Nonlinear apodization for sidelobe control in SAR imagery

Synthetic aperture radar (SAR) imagery often requires sidelobe control, or apodization, via weighting of the frequency domain aperture. This is of particular importance when imaging scenes containing objects such as ships or buildings having very large radar cross sections. Sidelobe improvement using spectral weighting is invariably at the expense of mainlobe resolution presented here is a class of nonlinear operators which significantly reduce sidelobe levels without degrading mainlobe resolution implementation is via sequential nonlinear operations applied to complex-valued (undetected) SAR imagery. SAR imaging is used to motivate the concepts developed in this work. However, these nonlinear apodization techniques have potentially broad and far-ranging applications in antenna design, sonar, digital filtering etc., i.e., whenever data can be represented as the Fourier transform of a finite-aperture signal     

环管形燃烧室燃气导管造型计算方法

本文对航空燃气轮机环管形燃烧室火焰筒燃气导管造型问题进行了研究。给出了一种基于沿轴向面积变化规律、上下素线和上下圆弧半径变化规律的半经验剖切造型计算方法。应用这种方法,以斯贝发动机燃气导管为例进行了计算。计算结果与技术图纸吻合良好。      

Millimeter Radar Instrumentation

Millimeter wavelength radars are used to study plasma effects associated with ionized flow fields of projectiles launched at hypersonic speeds into a free-flight ballistic range. Two CW Doppler radars, at frequencies of 35 and 70 Gc/s, measure the nose-on backscattering radar cross sections during flight. The design and performance of the two radars are described in detail. A signal simulator provides absolute calibration. The purpose is to measure changes that occur in the radar cross sections of hypersonic projectiles caused by highly ionized flow fields. Under certain conditions the nose-on backscattering radar cross section of a blunt-nosed metal projectile decreases drastically when a thin, shock-produced layer of ionized gas covers the projectile. A theoretical analysis of this effect is given. Comparisons between theoretical predictions and experimental data show good correlation.     

并联硅基扩缩微通道热沉强化传热特性

采用去离子水作为冷却工质,实验研究了并联硅基扩缩微通道热沉内的流体流动与强化传热特性.基于微尺度强化传热机理,设计加工了两种并联硅基扩缩微通道热沉.通过测量流体的体积流量、进出口压降与温度、热沉底面加热膜温度,并以传统矩形直通道热沉为参照,获得了并联硅基扩缩微通道热沉在恒热流边界条件和不同体积流量工况下流体流动与对流传热特性参数.结果显示:相对于矩形直通道,并联硅基扩缩微通道热沉的表面传热系数可提高12.5%~85.1%,但摩擦因数只增加了-9.2%~31.4%.表明并联硅基扩缩微通道热沉具有优越的强化传热特性.      

Probability of Detecting a Swerling I Targetet on Two Correlated Observations

Modern radars characterized by electronically steered beams, frequency agility, and range-ambiguous waveforms can use a processing technique whereby each new detection is followed by a second dwell to verify the initial detection. The second dwell is used to minimize false alarms and to resolve range and/or Doppler ambiguities. Under the assumption of a Swerling I target model, the target cross section remains constant when both dwells occur on the same scan and the same transmission frequency is used. Analytic expressions have been developed for the probability of detecting a Swerling I target on both of the correlated dwells (same target cross section). These expressions are used to calculate the probability of a second dwell detection given a first dwell detection and the probability of at least one detection on two dwells. These probabilities are compared with those of independent dwells (independent target cross sections) which result when two transmission frequencies of sufficient separation are utilized.     

Hybrid calibration method for six-component force/torque transducers of wind tunnel balance based on support vector machines

A hybrid calibration approach based on support vector machines (SVM) is proposed to characterize nonlinear cross coupling of multi-dimensional transducer. It is difficult to identify these unknown nonlinearities and crosstalk just with a single conventional calibration approach. In this paper, a hybrid model comprising calibration matrix and SVM model for calibrating linearity and nonlinearity respectively is built up. The calibration matrix is determined by linear artificial neural network (ANN), and the SVM is used to compensate for the nonlinear cross coupling among each dimension. A simulation of the calibration of a multi-dimensional sensor is conducted by the SVM hybrid calibration method, which is then utilized to calibrate a six-component force/torque transducer of wind tunnel balance. From the calibrating results, it can be indicated that the SVM hybrid calibration method has improved the calibration accuracy significantly without increasing data samples, compared with calibration matrix. Moreover, with the calibration matrix, the hybrid model can provide a basis for the design of transducers.     

