A DOMAIN DECOMPOSITION ALGORITHM WITH FINITE ELEMENT-BOUNDARY ELEMENT COUPLING

A domain decomposition algorithm coupling the finite element and the boundary element was presented. It essentially involves subdivision of the analyzed domain into sub-regions being independently modeled by two methods, i.e., the finite element method （FEM） and the boundary element method （BEM）. The original problem was restored with continuity and equilibrium conditions being satisfied on the interface of the two sub-regions using an iterative algorithm. To speed up the convergence rate of the iterative algorithm, a dynamically changing relaxation parameter during iteration was introduced. An advantage of the proposed algorithm is that the locations of the nodes on the interface of the two sub-domains can be inconsistent. The validity of the algorithm is demonstrated by the consistence of the results of a numerical example obtained by the proposed method and those by the FEM, the BEM and a present finite element-boundary element （FE-BE） coupling method.

THE COUPLING OF BOUNDARY ELEMENT AND FINITE ELEMENT METHODS FOR THE EXTERIOR NONSTATIONARY NAVIER-STOKES EQUATIONS

In this paper, we represent a new numerical method for solving the nonstationary Stokes equations in an unbounded domain. The technique consists in coupling the boundary integral and finite element methods. The variational formulation and well posedness of the coupling method are obtained. The convergence and optimal estimates for the approximation solution are provided.

THE COUPLING METHOD OF FINITE ELEMENTS AND BOUNDARY ELEMENTS FOR RADIATION PROBLEM

The paper presents the variational formulation and well posedness of the coupling method offinite elements and boundary elements for radiation problem. The convergence and optimal errorestimate for the approximate solution and numerical experiment are provided.

COUPLING OF FINITE ELEMENT AND BOUNDARY ELEMENT METHODS FOR THE SCATTERING BY PERIODIC CHIRAL STRUCTURES

Consider a time-harmonic electromagnetic plane wave incident on a biperiodic structure in R^3. The periodic structure separates two homogeneous regions. The medium inside the structure is chiral and nonhomogeneous. In this paper, variational formulations coupling finite element methods in the chiral medium with a method of integral equations on the periodic interfaces are studied. The well-posedness of the continuous and discretized problems is established. Uniform convergence for the coupling variational approximations of the model problem is obtained.

MULTIGRID ALGORITHM FOR THE COUPLING SYSTEM OF NATURAL BOUNDARY ELEMENT METHOD AND FINITE ELEMENT METHOD FOR UNBOUNDED DOMAIN PROBLEMS

In this paper, some V-cycle multigrid algorithms are presented for the coupling system arising from the discretization of the Dirichlet exterior problem by coupling the natural boundary element method and finite element method. The convergence of these multigrid algorithms is obtained even with only one smoothing on all levels. The rate of convergence is found uniformly bounded independent of the number of levels and the mesh sizes of all levels, which indicates that these multigrid algorithms are optimal. Some numerical results are also reported.

THE COUPLING METHOD FOR TORSION PROBLEM OF THE SQUARE CROSS-SECTION BAR WITH CRACKS

By coupling natural boundary element method (NBEM) with FEM based on domain decomposition, the torsion problem of the square cross-sections bar with cracks have been studied, the stresses of the nodes of the cross-sections and the stress intensity factors have been calculated, and some distribution pictures of the stresses have been drawn. During computing, the effect of the relaxed factors to the convergence speed of the iterative method has been discussed. The results of the computation have confirmed the advantages of the NBEM and its coupling with the FEM.

Time-domain Finite Element Method for Transient Electric Field and Transient Charge Density on Dielectric Interface

This paper is devoted to solving the transient electric field and transient charge density on the dielectric interface under the electroquasistatic(EQS)field conditions with high accuracy.The proposed method is suitable for both 2-D and 3-D applications.Firstly,the governing equations represented by scalar electric potential are discretized by the nodal finite element method(FEM)in space and the finite difference method in time.Secondly,the transient constrained electric field equation on the boundary(TCEFEB)is derived to calculate the normal component of the transient electric field intensities on the Dirichlet boundary and dielectric interface as well as the transient charge density on the dielectric interface.Finally,a 2-D numerical example is employed to demonstrate the validity of the proposed method.Furthermore,the comparisons of the numerical accuracy of the proposed method in this paper with the existing FEMs for electric field intensity and charge density on the dielectric interface are conducted.The results show that the numerical accuracy of the proposed method for calculating the normal component of transient electric field intensities on the Dirichlet boundary and dielectric interface as well as the transient charge density on the dielectric interface is close to that of nodal electric potential and an order of magnitude higher than those of existing FEMs.

