The Local Artificial Boundary Conditions for Numerical Simulations of the Flow Around a Submerged Body

We consider in this work the numerical approximations of the two-dimensional steady potential flow around a body moving in a liquid of finite constant depth at constant speed and distance below a free surface. Several vertical segments are introduced as the upstream and the downstream artificial boundaries, where a sequence of high-order local artificial boundary conditions are proposed. Then the original problem is solved in a finite computational domain, which is equivalent to a variational problem. The numerical approximations for the original problem are obtained by solving the variational problem with the finite element method. The numerical examples show that the artificial boundary conditions given in this work are very effective.     

Boundary Conditions and Estimates for the Steady Stokes Equations on Staggered Grids

We consider the steady state Stokes equations, describing low speed flow and derive estimates of the solution for various types of boundary conditions. We formulate the boundary conditions in a new way, such that the boundary value problem becomes non-singular. By using a difference approximation on a staggered grid we are able to derive a non-singular approximation in a direct way. Furthermore, we derive the same type of estimates as for the continuous case. Numerical experiments confirm the theoretical results.     

感应电动机传动系统建模及仿真的一般方法

感应电动机传动系统是目前应用最广泛的交流传动系统,对其性能的计算机模拟和仿真具有重要的实际指导意义。结合MATLAB语言和环境,介绍在任息速参考坐标系下,基于感应电动机传动系统不同的控制策略和运行状态,导出其由状态变量方程描述的统一时域建模方法,详述仿真工具和方法的选择、逆变器的处理、及其完整的流程。对三相异步电动机的直接转矩控制（DTC）系统的仿真结果表明,该文所提出的仿真方法是有效和可行的。     

Boundary value methods: The third way between linear multistep and Runge-Kutta methods

It is well known that the approximation of the solutions of ODEs by means of k-step methods transforms a first-order continuous problem in a k^th-order discrete one. Such transformation has the undesired effect of introducing spurious, or parasitic, solutions to be kept under control. It is such control which is responsible of the main drawbacks (e.g., the two Dahlquist barriers) of the classical LMF with respect to Runge-Kutta methods. It is, however, less known that the control of the parasitic solutions is much easier if the problem is transformed into an almost equivalent boundary value problem. Starting from such an idea, a new class of multistep methods, called Boundary Value Methods (BVMs), has been proposed and analyzed in the last few years. Of course, they are free of barriers. Moreover, a block version of such methods presents some similarity with Runge-Kutta schemes, although still maintaining the advantages of being linear methods. In this paper, the recent results on the subject are reviewed.     

SPH中一种层流入口、出口边界的处理方法

提出一种替代周期边界条件的对入口、出口边界的处理方法,这种方法使用跟随虚粒子处理入口、出口边界。跟随虚粒子设置于入口、出口的外侧,这些虚粒子的速度、位置根据对应的内部粒子的速度、位置进行更新。该方法建立在对层流特性的分析的基础上,适用于具有层流入口或出口的低雷诺数流场中。利用该入、出口边界处理新方法,分别对Poiseuille流和渐扩管平面流进行数值摸拟,数值结果与理论解吻合良好。     

基于格子Boltzmann方法和大涡模拟的颈动脉分叉狭窄流动并行计算

颈动脉斑块的形成与复杂的血流动力学因素密切相关,血液流动状况的精确模拟对颈动脉斑块的临床诊断具有重要意义。为了精确模拟脉动流场,在格子Boltzmann方法（LBM）的基础上,添加大涡模拟（LES）模型,建立了LBM-LES颈动脉模拟算法。利用医学图像重构软件,建立颈动脉狭窄真实几何模型,对颈动脉狭窄脉动流动进行了数值模拟,通过计算血液流动速度、壁面剪切应力（WSS）等,得出了有意义的流动结果,验证了LBM-LES对颈动脉狭窄后段血液流动研究的有效性。基于OpenMP编程环境,在高性能集群机全互联胖节点上进行了千万量级网格的并行计算,结果表明LBM-LES颈动脉模拟算法具有较好的并行性能。     

On Parabolic Boundary Layers for Convection–Diffusion Equations in a Channel: Analysis and Numerical Applications

In this article we discuss singularly perturbed convection–diffusion equations in a channel in cases producing parabolic boundary layers. It has been shown that one can improve the numerical resolution of singularly perturbed problems involving boundary layers, by incorporating the structure of the boundary layers into the finite element spaces, when this structure is available; see e.g. [Cheng, W. and Temam, R. (2002). Comput. Fluid. V.31, 453–466; Jung, C. (2005). Numer. Meth. Partial Differ. Eq. V.21, 623–648]. This approach is developed in this article for a convection–diffusion equation. Using an analytical approach, we first derive an approximate (simplified) form of the parabolic boundary layers (elements) for our problem; we then develop new numerical schemes using these boundary layer elements. The results are performed for the perturbation parameter ε in the range 10⁻¹–10⁻¹⁵ whereas the discretization mesh is in the range of order 1/10–1/100 in the x-direction and of order 1/10–1/30 in the y-direction. Indications on various extensions of this work are briefly described at the end of the Introduction.Dedicated to David Gottlieb on his 60th birthday.     

Vector Fields: an Interactive Tool for Animation, Modeling and Simulation with Physically Based 3D Particle Systems and Soft Objects

Vectorfields have traditionally been used in computer graphics as a means of visualizing models over time. This paper presents a system which extends the use of vector fields as an interactive tool for physically based three dimensional particle systems and soft objects. The techniques implemented in the system provide the user with new flexibility in animation, modeling and simulation. This paper describes bounded interactive vectorfields and how they can be used to manipulate particle systems and a class of soft objects. Applications to animation, modeling and simulation are also presented.     

Fusion of Region and Boundary/Surface-Based Computer Vision and Pattern Recognition Techniques for 2-D and 3-D MR Cerebral Cortical Segmentation (Part-II): A State-of-the-Art Review

Extensive growth in functional brain imaging, perfusion-weighted imaging, diffusion-weighted imaging, brain mapping and brain scanning techniques has led tremendously to the importance of cerebral cortical segmentation both in 2-D and 3-D from volumetric brain magnetic resonance imaging data sets. Besides that, recent growth in deformable brain segmentation techniques in 2-D and 3-D has brought the engineering community, such as the areas of computer vision, image processing, pattern recognition and graphics, closer to the medical community, such as to neuro-surgeons, psychiatrists, oncologists, neuro-radiologists and internists. In Part I of this research (see Suri et al [1]), an attempt was made to review the state-of-the-art in 2-D and 3-D cerebral cortical segmentation techniques from brain magnetic resonance imaging based on two main classes: region- and boundary/surface-based. More than 18 different techniques for segmenting the cerebral cortex from brain slices acquired in orthogonal directions were shown using region-based techniques. We also showed more than ten different techniques to segment the cerebral cortex from magnetic resonance brain volumes using boundary/surface-based techniques. This paper (Part II) focuses on presenting state-of-the-art systems based on the fusion of boundary/surface-based with region-based techniques, also called regional-geometric deformation models, which takes the paradigm of partial differential equations in the level set framework. We also discuss the pros and cons of these various techniques, besides giving the mathematical foundations for each sub-class in the cortical taxonomy. Special emphasis is placed on discussing the advantages, validation, challenges and neuro-science/clinical applications of cortical segmentation. Received: 25 August 2000, Received in revised form: 28 March 2001, Accepted: 28 March 2001