车模比例和风洞截面对轿车气动性能影响

采用三维不可压缩N-S(Navier-Stokes)方程和RNG k-ε(renormalization group k-ε)湍流模型仿真计算汽车流场特性.控制界面的物理量应用二阶迎风差分格式获得,并运用SIMPLEC压力修正法进行迭代.选用不同车模比例和风洞截面形状进行仿真计算,分析了车模比例和风洞截面形状对轿车气动性能的影响.结果表明:车模比例在一定范围内,汽车气动参数变化平缓;选用不同的车模比例在三种典型风洞模型中进行仿真计算,结果相差很大.      

Bistatic Radar Cross Sections of Chaff

Bistatic cross sections applicable to scattering from a cloud of randomly positioned and randomly oriented resonant dipoles, or chaff, are found. The chaff cloud can have an arbitrary location relative to an illuminating radar and the radar antenna can have an arbitrarily specified polarization. The receiver can be located arbitrarily in relation to the radar and chaff cloud and can also have arbitrary polarization (different from the transmitter antenna). Average cross sections are found for a preferred receiver polarization and the corresponding orthogonal polarization. Results are reduced to simple, easily applied expressions, and several examples are developed to illustrate the ease with which the general results can be applied in practice.     

翼型及钝体的气动导纳

自20世纪60年代Davenport提出钝体气动导纳概念之后,气动导纳一直是钝体空气动力学研究的焦点之一.本文通过大量风洞试验,研究了湍流场中NACA0012翼型和七种典型钝体截面的非定常气动力,试验中首次采用同步脉动压力测量技术,气动导纳的识别采用统计理论.试验得到如下两个主要结论:对于某一给定的几何断面,非定常气动力与湍流各脉动分量存在着不同的气动导纳:流线型体的气动导纳对攻角变化较敏感,而对钝体的影响不太明显.     

热障涂层喷涂质量微焦点CT检测

采用微焦点X射线源对热障涂层样品进行了高分辨率X射线成像研究,采用多尺度对比度增强算法提高了热障涂层与基体之间的对比度.X射线图像显示热障涂层各处厚度差异较大,热障涂层内部可能存在大量孔隙.通过微焦点计算机层析成像(CT)对样品进行了高分辨率CT扫描与重建,得到了样品的CT重建图像.从热障涂层CT图像观察到热障涂层内部存在大量孔隙,且不同层孔隙尺寸、数量不同.结合3个坐标方向的切片图像分析了热障涂层深度方向的热障涂层厚度,得到了热障涂层厚度的二维分布,测量得到热障涂层平均厚度约为40μm.对沿热障涂层深度方向850~1131层切片图像,利用可视化软件VGstudio MAX得到了热障涂层内部孔隙三维分布.研究结果表明:微焦点CT可用于热障涂层制备工艺质量检测.      

Numerical Modeling of the Compression Process of Elastic Open-cell Foams

The random models of open-cell foams that can reflect the actual cell geometrical properties are constructed with the Voronoi technique. The compression process of elastic open-cell foams is simulated with the nonlinear calculation module of finite element analysis program. In order to get the general results applicable to this kind of materials, the dimensionless compressive stress is used and the stress-strain curves of foam models with different geometrical properties are obtained. Then, the influences of open-cell geometrical properties, including the shape of strut cross section, relative density and cell shape irregularity, on the compressive nonlinear mechani- cal performance are analyzed. In addition, the numerical results are compared with the predicted results of cubic staggering model. Numerical results indicate that the simulated results reflect the compressive process of foams quite well and the geometrical properties of cell have significant influences on the nonlinear mechanical behavior of foams.     

