Modelling flow over a backward–facing step using the F.E.M. and the two-equation model of turbulence

Flow over a downstream-facing step is predicted using the F.E.M. A two-equation model of turbulence is employed where the transport of turbulence kinetic energy and dissipation rate are depicted using transport-type equations, i.e. the two-equation model of turbulence. The results obtained are compared with other models and experimental results. Generally, the model was found to be under-predictive with regard to the reattachment length when previous empirical data was used in the transport equations.     

串级热声发动机的机理分析及数值模拟

简要综述了热声发动机的发展历程,把三种典型的发动机形式作了比较分析,在此基础上,对国际上最近提出的串级型发动机进行了定性讨论,并结合一定的几何结构,运用线性热声理论对该类型热声机进行了数值模拟。     

Numerical radius distance-preserving maps on

Let be a complex Hilbert space, be the algebra of all bounded linear operators on , be the subset of all selfadjoint operators in and or . Denote by the numerical radius of . We characterize surjective maps that satisfy for all without the linearity assumption.

     

Non-destructive testing of tubes using a time reverse numerical simulation (TRNS) method

A method for the detection of defects in cylindrical structures and the determination of their positions and orientations is presented in this paper. The scattered field, which is generated by the interaction of excited guided waves with a defect, is evaluated with an approach named time reverse numerical simulation method (TRNS). Since the excited waves and the scattered field propagate along the sample, the time-consuming scanning of the whole tube can be eliminated. The scattered displacement field is measured in three dimensions over time with a laser vibrometer at different locations distributed equally around the circumference at a fixed axial coordinate far away from the defect. Instead of analyzing the complicated time signals directly, they are played back in time. If the recorded displacement histories of the scattered field are reversed in time and played back in an identical structure, the waves travel back the same path and interfere to a maximum at their origin. The result is an amplitude increase at the position of the defect where the scattered field was generated. Instead of playing back the recorded time signals in an experiment, this step is replaced by a numerical simulation. Only this enables the visualization and detection of the amplitude increase. As long as the simulation is of high accuracy, the position of the maximum interference corresponds exactly to the location of the defect in the experiment, although no defect is implemented in the simulation.     

Reduction of spin glasses applied to the Migdal-Kadanoff hierarchical lattice

A reduction procedure to obtain ground states of spin glasses on sparse graphs is developed and tested on the hierarchical lattice associated with the Migdal-Kadanoff approximation for low-dimensional lattices. While more generally applicable, these rules here lead to a complete reduction of the lattice. The stiffness exponent governing the scaling of the defect energy E with system size L, (E) ~L ^y, is obtained as y ₃ = 0.25546(3) by reducing the equivalent of lattices up to L = 2¹⁰⁰ in d = 3, and as y ₄ = 0.76382(4) for up to L = 2³⁵ in d = 4. The reduction rules allow the exact determination of the ground state energy, entropy, and also provide an approximation to the overlap distribution. With these methods, some well-know and some new features of diluted hierarchical lattices are calculated.     

Modelling of magnetic effects in near-field optics

Green's dyadic technique represents a powerful tool for calculations in electrodynamics, especially in modelling optical properties of nanoscopic objects. The method does not only provide field distributions, but also maps of susceptibilities and densities of states. Whereas the formalism is well established for dielectrics and electric fields, I present here a straight forward extension to tensors of both electric and magnetic type as well as mixed ones and furthermore to the situation where objects with dielectric and magnetic permeabilities are present together. As examples, characteristic field patterns are compared for elementary dielectric and magnetic perturbations. Green's tensors calculated for a coral structure reveal that mixed susceptibilities can exhibit other symmetries than pure electric or magnetic ones. Maps of all tensor components can thus give essential clues to the interpretation of near-field images. Received 15 December 2002 Published online 20 June 2003 RID="a" ID="a"Files “maths.ps” and “tensors.ps” are only available in electronic form at http://www.edpsciences.org RID="b" ID="b"e-mail: Ursula.Schroeter@uni-konstanz.de     

Quantum topological excitations: from the sawtooth lattice to the Heisenberg chain

From the recently determined structure of the delafossite YCuO_2.5, we argue that the Cu-O network has nearly independent Δ chains but with different interactions between the s = 1/2 spins. Motivated by this observation, we study the Δ chain for different ratios of the base-base and base-vertex interactions, J _bb/J _bv. By exact diagonalization and extrapolation, we show that the elementary excitation spectrum is the same for total spins S _tot = 0 and 1, but not for S _tot = 2, and has a gap only in the interval 0.4874(1) ⩽ J _bb/J _bv ⩽ 1.53(1). The gap, known to be dispersionless for J _bb = J _bv, is found to acquire increasing k-dependence as J _bb/J _bv moves away from unity. Received 29 October 2002 / Received in final form 14 January 2003 Published online 6 March 2003 RID="a" ID="a"e-mail: sblundell@cea.fr RID="b" ID="b"e-mail: nunezreg@lps.u-psud.fr     

自调Q光纤激光器的理论分析和数值模拟

基于自调Q光纤激光器的基本理论,结合速率方程理论与自调Q特点,借助简化的环形腔掺Er3+光纤激光器模型,分析了泵浦功率与自调Q脉冲周期、峰值功率的关系.随着泵浦功率的增加,光纤激光器自调Q脉冲的周期减小,峰值功率变大；当信号光强足够大时,激光器的自调Q现象消失,变为准连续激光输出.通过MATLAB编程对光纤激光器输出特性进行了数值模拟,模拟结果与实验结果基本相符.     

射流式单重态氧发生器的气液两相耦合模型

 将气液两相耦合模型应用于射流式单重态氧发生器，通过数值模拟和实验的比较验证了该模型的可行性。分析了发生器工作参数对其动力学过程的影响，发现在保持其它参数不变的情况下，氯气和BHP溶液的相对浓度决定了发生器动力学过程的类型。给出了利用氯气吸收速率变化曲线判定发生器动力学过程类型的方法。在原有模型基础上考虑了热效应及其对模型参数的影响，计算了水蒸气的含量。模型改进后，模拟结果更接近于实验值。     

Rod-pinch 二极管理论及数值模拟

 介绍了Rod-pinch二极管的基本结构和工作原理，采用Laminar模型分析了Rod-pinch二极管中的粒子运动过程及其阻抗特性。考虑背景空间离子电荷的影响，用1维Laminar方程分析Rod-pinch二极管中电子的自箍缩过程，并且利用Magic程序对其中的粒子运动进行数值模拟，求解二极管中的电压和电流，最终得出二极管的阻抗特性，在较低电压下，负极性RPD的性能明显不如正极性RPD。根据临界电流经验公式，初步验证Laminar理论模型的可行性。