Late stages of stellar evolution – Herschel’s contributions

Cool objects glow in the infrared. The gas and solid-state species that escape the stellar gravitational attraction of evolved late-type stars in the form of a stellar wind are cool, with temperatures typically ?1500 K, and can be ideally studied in the infrared. These stellar winds create huge extended circumstellar envelopes with extents approaching 10¹⁹${10}^{19}$ cm. In these envelopes, a complex kinematical, thermodynamical and chemical interplay determines the global and local structural parameters. Unraveling the wind acceleration mechanisms and deriving the complicated structure of the envelopes is important to understand the late stages of evolution of ∼97% of stars in galaxies as our own Milky Way. That way, we can also assess the significant chemical enrichment of the interstellar medium by the mass loss of these evolved stars. The Herschel Space Observatory is uniquely placed to study evolved stars thanks to the excellent capabilities of the three infrared and sub-millimeter instruments on board: PACS, SPIRE and HIFI. In this review, I give an overview of a few important results obtained during the first two years of Herschel observations in the field of evolved low and intermediate mass stars, and I will show how the Herschel observations can solve some historical questions on these late stages of stellar evolution, but also add some new ones.     

机器人镗孔加工系统稳定性分析

为了解决大型飞机装配现场主起落架交点孔的镗孔精加工问题,提出了采用六自由度工业机器人及专用末端执行器组合的创新解决方案。压脚是抑制机器人镗孔加工系统颤振的核心环节。通过对施加压脚前后的机器人镗孔加工系统进行动力学建模,对系统的稳定性进行分析,得出压脚装置对加工系统稳定性叶瓣图的影响。最后通过实际镗孔加工实验验证了机器人镗孔系统在不同压脚压力下的加工稳定性,表明合理的压脚压力可提高稳定切深,拓展加工稳定区域,有效避免加工颤振。     

基于舵面配平的飞机续航飞行航迹优化方法

舵面偏转除了能够提供保证飞机稳定飞行的配平力矩,还将改变飞机的升阻比,从而影响其续航性能。在基于飞机质点假设的传统航迹优化方法的基础上,研究了飞机的升阻特性对其续航性能的影响:在上升段、巡航段及下降段前期增大升阻比,在下降段后期减小升阻比,有利于提高飞机的续航性能;提出了基于舵面配平的续航飞行航迹优化方法,以获得更接近于发挥飞机实际潜力的最优航迹与最大航程。对于多操纵面布局飞机,通过该方法能够确定其最优舵面组合配平规律。算例飞机的优化结果表明:相比单一舵面配平,最优舵面组合配平能够使算例飞机的总航程最大提高7.5%。      

真空紫外辐照引起的聚合物材料质量损失模型及实验验证

基于扩散理论及辐射化学反应理论,建立真空紫外(VUV)辐照引起的聚合物材料质量损失数学模型。采用分离变量法与特征函数展开法,获得材料中挥发物质量随辐照剂量变化的函数关系式。通过实验得到聚酰亚胺(PI)薄膜和聚酯(PET)薄膜的质量损失数据,拟合求解模型中的参数,验证模型的适用性。实验结果表明,该质量损失模型适用于PI薄膜及PET薄膜,借助该模型可预估VUV辐照引起的材料的质量损失。     

尖楔前体飞行器FADS系统驻点压力对神经网络算法精度的影响

针对尖楔前体（类乘波体）飞行器用嵌入式大气数据传感（FADS）系统存在建模困难及解算模型精度低的问题,首先采用BP神经网络建模代替传统的FADS系统空气动力学建模的方法,建立含有双隐含层的四层神经网络模型,然后通过合理选择网络结构参数及训练验证,对FADS系统的攻角进行解算,最后对驻点压力对算法精度的影响进行研究。结果表明,本文建立的含有双隐含层的四层神经网络模型精度较高,攻角测试误差小于 0.25°; 驻点压力是否作为输入参数对FADS系统神经网络算法求解精度影响较大,攻角测试误差相差达0.1°。在飞行器前缘半径允许的情况下,应尽量得到驻点压力用于解算攻角,提高解算精度。     