Application of semi-analytical finite element method coupled with infinite element for analysis of asphalt pavement structural response

A specific computational program SAFEM was developed based on semi-analytical finite element （FE） method for analysis of asphalt pavement structural responses under static loads. The reliability and efficiency of this FE program was proved by comparison with the general commercial FE software ABAQUS. In order to further reduce the computational time without decrease of the accuracy, the infinite element was added to this program. The results of the finite-infinite element coupling analysis were compared with those of finite element analysis derived from the verified FE program, The study shows that finite-infinite element coupling analysis has higher reliability and efficiency.

Methodology for Comparing Coupling Algorithms for Fluid-Structure Interaction Problems

The multi-physics simulation of coupled fluid-structure interaction problems, with disjoint fluid and solid domains, requires one to choose a method for enforcing the fluid-structure coupling at the interface between solid and fluid. While it is common knowledge that the choice of coupling technique can be very problem dependent, there exists no satisfactory coupling comparison methodology that allows for conclusions to be drawn with respect to the comparison of computational cost and solution accuracy for a given scenario. In this work, we develop a computational framework where all aspects of the computation can be held constant, save for the method in which the coupled nature of the fluid-structure equations is enforced. To enable a fair comparison of coupling methods, all simulations presented in this work are implemented within a single numerical framework within the deal.ii [1] finite element library. We have chosen the two-dimensional benchmark test problem of Turek and Hron [2] as an example to examine the relative accuracy of the coupling methods studied;however, the comparison technique is equally applicable to more complex problems. We show that for the specific case considered herein the monolithic approach outperforms partitioned and quasi-direct methods;however, this result is problem dependent and we discuss computational and modeling aspects which may affect other comparison studies.

Improved Simulation Method for Soil-Geogrid Interaction of Reinforced Earth Structure in FEM

The interaction between geogrid and soil is crucial for the stability of geogrid-reinforced earth structure. In finite element (FE) analysis, geogrids are usually assumed as beam or truss elements, and the interaction between geogrid and soil is considered as Coulomb friction resistance, which cannot reflect the true stress and displacement developed in the reinforcement. And the traditional Lagrangian elements used to simulate soil always become highly distorted and lose accuracy in high-stress blocks. An improved geogrid model that can produce shear resistance and passive resistance and a soil model using the Eulerian technique, in combination with the coupled Eulerian-Lagrangian (CEL) method, are used to analyze the interaction between geogrid and soil of reinforced foundation test in ABAQUS. The stress in the backfill, resistance of geogrid, and settlement of foundation were computed and the results of analysis agree well with the experimental results. This simulation method is of referential value for FE analysis of reinforced earth structure.

Coupled BEM-FEM Analysis of the Large-Diameter Cylinder Structure System

A method of coupled BEM-FEM analysis for the elastic spatial structure system is presented. It can be applied to the calculation of the stress and deformation of the large-diamater cylinder structure system and it is suitable for symmetric or non-symmetric structures under the distributed or concentrated load. Numerical examples show that the proposed method and computer program BEFEM are quite efficient in the analysis of the large-diameter cylinder structure problems in ocean engineering.

DYNAMIC RESPONSE OF UNDERGROUND STRUCTURES BY TIME DOMAIN SBEM AND SFEM

The dynamic interaction problems of three-dimensional lineqr elastic structures with arbitrary shaped section embedded in a homogeneous, isotropic and linear elastic half space under dynamic disturbances are numerically solved. The numerical method employed is a combination of the time domain semi-analytical boundary element method (SBEM) used for the semi-infinite soil medium and the semi-analytical finite element method (SFEM) used for the three-dimensional structure. The two methods are combined through equilibrium and compatibility conditions at the soil-structure interface. Displacements, velocities, accelerations and interaction forces at the interface between underground structure and soil medium produced by the diffraction of wave by an underground structure for every time step are obtained. In dynamic soil-structure interaction problems, it is advantageous to combine the SBEM and the SFEM in an effort to produce an optimum numerical hybrid scheme which is characterized by the main advantages of the two methods. The effects of the thickness, the ratio of length and diameter of underground structure and the soil medium on dynamic responses are discussed.

