compute the higher order derivatives that arise in the stabilization terms, specifically derivatives of the stress tensor. Linearization of the weak form is derived to enable a Newton–Raphson solution procedure of the resulting non-linear equations. Numerical experiments using the stabilization method with equal order shape functions for the displacement and pressure fields in hyperelastic problems show that the stabilized method is effective for some non-linear finite deformation problems. Finally, conclusions are inferred and extensions of this work are discussed.     

A finite element formulation for transient incompressible viscous flows stabilized by local time-steps

Stabilized finite element formulations are well suited for convection dominated flows and for the solution of the incompressible Navier–Stokes equations in primitive variables. In this paper, we present a method where the structure of stabilization terms appear naturally from a least-squares minimization of the time-discretized momentum balance. Local time-steps, chosen according to the time-scales of convection–diffusion of momentum, play the role of stabilization parameters. Numerical solutions of incompressible viscous flows demonstrate the usefulness of the proposed stabilized formulation.     

A subgrid stabilization finite element method for incompressible magnetohydrodynamics

This paper studies a numerical scheme for approximating solutions of incompressible magnetohydrodynamic (MHD) equations that uses eddy viscosity stabilization only on the small scales of the fluid flow. This stabilization scheme for MHD equations uses a Galerkin finite element spatial discretization with Scott-Vogelius mixed finite elements and semi-implicit backward Euler temporal discretization. We prove its unconditional stability and prove how the coarse mesh can be chosen so that optimal convergence can be achieved. We also provide numerical experiments to confirm the theory and demonstrate the effectiveness of the scheme on a test problem for MHD channel flow.     

A mixed finite element method with exactly divergence-free velocities for incompressible magnetohydrodynamics

We introduce and analyze a mixed finite element method for the numerical discretization of a stationary incompressible magnetohydrodynamics problem, in two and three dimensions. The velocity field is discretized using divergence-conforming Brezzi–Douglas–Marini (BDM) elements and the magnetic field is approximated by curl-conforming Nédélec elements. The H¹-continuity of the velocity field is enforced by a DG approach. A central feature of the method is that it produces exactly divergence-free velocity approximations, and captures the strongest magnetic singularities. We prove that the energy norm error is convergent in the mesh size in general Lipschitz polyhedra under minimal regularity assumptions, and derive nearly optimal a priori error estimates for the two-dimensional case. We present a comprehensive set of numerical experiments, which indicate optimal convergence of the proposed method for two-dimensional as well as three-dimensional problems.     

A stabilized mixed finite element method for Darcy flow

We develop new stabilized mixed finite element methods for Darcy flow. Stability and an a priori error estimate in the “stability norm” are established. A wide variety of convergent finite elements present themselves, unlike the classical Galerkin formulation which requires highly specialized elements. An interesting feature of the formulation is that there are no mesh-dependent parameters. Numerical tests confirm the theoretical results.     

An anisotropic pressure-stabilized finite element method for incompressible flow problems

We consider a pressure-stabilized, finite element approximation of incompressible flow problems in primitive velocity–pressure variables, which is based on a projection of the gradient of the discrete pressure onto the space of discrete functions. Equal order interpolation for the velocity and the pressure can be employed with this formulation. The method introduced here is specially developed to be used on anisotropic finite element meshes with large element aspect ratios.     

Theory and implementation of a semi-implicit finite element method for viscous incompressible flow

A numerical procedure for solving the time-dependent, incompressible Navier-Stokes equations is derived based on the operator-splitting technique. This operator split allows separate operations on each of the variable fields to enable pressure-velocity coupling. Discretizations of the equations are formed on a nonstaggered finite element mesh and the solutions are obtained in a time-marching fashion. Several benchmark problems, including a standing vortex problem, a lid-driven cavity and a flow around a rectangular cylinder, are studied to demonstrate the robustness and accuracy of the present algorithm.     

A stabilized finite element method for modeling of gas discharges

An efficient stabilized finite element method for modeling of gas discharge plasmas is represented which provides wiggle-free solutions without introducing much artificial diffusion. The stabilization is achieved by modifying the standard Galerkin test functions by means of a weighted quadratic term that results in a consistent Petrov-Galerkin formulation of the charge carriers in the plasma. Using the example of a glow discharge plasma in argon, it is shown that this efficient method provides more accurate results on the same spatial grid than the widely used finite difference approach proposed by Scharfetter-Gummel if the weighting factor is determined in dependence on the local Péclet number and the modified test functions are consistently applied to all terms of the governing equations.     