基于有限元法的金属材料静强度破坏准则应用研究

根据1种基于有限元法的金属材料静强度破坏准则,设计了2种净截面积相同的缺口试件,用弹塑性、大应变、大变形的非线性有限元方法对试件在静载荷作用下的破坏载荷进行了计算,并进行了试验验证。计算结果与试验结果相比误差很小,很好地验证了基于有限元法的金属材料静强度破坏准则。     

非线性混合回归演化算法研究

基于最小二乘法的常规非线性回归算法的缺点,提出了方程结构与系数的混合回归演化策略方法。在指定最大回归项数的前提下,该混合回归演化策略先找到一些基本满足拟合要求（回归项数与精度）的初始种子回归方程,再以回归项数逐步减少、拟合精度逐步提高为准则,对初始种子回归方程进一步混合回归演化,最后得到回归项数最少、拟合精度最高的最佳回归方程。此外,利用该混合回归演化策略对液体火箭发动机常见的几个经验关系式进行了非线性回归拟合实例分析,拟合残差分布表明该混合回归演化策略方法是有效的。     

考虑非线性扭转影响的薄壁梁翘曲特性分析

 考虑翘曲和大扭角变形的影响,研究薄壁梁结构轴向与扭转非线性耦合弹性变形问题。忽略高阶非线性的影响,结合升阶谱有限元方法,形成对称的非线性几何矩阵,从而建立一个稳定的非线性方程数值解模型。针对矩形截面薄壁梁结构,通过与线性结果对比,分析非线性对截面翘曲正应力、自由端转角的影响。     

Bistatic Radar Cross Sections of Horizontally Oriented Chaff

Bistatic radar cross sections are determined for scattering from a cloud of randomly positioned resonant dipoles (chaff). Dipoles are assumed to be horizontally oriented with axes randomly oriented in the horizontal plane. The cloud is arbitrarily located relative to an illuminating source having an arbitrary (elliptical) polarization. Cloud cross section is found for an arbitrarily located receiver that views the cloud with an antenna of arbitrary polarization. Cross section applicable to the receiver's orthogonal polarization is also found.     

Passive Array Tracking of a Continuous Wave Transmitting Projectile

Differential Doppler measurements by a passive array are used to track an unstable continuous wave (CW) source moving in a ballistic trajectory, e.g., a projectile carrying a proximity fuse. The ballistic equations of motion couple frequency measurements at various sections along the track with the track parameters at any arbitrary time, e.g., at impact. A nonlinear weighted leastsquare method is used to estimate the track parameters, and the resulting error covariance matrix is derived. A numerical example demonstrates the relative contributions of various frequency measurements to the estimation accuracy.     

箔条云团的后向雷达散射截面积

用矩量法计算了３种箔条云团模型的后向雷达散射截面积。计算结果表明平均间距较大时（大于２λ）互耦对ＲＣＳ影响不大,后向散射截面积的概率分布无论考虑互耦与否都满足指数分布,只是其数学期望有所不同。     

均匀湍流场下矩形断面流场及气动参数数值模拟

采用大涡模拟方法数值模拟了两种矩形断面(B/H=1,B/H=2)在不同湍流来流下的流场及气动参数,采用谐波合成方法生成与目标谱一致的脉动入口风速.数值计算结果表明来流湍流对断面的流场及气动参数有较明显的影响,通过与均匀来流的计算结果作对比可以看出,湍流导致断面的阻力系数减小明显,其主要原因为背压区负压系数的增加,与实验的结果较一致;从流场分布看湍流主要影响断面尾流涡的形成,这种影响会影响尾流涡的形成.     

槽壁试验段低超声速流场特性数值模拟

 在跨声速风洞中通过开槽和抽气可以建立低超声速流场,由于槽壁试验段设计参数多,流场结构复杂,为提高设计准确性,通过数值模拟对槽壁试验段低超声速流场特性进行了研究。首先根据槽壁试验段的一般设计准则进行了气动设计,给出了槽壁尺寸和不同马赫数所对应的抽气量。基于设计结果,通过数值模拟对流场特性进行了研究,计算表明：通过抽气可以建立均匀的低超声速流场,抽气量对试验段马赫数均匀区长度有较大影响;随后对不同气动外形进行了比对,结果表明:抽气口位置、壁板厚度、驻室容积、开闭比及槽壁外形等对试验段的气流质量有影响,开闭比和槽型的影响尤为显著;最后对槽壁的通流特性进行了分析,探讨了槽型对试验段流场影响的成因。     

均匀受压三铰圆拱的屈曲

 本文基于经典线性理论,同时考虑了屈曲的径向位移和周向位移分量,由Trefftz稳定准则导得屈曲平衡微分方程及中间铰点的联接条件,并进一步导得求解非对称,不等截面均匀受压三铰圆拱屈曲载荷的超越方程,从而扩大了解题范围。将对称等截面情况的计算结果与Dinnik的结果相比较表明:忽略屈曲的周向位移分量,导致偏高的计算结果。