Numerical Simulation of Characteristics of CEX Ions in Ion Thruster Optical System

Charge exchange(CEX) ions could inflict severe damages on the ion thruster optical system. This article is aimed at investigating the characteristics of the CEX ions and their influences upon the optical system by means of particle-in- cell(PIC) ion simulation and Monte Carlo collision(MCC) methods. The results from numerical simulation indicate that despite the fact that CEX ions appear in the entire beamlet region near the ion optical system, the ones that present themselves downstream of the accelerator grid have good reason for attracting more attention. As their trajectories are significantly affected by the local electric field, a great number of CEX ions are accelerated toward grids resulting in sputtering erosion. When the influences of the CEX ions are considered in the numerical simulation, there could hardly be observed augments in the screen grid current, but the accelerator grid current increases from zero to 1. 4% of the beamlet current. It can be understood from the numerical simulation that the CEX ions formed in the region far downstream of the accelerator grid should be blamed for the erosion on the downstream surface of the accelerator grid.     

粒子拖尾长度对高频DPIV测量误差的影响

基于连续激光源的高采样频率DPIV系统在进行曝光时可能会产生粒子拖尾现象。明渠湍流试验和数值模拟试验结果显示，粒子拖尾长度的增加会导致平均流速减小和湍流强度增大。分析表明，导致平均流速误差的原因之一是诊断窗口边缘局部粒子图像的缺失，且图像缺失程度会随着粒子拖尾长度增加而增大，从而使平均速度随粒子拖尾长度增加而减小；粒子拖尾的另一个直接后果是增大了粒子图像的有效粒径，从而导致湍流强度增大。通过增大第二个诊断窗口的尺寸，进行相关函数标准化和相关系数补偿等方法，可以消除粒子拖尾长度引起的误差。     

Perspective on space radiation for space flights in 2020–2040

The Sun undergoes several well known periodicities in activity, such as the Schwabe 11 year cycle, the Gleissberg 80–90 year cycle, the Suess 200–210 year cycle and the Halstatt 2200–2300 year cycle. In addition, there is evidence that the 20th century levels of solar activity are unusually high. The years 2020–2040 are expected to coincide with increased activity in human space flight beyond low Earth orbit. The solar cycles and the present level of solar activity are reviewed and their activities during the years 2020–2040 are discussed with a perspective on space radiation and the future program of space flight. It is prudent to prepare for continuing levels of high solar activity as well as for the low levels of the current deep minimum, which has corresponded to high galactic cosmic ray flux.     

Ground-Based Cloud Using Exponential Entropy/Exponential Gray Entropy and UPSO

Objective and accurate classification model or method of cloud image is a prerequisite for accurate weather monitoring and forecast. Thus safety of aircraft taking off and landing and air flight can be guaranteed. Thresholding is a kind of simple and effective method of cloud classification. It can realize automated ground-based cloud detection and cloudage observation. The existing segmentation methods based on fixed threshold and single threshold cannot achieve good segmentation effect. Thus it is difficult to obtain the accurate result of cloud detection and cloudage observation. In view of the above-mentioned problems, multi-thresholding methods of ground-based cloud based on exponential entropy/exponential gray entropy and uniform searching particle swarm optimization （UPSO） are proposed. Exponential entropy and exponential gray entropy make up for the defects of undefined value and zero value in Shannon entropy. In addition, exponential gray entropy reflects the relative uniformity of gray levels within the cloud cluster and background cluster. Cloud regions and background regions of different gray level ranges can be distinguished more precisely using the multi-thresholding strategy. In order to reduce computational complexity of original exhaustive algorithm for multi-threshold selection, the UPSO algorithm is adopted. It can find the optimal thresholds quickly and accurately. As a result, the real-time processing of segmentation of groundbased cloud image can be realized. The experimental results show that, in comparison with the existing groundbased cloud image segmentation methods and multi-thresholding method based on maximum Shannon entropy, the proposed methods can extract the boundary shape, textures and details feature of cloud more clearly. Therefore, the accuracies of cloudage detection and morphology classification for ground-based cloud are both improved.     

