应用小波变换剔除超声多重反射的研究

利用小波变换具有空间局部化的特性和表征信号奇异性的能力,在适当尺度下刻画待检对象表面第1次超声回波信号的极大值点,从中选择最大值点作为脉冲重复频率(PRF)技术“与操作”的基准点,以满足PRF技术在消除超声多重反射时对“与操作”基准点需高精度定位的要求,从而解决传统方法对检测设备和操作条件严格一致的弊端。计算机仿真与实验研究表明提出的方法是可行的,并具有较强的鲁棒性和适用性。     

RANGE AMBIGUITY DISTRIBUTION CHARACTERISTIC OF SPACEBORNE SAR AND ITS DESIGN CONSIDERATION

A new approach is proposed which can satisfy the range ambiguity requirement while keep the antenna width not to be enlarged and get the high quality image also. Here the chirp slope of the transmitted linear frequency modulated pulses is reversed alternately. Therefore, during the range compression significant portion of the ambiguous return which is due to the mismatching with the signal reference function is suppressed. The suggested method provides significant improvement in range ambiguity ratio and can be utilized in attaining wider swath.     

Investigations on surrogate fuels for high-octane oxygenated gasolines

Gasoline is a complex mixture that possesses a quasi-continuous spectrum of hydrocarbon constituents. Surrogate fuels that decrease the chemical and/or physical complexity of gasoline are used to enhance the understanding of fundamental processes involved in internal combustion engines (ICEs). Computational tools are largely used in ICE development and in performance optimization; however, it is not possible to model full gasoline in kinetic studies because the interactions among the chemical constituents are not fully understood and the kinetics of all gasoline components are not known. Modeling full gasoline with computer simulations is also cost prohibitive. Thus, surrogate mixtures are studied to produce improved models that represent fuel combustion in practical devices such as homogeneous charge compression ignition (HCCI) and spark ignition (SI) engines. Simplified mixtures that represent gasoline performance in commercial engines can be used in investigations on the behavior of fuel components, as well as in fuel development studies. In this study, experimental design was used to investigate surrogate fuels. To this end, SI engine dynamometer tests were conducted, and the performance of a high-octane, oxygenated gasoline was reproduced. This study revealed that mixtures of iso-octane, toluene, n-heptane and ethanol could be used as surrogate fuels for oxygenated gasolines. These mixtures can be used to investigate the effect of individual components on fuel properties and commercial engines performance.     

数字图像合成器欺骗式干扰的识别方法

分析了数字图像合成器（DIS）生成的目标图像与真实目标图像之间的异同点,经过推导,得出识别数字图像合成器欺骗式干扰的方法：由于相位增量量化导致欺骗式假目标在横向多普勒方向上图像不连续;脉冲重复频率不匹配时,由DIS生成的欺骗式假目标经ISAR处理后在成像视窗内目标数量会发生变化。从而发现了数字图像合成器欺骗式干扰的缺陷,对识别这种干扰的假目标具有决定意义。     

汽油压燃发动机中燃烧化学动力学特性的模拟

利用PRF70掺混燃料作为汽油表征燃料进行仿真研究,将三维计算软件CONVERGE和Chemkin结合,研究了汽油压燃燃烧反应过程的主要放热反应并对其进行了相关的化学反应路径分析。结果表明:在汽油压燃燃烧反应过程中,不同反应对总放热率的贡献不同,由HCO+O_2CO+HO_2、CH2O+OHHCO+H2O、CH2CCH2OH+O_2CH2OH+CO+CH2O是燃料燃烧过程中对放热贡献最大的3个反应,其放出的热量远大于其他反应;汽油压燃燃烧反应过程中参与夺氢反应的自由基主要有4种,分别是HO_2、OH、H、O,在不同曲轴转角处上述4种自由基参与夺氢反应的重要性不同,HO_2夺氢反应所占比例始终领先其他3种,H、O参与的夺氢反应所占比例随曲轴转角的增加而增加,相应的OH参与夺氢反应所占比例减少;汽油压燃燃烧反应过程中,随着曲轴转角的变化,缸内温度升高,异辛烷发生高温裂解的比例增加。     

SAR方位模糊分析与抑制

方位模糊是合成孔径雷达特别是星载合成孔径雷达应用中所要解决的一个重要问题.本文给出了合成孔径雷达回波模型,分析了回波信号的特点,以及方位模糊产生的原因.从图像处理的角度出发提出了一种抑制方位模糊的方法,该方法主要思路是根据成像原理和成像参数,以及带有方位模糊的回波信号的特点,构建一个滤波器,用这个滤波器对压缩后的信号进行反卷积运算,从而达到抑制方位模糊的效果.通过点目标的仿真,验证了该方法的可行性.     