The analysis of incompressible hyperelastic bodies by the finite element method

A method is developed for the finite element analysis of problems involving incompressible hyperelastic bodies; the constitutive relation is based on a class of strain-energy functions due to Ogden [4], which involve sums of real powers of principal stretches. Incremental equilibrium equations are derived from a rate form of the principle of virtual work and an additional set of equations which express the condition of incompressibility in an average manner, is appended to the equilibrium equations. Examples of solutions are given and compared either with closed-form solutions or with numerical solutions found using conventional approaches.     

A finite element method for plane strain deformations of incompressible solids

A finite element method for incompressible deformation is formulated from the virtual work equation based on deviatoric quantities. Particular emphasis is given to nonlinear material behavior. The incompressibility constraint is imposed on the admissible displacement field by direct elimination of nodal displacements. The resulting stiffness matrix is symmetric and positive definite. Once a convergent solution for the displacement is obtained, the hydrostatic stress is determined from the principle of virtual work. A variety of illustrative examples are presented. The efficiency, economy and limitations of the method are discussed.     

Direct numerical simulation of turbulent channel flows using a stabilized finite element method

Direct numerical simulations (DNS) of incompressible turbulent channel flows at Re_τ = 180 and 395 (i.e., Reynolds number, based on the friction velocity and channel half-width) were performed using a stabilized finite element method (FEM). These simulations have been motivated by the fact that the use of stabilized finite element methods for DNS and LES is fairly recent and thus the question of how accurately these methods capture the wide range of scales in a turbulent flow remains open. To help address this question, we present converged results of turbulent channel flows under statistical equilibrium in terms of mean velocity, mean shear stresses, root mean square velocity fluctuations, autocorrelation coefficients, one-dimensional energy spectra and balances of the transport equation for turbulent kinetic energy. These results are consistent with previously published DNS results based on a pseudo-spectral method, thereby demonstrating the accuracy of the stabilized FEM for turbulence simulations.     

Symbolic integration of multibody system dynamics with the finite element method

With the widespread use of computer-aided engineering (CAE) to solve computational mechanics problems, engineering design has become more accurate and efficient. The integration of the finite element method (FEM) and flexible multibody dynamics (FMD) is a typical application of computational mechanics. It constitutes an important contribution to engineering development, but its potential is restrained by numerical computation. Computational time is a critical factor that influences the efficiency and cost of design and analysis. The advent of symbolic computation enables faster simulation code, but the symbolic integration of FEM and FMD is at the initial stages. A general symbolic integration procedure is presented in this paper. The performance of the symbolic model is compared with models from the literature and numerically-based commercial software.     

Computation of incompressible thermal flows using Hermite finite elements

Using Hermit basis functions with the finite element method offers a remarkably simple way to compute non-isothermal buoyancy-driven incompressible flow. The Hermite bases we use simplify the governing equation and strongly enforce the continuity equation. For this problem, we use a fourth-order C¹ stream function defined on rectangles here, but other higher and lower-order Hermite elements on rectangles and triangles can easily be derived or modified from elements found in the plate-bending literature. Hermite elements are also used for the temperature and pressure. We conclude with results from application of the method to the square thermal cavity at moderate to high Rayleigh numbers.     

二维不可压缩Navier-Stokes方程的并行谱有限元法求解

针对不可压缩Navier-Stokes （N-S）方程求解过程中的有限元法存在计算网格量大、收敛速度慢的缺点,提出了基于面积坐标的三角网格剖分谱有限元法（TSFEM）并进一步给出了利用OpenMP对其并行化的方法。该算法结合谱方法和有限元法思想,选取具有无限光滑特性的指数函数取代传统有限元法中的多项式函数作为基函数,能够有效减少计算网格数量,提高算法的精度和收敛速度;利用面积坐标便于三角形单元计算的特点,选取三角单元作为计算单元,增强了适用性;在顶盖方腔驱动流问题上对该算法进行验证。实验结果表明,TSFEM较传统有限元法（FEM）无论是收敛速度还是计算效率都有了显著提高。