The Galactic Halo from Microlensing

The status of the microlensing search for galactic dark matter in the form of massive astronomical compact halo objects (machos) is reviewed. Unresolved issues are discussed, as well as possible ways to solve these.     

The time delay of a solar grazing photon

Moving clocks and clocks in a gravitational field slow down, not due to the effects of special relativity nor to the space-time curvature of general relativity, but due only to the principle of equivalence and the conservation of energy. However, some might argue that there has been a further “test” of the effect of gravity on time, namely, the measurement of the time-delay of a round-trip, solar-grazing radar beacon performed by Shapiro in the 1960s. In this test, Shapiro bounced a radar pulse off Mars at superior conjunction (a feat in itself for the time), and compared the measured round-trip travel time of this pulse with the expected round-trip time of a signal traveling at c for the entire trip, as determined from highly accurate planetary ephemerides. Shapiro had predicted this time-delay long before being technologically able to make such a measurement. While general relativity can be used to correctly obtain the magnitude of this delay, it is not the only explanation. As earlier with the analysis of clocks in motion and of clocks in a gravitational field, this paper derives the same result without invoking the space-time curvature of general relativity     

空域网格化方法及其在空管中的应用研究

传统的空域理论方法面临交通密度快速增长、空域管制对象复杂多类异构等一系列问题,研究空域数字化建模,发展出新的空中交通四维时空框架,在此基础上开展全新的空域与空中交通流量协同管理是当前的迫切需要。本文聚焦包括平面格网模型以及空域空间格网模型的空域网格化方法的总结,并对空域网格化方法的空管应用研究进行综述,在此基础上,对空域网格化及数字化研究重点及发展趋势进行综合分析。本文研究能够为空域网格化及数字化理论与应用的可持续发展提供科学指引。     

基于飞行仿真平台的相关坐标变换模型

飞行力学中相关的飞行器运动方程都是在地面坐标系中建立的,地面坐标系的前提假设"平面大地"与地球表面具有曲率不符,经纬度能真实反映飞行器的飞行情况,但不能直接运用于飞行器运动方程.针对以上情况,提出了能够运用于地面坐标与经纬度相互转换的模型.在飞行仿真平台上,由飞行器运动方程建立相关的飞行机动动作模型,再由提出的坐标变换模型将运动方程中的相关参数转换为经纬度.实验结果表明,坐标变换模型是合理的,具有实用性,能运用于相关的飞行仿真平台中.     

MTI data clustering and formation recognition

Image exploitation technology approaches have generally focused on the detection and spatial analysis of stationary groups of objects on the ground using various sensors. While spatial arrangement is clearly necessary in analyzing military formations, it is usually not sufficient. Typically the arrangement must be examined within some context in order to interpret a pattern of deployment. For moving objects the spatial arrangement of the group relative to the direction of motion is key to recognizing the formation. By examining ground moving target indicator (MTI) radar data over time, motion can be inferred and used to establish a context for interpreting the spatial arrangement of the data. New techniques that exploit the multitemporal nature of MTI data are described. The first is a space-time clustering technique that locates compact groups of objects that persist in time. The technique Is an application of Marr and Hildreth's edge detection methodology to the dual problem of region segmentation, or more accurately, volumetric segmentation of space-time. The second technique is based on the use of the Hough transform for recognizing moving formations such as columns, wedges, and lines abreast by analyzing the shape of clustered MTI detections (specifically the orientation of linear arrangements within the group) with respect to their direction of motion. Preliminary results from simulated MTI data sets are presented     

曲壁气膜冷却数值计算的研究

 本文在一般二维边界层流动和换热计算方法的基础上,引入湍流混合长度的修正模型,用以考虑壁面曲率和注入冷气对换热的影响,研究计算了壁面的弯曲部分和曲率恢复部分换热的变化,并对曲面上的气膜冷却问题作了计算,取得较满意的结果。     

电大尺寸散射体的RCS计算方法研究

为了更加有效的求解电大目标散射计算问题,在避免谐振区效应情况下获得复杂目标的隐身雷达散射截面(RCS)特性,引用混合场积分方程(CFIE),在快速算法的求解迭代过程中采用共轭梯度算法(CG)的收敛技术,能够稳定的求解电大尺寸的RCS。计算和分析了金属球双站RCS和金属立方体双站RCS,并和精确计算、相关参考文献进行了比较,证明了方法的正确性,计算结果稳定,在工程实际上有较大的应用价值。     

基于曲率分布控制的叶型前缘设计方法

为了实现对压气机叶片的优化,提出了一种基于曲率分布控制的前缘造型方法,实现了对叶型前缘曲率的直接、精确控制。将该造型方法应用于某工业级压气机的可控扩散叶型（CDA）上,通过数值仿真方法计算了叶型在设计来流马赫数下的全攻角工况性能。结果显示增加前缘曲率能有效拓宽许用攻角范围,减小尖峰扩散因子,在相同攻角下能削弱前缘吸力峰,抑制甚至消除前缘分离泡,避免提前转捩的发生。同时,调整曲率分布使其在靠近前缘点处尽可能"饱满"、减缓曲率下降速度,也有同样的效果。理论分析发现前缘曲率通过调整静压分布影响边界层发展起始流态,从而影响叶型性能。设计前缘几何形状时需要确保曲率连续性,调整曲率分布以减小前缘吸力峰的强度,避免分离诱导转捩的出现。     

