超细磨粒砂轮修整后表面轮廓特征的实验研究

本文以不同的修整条件修整砂轮,用表面粗糙度仪测量砂轮表面轮廓曲线数据,通过FFT软件分析其频谱变化.实验结果表明,当修整深度较小时,砂轮表面出现明显的修整特征频率;当修整深度增大时,修整特征频率不明显.而修整进给率较小时,修整特征频率不明显;当修整进给率较大时,砂轮表面出现明显的修整特征频率.     

基于BP神经网络的GNSS-R海面风速反演

针对GNSS-R进行海面风速反演过程中时频域相关物理量较多，数据耦合性强等问题，提出了基于反向传播（BP）神经网络反演海面风速的方法。建立反演过程中相关观测量与风速的对应关系，选取多观测量作为输入，对输入数据进行处理，设置神经元与激励函数，使用BP神经网络自适应调整拟合参数，将风速作为神经网络输出端的特征量提取。反演结果，风速≤ 20 m/s时，反演均方根误差RMSE=1.21 m/s，风速>20 m/s时反演均方根误差RMSE=2.54 m/s，反演结果优于使用时延相关曲线前沿斜率（LES）和时延多普勒相关功率均值（DDMA）方法得到的反演结果，且迭代次数较少，复杂度较低，证明该方法可以应用于GNSS-R海面风速反演。     

Love Waves in Layered Graded Composite Structures with Imperfectly Bonded Interface

Theoretical analysis and numerical calculations of Love wave propagation in layered graded composites with imperfectly bonded interface are described in this paper. On the basis of WKB method, the approximate analytic solutions for Love waves are obtained. By the interface shear spring model, the dispersion relations for Love waves in layered graded composite structures with rigid, slip, and imperfectly bonded interfaces are given, and the effects of the interface conditions on the phase velocities of Love waves in SiC/Al lay- ered graded composites are discussed. Numerical analysis shows that the phase velocity decreases when the defined flexibility parameter is greater. For the general imperfectly bonded interface, the phase velocity changes in the range of the velocities for the rigid and slip interface conditions.     

Urban growth assessment around Winam Gulf of Kenya based on satellite imagery

Urban growth and population dynamics are among the most critical information needed for future economic development planning, natural resources allocation and environmental management. In the present work, two methods, the first based on night-time images produced by NOAA and population maps provided by Oak Ridge National Laboratory's (ORNL) LandScan, and the second one on SAR imagery, were used in order to assess the expansion of urban areas surrounding the Winam Gulf (Lake Victoria, Kenya) at different scales. In the time covered by night-time lights imagery, the study highlighted a period of constant growth rate between 2002 and 2006 and a negative trend after 2006 and 2008. This decrease may be related to two main events occurring in the study area between 2006 and 2007: the decline of the Lake Victoria level and the abnormal proliferation of the floating weeds within the Winam Gulf. Meanwhile, the urban feature extraction obtained at a different scale within a particular district from 1997 up to 2008 results in a constant growth rate. Population movements around this zone explain different dynamics that should be studied in detail in order to understand their particular roots.     

热解气体燃烧对炭化复合材料烧蚀热响应影响规律

在近空间高超声速飞行器飞行时间长、马赫数不断增加的发展趋势下，热防护与轻量化的矛盾越来越突出。基于此，开展了热解气体燃烧对炭化复合材料表面烧蚀影响的相关数值模拟研究，并与风洞试验结果进行了对比。结果表明：热解气体的燃烧可降低炭化复合材料表面的烧蚀厚度，并且随着气动热的增加，热解气体燃烧对材料表面碳的保护作用越来越明显。研究成果可为下一代近空间高超声速飞行器热防护系统的优化设计提供技术支撑。     

