Improved bolometric corrections for WR stars

Evolutionary models allow an assignment of both a mass and a luminosity to a Wolf-Rayet (WR) star in a cluster, and hence allow a determination of the Bolometric Correction (B.C.). The B.C.'s derived for WN stars range from –4.0 to –6.0 with the expected trend of larger values (in absolute values) for stars with higher excitation spectra. For WC stars, there is little evidence for a similar trend; most observations presented here are consistent with B.C.=–4.5, as found by Smith and Maeder (1989). The convergence of B.C. values derived from evolutionary and atmospheric models is extremely satisfactory, giving increased confidence in both methods.     

AC/DC converter topologies for the Space Station

A new class of AC/DC converter topologies (Type-1 converters) is described, suitable for use in an advanced single-phase sine-wave voltage, high-frequency power distribution system, of the type that was proposed for a 20 kHz Space Station primary electrical power distribution system. The converter comprises a transformer, a resonant network, a current controller, a diode rectifier, and an output filter. The input AC voltage source is converted into a sinusoidal current source using the resonant network. The output of this current source is rectified by the diode rectifier and is controlled by the current controller. The controlled rectified current is then filtered by the output filter to obtain a constant voltage across the load. Three distinct converter topologies, Type-1A, Type-1B, and Type 1-C, are described, and their performance characteristics are presented. All three types have a close-to-unity rated power factor (greater than 0.98), low total harmonic distortion in input current (less than 5%), and high conversion efficiency (greater than 96%)     

Coarse attitude determination from Earth albedo measurements

Attitude estimation algorithms for the Thrusted Vector Mission which determine attitude based on Sun sensor and very coarse albedo sensor measurements are presented. On the basis of these measurements, it has been demonstrated by comparison with more accurate gyro-based attitude that it is possible to estimate three-axis attitude with an average error per axis of 11 deg. Most of this error is about the Sun direction. Both deterministic quick-look and optimal estimates are examined     

Image texture synthesis-by-analysis using moving-average models

A synthesis-by-analysis model for texture replication or simulation is presented. This model can closely replicate a given textured image or produce another image that although distinct from the original, has the same general visual characteristics and the same first and second-order gray-level statistics as the original image. The texture synthesis algorithm, proposed contains three distinct components: a moving-average (MA) filter, a filter excitation function, and a gray-level histogram. The analysis portion of the texture synthesis algorithm derives the three from a given image. The synthesis portion convolves the MA filter kernel with the excitation function, adds noise, and modifies the histogram of the result. The advantages of this texture model over others include conceptually and computationally simple and robust parameter estimation, inherent stability, parsimony in the number of parameters, and synthesis through convolution. The authors describe a procedure for deriving the correct MA kernel using a signal enhancement algorithm, demonstrate the effectiveness of the model by using it to mimic several diverse textured images, discuss its applicability to the problem of infrared background simulation, and include detailed algorithms for the implementation of the model     

Performance analysis for DOA estimation algorithms: unification,simplification, and observations

Subspace based direction-of-arrival (DOA) estimation has motivated many performance studies, but limitations such as the assumption of an infinite amount of data and analysis of individual algorithms generally exist in these performance studies. The authors have previously proposed a unified performance analysis based on a finite amount of data and achieved a tractable expression for the mean-squared DOA estimation error for the multiple signal classification (MUSIC). Min-Norm, estimation of signal parameters using rotational invariance techniques (ESPRIT), and state-space realization algorithms. However, this expression uses the singular values and vectors of a data matrix, which are obtained by the highly nonlinear transformation of the singular value decomposition (SVD). Thus the effects of the original data parameters such as numbers of sensors and snapshots, source coherence and separations were not explicitly analyzed. The authors unify and simplify this previous result and derive a unified expression based on the original data parameters. They analytically observe the effects of these parameters on the estimation error     

Multiposition alignment of strapdown inertial navigation system

The authors demonstrate that the stationary alignment of strapdown inertial navigation system (SDINS) can be improved by employing the multiposition/technique. Using an observability analysis, it is shown that an optimal two-position alignment not only satisfies complete observability conditions but also minimizes alignment errors. This is done by analytic rank testing of the stripped observability matrix and numerical calculation of the error covariance. It is also shown that an optimal three-position alignment accelerates the convergence of the alignment error compared with two-position alignment     

航空发动机润滑系统通用分析软件开发

发展了一种全新的航空发动机润滑系统通用分析软件.采用流动换热的网络算法, 将发动机润滑系统分解成由相应的典型元件和节点组成的网络, 用有限的元件和流动介质类型就可描述通用的发动机润滑系统.元件分为简单元件和组合元件, 并和润滑系统结构一一对应, 方便了元件的初始化.软件前台采用图形建模方式生成计算网络, 后台采用了网络自动识别技术, 方便了复杂网络的建立.可视化界面及支撑数据库, 提高了软件的易用性.      

航空发动机数控系统中LVDT传感器信号处理及在线故障检测

通过对线位移差动变压器(LVDT)信号处理电路的深刻研究, 给出了芯片发热、温漂、信号精度等问题的一般解决方法, 提出了一种通过软、硬件结合, 在不增加外围器件的情况下对LVDT传感器进行在线故障检测的定值判断法.发动机台架试验结果表明, 所提出的方法是行之有效的.      

高维小失效概率下的改进线抽样方法

 针对隐式极限状态方程的可靠性分析,提出了一种基于失效域样本模拟的高效线抽样法。该法采用马尔可夫链来快速得到失效域中的条件样本,利用这些失效域中的样本可以得到线抽样的重要方向,并且这些样本还可以作为线抽样的样本,来得到隐式极限状态方程失效概率的估计值。由于该法可以较准确地得到线抽样方法的重要方向,因此线抽样的效率可以得到提高,另外失效域的样本又可以作为线抽样的样本,从而进一步降低所提方法的计算工作量。算例表明所提算法的计算精度和效率均高于传统线抽样方法。     

A Possible Transfer Mechanism for the 11-Year Solar Cycle to the Lower Stratosphere

Observational evidence of the 11-year solar cycle (SC) modulation of stratosphere temperatures and winds from the ERA-40 dataset is reviewed, with emphasis on the Northern winter hemisphere. A frequency modulation of sudden warming events is noted, with warmings occurring earlier in solar minimum periods than in solar maximum periods. The observed interaction between the influence of the SC and the quasi biennial oscillation (QBO) on the frequency of sudden warmings is noted as a possible clue for understanding their mechanism of influence. A possible transfer route for the 11-year solar cycle from the equatorial stratopause region to the lowest part of the stratosphere is proposed, via an influence on sudden warming events and the associated induced meridional circulation. SC and QBO composites of zonal wind anomalies show anomalous wind distributions in the subtropical upper stratosphere in early winter. Mechanistic model experiments are reviewed that demonstrate a sensitivity of sudden warmings to small wind anomalies in this region. Various diagnostics from these experiments are shown, including EP fluxes and their divergence and also the synoptic evolution of the polar vortex, in order to understand the mechanism of the influence. Some recent GCM experiments to investigate the SC/QBO interaction are also described. They simulate reasonably well the observed SC/QBO interaction of sudden warming events and appear to support the hypothesis that tropical/subtropical upper stratospheric wind anomalies are an important influence on the timing of sudden warmings.