A new simpler rotation/curvature correction method for Spalart-Allmaras turbulence model

 A new and much simpler rotation and curvature effects factor, which takes the form of Richardson number suggested by Hellsten originally for SST k-x model, is presented for Spalart and Shur's rotation and curvature correction in the context of Spalart-Allmaras (SA) turbulence model. The new factor excludes the Lagrangian derivative of the strain rate tensor that exists in the SARC model, resulting in a simple, efficient and easy-to-implement approach for SA turbulence model (denoted as SARCM) to account for the effects of system rotation and curvature, techniquely. And then the SARCM is tested through turbulent curved wall flows: one is the flow over a zeropressure- gradient curved wall and the other is the channel flow in a duct with a U-turn. Predictions of the SARCM model are compared with experimental data and with the results obtained using original SA and SARC models. The numerical results show that SARCM can predict the rotation-curvature effects as accurately as SARC, but considerably more efficiently. Additionally, the accuracy of SARCM might strongly depend on the rotation-curvature model constants. Suggesting values for those constants are given, after some trials and errors.     

简单二体引力模型用于空间目标轨道面交线确定的效果分析

在空间目标碰撞预警分析中，准确地计算出空间目标的轨道面交线是进行地心距筛选、时间差筛选的前提，目前较多使用的快速确定轨道面交线的方法为简单二体引力模型。深入分析该模型，比较了其计算的空间目标轨道面交线与STK计算结果的差异，指出了简单二体引力模型在计算空间目标轨道面交线时的局限性，认为轨道摄动是影响轨道面交线计算准确性的主要原因，应该采用更能反映空间目标实际运动规律的改进的二体引力模型的方法。     

反映流动曲率影响的非线性湍流模式

本文从一般曲线坐标系下的雷诺应力模式出发，在雷诺应力模式向代数应力模式简化过程中，保留工率项对雷诺应力输运的影响，得到了曲率修正的代数应力模式和非线性κ-ε模式。并且该模式对一典型曲率流动－－U型管道内的湍流流动进行了数值模拟与分析。计算结果显示,率修正的代数应力模式较好地模拟出了凸曲率地雷诺剪切应力的阻滞作用和凹曲率对剪切应力的增强作用，捕捉到了曲率流动的主要特征，是对雷诺应力模式的一个合理而有效的简化。     

多级轴流压气机湍流和二次流径向掺混的简单统一模型及其应用

本文构造出一套新的统一表示湍流和二次流径向掺混作用的多级轴流压气机子午通流流场的模型方程组。除了掺混系数外,新的方程组没有其它的未知关联项,使得含径向掺混的多级轴流压气机子午通流的计算简单得多,相应计算机程序的计算时间只是无粘计算机时的约1.5倍。新方程组的数值结果与试验结果以及与则Gallimore掺混理论结果比较,表明新方程组的合理性和优越性。新方程组有严格的理论基础,不仅能用于亚音压气机,还可能用于跨音和超音压气机。      

Design Principles of MIMO Radar Detectors

This paper considers the problem of multiple-input multiple-output (MIMO) radars employing space-time coding (STC) to achieve diversity. To this end, after briefly outlining the model of the received echo, a suitable detection structure is derived, and its performance is expressed in closed form as a function of the clutter statistical properties and of the space-time code matrix. Interestingly, this receiver requires prior knowledge of the clutter covariance, but the detection threshold is functionally independent thereof. At the transmitter design stage, we give two criteria for code construction: the first is based on the classical Chernoff bound, the second is an information-theoretic criterion. Interestingly, the two criteria lead to the same condition for code optimality, which in turn specializes, under the assumption of uncorrelated clutter and square code matrix, in some well-known full-rate space-time codes. A thorough performance assessment is also given, so as to establish the optimum achievable performance for MIMO radar systems.     

Space-time-frequency processing of synthetic aperture radar signals

The subject of this work is the detection and high resolution microwave imaging of objects moving on the ground and observed by an airborne radar. The proposed approach is based on a combined space-time and time-frequency processing. The space-time processing makes use of a linear array antenna and exploits the radar motion for filtering the received echoes in order to improve as much as possible the signal-to-disturbance ratio. The signal is then mapped onto the time-frequency domain, by computing its Wigner-Ville distribution, for a further filtering and for estimating its instantaneous frequency, necessary for the formation of a high resolution image of the moving object     

适合机动飞行的旋翼气动模型研究

为改善对旋翼空气动力他轴响应的预测,在旋翼气动模型中引入时变的尾迹弯曲参数K_R,以体现机动旋翼的尾迹畸变效应.与已有方法相比,该模型的计算结果更为接近实验数据.还建立了动态尾迹畸变模型,并分析了计入尾迹收缩效应的必要性.结果表明,为了正确计算机动飞行中的旋翼气动力,特别是操纵的他轴响应,应当在模型中采用随状态变化的尾迹弯曲参数K_R.      

