High Order Finite Difference Approximations of Electromagnetic Wave Propagation Close to Material Discontinuities

Electromagnetic wave propagation close to a material discontinuity is simulated by using summation by part operators of second, fourth and sixth order accuracy. The interface conditions at the discontinuity are imposed by the simultaneous approximation term procedure. Stability is shown and the order of accuracy is verified numerically.     

A Fourth Order Accurate Finite Difference Scheme for the Elastic Wave Equation in Second Order Formulation

We present a fourth order accurate finite difference method for the elastic wave equation in second order formulation, where the fourth order accuracy holds in both space and time. The key ingredient of the method is a boundary modified fourth order accurate discretization of the second derivative with variable coefficient, (??(x)u _x ) _x . This discretization satisfies a summation by parts identity that guarantees stability of the scheme. The boundary conditions are enforced through ghost points, thereby avoiding projections or penalty terms, which often are used with previous summation by parts operators. The temporal discretization is obtained by an explicit modified equation method. Numerical examples with free surface boundary conditions show that the scheme is stable for CFL-numbers up to 1.3, and demonstrate a significant improvement in efficiency over the second order accurate method. The new discretization of (??(x)u _x ) _x has general applicability, and will enable stable fourth order accurate approximations of other partial differential equations as well as the elastic wave equation.     

A High Order Compact Time/Space Finite Difference Scheme for the Wave Equation with Variable Speed of Sound

We consider fourth order accurate compact schemes, in both space and time, for the second order wave equation with a variable speed of sound. We demonstrate that usually this is much more efficient than lower order schemes despite being implicit and only conditionally stable. Fast time marching of the implicit scheme is accomplished by iterative methods such as conjugate gradient and multigrid. For conjugate gradient, an upper bound on the convergence rate of the iterations is obtained by eigenvalue analysis of the scheme. The implicit discretization technique is such that the spatial and temporal convergence orders can be adjusted independently of each other. In special cases, the spatial error dominates the problem, and then an unconditionally stable second order accurate scheme in time with fourth order accuracy in space is more efficient. Computations confirm the design convergence rate for the inhomogeneous, variable wave speed equation and also confirm the pollution effect for these time dependent problems.     

利用有限差分法计算真实头模型脑电正问题

脑电研究领域的两个关键问题是脑电正问题和脑电逆问题,脑电正问题是脑电逆问题的基础.由于复杂、非规则真实头模型中的脑电正问题不存在解析解,因此脑电源分析依赖于正问题数值算法的精度和效率.文章首先详细推导了有限差分算法求解三维各向同性脑电正问题的数学模型,然后在三层同心球模型上通过与解析解比较验证了该算法的精度和效率,最后将该算法应用于真实头模型.仿真结果表明,有限差分法可以有效地处理任意形状几何体的电位场分布问题,是模拟计算真实头模型中脑电正问题的有力工具.     

A Comparison of Mamdani and Sugeno Fuzzy Based Packet Scheduler for MANET with a Realistic Wireless Propagation Model

 The mobile nature of the nodes in a wireless mobile ad-hoc network (MANET) and the error prone link connectivity between nodes pose many challenges. These include frequent route changes, high packet loss, etc. Such problems increase the end-toend delay and decrease the throughput. This paper proposes two adaptive priority packet scheduling algorithms for MANET based on Mamdani and Sugeno fuzzy inference system. The fuzzy systems consist of three input variables: data rate, signal-to-noise ratio (SNR) and queue size. The fuzzy decision system has been optimised to improve its efficiency. Both fuzzy systems were verified using the Matlab fuzzy toolbox and the performance of both algorithms were evaluated using the riverbed modeler (formally known as OPNET modeler). The results were compared to an existing fuzzy scheduler under various network loads, for constant-bit-rate (CBR) and variable-bit-rate (VBR) traffic. The measuring metrics which form the basis for performance evaluation are end-to-end delay, throughput and packet delivery ratio. The proposed Mamdani and Sugeno scheduler perform better than the existing scheduler for CBR traffic. The end-to-end delay for Mamdani and Sugeno scheduler was reduced by an average of 52% and 54%, respectively. The performance of the throughput and packet delivery ratio for CBR traffic are very similar to the existing scheduler because of the characteristic of the traffic. The network was also at full capacity. The proposed schedulers also showed a better performance for VBR traffic. The end-to-end delay was reduced by an average of 38% and 52%, respectively. Both the throughput and packet delivery ratio (PDR) increased by an average of 53% and 47%, respectively. The Mamdani scheduler is more computationally complex than the Sugeno scheduler, even though they both showed similar network performance. Thus, the Sugeno scheduler is more suitable for real-time applications.     

