On the Incorporation of Residual Stresses in Arterial Walls

The present work deals with the incorporation of residual stresses existing in circumferential direction of arterial walls. For the consideration of the residual stresses a novel model will be presented. This model is based on the assumption that residual stresses decrease the stress gradients through the thickness of the arterial wall. Since arterial walls exhibit a pronounced material behavior in fiber direction, the radial gradients of the fiber stresses are considered for the definition of the residual stresses. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Accurate Computation of Traffic Workload Distributions

In traffic modeling, the computation of workload distributions plays a prominent role, since they influence the quality of service parameters. This paper deals with different ways to determine the workload distributions of a GI/GI/1 service system accurately. We discuss two different techniques, the polynomial factorization approach and the Wiener–Hopf factorization. These methods factorize the server system's characteristic polynomial to compute the workload. Starting with classical algorithms to obtain initial approximations, we enhance them using verification techniques to guarantee the correctness of the results.     

Modeling of Anisotropic Damage in Arterial Walls

In a certain range of overexpansion arterial walls are characterized by an orthotropic elastic material behavior. Due to different stabilities of the helically arranged fibers, i.e. breakage of collagen crosslinks between the fibers, damage effects are observed in experiments. Because of the fibrous composition it is assumed that damage mainly occurs in the fiber direction. The proposed damage model is extended to arterial wall applications by introducing a referential damage state. The damage approach is applied to a polyconvex model for the hyperelastic behavior of arteries in order to obtain a materially stable model, which guarantees the existence of solutions of the underlying boundary value problem. The performance of the proposed model is presented in a numerical example, where the overexpansion of an atherosclerotic artery is simulated. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical Simulations of Arterial Walls Based on IVUS-Data

Atherosclerosis has become one of the most frequent causes of death in the last decades. So the analysis and improvement of corresponding medical treatments, e.g., balloon-angioplasty or stenting, are significant tasks in the fields of biomechanics and biomedical engineering. One important component of the simulation of arterial walls, especially of atherosclerotically degenerated ones, is the knowledge of their anatomical and physiological composition. The intravascular ultrasound and the interpretation by the Virtual Histology provide cross-sectional images of the diseased arterial walls where the different components are characterized by color-coding. The topic of this contribution is the 3D reconstruction and discretization of the artery using these color-coded images and the biplane angiography. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical Calculation of Fiber Orientation in Three-Dimensional Arterial Walls

In this contribution an approach for the fiber reorientation in three-dimensional arterial walls is presented. In detail the load-bearing capacity of the tissue is increased by re orienting the fibers with respect to the principal stresses, cf. [1]. The improved fiber reorientation algorithm is combined with the polyconvex nonlinear anisotropic material model presented in [3]. The results of a three-dimensional finite element simulation, where the reorientation approach is applied to a short segment of a patient-specific arterial geometry, are presented. (© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Iso-surface Computation in 3D using a graph-theoretical Approach

The existing combinatorial methods for iso-surface computation are efficient for pure visualization purposes, but it is known that the resulting iso-surfaces can have holes, and topological problems like missing or wrong connectivity can appear. To avoid such problems, we introduce a graph-theoretical method for the computation of iso-surfaces on cuboid meshes in ℝ³. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

三维爆炸与冲击问题的大规模高性能计算研究

爆炸与冲击问题常常涉及到材料的大变形、多种物质交界面处理及各种强间断的处理,对此类问题采用Euler方法具有显著的优势．对于Euler方法的并行计算,由于在扰动未到达之前某些网格的物理量是不发生变化的,采用固定分区方法并行策略容易出现负载不均衡,且计算效率不高．为解决这一问题,采用动态并行策略,即计算域随着扰动传播而扩充,并采用国际上通用的MPI并行模式设计出相应的动态并行程序．从程序测试结果来看,动态并行程序结果与原有并行程序结果一致,且与实际情况也是一致的．     

三角翼大迎角流场结构和气动力特性的计算分析研究

采用数值计算方法对亚音速三角翼纵向及带有小侧滑和横侧小扰动情况下的流场结构进行了计算,利用数值计算所得到的大迎角流动流场数据,结合相关的实验研究结果,建立了对大迎角旋涡流场结构进行定量分析的方法．给出了三角翼大迎角情况下相应的气动力、力矩系数,以及机翼前缘分离涡轴线位置和旋涡破裂位置随迎角的变化规律,并对带有小侧滑和横侧小扰动情况下对横侧力矩的影响进行了计算与分析．计算结果表明,在前缘分离涡破裂前的上游旋涡区内,前缘分离涡轴线基本保持为直线,且随着迎角增加,前缘分离涡轴线位置愈靠近翼根,并远离翼面;在前缘分离涡破裂的初始阶段,于旋涡轴线处,压力系数会迅速增加,沿涡轴线方向速度迅速减小,在垂直于流向的截面内,愈靠近涡轴线处,沿涡轴线方向速度愈小,甚至出现负值,说明沿涡轴线方向出现回流．当绕机翼上表面前缘分离涡破裂后,将会导致横侧运动不稳定,如果受到小扰动,将产生横侧力矩发散．     

