Design and Evaluation of an Adaptive MTI Filter

The design and evaluation of an adaptive moving target indicator (MTI) filter, the adaptive canceler for extended clutter (ACEC) is dealt with, taking into consideration adaptivity to clutter mean Doppler frequency. This consideration is one of the most important operational requirements in adaptive MTI's and permits a relatively simple hardware implementation as compared to more general optimization and adaptivity criteria (briefly described). The ACEC's algorithm compensates in real time for the clutter mean Doppler frequency. Performances have been obtained by digital computer simulation in various operational conditions.     

P-S-N曲线试验方案的统计模拟研究

 本文应用蒙特卡洛方法模拟测定材料P-S-N曲线的疲劳试验,确定丁做P-S-N曲线的最佳试验方案。节省大量试验时间和试样。     

Detection Performance of the Cell Averaging LOG/CFAR Receiver

The cell averaging LOG/CFAR receiver is a special implementation of a constant-false-alarm-rate (CFAR) receiver in which the noise level estimate is derived from a set of contiguous time samples of the output of a logarithmic (LOG) detector as obtained from a tapped delay line. This CFAR receiver is capable of operating over a larger dynamic range of noise levels than a conventional cell averaging CFAR receiver, but with somewhat poorer detectability. The performance in stationary Gaussian noise of the cell averaging LOG/CFAR receiver with no post-detection integration is determined in this paper. For a small number of reference noise samples, results were obtained by a Monte Carlo simulation using the technique of importance sampling. For a large number of reference noise samples, a second moment analysis gave the desired results. Both these results can be summarized in the following simple formula, NLOG = 1.65NLIN - 0.65, which relates the number of reference samples required by each of the two receivers for equivalent performance. Thus, for the cell averaging LOG/CFAR receiver to give the same detection performance as the conventional cell averaging CFAR receiver, the number of reference noise samples has to be increased by up to 65 percent.     

基于BP神经网络的多应力加速寿命试验预测方法

 针对多应力加速寿命试验(ALT)中传统的寿命预测方法存在建立加速模型及求解多元似然方程组困难的缺点,基于反向传播(BP)人工神经网络(ANN),利用BP神经网络良好的预测特性,建立了多应力恒定加速寿命试验寿命预测模型。首先,以加速寿命试验中的加速应力水平和通过经验分布得到的可靠度作为网络训练输入向量;以非线性最小二乘法对原始失效数据进行拟合并得到回归方程,利用回归方程生成大量的仿真数据作为训练目标向量;然后,建立3层BP神经网络并对网络进行训练。最后,把正常应力水平和设定的可靠度输入训练好的网络,得到预测的失效时间,进而给出可靠度函数的预测曲线。通过仿真算例对本方法进行验证,预测值和仿真值相比较表明,所建立的网络能反映应力水平、可靠度与寿命的关系,为多应力加速寿命试验的寿命预测提供了新的思路。     

Unsteady pressure fluctuations in the boattail region of a heat shield

Unsteady, turbulent, compressible, axisymmetric, Reynolds-averaged Navier–Stokes equations are solved for the flow past a bulbous payload shroud for a freestream Mach number of 0.95 and Reynolds number of 16.40×10⁶. A time-dependent numerical simulation is carried out using a finite-volume discretization technique in conjunction with a multistage Runge–Kutta time-stepping scheme. The closure of the system of equations is achieved using the Baldwin–Lomax turbulence model. Comparisons have been made with experimental results such as the schlieren picture and surface pressure distribution. A good agreement is found between them. A separated flow zone on the boattail region is observed and is found to be highly unsteady. Standard deviation, higher-order moments, histograms, and spectrum of surface pressure levels are analysed in the separated region of the boattail.     

某民用涡扇发动机飞行包线内吸雨量计算分析

分析吸雨对发动机工作性能的影响,首要的是确定能被发动机吸入的雨量。通过综合考虑适航规章要求的大气雨水分布、环境温度、飞行速度、发动机功率和聚集效应对发动机吸雨的影响,结合完整的推力调节计划,计算分析了某民用大涵道比涡扇发动机在不同推力等级下全飞行包线内风扇进口水气比(WAR)。结果表明:在同一马赫数下,风扇进口水气比在6 100m处达到最大;在该高度以下,水气比随着高度的增加而增大;在该高度以上,水气比随着高度的增加而减小;飞行马赫数越大、环境温度越高,水气比也越大;并且标准天空中慢车推力下的风扇进口水气比最大可达到7.38%。该计算方法可为民用涡扇发动机吸雨适航取证的关键点分析提供参考。     

Inflatable Wing Design Parameter Optimization Using Orthogonal Testing and Support Vector Machines

The robust parameter design method is a traditional approach to robust experimental design that seeks to obtain the optimal combination of factors/levels. To overcome some of the defects of the inflatable wing parameter design method, this paper proposes an optimization design scheme based on orthogonal testing and support vector machines (SVMs). Orthogonal testing design is used to estimate the appropriate initial value and variation domain of each variable to decrease the number of iterations and improve the identification accuracy and efficiency. Orthogonal tests consisting of three factors and three levels are designed to analyze the parameters of pressure, uniform applied load and the number of chambers that affect the bending response of inflatable wings. An SVM intelligent model is established and limited orthogonal test swatches are studied. Thus, the precise relationships between each parameter and product quality features, as well the signal-to-noise ratio (SNR), can be obtained. This can guide general technological design optimization.     