Composite profile of the Fe Kα spectral line emitted from a binary system of supermassive black holes

We used a model of a relativistic accretion disk around a supermassive black hole (SMBH), based on ray-tracing method in the Kerr metric, to study the variations of the composite Fe Kα line emitted from two accretion disks around SMBHs in a binary system. We assumed that the orbit of such a binary is approximately Keplerian, and simulated the composite line shapes for different orbital elements, accretion disk parameters and mass ratios of the components. The obtained results show that, if observed in the spectra of some SMBH binaries during their different orbital phases, such composite Fe Kα line profiles could be used to constrain the orbits and several properties of such SMBH binaries.     

基于闭环准则的LQG/LTR飞行控制律优化设计

针对LQG/LTR飞行控制律设计中加权矩阵的选取问题,提出了一种基于闭环准则的LQG/LTR飞行控制律优化设计方法。以俯仰姿态控制律优化设计为例,首先基于闭环准则选取了满足一级飞行品质要求的参考模型,并基于该参考模型使用粒子群优化算法优化得到了两个加权矩阵,从而得到了卡尔曼滤波器和相应的目标反馈回路。进而借鉴飞机等效系统拟配的方法,优化得到了另两个加权矩阵,从而得到了LQG最优控制器,完成了回路传递恢复的优化设计。仿真结果表明,通过该方法能够快速自动地选取合适的加权矩阵,使得优化设计的LQG/LTR飞行控制律较好地满足了俯仰姿态控制的要求。     

SiCP/Al复合材料棱镜组件研磨工艺方法研究

通过对SiCp/Al复合材料棱镜组件研磨工艺试验及其结果分析,总结、优化了研磨工艺参数、研磨砂种类、粒度、研磨时间、研磨工艺过程等参数,研磨出的棱镜组件符合其精度要求,为棱镜组件在平台上的安装精度提供了基础.     

连续变速颤振试验的自适应粒子滤波算法

连续变速颤振试验的采集信号通常为非平稳信号,其频率和幅值随时间变化,尤其在亚临界状态下, 变化程度十分剧烈。常用的非平稳信号时变参数建模分析方法,在信号非平稳程度较高的情况下难以对信号 的模态进行准确地分析和跟踪。为了解决这一问题,结合信号非平稳度量计算方法,提出一种改进的自适应粒 子滤波算法,并通过仿真实验数据对所提算法在高非平稳度情况下的跟踪性能进行验证。结果表明:与一般粒 子滤波算法相比,本文方法在高非平稳度情况下具有更高的跟踪精度。     

MHD Spectroscopy of Transonic Flows

In previous publications (Keppens et al.: 2002, Astrophys. J. 569, L121; Goedbloed et al.: 2004a, Phys. Plasmas 11, 28), we have demonstrated that stationary rotation of magnetized plasma about a compact central object permits an enormous number of different MHD instabilities, with the well-known magneto-rotational instability (Velikhov, E. P.: 1959, Soviet Phys.–JETP Lett. 36, 995; Chandrasekhar, S.: 1960, Proc. Natl. Acad. Sci. U.S.A. 46, 253; Balbus, S. A. and Hawley, J. F.: 1991, Astrophys. J. 376, 214) as just one of them. We here concentrate on the new instabilities found that are driven by transonic transitions of the poloidal flow. A particularly promising class of instabilities, from the point of view of MHD turbulence in accretion disks, is the class of trans-slow Alfv’en continuum modes, that occur when the poloidal flow exceeds a critical value of the slow magnetosonic speed. When this happens, virtually every magnetic/flow surface of the disk becomes unstable with respect to highly localized modes of the continuous spectrum. The mode structures rotate, in turn, about the rotating disk. These structures lock and become explosively unstable when the mass of the central object is increased beyond a certain critical value. Their growth rates then become huge, of the order of the Alfv’en transit time. These instabilities appear to have all requisite properties to facilitate accretion flows across magnetic surfaces and jet formation.     

应用粒子激光散射测量混合反应器分散相浓度场分布

研究了圆管反应器中四束侧向对撞射流与轴向流形成的混合流动.应用激光粒子散射成像测量了侧向分散相在混合流中的浓度场分布,得到了不同的浓度分布图形随侧向流与轴向流速比关系,揭示了内在有规律的独特的浓度分布结构.