Fast and Robust Sixth Order Multigrid Computation for 3D Convection Diffusion Equation

We present a sixth-order explicit compact finite difference scheme to solve the three-dimensional (3D) convection-diffusion equation. We first use a multiscale multigrid method to solve the linear systems arising from a 19-point fourth-order discretization scheme to compute the fourth-order solutions on both a coarse grid and a fine grid. Then an operator-based interpolation scheme combined with an extrapolation technique is used to approximate the sixth-order accurate solution on the fine grid. Since the multigrid method using a standard point relaxation smoother may fail to achieve the optimal grid-independent convergence rate for solving convection-diffusion equations with a high Reynolds number, we implement the plane relaxation smoother in the multigrid solver to achieve better grid independency. Supporting numerical results are presented to demonstrate the efficiency and accuracy of the sixth-order compact (SOC) scheme, compared with the previously published fourth-order compact (FOC) scheme.     

On the Reconstruction and Computation of Dual-Phase Steel Microstructures Based on 3D EBSD Data

In this contribution we reconstruct the three dimensional microstructure of a dual-phase steel based on the tomographic experimental data provided by the 3D electron backscatter diffraction (3D EBSD) method. The cross sections of the resulting microstructure are compared to the 2D reconstruction, which are obtained directly from the 3D EBSD data. We also perform FE-simulations based on these geometries and observe comparable results in 2D and 3D. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Uniqueness and Stability of Determining the Residual Stress by One Measurement

In this paper we prove a Hölder and Lipschitz stability estimates of determining the residual stress by a single pair of observations from a part of the lateral boundary or from the whole boundary. These estimates imply first uniqueness results for determination of residual stress from few boundary measurements.     

D6等变非线性分歧问题的计算

本文研究D6对称群的分歧子群的分类,由此得到相应对称破缺分歧的计算方法,并应用到D6等变的Brusselator反应问题。     

焊接过程瞬态温度场与残余应力场

本文根据焊接热传导及热弹塑性的研究现状,对如下两个问题进行了探讨.首先,采用非线性问题线性化的方法对非线性非定常温度场问题的边界元法做了改进,并将其用于焊接热传导分析:其次,提出“等放线膨胀系数法”考虑相变对于应力场的影响,并将其用于焊接热弹塑性分析.实例的数值计算结果与实测数据的对比分析表明本文方法行之有效.     

Residual a posteriori error estimates for a 3D mortar finite-element method: the Stokes system

We consider a non-conforming domain decomposition techniquefor the discretization of the three-dimensional Stokes equationsby the mortar finite-element method. Relying on the velocity–pressureformulation of the system, we perform the numerical analysisof residual error indicators for this problem and we prove thatthe error estimators provide upper and lower bounds for theenergy norm of the mortar finite-element solution.     

球壳中球形夹杂对SH波的三维散射与动应力集中

研究了一般情况下球壳中球形夹杂(包括孔洞)引起的SH波三维散射与动应力集中现象.根据球壳与夹杂的几何特点,分别以球壳和夹杂中心建立球坐标,用于描述球壳中的入射波、散射波和夹杂中的驻波势函数,并采用球波函数的加法公式,实现了不同坐标下球波函数的变换,推导出位移、应力分量的解析解.结合球壳的边界条件和夹杂界面的连续条件,求解了不同材料属性夹杂,以及空洞情况下弹性波的散射和动应力集中因子分布情况,并分析了频率以及夹杂中心位置对动应力集中因子的影响.文中的研究为球壳结构的力学性能分析以及无损检测提供了理论支持.     

双材料界面裂纹平面问题的半权函数法

应用半权函数法求解双材料界面裂纹的平面问题．由平衡方程、应力应变关系、界面的连续条件以及裂纹面零应力条件推导出裂尖的位移和应力场,其特征值为lambda及其共轭．设置特征值为lambda的虚拟位移和应力场,即界面裂纹的半权函数A·D2由功的互等定理得到应力强度因子KⅠ和KⅡ以半权函数与绕裂尖围道上参考位移和应力积分关系的表达式．数值算例体现了半权函数法精度可靠、计算简便的特点．     

Effect of the Dimensions of Mines and Angles of Inclination of Beds on the Stress Distribution in Mountain Masses

The effect of the angle of inclination of a bed and the geometric dimensions of a mine with unloading crevices on the stressed state of a massif is studied numerically. A mine with a vaulted cross section and different angles of inclination of the sides to the base plane is considered.