Spatially Heterogeneous Vaccine Coverage and Externalities in a Computational Model of Epidemics

Computational Economics - We develop an agent-based computational model of epidemics with varying amounts of heterogeneity in regional vaccination coverage. We examine the severity of the resulting...     

The Use of Curved Elements in the Finite Element Approximation of Thin Plates by High Order p and hp Methods

In this article we focus on the approximation of the Reissner–Mindlin (R–M) plate model by high-order p and hp versions of the Finite Element Method (FEM), in the case when the mesh includes curved elements; this is of particular interest when plates with smooth boundary are considered. Our main goal is to verify that an appropriately defined Mixed Interpolated Tensorial Components (MITC) FEM performs well, in the presence of curved elements, when the energy norm is used as an error measure, as well as when quantities of engineering interest are computed. Comments on the performance of the standard FEM formulation are also made     

基于SI1SI2R模型的异质群体重复感染的传染病建模与仿真

具有重复感染可能性的传染病由于不受抗体或疫苗等手段的阻碍,其传播范围会比普通传染病更加广泛,严重危害人类的身心健康,影响社会的和谐稳定,有必要对其传播规律进行探究,以便于更有针对性地制定应对策略。基于经典的传染病模型,本文构建考虑了传染病重复感染特征的SI1SI2R模型,定义异质性个体之间的交互及传播规则,并利用ABM仿真建模的方法探究具有重复性感染特征的传染病在异质性群体间的传播规律。仿真实验结果表明,此类传染病感染范围会随二次感染率的提升而增加,而异质性传播群体的组成不仅会影响感染人数的比例,还会影响其到达峰值的时间。因此,不仅要减小此类疾病反复感染的可能,还要根据传播群体的异质性提出更有效率的管理方案。     

高频斩波串级调速系统建模与动、静态特性分析

与传统控制方式不同,斩波串级调速在回路拓扑结构和主回路工作方式上变化较大,因此需要详细分析其调速机理和系统动、静态特性,从而为其工程设计提供理论依据。首先介绍了高频斩波串级调速系统的工作原理,利用状态空间平均法分析了主回路各参量基本输入输出关系,得到了平均状态下的主回路简化数学模型,结合串级调速下负载电流与转矩关系以及异步电动机机电运动方程,得出了高频斩波串级调速系统的简化动态模型,并在此基础给出了系统调速的开环动态结构图,分析了斩波串级调速系统的动、静态特性,为进一步分析系统闭环控制及工程设计提供了理论基础。     

移动计算环境下异构数据快速同步模型的设计与研究

针对移动计算环境下的普遍应用情况,提出"在低端加强移动性,在高端提供高性能"的创新思想,结合移动数据库容量小,断接频繁;主存数据库响应时间短,存取数据快的特点,遵循SyncML协议,搭建起一个移动客户端使用移动数据库技术,服务器端采用定制主存数据库技术的异构数据快速同步模型.     

移动计算环境下异构数据快速同步模型的设计与研究

针对移动计算环境下的普遍应用情况,提出“在低端加强移动性,在高端提供高性能”的创新思想,结合移动数据库容量小,断接频繁;主存数据库响应时间短,存取数据快的特点,遵循SyncML协议,搭建起一个移动客户端使用移动数据库技术,服务器端采用定制主存数据库技术的异构数据快速同步模型。     

移动计算环境下异构数据快速同步模型的设计与研究

针对移动计算环境下的普遍应用情况,提出“在低端加强移动性,在高端提供高性能”的创新思想,结合移动数据库容量小,断接频繁;主存数据库响应时间短,存取数据快的特点,遵循SyncML协议,搭建起一个移动客户端使用移动数据库技术,服务器端采用定制主存数据库技术的异构数据快速同步模型。