EAST低杂波天线耦合特性研究

采用数值模拟方法研究了EAST低杂波天线的耦合特性。基于二维线性耦合理论,计算了天线功率反射系数和方向性系数与等离子体参数的关系,分析了无源波导对天线耦合性能的影响,并给出了不同参数下天线的功率谱。结果表明,无源波导使天线的方向性略有下降,但减小了天线的功率反射系数,这有利于天线与等离子体的耦合。     

被动传感器阵列中基于粒子滤波的目标跟踪

针对被动传感器阵列中的机动目标跟踪问题,该文提出了一种基于多模Rao-Blackwellized粒子滤波的机动目标跟踪新方法。算法首先基于Rao-Blackwellization理论将机动目标跟踪问题划分为模型选择和目标跟踪两个子问题;采用多模Rao-Blackwellized粒子滤波对目标运动模型进行选择,扩展Kalman滤波对目标进行更新,有效降低了抽样粒子状态维数,节省了计算时间;最后,建立了被动传感器阵列的非线性观测模型。实验结果表明,提出方法可以有效地对目标模型进行选择,算法的跟踪性能及稳定性要好于交互多模型(IMM)方法。     

Surface characterisation and crevice corrosion behaviour of nickel-based alloys in the paper industry

Recycled water in the paper industry acts as a stronger electrolyte as its concentration and temperature are quite high. The lower pH that exists in these solutions enhances the corrosion and the high concentration of solids leads to leaching of metal ions from surface. Crevice corrosion is a form of localised attack, which can quickly induce metal perforation even with high grade stainless steels like 316L when the protective chromium oxide film is damaged. A specially designed crevice cell assembly was used to study the electrochemical aspects of nickel-based alloys with the commonly used 316L SS. The mounted specimen with its crevice was anodically polarised in a white water medium, which is normally encountered in the paper machine section. The nickel-based alloys show remarkable crevice corrosion resistance. X-ray photoelectron spectroscopy studies revealed that the enhanced corrosion resistance of these alloys is due to the contributing nature of the individual elements Cr, Ni and Mo and their presence in the form of various oxidised species on the passive films. Received: 22 February 1999 / Accepted: 6 June 1999     

非互易网络的无耗约束条件

该文论述了在非互易条件下,无源无耗网络的各种约束表征:例如,阻抗[Z]矩阵可以存在实部。它并不与能量定律相悖,即在这种情况下,阻抗矩阵中的电阻并不代表损耗,其负阻也不表示有源增益;传输[A]矩阵也反映出|det[A]|=1的似互易特性。文中给出了具体实例。     

DEM analysis of passive arching in a shallow trapdoor under eccentric loading

This study analyzed the passive arching effect under eccentric loading by developing a series of trapdoor discrete numerical models. The numerical models were validated by comparison with laboratory test results. The deformation pattern, soil arching ratio, force chain distribution, and coordination number under various surcharge magnitudes and deviation distances were analyzed. The numerical results showed that the deformation diagram of soil particles can be divided into three zones: principal displacement zone, transition zone, and static zone. With an increase in the surcharge magnitude, the range of the principal displacement zone decreased, but the range of the transition region increased. The curve of the soil arching ratio on the trapdoor can be divided into three phases, which can be well characterized by the tangent modulus. The passive arching effect is degraded by a surcharge. The ultimate soil arching ratio could be approximated as a W-shaped distribution along the +x-direction. With an increase in the trapdoor displacement, the force chain on the trapdoor gradually expanded outward to form an inverted funnel shape. The most powerful force on the trapdoor was mainly distributed on its edge. The average coordination number decreased gradually as the trapdoor moved upward.     

DNS of a spatially developing turbulent boundary layer with passive scalar transport

A direct numerical simulation (DNS) of a spatially developing turbulent boundary layer over a flat plate under zero pressure gradient (ZPG) has been carried out. The evolution of several passive scalars with both isoscalar and isoflux wall boundary condition are computed during the simulation. The Navier–Stokes equations as well as the scalar transport equation are solved using a fully spectral method. The highest Reynolds number based on the free-stream velocity U_∞ and momentum thickness θ is Re_θ=830, and the molecular Prandtl numbers are 0.2, 0.71 and 2. To the authors’ knowledge, this Reynolds number is to date the highest with such a variety of scalars. A large number of turbulence statistics for both flow and scalar fields are obtained and compared when possible to existing experimental and numerical simulations at comparable Reynolds number. The main focus of the present paper is on the statistical behaviour of the scalars in the outer region of the boundary layer, distinctly different from the channel-flow simulations. Agreements as well as discrepancies are discussed while the influence of the molecular Prandtl number and wall boundary conditions is also highlighted. A Pr scaling for various quantities is proposed in outer scalings. In addition, spanwise two-point correlation and instantaneous fields are employed to investigate the near-wall streak spacing and the coherence between the velocity and the scalar fields. Probability density functions (PDF) and joint probability density functions (JPDF) are shown to identify the intermittency both near the wall and in the outer region of the boundary layer. The present simulation data will be available online for the research community.     

Passive control of wake flow by two small control cylinders at Reynolds number 80

Passive control of the wake behind a circular cylinder in uniform flow is studied by numerical simulation at Re_D=80. Two small control cylinders are placed symmetrically along the separating shear layers at various stream locations. In the present study, the detailed flow mechanisms that lead to a significant reduction in the fluctuating lift but maintain the shedding vortex street are clearly revealed. When the stream locations lie within 0.8≤X_C/D≤3.0, the alternate shedding vortex street remains behind the control cylinders. In this case, the symmetric standing eddies immediately behind the main cylinder and the downstream delay of the shedding vortex street are the two primary mechanisms that lead to a 70–80% reduction of the fluctuating lift on the main cylinder. Furthermore, the total drag of all the cylinders still has a maximum 5% reduction. This benefit is primarily attributed to the significant reduction of the pressure drag on the main cylinder. Within X_C/D>3.0, the symmetry of the standing eddy breaks down and the staggered vortex street is similar to that behind a single cylinder at the same Reynolds number. In the latter case, the mean pressure drag and the fluctuating lift coefficients on the main cylinder will recover to the values of a single cylinder.     

基于多维指派算法的被动多目标跟踪方案

该文针对无源测向交叉定位系统提出了基于多维指派算法的被动多目标跟踪方案。该方案是将三维空间的基准线最小代价算法与动态2D指派算法相结合。该方案的特点是:三维空间的基准线最小代价算法是一种快速数据关联算法,它能够快速削减候选关联集合并简化关联代价的计算,完成航迹起始任务;动态2D指派算法则能够简单而准确地实现观测与航迹的关联。仿真结果证明该方案可以有效地完成无源测向交叉定位系统中的多目标跟踪任务。     

机载FTIR被动遥测大气痕量气体

介绍了以各种地物为背景的机载傅里叶变换红外光谱法(FTIR)被动遥测大气中痕量气体飞行测量试验,讨论了相应的被动下视遥测技术,复杂背景下大气痕量气体红外特征光谱信息获取方法和浓度反演算法模型,定量分析了飞行试验区域内高度1 000 m以下边界层内大气中痕量气体CO和N₂O的平均浓度。这种遥测量技术和数据分析方法可在不预先测量背景辐射光谱的情况下对大尺度区域内大气痕量气体进行快速、机动遥感遥测,以及突发性大气污染事故的应急监测。