Stability of Parameter Estimates for a Gaussian Process

Maximum-likelihood estimates for the levels of the mean value function and the covariance function of a Gaussian random process are investigated. The stability of these estimates is examined as the actual covariance function of the process deviates from the form assumed in the estimators. It is found that the time-bandwidth product for stationary processes represents an upper bound on the number of estimator terms that can be safely used when estimating with uncertainty about the process covariance function. This result is consistent with other interpretations of the time-bandwidth product and tempers the conclusion that, in principle, an infinite number of estimator terms can be used to obtain a perfect estimate of the covariance level. In practice, the estimate of the level can never be perfect, and the accuracy of the estimate depends on the observation interval. Finally, conditions are established to ensure asymptotic stability of the estimates and physical interpretations are presented.     

Simplification approaches for multi-level load spectra by using equivalent damage rule

Reasonable and effective fatigue load spectrum is key of aircraft structural fatigue test. Two kinds of load spectra simplification approaches based on statistical consistent fatigue damage model are proposed in the paper. One method is to simplify the original multi-level load spectra to spectral load with fewer level and was verified by the experimental data of five levels and seven levels load spectra. The equivalent damage here is not equivalent fatigue life, but equivalent number of spectral blocks. Moreover, it was proved that a section of low stress load cannot be converted into the highest stress load considering equivalent damage. The other is to convert the life distribution under multi-level load spectra into that of constant amplitude spectrum according to a certain principle, that is, equivalent damage is achieved through equivalent fatigue life, and two groups of three levels spectral data were used for experimental verification. The results show that equivalent damage simplification approach of multi-level load spectra is feasible, and it achieve that the damage of one loading block before and after the simplification is equal in probability statistics. The life distribution of load simplified as constant amplitude spectrum is consistent with test data. The simplification method presented in this paper can be generalized and is a predictive engineering method which does not depend on experimental data.     

Application of a Highly Parallel Processor to Radar Data Processing

The application of a highly parallel computer known as PEPE (Parallel Element Processing Ensemble) to radar data processing is described. The PEPE computer consists of a large number of identical processing elements which operate in parallel and are controlled by a common control unit. Each of the processing elements is assigned to a particular radar target such that many targets can be tracked in parallel. PEPE is designed to augment a conventional computer rather than to stand alone. The total data processing load associated with a radar tracking system is distributed between PEPE and the sequential machine in a manner than maximizes the overall system efficiency and desensitizes the system performance to fluctuations in traffic levels. The use of PEPE provides very high data processing throughput potential to a radar data processing system.     

表面热流可辨识性初步分析

表面热流的可辨识性分析可用于飞行器防热层内温度测量精度和测温点位置的确定,在工程上有较强的实用意义。从无量纲分析和仿真辨识出发,根据防热层材料热物性系数、测点位置、表面热流的频域特性等参数对表面热流辨识结果的影响规律,总结出了表面热流辨识问题的相似参数：基于表面热流频率参数的傅立叶数。此后,以这一傅立叶数为判据,针对不同测量误差值的情况初步建立起了表面热流可辨识性的准则和分析方法。     

Efficient aerodynamic shape optimization using variable-fidelity surrogate models and multilevel computational grids

A variable-fidelity method can remarkably improve the efficiency of a design optimization based on a high-fidelity and expensive numerical simulation, with assistance of lower-fidelity and cheaper simulation(s). However, most existing works only incorporate “two” levels of fidelity, and thus efficiency improvement is very limited. In order to reduce the number of high-fidelity simulations as many as possible, there is a strong need to extend it to three or more fidelities. This article proposes a novel variable-fidelity optimization approach with application to aerodynamic design. Its key ingredient is the theory and algorithm of a Multi-level Hierarchical Kriging (MHK), which is referred to as a surrogate model that can incorporate simulation data with arbitrary levels of fidelity. The high-fidelity model is defined as a CFD simulation using a fine grid and the lower-fidelity models are defined as the same CFD model but with coarser grids, which are determined through a grid convergence study. First, sampling shapes are selected for each level of fidelity via technique of Design of Experiments (DoE). Then, CFD simulations are conducted and the output data of varying fidelity is used to build initial MHK models for objective (e.g. C_D) and constraint (e.g. C_L, C_m) functions. Next, new samples are selected through infill-sampling criteria and the surrogate models are repetitively updated until a global optimum is found. The proposed method is validated by analytical test cases and applied to aerodynamic shape optimization of a NACA0012 airfoil and an ONERA M6 wing in transonic flows. The results confirm that the proposed method can significantly improve the optimization efficiency and apparently outperforms the existing single-fidelity or two-level-fidelity method.     