二尖瓣运动的流固耦合仿真

目的 建立包含左心和血液的二尖瓣理想模型,用流固耦合仿真研究二尖瓣在血流中的运动特性。方法 基于解剖学参数建立二尖瓣、左心和血液模型,流固耦合仿真采用有限元结合浸没边界法,使用有限元软件LS-DYNA模拟二尖瓣运动,获取形态学、力学和血液动力学参数,并与结构仿真结果进行对比。结果 两种仿真下二尖瓣形态学结果差异较大,流固耦合结果与超声影像吻合。流固耦合仿真和结构仿真的瓣叶应力分布结果一致,最大第一主应力分别为1.48、1.53 MPa,相对误差为3.27%。左心流场有较为复杂的涡旋结构,舒张期二尖瓣最大流速为1.02 m/s,与健康人体生理数据(0.89±0.15) m/s相吻合。结论 二尖瓣流固耦合仿真可以获取更贴近于生理的形态学结果;流固耦合仿真可以提供临床诊断不可或缺的流场参数信息;单研究瓣叶应力分布问题时,结构仿真更高效。

建立基于浸入边界有限元法的三维静脉瓣流固耦合数值模型

目的 探究静脉内血液与瓣膜之间的流固耦合动态过程和保证血液单向回流的生理机制。方法 基于浸入边界有限元法,结合人体下肢静脉医学图像及牛大隐静脉的解剖结构和尺寸,采用超弹性本构模型描述静脉瓣膜在生理条件下不可压缩、非线性和超弹性力学响应,构建静脉血管和静脉瓣膜的三维数值模型。结果 研究结果可视化地展示静脉动态运输血液的过程以及静脉瓣膜防止反流的功能机制,再现静脉内瓣膜运动和血液流动的周期性特点,讨论和量化了整个心动周期内的重要生理数据,包括静脉内血液的压力、流速和流量以及静脉瓣膜的开口面积、静脉瓣膜表面的应力和应变分布等。结论 三维流固耦合模型可数值再现静脉内生理动态过程,为进一步揭示静脉疾病相关机制提供重要的参考和指导意义。

海流作用下坠物沉降及撞击海底管道数值模拟研究

采用基于浸入边界有限元法的流固耦合数值模拟方法,对海流作用下坠物撞击海底管道完整过程进行了数值模拟研究。数值结果表明:在自由沉降时,坠物的流向速度与海流速度基本一致,不同海流速度下相同质量的坠物垂向速度基本一致,但在较低的坠物质量和海流速度下会出现冲跃现象,球形坠物的垂向阻力系数比立方形坠物小,并且阻力系数随坠物质量的变化而变化,坠物沉降角度随海流速度的增加而增大;在坠物接近海底管道时,流向速度逐渐减小,垂向速度均为先增大后减小,立方形坠物减小程度要比球形坠物大,质量大的坠物垂向速度减小的程度也要比质量小的大;在坠物撞击管道时,球形坠物撞击速度比立方形大,撞击速度比自由沉降速度大,撞击角度比自由沉降角度小,最大应力和凹痕均比无海流作用时小,球形坠物应力减小幅度为4.44%~16.71%,立方形坠物应力减小幅度为9.29%~12.14%,球形坠物凹痕减小幅度为4.17%~30.58%,立方形坠物凹痕减小幅度为10.41%~14.44%。

Numerical Simulation on the Resistance Performance of Ice-Going Container Ship Under Brash Ice Conditions