Detection of small objects in clutter using a GA-RBF neural network

Detection of small objects in a radar or satellite image is an important problem with many applications. Due to a recent discovery that sea clutter, the electromagnetic wave backscatter from a sea surface, is chaotic rather than purely random, computational intelligence techniques such as neural networks have been applied to reconstruct the chaotic dynamic of sea clutter. The reconstructed sea clutter dynamical system which usually takes the form of a nonlinear predictor does not only provide a model of the sea scattering phenomenon, but it can also be used to detect the existence of small targets such as fishing boats and small fragments of icebergs by observing abrupt changes in the prediction error. We applied a genetic algorithm (GA) to obtain an optimal reconstruction of sea clutter dynamic based on a radial basis function (RBF) neural network. This GA-RBF uses a hybrid approach that employes a GA to search for the optimum values of the following RBF parameters: centers, variance, and number of hidden nodes, and uses the least square method to determine the weights. It is shown here that if the functional form of an unknown nonlinear dynamical system can be represented exactly using an RBF net (i.e., no approximation error), this GA-RBF approach can reconstruct the exact dynamic from its time series measurements. In addition to the improved accuracy in modeling sea clutter dynamic, the GA-RBF is also shown to enhance the detectability of small objects embedded in the sea. Using real-life radar data that are collected in the east coast of Canada by two different radar systems: a ground-based radar and a satellite equipped with synthetic aperture radar (SAR), we show that the GA-RBF network is a reliable detector for small surface targets in various sea conditions and is practical for real-life search and rescue, navigation, and surveillance applications     

Model-based orientation-independent 3-D machine vision techniques

Orientation-dependent techniques for the identification of a three-dimensional (3-D) object by a machine vision system are represented in parts. In the first part, the data consist of intensity images of polyhedral objects obtained by a single camera, while in the second part, the data consist of range images of curved objects obtained by a laser scanner. In both cases, the attributed graph representation of the 3-D object surface is used to drive the respective algorithm. In this representation, a graph node represents a surface path and a link, the adjacency between two patches. The attributes assigned to nodes are moment invariants of the corresponding face for polyhedral objects. For range images, the Gaussian curvature is used as a segmentation criterion for providing symbolic shape attributes. Identification is achieved by an efficient graph-matching algorithm used to match the graph obtained from the data to a subgraph of one of the model graphs stored in the computer memory     

Space-time autoregressive filtering for matched subspace STAP

Practical space-time adaptive processing (STAP) implementations rely on reduced-dimension processing, using techniques such as principle components or partially adaptive filters. The dimension reduction not only decreases the computational load, it also reduces the sample support required for estimating the interference statistics. This results because the clutter covariance is implicitly assumed to possess a certain (nonparametric) structure. We demonstrate how imposing a parametric structure on the clutter and jamming can lead to a further reduction in both computation and secondary sample support. Our approach, referred to as space-time autoregressive (STAR) filtering, is applied in two steps: first, a structured subspace orthogonal to that in which the clutter and interference reside is found, and second, a detector matched to this subspace is used to determine whether or not a target is present. Using a realistic simulated data set for circular array STAP, we demonstrate that this approach achieves significantly lower signal-to-interference plus noise ratio (SINR) loss with a computational load that is less than that required by other popular approaches. The STAR algorithm also yields excellent performance with very small secondary sample support, a feature that is particularly attractive for applications involving nonstationary clutter.     

球柱联合转台轴承摩擦力矩特性

为研究球柱联合转台轴承摩擦力矩特性,建立了轴承的力学模型,采用Newton-Raphson法对模型进行求解。分析了轴承摩擦力矩产生机理,建立了摩擦力矩计算模型,利用试验机对模型进行了试验验证,研究了不同结构参数对轴承摩擦力矩的影响。结果表明:所建立的模型可较好的实现轴承摩擦力矩预测,轴承摩擦力矩与轴向载荷、倾覆力矩和轴承转速呈正相关;减小初始轴向游隙可显著降低轴承摩擦力矩;钢球数量存在一个最优值使轴承摩擦力矩最小;沟曲率半径存在合理区间使轴承综合性能最优;适当减小保持架质量可有效降低轴承摩擦力矩。研究结果为球柱联合转台轴承的优化设计提供了理论依据。     

湍流模型对三维翼梢涡流场数值模拟的影响

 梢涡流场数值模拟对减少梢涡的不良影响意义深远。应用雷诺平均Navier-Stokes(RANS)方法建立了三维翼梢涡流场数值模型,选用的湍流模型包括两种经典两方程模型和一种代数雷诺应力模型。计算内容包括翼梢涡流场速度分布、压力分布及涡核位置,为了准确模拟梢涡涡核内系统旋转和流线曲率影响,将旋转和曲率修正方法应用于选用的湍流模型中。计算时对翼梢涡涡核处网格进行了加密处理。将计算结果与实验结果对比后发现,修正后的湍流模型较标准湍流模型明显提高了预报精度,与实验结果吻合较好,具有工程应用价值。