脉冲激光刻蚀作用下冷喷涂沉积过程数值模拟

脉冲激光会烧蚀材料形成表面微纳结构,研究该刻蚀作用下的冷喷涂沉积过程,以提高表面修复质量与效率。首先,结合脉冲激光作用过程与实验观测结果建立了单/多粒子与基板的沉积模型;其次,运用热-结构耦合算法对存在表面微纳结构时的粒子/基板碰撞过程开展数值模拟,研究了脉冲激光刻蚀作用对冷喷涂材料沉积过程的影响;最后,分析了脉冲激光-冷喷涂复合修复过程中材料的沉积机理。结果表明,刻蚀作用形成的微纳结构使得粒子与基板的温度分布更加均匀,形变更加复杂,特别是碰撞中心区域的形变得到显著提升;且粒子碰撞在基板表面微纳结构的波峰处时应变、温升最大,波谷处时最小;脉冲激光-冷喷涂的沉积机理为塑性射流处的机械-物理结合与激光熔化区的冶金焊合。     

Observations of low-energy electrons from AE-C in the south polar cusp during the geomagnetic storm of September 21, 1977

The AE-C spacecraft skimmed through the southern polar cusp at a 400 km altitude during a large geomagnetic storm on September 21, 1977. This period has been designated as a special IMS period, and the AE-C data were acquired close to the times that data were acquired by the DMSP satellite at nearly the same location over the southern polar cap, and by the GEOS satellite located near the noon-meridian in the northern hemisphere. Low energy electrons (1-500 eV) were measured with the photoelectron spectrometer experiment experiment onboard AE-C. This instrument was operated in the mode which measured precipitating electron fluxes and backscattered electron fluxes in alternating 4s intervals with two sensors. A region of intense precipitating electron fluxes was observed near 0924 UT on September 21, 1977 extending from 69 degree invariant latitude at 1100 MLT to 72 degree invariant latitude at 1152 MLT. From the spectra of the precipitating electrons, this region is identified as the southern polar cusp. Since the K _p equals 7- during this time, the displacement of the cusp down to these low latitudes is not unreasonable. Particle data obtained from the DMSP satellite on orbits close to AE-C, confirm that the position of the cusp was rapidly changing during this period, and was displaced to latitudes equatorward of the quiet time position. A second region of intense fluxes of precipitating electron was observed by AE-C at approximately 0933 UT from 69 degree invariant latitude near 1700 MLT to 66 degree invariant latitude near 1730 MLT. This region of low energy electron fluxes is characterized by slightly harder energy spectra and is interpreted as being the afternoon auroral zone. The remarkable and fortunate location of the AE-C, DMSP, and GEOS spacecraft during this special IMS period will allow future correlative studies aimed at the determination of the shape of the magnetosphere during very disturbed conditions.     

楔形夹缝内部液体核态沸腾特性

针对楔形夹缝内部液体核态沸腾状况进行可视化实验研究。实验利用表面平整均匀的铜板作为加热平板,用玻璃作为盖板形成楔形夹缝。实验现象表明沸腾核化现象比较容易在狭缝中相对较大的空间中形成,而在狭缝中相对较小的空间里形成沸腾需要更高的热流密度。实验现象还表明,在同一空间尺度条件处的核化和气泡演化情况基本相同,气泡的生长速率在无量纲参数D≥0.1时受到空间尺度的明显影响。      

材料表面改性的一种新技术

 本文提出了一种高功率脉冲等离子体束使材料表面改性的新技术。脉冲等离子体束的功率密度约为(1～40)×10~4W/cm~2,运动能为1～5KeV,沉积能为1～25J/cm~2,脉冲时间为60μs。碳钢经脉冲等离子体束处理后,表面有新的氮化物和碳化物相(Fe_3N,(Cr,Fe)_7C_3,Cr_(23)C_6)产生;SEM金相照片表明,在钢表面区域有一层白亮层,它是碳钢高速淬火后所具有的特征;注入元素浓度沿深度的公布近似为高斯分布。表面显微硬度有了显著的提高。     

Tetrahedral Robotics for Space Exploration

A reconfigurable space filling robotic architecture has a wide range of possible applications. One of the more intriguing possibilities is mobility in very irregular and otherwise impassable terrain. NASA Goddard Space Flight Center is developing the third generation of its addressable reconfigurable technology (ART) tetrahedral robotics architecture. An ART-based variable geometry truss consisting of 12 tetrahedral elements made from 26 smart struts on a wireless network has been developed. The primary goal of this development is the demonstration of a new kind of robotic mobility that can provide access and articulation that complement existing capabilities. An initial set of gaits and other behaviors are being tested, and accommodations for payloads such as sensor and telemetry packages are being studied. Herein, we describe our experience with the ART tetrahedral robotics architecture and the improvements implemented in the third generation of this technology. Applications of these robots to space exploration and the tradeoffs involved with this architecture will be discussed.     