Ice resistance prediction is a critical issue in the preliminary design of ships navigating brash ice conditions, which is closely related to the safety of a ship to navigate encounter brash ice, and has significant effects on the kinds of propellers and motor power needed. In research on this topic, model tests and full-scale tests on ships have thus far been the primary approaches. In recent years, the application of the finite element method(FEM) has also attracted interest. Some researchers have conducted numerical simulations on ship–ice interactions using the fluid–structure interaction(FSI) method. This study used this method to predict and analyze the resistance of an ice-going ship, and compared the results with those of model ship tests conducted in a towing tank with synthetic ice to discuss the feasibility of the FEM. A numerical simulation and experimental methods were used to predict the brash ice resistance of an ice-going container ship model in a condition with three concentrations of brash ice(60%, 80%, and 90%). A comparison of the results yielded satisfactory agreement between the numerical simulation and the experiments in terms of both observed phenomena and resistance values, indicating that the proposed numerical simulation has significant potential for use in related studies in the future.

移动荷载作用下层状横观各向同性饱和土中排桩的隔振效应

利用有限元-边界元耦合法评估了移动荷载下层状横观各向同性饱和土体中排桩的隔振效果。利用有限元法将排桩离散成单桩以及桩单元,基于Bernoulli-Euler梁理论得到桩的有限元矩阵方程;在桩-土边界,土体单元与桩单元进行了等节点离散,并以层状饱和地基的解析层元基本解作为核函数,利用边界积分法得到桩-土界面处地基的柔度矩阵;基于两阶段理论,将侧摩阻力响应与移动荷载直接引起的振动进行耦合,结合边界元法得到饱和地基的边界元方程;考虑Bernoulli-Euler梁和土体之间不发生相对滑移和脱开的位移协调条件,耦合有限元和边界元方程,得到排桩的动力响应方程;计算有无排桩隔振下某一观测点的位移,即可结合隔振理论分析排桩的隔振效率。在验证所提方法准确性的基础上,分析了桩长、桩身刚度、移动荷载速度以及横观各向同性参数对排桩隔振效应的影响。结果表明:最优桩长约等于2倍瑞利波长,超过该值隔振效果提高不大;桩与地基的刚度差越大,隔振效果越好;荷载速度超过剪切波速,桩基隔振表现反而更好。

Transient Hydroelastic Response of VLFS by FEM with Impedance Boundary Conditions in Time Domain

A time-dependent finite element method (FEM) is developed to analyze the transient hydroelastic responses of very large floating structures (VLFS) subjected to dynamic loads. The hydrodynamic problem is formulated based on the linear theory of fluid and the structural response is analyzed based on the thin plate theory. The FEM truncates the unbounded fluid domain by introducing an artificial boundary surface, thus defining a finite computational domain. At this boundary surface an impedance boundary conditions are applied so that no wave reflections occur. In the proposed scheme, all of the procedures are processed directly in time domain, which is efficient for nonlinear analyses of structure floating on unbounded fluid. Numerical results indicate acceptable accuracy of the proposed method.

某小口径火炮弹丸挤进过程动力学特性分析

为研究某小口径火炮不同坡膛结构下弹丸挤进过程的动力学特性,建立了考虑挤进阻力变化的内弹道方程组并通过vuamp子程序引入到有限元求解中,实现了挤进过程中内弹道计算与弹丸挤进有限元仿真耦合求解。通过对弹丸全膛运动的仿真分析验证了模型的可行性,随后得到并分析不同坡膛锥度、膛线深度及阴线宽度下挤进过程中弹丸的速度、加速度、挤进阻力和膛压曲线及变化影响规律。结果表明,坡膛锥度对挤进过程中弹丸的动力学参数影响不是单调的,而膛线深度及阴线宽度的改变则单调地影响弹丸动力学参数,对优化坡膛结构及弹带设计具有一定的指导意义。

Simulation of the Whole Process of Ship-Bridge Collision

The simulation of the whole ship-bridge collision process can be effectively carried out by nonlinear dynamic finite element method. Based on the simple description of the theory, a scenario of a 40000 DWT oil tanker colliding with a bridge across the Yangtze River is designed for simulation. The technology of structure modeling and the determination of related parameters are introduced. The deformation of the bulb bow, the history,of collision force change, the exchange of collision energy and the stress distribution. of the bridge pier-are described in detail, which: are of great value to bridge design and bridge pier damage estimation. mechanical characters in the process of ship-bridge collision are described. More accurate results can be produced by finite element method than that by empirical formulas and simplified analytical methods.