焊缝补强方式对大直径贮箱箱底承载性能影响

基于有限元法研究了典型10 m级直径贮箱箱底焊缝在双面对称补强、内表面补强及外表面补强三种情况下的内压承载性能。数值计算采用二维对称平面模型,考虑了筒段和短壳真实边界条件及焊缝对承载能力的影响,构建的考虑材料塑性的非线性数值分析模型准确地获得了贮箱箱底内外表面经向应力。结果表明:顶盖和瓜瓣焊缝区域是箱底的薄弱位置,使用内压下,双面对称补强的加强区内外表面经向应力差较小,结构均未进入塑性,单面补强的加强区内外表面经向应力差远大于双面对称补强,局部进入塑性,且外表面补强的加强区最大和最小经向应力差相对于内表面补强分别增加了9.7%和27.2%,变形不协调；设计内压下,箱底内外表面均有局部进入塑性,材料塑性对内外表面经向应力差有一定的缓解作用,能够显著缓解内外表面应力差造成的附加弯矩；双面对称补强优于内表面补强,内表面补强优于外表面补强,单面补强易产生附加弯矩,不利于箱底的均匀承载和变形协调。该研究结果指导了大直径贮箱箱底结构的优化设计。     

MHD surface waves – a source of global magnetospheric modes (resonances)?

Various mechanisms have been proposed for explanation of the global magnetospheric modes whose frequency peaks are in the frequency range 1–4 mHz. Recent papers claim: basic characteristics of the 1–4 mHz activity events observed on ground give evidences for an existence of MHD surface mode excited on the Earth magnetopause. The discrete frequencies of such MHD surface wave modes suggest an emergence of standing wave structures along the magnetic field lines lying on the magnetopause. Such discrete frequencies of MHD surface waves on magnetopause however, are not stable, at all. Contrariwise, MHD surface wave modes supported by the two plasma boundaries – the magnetopause and the plasmapause, are in accordance with existing experimental facts: discrete set of almost stable frequencies, field amplitude peaks and positions, energy dissipation, and field distribution from high to low latitudes. Mechanisms of the global magnetospheric mode resonance are pointed out as well as tools for their identification and discrimination.     

位错对形变304亚稳态不锈钢中奥氏体的电化学性能的影响

AISI 304亚稳态奥氏体不锈钢形变时容易发生相变,为避免产生相变马氏体而专注于研究位错的影响,采用局部电化学技术研究了拉伸应变分别为0%、10%、20%、30%和40%时的AISI 304不锈钢中奥氏体晶粒的电化学性能的变化规律,并分析了位错密度及位错组态对阻抗谱和动电位极化曲线等电化学性能的影响机理。研究结果表明:奥氏体的阻抗随位错密度的增大而降低,在低应变水平下位错密度的增大对阳极电流密度的降低有着重要作用;阳极电流密度随应变水平增加而增加,达到一个最大值后显著下降。通过扫描开尔文探针(SKP)的测量结果,计算得知位错堆积数比位错密度对阳极电流密度的影响更为显著,尤其是对于高应变水平不锈钢。     

风扇出口导向叶片低噪声设计Ⅰ:方法与优化

作为"风扇出口导向叶片（Outlet Guide Vanes，OGV）低噪声设计"系列文章的第1篇，本文在现有压气机气动设计流程的基础上，加入了噪声评估过程，建立了基于通流设计的气动/声学一体化设计方法。为提高设计阶段的评估速度，以三维升力面理论与管道声学理论为基础，从通流设计和造型设计输出中提取参数，结合转子尾迹模型，建立了转/静干涉噪声的解析预测模型。以现代大涵道比涡扇发动机风扇/增压级为对象，采用该模型系统分析了OGV轴向掠形与周向倾斜对转/静干涉噪声的影响，获取了轴向掠形角与周向倾斜角等三维设计参数与风扇噪声的关系图谱，初步确定了低噪声设计较优降噪量的掠和倾组合方案。以此为基础，在保持叶尖子午投影位置和弦长不变的前提下，将叶片前缘和径向积叠进行参数化，采用遗传算法进一步开展了OGV的低噪声优化设计，最终获得了2个优化方案，预估的降噪量达到了8 dB。