发散孔纵向波纹隔热屏气膜冷却特性

对燃烧室内开有发散孔的纵向波纹隔热屏进行了数值模拟.研究隔热屏的四种结构参数开孔率、波纹板高度、孔径和冷却通道高度的改变对隔热屏冷却效果的影响.研究表明:在气膜孔出流总量相同的情况下,3%开孔率比6%开孔率的隔热屏平均冷却效率较高;波纹板高度对隔热屏冷却效果影响较大,波纹板无量纲高度为1%比波纹板无量纲高度为2%和3.33%的隔热屏平均冷却效率高;孔径和冷却通道高度的改变对隔热屏冷却效果几乎没有影响.      

不同矢量偏角下二元喷管内流特性的数值研究

应用多重网格MacCormack显式格式对不同矢量角下二元喷管的内部流动进行了模拟分析。计算时, 在细网格上采用了雷诺平均N-S方程与k-ε湍流模型方程;在粗网格上则采用Euler方程。数值计算给出了较清晰的流谱, 壁面压力分布的计算也与实验结果相吻合。通过计算结果分析了矢量喷管几何参数对流场的影响, 并对矢量喷管的推力特性进行了数值研究。      

脉冲射流冲击平直表面的对流换热实验

利用红外热像仪测试了脉冲频率为10 Hz和占空比为50%的脉冲射流冲击平直表面的对流换热特性,在射流雷诺数为5 000~20 000、冲击间距比为2~8的范围内,与连续射流冲击换热进行了对比分析。研究结果表明,脉冲射流冲击对流换热系数依然具有随射流雷诺数增加而提高、沿径向急剧降低等基本特征,但是与连续射流相比,脉冲射流冲击引起的射流驻点和壁面射流区的对流换热存在差异,其影响与射流雷诺数和射流冲击间距密切相关;一般而言,在较大的射流冲击间距比下,脉冲射流体现出传热增强的效果,随着射流雷诺数的增加,脉冲射流较连续射流的优势更为明显;而在小射流冲击间距比下,连续射流则更具优势,在Re=20 000下仅当以3倍以上射流管直径作为区域半径进行平均时,脉冲射流才具有略高于连续射流冲击换热的作用效果。     

Investigating suitable orbits for the Swarm constellation mission – The frozen orbit

This paper proposes a suitable orbit design for the lower pair of ESA's Swarm constellation mission, flying side-by-side in near-polar and circular orbits with a separation of only 1.4° at ascending node. Both orbits are suggested to be frozen orbits to minimize the evolution, and an along-track separation strategy is applied to avoid collision risk. The characteristics of the proposed orbit type are examined through numerical techniques including high-fidelity perturbation models. The prime change from the initial configuration is an along-track separation. The perturbations causing the along-track drift are analyzed by switching on/off certain perturbations. The results indicate that the tesseral harmonics and the atmospheric drag yield dominant effects. The atmospheric drag effect shows a dependence on the local time of the ascending node. From two months of orbit propagation for the altitude 300 km the maximum along-track drift we obtain is about 80 km, which is still within the measurement requirement range. Several maneuver strategies for maintaining the proposed orbit design are suggested. The results analyzed for the proposed orbit design show that collision risk can be avoided by along-track separation within the frozen orbit design. Consequently, this combination is considered as a suitable approach for Swarm's lower pair.     

Coherent spatial filtering for SAR detection of stationary targets

Analysis of UHF synthetic aperture radar (SAR) images has revealed a spatially correlated phase structure over a resolved target such as a long wire or a large truck. This phase variation, approximated by a simple, range-dependent factor 4πr/λ, results in a line-shaped image spatial spectrum. Such image phase and spectral features can be exploited by coherent spatial filtering to improve target-to-clutter ratios of large targets and stationary target detectability in a strong clutter environment