Application of focused laser differential interferometer to hypersonic boundary-layer instability study

Accurate prediction of hypersonic boundary-layer transition plays an important role in thermal protection system design of hypersonic vehicles. Restricted by the capability of spatial diagnostics for hypersonic boundary-layer study, quite a lot of problems of hypersonic boundary-layer transition, such as nonlinearity and receptivity, remain outstanding. This work reports the application of focused laser differential interferometer to instability wave development across hypersonic boundary-layer on a flared cone model. To begin with, the focused laser differential interferometer is designed and set up in a Mach number 6 hypersonic quiet wind tunnel with the focal point in the laminar boundary-layer of a 5 degree half-angle flared cone model. Afterwards, instability experiments are carried out by traversing the focal point throughout the hypersonic boundary-layer and the density fluctuation along the boundary-layer profile is measured and analyzed. The results show that three types of instability waves ranging from 10 kHz to over 1 MHz are co-existing in the hypersonic boundary-layer, indicating the powerful capability of focused laser differential interferometer in dynamic response resolution for instability wave study in hypersonic flow regime; furthermore, quantitative analyses including spectra and bicoherence analysis of instability waves throughout the hypersonic boundary-layer for both cold and heated cone models are performed.     

空间红外遥感相机制冷机微振动对MTF影响分析

空间红外遥感相机内部活动部件微振动载荷会引起相机光学元件产生刚体位移和面形变化,从而影响相机的在轨光学成像品质。文章基于某空间红外遥感相机内部扰振源的测试结果,建立了适用于微振动分析的相机精细有限元模型,经分析计算,确定了基于Zernike多项式曲面拟合和调制传递函数(MTF)分析的方法和流程;最后结合某空间红外遥感相机进行MTF影响分析,结果显示制冷机微振动对该相机MTF影响在工程可接受的范围内。     

地月第二拉格朗日点卫星激光测距技术研究

介绍了月球/中继星激光测距的科学意义、研究状况和发展趋势;研究了地月第二拉格朗日点(L2点)纯反射式激光测距技术和任务设计,主要包含单体大孔径激光角锥反射器的设计与研制,以及基于云南天文台1.2 m望远镜的月球/中继星激光测距系统研究。研究结果表明:采用单脉冲能量3 J和10 ns脉宽脉冲激光器,预期系统能接收到的单脉冲回波光电子数约为0.74,单光子测距精度优于1 m。     

Rationale and systems architecture for a near-term human lunar return

Early human missions to the Moon have landed on six different sites on the lunar surface. These have all been in the low-latitude regions of the near side of the Moon. Early missions were designed primarily to assure crew safety rather than for scientific value. While the later missions added increasingly more challenging science, they remained restricted to near-side, low-latitude sites. Since the 1970s, we have learned considerably more about lunar planetology and resources. A return within the next five to ten years can greatly stimulate future human space exploration activities. We can learn much more about the distribution of lunar resources, especially about hydrogen, hydrated minerals, and water ice because they appear to be abundant near the lunar poles. The presence of hydrogen opens the possibility of industrial use of lunar resources to provide fuel for space transportation throughout the solar system.This paper discusses the rationale for near-term return of human crews to the Moon, and the advantages to be gained by selecting the Moon as the next target for human missions beyond low-Earth orbit. It describes a systems architecture for early missions, including transportation and habitation aspects. Specifically, we describe a primary transportation architecture that emphasizes existing Earth-to-orbit transportation systems, using expendable launch vehicles for cargo delivery and the Space Shuttle and its derivatives for human transportation. Transfer nodes should be located at the International Space Station (ISS) and at the Earth-Moon L1 (libration point).Each of the major systems is described, and the requisite technology readiness is assessed. These systems include Earth-to-orbit transportation, lunar transfer, lunar descent and landing, surface habitation and mobility, and return to Earth. With optimum reliance on currently existing space systems and a technology readiness assessment, we estimate the minimum development time required and perform order-of-magnitude cost estimates of a near-term human lunar mission.     