Phase quantized SAR signal processing: theory and experiments

A complete theory of synthetic aperture radar (SAR) processing from phase-only data is presented, for both analogue and coded raw signals and reference functions. Appropriate processing architectures are presented as well as quality indexes to judge performance of the selected coding. The analysis encompasses both images and interferometric fringes for interferometry SAR (IFSAR) application. Theoretical results are validated by a large number of experiments, comparing the performance of the suggested coding levels, and their combinations between raw and reference functions. Experiments are extended until the comparison of final digital elevation models     

疲劳断口形貌与材料性能的关系

1.疲劳断口分析概述 当金属材料承受循环载荷时,随材料的初始状态以及应力-应变水平不同,将在整个体积内发生循环硬化或循环软化现象。开始时,这些变化是在受载金属中均匀发生的。当循环周次达到一定的数值时,某些变化将集中在局部区域进行,并导致出现微观疲劳裂     

基于高阶耗散紧致格式的GMRES方法收敛特性研究

计算效率较低是当前限制高阶精度计算方法应用的重要因素。为了提高高阶精度混合型耗散紧致格式（HDCS）的计算效率,发展了适合多块对接网格的广义最小残值（GMRES）方法,并利用GMRES方法开展了HDCS格式的加速收敛研究。首先研究了GMRES的预处理方法、CFL数和内层迭代步数对HDCS数值模拟收敛特性的影响,计算结果显示：点松弛方法是一种高效的预处理方法;CFL数对计算收敛速度影响较大;GMRES方法存在最优的内层迭代步数。利用GMRES方法完成了NACA 0012翼型绕流、NLR 7301翼型绕流和DLR-F4翼身组合体绕流的数值模拟,并与其他隐式时间推进方法进行了对比,GMRES方法计算更加稳定,并且计算效率相对LU-SGS（Lower-Upper Symmetric Gauss-Seidel）方法可以提高5倍以上。研究结果表明,本文发展的GMRES方法在多块对接网格中具有良好的计算稳定性,计算结果的残差可以收敛到更低的量级,并且可以较大幅度地提高高阶精度数值模拟的计算效率。     

试验设计法与BP-GA算法在复合材料结构优化中的应用

试验设计法具有均衡分散性和整齐可比性,通过试验设计进行响应面的样本选取可以大大减少样本的数目,同时保证响应面的训练精度。响应面与遗传算法相结合在工程应用中已经形成一套适用于复杂结构设计的高效优化方法。本文以复合材料加筋蒙皮的质量优化问题为例,通过正交试验进行响应面训练样本选取,通过PATRAN/NASTRAN进行有限元分析得出样本的数值,应用神经网络(BP)训练、构建优化的目标函数和强度、稳定性约束响应面,结合其他常规约束条件通过遗传算法(GA)进行优化,验证了这套算法的有效性和实用性。     

Performance analysis of conformal conical arrays for airborne vehicles

Conformal array apertures have great potential for providing high performance, low weight systems with little or no impact to the aerodynamic design of the air vehicle. A performance analysis of conformal conical arrays for a national airborne radar application is presented. The conical array geometry is chosen for its similarity to an aircraft or missile nosecone. Performance capabilities are analyzed for a number of antenna performance parameters including scan volume, sidelobe levels, grating lobes, beamwidth, directivity, element count, and cross-polarization     

一种面向飞机族的结构优化方法

 飞机族是一组共享通用部件或子系统的、但性能或使用要求不同的相关飞机产品的集合。针对通用模块化飞机族结构优化问题,提出一种基于代理模型的二级优化方法。该方法将结构优化分为两个层次：通用模块结构优化层次和专用模块结构优化层次。这种方法不但流程简单,且各层次的设计变量和约束较少,并且专用模块优化层次之间具有独立性,易于实现并行设计。以某支线客机族的机翼结构优化问题为例,首先阐述飞机族结构优化模型,然后应用本文提出的二级优化方法求解支线客机族的机翼结构优化问题。优化结果表明该方法能成功解决飞机族的结构优化设计问题,能同时优化飞机族中各型号的结构参数。     

Data and performances of selected aircraft and rotorcraft

The purpose of this article is to provide a synthetic and comparative view of selected aircraft and rotorcraft (nearly 300 of them) from past and present. We report geometric characteristics of wings (wing span, areas, aspect-ratios, sweep angles, dihedral/anhedral angles, thickness ratios at root and tips, taper ratios) and rotor blades (type of rotor, diameter, number of blades, solidity, rpm, tip Mach numbers); aerodynamic data (drag coefficients at zero lift, cruise and maximum absolute glide ratio); performances (wing and disk loadings, maximum absolute Mach number, cruise Mach number, service ceiling, rate of climb, centrifugal acceleration limits, maximum take-off weight, maximum payload, thrust-to-weight ratios). There are additional data on wing types, high-lift devices, noise levels at take-off and landing. The data are presented on tables for each aircraft class. A graphic analysis offers a comparative look at all types of data. Accuracy levels are provided wherever available.