Efficient computation of mean drag for the subcritical flow past a circular cylinder using general Galerkin G2

General Galerkin (G2) is a new computational method for turbulent flow, where a stabilized Galerkin finite element method is used to compute approximate weak solutions to the Navier–Stokes equations directly, without any filtering of the equations as in a standard approach to turbulence simulation, such as large eddy simulation, and thus no Reynolds stresses are introduced, which need modelling. In this paper, G2 is used to compute the drag coefficient c_D for the flow past a circular cylinder at Reynolds number Re=3900, for which the flow is turbulent. It is found that it is possible to approximate c_D to an accuracy of a few percent, corresponding to the accuracy in experimental results for this problem, using less than 10⁵ mesh points, which makes the simulations possible using a standard PC. The mesh is adaptively refined until a stopping criterion is reached with respect to the error in a chosen output of interest, which in this paper is c_D. Both the stopping criterion and the mesh‐refinement strategy are based on a posteriori error estimates, in the form of a space–time integral of residuals times derivatives of the solution of a dual problem, linearized at the approximate solution, and with data coupling to the output of interest. Copyright © 2008 John Wiley & Sons, Ltd.     

Simple Singularities for Hamilton-Jacobi Equations with Max-Concave Hamiltonians and Generalized Characteristics

We consider piecewise smooth viscosity solutions to a Hamilton-Jacobi equation, for which the Hamiltonian is the maximum of a finite number of smooth concave functions. We describe the possible types of “basic” singularities, namely jump discontinuities in either the derivative of the solution or the Hamiltonian, which occur across a smooth hypersurface. Each such type of singularity is described in terms of the properties of the Hamiltonian and the classical characteristics in the regions on either side of the singularity. Where appropriate, singular characteristic equations of the types developed by Melikyan are formulated which can be used in constructions.     

Chaos,transport and mesh convergence for fluid mixing

Chaotic mixing of distinct fluids produces a convoluted structure to the interface separating these fluids. For miscible fluids （as considered here）, this interface is defined as a 50% mass concentration isosurface. For shock wave induced （Richtmyer-Meshkov） instabilities, we find the interface to be increasingly complex as the computational mesh is refined. This interfacial chaos is cut off by viscosity, or by the computational mesh if the Kolmogorov scale is small relative to the mesh. In a regime of converged interface statistics, we then examine mixing, i.e. concentration statistics, regularized by mass diffusion. For Schmidt numbers significantly larger than unity, typical of a liquid or dense plasma, additional mesh refinement is normally needed to overcome numerical mass diffusion and to achieve a converged solution of the mixing problem. However, with the benefit of front tracking and with an algorithm that allows limited interface diffusion, we can assure convergence uniformly in the Schmidt number. We show that different solutions result from variation of the Schmidt number. We propose subgrid viscosity and mass diffusion parameterizations which might allow converged solutions at realistic grid levels.     

Multistable current–voltage characteristics a fingerprints of growth-related imperfections in semiconductor superlattices

The influence of imperfections and growth-related disorder on the current–voltage characteristics of doped superlattices is investigated theoretically. It is demonstrated that the location of a defect can be directly identified from the current measurements. Such measurements can also be used as an estimate for the degree of microscopic disorder.     

Modelling the effects of emergent vegetation on an open‐channel flow using a lattice model

A two‐dimensional lattice model has been developed to describe the influence of vegetation on the turbulent flow structure in an open channel. The model includes the influence of vegetation density on the frictional effect of the channel bed and walls. For the walls, a semi‐slip boundary condition has been considered as an alternative to overcome the no‐slip boundary condition limitations in turbulent flows. The drag stress exerted by the flow on the vegetation as well as the gravity effect has also been taken into account. The proposed lattice model has been used to simulate the experimental results reported from the study of the influence of alternate vegetated zones on the open‐channel flow. The results show that the lattice model approach is a valid tool for describing these kinds of flows. Copyright © 2007 John Wiley & Sons, Ltd.     

电力变压器铁心设计的节材措施

电力变压器的节材与节能始终是变压器制造业的重要发展方向，由于铁心截面设计是变压器设计中的重要环节，因此，铁心设计采取节材措施具有重要意义，采用优化设计方法以便在相同的铁心截面半径条件下获取最大的铁心截面有效面积，通过与传统方法对比，可使铁心有效截面积平均增大4％左右，达到了节材与节能的设计目的。另一方面，将传统的圆形铁轭形状改造为D形形状，通过缩短主级之外的各级叠片长度，使铁心总重降低2％左右，提出的铁心设计节材措施具有一定的应用与推广价值。     

山背波:对飞机的潜在威胁

山背波对飞机的潜在威胁现已毋庸置疑。为了提高在这方面作出更确切判断的能力并能进一步找出实践中可行的相应措施，首要的问题就是必须对山背波本身有足够的认识，而这正是当前国内文献中极为薄弱的环节。有鉴于此，本文系统地、较全面地介绍了山背波这种独特的次声波，对其发生机理和基本特征、以及作为其直接衍生物的相关云相和晴空湍流－－后者正是对飞机的主要潜在威胁所在－－作出简明扼要的评述。     

网络远程教育发展与应用的思考

网络远程教育是近年来高等教育领域讨论的热点之一，对于网络远程教育的优势和不足、定位及发展前景众说纷纭，本文试从网络远程教育发生发展的背景出发，通过与传统教育进行对比分析，指出网络远程教育的特点与优势，并就现阶段在开展网络远程教育过程中出现的一些问题给出建议，旨在探讨网络远程教育良性发展的对策。     

反全球化背景下的国际政治特性

全球化催生了反全球化，随着反全球化运动的“国际化”，反全球化日益走进我们的日常生活，并对未来世界的发展模式起到一定的导向作用。反全球化背景下的国际政治发生了深刻的变革，国际政治将在国家主权、内政自法、国际政治主体、政府的代表性和民族文化的同一性等方面应对反全球化的挑战。     

虚拟实践及其哲学

虚拟实践是一种崭新的社会实践形成，具有开放性、虚拟性、交互性、自主性和兼容性等新特点。虚拟实践是对现实实践的突破和超越，它扩大了实践的范围，大大减少了社会发展的代价，促进了人自身的发展。虚拟实践与现实实践是一种辩证统一关系。