USA-193 decay predictions using public domain trajectory data and assessment of the post-intercept orbital debris cloud

In early 2008, the need arose to predict the orbital decay of the American spacecraft USA-193, whose characteristics, function and orbit were classified information. With no orbit data and independent Italian tracking capability available, we turned our attention on the orbits determined by a worldwide network of about 20 visual satellite observers. The orbits of USA-193 obtained from their visual observations were therefore used as the sole source of orbit information. Contrary to our expectations, this exercise was extremely successful and we learned a lot in the process. The orbits provided by the visual observers were very accurate for such a low satellite (although the minimum and very stable level of solar activity helped considerably); however, data gaps of a few days were sometimes possible, due to unfavorable pass geometry or weather and light conditions. In any case, the orbital period and the semimajor axis were so accurate that it was possible for us to obtain very good decay fits using special perturbation software, including various atmospheric density models together with all the other relevant perturbing accelerations. We were therefore able to estimate accurate values of the ballistic parameter and the resulting decay and reentry predictions were extremely stable. Amateur optical observations and images of USA-193 had also led to a rough estimation of the shape and sizes of the satellite, revealing that the solar arrays had never been deployed. With this information, and taking into account our estimates of the ballistic parameter, we obtained reasonable and consistent values of the spacecraft mass. Based on previous reentry fragmentation analyses, we were then able to guess the expected USA-193 casualty area, casualty expectancy, debris ground footprint and probability of impact in Italy. Lastly, after the decision by the US Government to destroy the satellite, we independently predicted the interception time windows and the post-event ground tracks. Following the successful spacecraft breakup, we analyzed the evolution of the resulting debris cloud and assessed its (very limited) adverse impact on the circumterrestrial environment.     

计算气动声学中的紧致滤波格式

从理论上分析了数值伪波产生的原因,要消除数值伪波可用滤波方法。为了使滤波器具有理想的截断特性,引入了修正的高斯函数。通过让滤波器的频响函数与修正的高斯函数逼近,利用序列二次规划(SQP)方法优化了五对角紧致滤波格式。优化的滤波格式提高了计算精度和效率,增强了数值稳定性,更易于实施。一维和二维算例体现了优化滤波格式的性能改进。     

带底部法兰筒形件切旋成形工艺研究

旋压成形工艺是一种少/无切削、成形力小的渐进成形技术,广泛应用于现代制造业。针对带底部法兰的筒形件提出一种切旋两步成形工艺,采用有限元方法对不同参数条件下的旋压成形过程进行数值模拟,得到旋轮倾角、旋轮圆角半径以及进给比等因素对成形过程中成形力大小和坯料的筒壁厚度分布以及应变分布的影响,并通过试验验证了工艺的可行性。     

一种两管径式脉动热管的流动与传热特性

为提高脉动热管的传热特性,提出了一种两管径式脉动热管结构,并基于质量、动量和能量守恒方程发展了适用的物理和数学模型。这种两管径式脉动热管对蒸发段和冷凝段取不同管径,两者的比值定义为直径比,应用上述理论模型分析了直径比对脉动热管运动规律和传热特性的影响。结果显示:采用两管径结构,可以有效提升脉动热管的自激振荡机制,特别是直径比小于1时的情况。而从传热特性而言,相比于传统等管径式脉动热管(直径比等于1),采用直径比小于1的结构可以使脉动热管的热阻明显减小,采用直径比大于1的结构却反而使传热特性下降。     

一种红外点目标图像高保真压缩方法

针对红外点目标图像的特点,融合目标检测与无损-近无损压缩技术,提出了一种目标-背景分类的高保真压缩方法。将红外图像分成若干子块,对每个子块进行基于"最大中值滤波"背景抑制算法的点目标检测,根据检测结果将图像子块分为包含疑似目标的目标子块和不含目标的背景子块,对目标子块图像无损压缩,对背景图像子块近无损压缩,从而提高红外遥感图像压缩比,降低数据传输量,减轻数据传输压力。实验结果表明:所提的方法与传统全图无损压缩方法相比,能在不损失点目标信息的高保真压缩前提下,使图像压缩比提高40%以上。该方法对星上实时处理系统、红外探测跟踪系统的设计具有一定的理论和工程应用价值。