对参差脉冲重复间隔脉冲列的重频分选

分析了一种常见的参差脉冲重复间隔脉冲列的固有性质,其子周期在某范围内可以构成一等差数列,并在此基础上提出了一种新的基于直方图统计的重频分选算法．并对脉冲列的抽取算法做了改进,通过使用变步长的方法来确定起始脉冲．该算法提高了确定起始脉冲的正确率和脉冲抽取的效率．仿真结果表明该算法非常适合于参差脉冲重复间隔脉冲列的分选．     

Pseudo Relevance Feedback for Searech-Based Image Annotation

Search-based image annotation （SBIA） is a promising way of Automatic image annotation （AIA）. In this paper, Pseudo relevance feedback （PRF） is employed to SBIA （referred as PRF-SBIA） for effectively promoting the annotation performance. Given an un-annotated image as the query, PRF is triggered out for collecting more relevant images from the annotated images. With these relevant images, PRF-based probability model is built up to characterize the hidden relation between the visual content of images and the textual keywords. Furthermore, based on the text-based retrieval technique, a regularized factor with respect to the reliability of keywords is pro- posed to re-rank the annotation list in each round of PRF. The experimental results reveal that high annotation accuracy can be achieved via PRF-SBIA.     

Direct numerical simulations of the ignition of lean primary reference fuel/air mixtures with temperature inhomogeneities

The effects of fuel composition, thermal stratification, and turbulence on the ignition of lean homogeneous primary reference fuel (PRF)/air mixtures under the conditions of constant volume and elevated pressure are investigated by direct numerical simulations (DNSs) with a new 116-species reduced kinetic mechanism. Two-dimensional DNSs were performed in a fixed volume with a two-dimensional isotropic velocity spectrum and temperature fluctuations superimposed on the initial scalar fields with different fuel compositions to elucidate the influence of variations in the initial temperature fluctuation and turbulence intensity on the ignition of three different lean PRF/air mixtures. In general, it was found that the mean heat release rate increases slowly and the overall combustion occurs fast with increasing thermal stratification regardless of the fuel composition under elevated pressure and temperature conditions. In addition, the effect of the fuel composition on the ignition characteristics of PRF/air mixtures was found to vanish with increasing thermal stratification. Chemical explosive mode (CEM), displacement speed, and Damköhler number analyses revealed that the high degree of thermal stratification induces deflagration rather than spontaneous ignition at the reaction fronts, rendering the mean heat release rate more distributed over time subsequent to thermal runaway occurring at the highest temperature regions in the domain. These analyses also revealed that the vanishing of the fuel effect under the high degree of thermal stratification is caused by the nearly identical propagation characteristics of deflagrations of different PRF/air mixtures. It was also found that high intensity and short-timescale turbulence can effectively homogenize mixtures such that the overall ignition is apt to occur by spontaneous ignition. These results suggest that large thermal stratification leads to smooth operation of homogeneous charge compression-ignition (HCCI) engines regardless of the PRF composition.     

PECVD制备Al-Al2O3复合阻氢涂层的研究

为了提高不锈钢的阻氢性能,利用PECVD(等离子体增强化学气相沉积)技术在不锈钢表面上制备了Al薄膜,经真空热氧化处理(480℃)获得了Al-Al2O3复合薄膜.用表面分析手段SEM、 XRD、 EDS和XPS对所制备涂层的表面形貌、相结构、表面成分、表面元素的化学态进行了分析,并对其进行了阻氘性能研究.结果表明:该技术能够获得均匀、致密复合薄膜;在低于350℃时,阻氘效果非常好,几乎没有渗透,在350℃到450℃之间,阻氘因子能够达到250左右,但在高于450℃后,渗透非常明显.