载人航天器大气环境控制系统性能集成分析

考虑到载人航天器大气环境控制系统设计参数和控制参数众多,文章建立了一种载人航天器大气环境控制系统性能集成仿真分析模型,包括舱体模块、航天员模块、舱压控制模块、温湿度控制模块和二氧化碳净化模块。利用该模型对载人航天器常规工作模式下大气环境控制系统性能进行了计算分析,得到了在不同热负荷水平下载人航天器密封舱空气各个参数随在轨时间的变化趋势,结果表明:氧分压控制、二氧化碳净化和人区温湿度控制之间存在着密切的相互影响关系,不可孤立地进行分析。此外,文章还分析确定了非常规工作模式下热负荷水平允许上限,为载人航天器工作模式的确定提供了依据。研究结果有助于载人航天器大气环境控制系统的设计和流程改进。     

Dynamical evolution of high area-to-mass ratio objects in Molniya orbits

For special demands, some notable orbit types have been developed by human, including the Molniya orbits, which have a relatively high eccentricity up to about 0.7, and a period of 12 h. Considering that space debris with high area-to-mass ratio (A/M) has been discovered, such objects may also exist in Molniya orbits due to spacecraft and upper stages fragmentation events. However, there are not sufficient studies of the complex dynamical phenomena of such orbits. These studies can enrich the knowledge about the long-term evolution of these orbits, be helpful to propose uncatalogued objects observation and identification, and also set the protected region as well as active debris removal. In this paper, the characteristics of 2:1 resonance of Molniya satellite orbits are studied. A large set of numerical simulations, including all the relevant perturbations, is carried out to further investigate the main characteristics, and special attention is payed to the dynamical evolution of objects with high A/M, particularly affected by the direct solar radiation pressure. The long-term dynamical evolution of orbital elements, as well as the dependency of lifetime on the A/M value, is discussed.     

下腹板长度对单边膨胀喷管性能的影响

在落压比为4~60范围内,对两个不同下腹板长度的单边膨胀喷管模型进行了试验研究,得到了其壁面静压分布,并对4种不同下腹板长度的喷管模型进行了数值模拟研究,获得了总体性能参数.结果表明:在整个落压比范围内,喷管流动都呈现很强的三维特征,而且在高落压比条件下上膨胀面两侧边气流出现了横向分离.喷管轴向推力系数峰值随下腹板长度增大而增加,峰值对应的落压比也逐渐增大并趋近设计点.在落压比为30~80范围内,轴向推力系数随着下腹板长度增大而增加,在设计点轴向推力系数的最大值和最小值相差约1.8%;在特定低落压比条件下,较短下腹板长度的喷管具有高的轴向推力系数.气流矢量角随下腹板长度增加而增大,在设计点最大值和最小值分别为6.03°和-1.83°.      

周向总压畸变进气下叶片前掠对转子稳定性的影响

以跨声速压气机NASA转子11为原型进行前掠改进,对得到的前掠转子与原型转子内流场进行了定常数值模拟,结果表明:在设计转速下前掠转子的性能有所提高,尤其是近失速流量有明显的降低.在周向总压畸变进气条件下,对转子11和前掠转子进行了非定常数值模拟,两者的性能较均匀进气条件下都出现了不同程度的下降,但前掠转子的近失速流量仍明显低于转子11,这是因为转子11叶尖区域处气流较前掠转子更易堵塞.      

两轮升压式空气动压轴承涡轮冷却器起浮压力计算

通过分析两轮升压式空气动压轴承涡轮冷却器的起浮原理,建立了计算起浮压力的方法.按照该方法编制程序计算了某型涡轮冷却器在不同条件下的起浮压力,计算结果与试验实测数据吻合较好,说明计算方法具有较高准确度.通过计算和分析可以看到涡轮出口背压和轴承预紧对轴承起浮影响较大,背压越低则起浮压力越低;轴承预紧力越大则起浮压力越高.