Alfvn Waves in the Solar Wind

Alfvn waves are found to be ubiquitous in the solar wind.Recent progress in observational studies of the waves is reviewed to formulate a microscopic picture for the Alfvenic fluctuations. The main aspects of the observational properties of these waves,including the wave intervals, propagation,evolution,origin and generation,are presented.Then Alfven wave heating and acceleration of the solar wind plasma are briefly introduced.The relation of the waves to rotational and tangential discontinuities,magnetic decreases,and other relatively large-scale structures such as flux tubes/ropes,magnetic clouds and interplanetary coronal mass ejections in the solar wind is particularly investigated.Finally,some remaining open questions are also indicated due to their fundamental importance of understanding of the physical nature of Alfven waves and the role of the waves in heating and accelerating the solar wind.     

Is the presence of oxygen on an exoplanet a reliable biosignature?

We revisit the validity of the presence of O(2) or O(3) in the atmosphere of a rocky planet as being a biosignature. Up to now, the false positive that has been identified applies to a planet during a hot greenhouse runaway, which is restricted to planets outside the habitable zone (HZ) of the star that are closer to the star. In this paper, we explore a new possibility based on abiotic photogeneration of O(2) at the surface of a planet that could occur inside the HZ. The search for such a process is an active field of laboratory investigation that has resulted from an ongoing interest in finding efficient systems with the capacity to harvest solar energy on Earth. Although such a process is energetically viable, we find it to be a very unlikely explanation for the observation of O(2) or O(3) in the atmosphere of a telluric exoplanet in the HZ. It requires an efficient photocatalyst to be present and abundant under natural planetary conditions, which appears unlikely according to our discussion of known mineral photochemical processes. In contrast, a biological system that synthesizes its constituents from abundant raw materials and energy has the inherent adaptation advantage to become widespread and dominant (Darwinist argument). Thus, O(2) appears to continue to be a good biosignature.     

Prebiotic significance of extraterrestrial ice photochemistry: detection of hydantoin in organic residues

The delivery of extraterrestrial organic materials to primitive Earth from meteorites or micrometeorites has long been postulated to be one of the origins of the prebiotic molecules involved in the subsequent apparition of life. Here, we report on experiments in which vacuum UV photo-irradiation of interstellar/circumstellar ice analogues containing H(2)O, CH(3)OH, and NH(3) led to the production of several molecules of prebiotic interest. These were recovered at room temperature in the semi-refractory, water-soluble residues after evaporation of the ice. In particular, we detected small quantities of hydantoin (2,4-imidazolidinedione), a species suspected to play an important role in the formation of poly- and oligopeptides. In addition, hydantoin is known to form under extraterrestrial, abiotic conditions, since it has been detected, along with various other derivatives, in the soluble part of organic matter of primitive carbonaceous meteorites. This result, together with other related experiments reported recently, points to the potential importance of the photochemistry of interstellar "dirty" ices in the formation of organics in Solar System materials. Such molecules could then have been delivered to the surface of primitive Earth, as well as other telluric (exo-) planets, to help trigger first prebiotic reactions with the capacity to lead to some form of primitive biomolecular activity.     

DIRECT NUMERICAL SIMULATIONS OF MULTIPHASE FLOWS

INTRODUCTIONMultiphase and multifluid flows are commonin many natural and technologically importantprocesses. Rain,spray combustion,spray paint-ing,and boiling heat transfer are just a few ex-amples.While it is the overall,integral charac-teristics of such flows that are of most interest,the global behavior is determined to a large de-gree by the evolution of the smallestscales in theflow.The combustion of sprays,for example,depends on the size and the number density ofthe drops.Generally…     

PARAMETRIC OPTIMIZATION AND STRUCTURAL DESIGN OF NLS

INTRODUCTIONIn general,nitrogen launching system[1,2 ] iscomprised of nitrogen cylinder,launching mecha-nism,framework and electrical system,withcompressed Nitrogen as launching energy re-sources.A physical and mathematical model isdeveloped for describing the relations betweenthe detached time and the force,volume,pres-sure,flow,displacement,velocity,acceleration ofnitrogen cylinder and launching mechanism.Thekey technique of NL S is to control detached pa-rameters by changing the press…     

液氧/液氢高压膨胀循环发动机初步试验

膨胀循环具有很好的性能和很高的可靠性,这一点已由普拉特·惠特尼公司的“半人马座”上面级推进系统RL10发动机证实,但是这种循环对燃烧室压力增加和推力增加都有限制作用,因为它仅仅利用了燃料在冷却燃烧室和喷管延伸段时吸收的热能,来推进燃料和氧化剂涡轮泵.建议采用具有内部热交换器的先进方案燃烧室,这种热交换器可以收集许多燃烧热量.叙述使用内部热交换器的高压膨胀循环发动机的主要性能,并给出了这种发动机的初步试验结果.热交换器中的平均热通量是Bartz相关计算值的70%.燃烧性能不会因装了内部热交换器而受到严重影响.     

固体推进剂的非线性结构模型

在大多数固体推进剂特性的线性和非线性理论中,一个主要的缺陷在于忽略了内部损伤。对于那些已引出损伤参数的非线性理论,数值常数与推进剂的物理性质无关。而且,这些理论具有如此复杂的性质以至于要完成一个庞大的实验测试程序。已经提出了一个表示固体推进剂性质的简单方法。这个方法以一个基本定律为基础,该定律用一个应力衰减函数修正粘弹表达式。根据单轴试验数据,可用衰减函数表示推进剂的非线性性质。在卸载期损伤的表达式中,这个方法比过去的方法显示出重大的改进。分别定义松弛和再加载为过去最大应力的正态函数。利用这种方法对复杂的过程进行了预测,而且实测应力与预测应力很接近。     

装有自由旋转尾翼的鸭式导弹的滚动发散

用解析的方法研究有自由旋转尾翼和鸭式控制面的导弹的滚动运动,确定了运动的平衡状态;探讨在所施加的铰链力矩不变时小扰动的情况;利用对平衡状态小偏差的线性运动方程,推导出稳定的显式条件,并用全运动方程数值解考察了大扰动情况和在俯仰、偏航上导弹自由度的影响,证明了大偏差的发散特性.研究结果表明,滚动运动在很大的参数范围内是发散的,产生发散的主要因素是气动力滚动阻尼的减少和离心力对控制面的作用.     

火箭发动机推力瞬变的测量

任何测力装置都能用一个质量弹簧系统来描述,最简单地说:即通过一个具有刚度 K 和质量 M 的简单弹簧来描述,其中它的阻尼为 c,位移为 x,于是有M(d~2x/dt~2) c(dx/dt) kx=T,这里 T 为外加力。而大多数系统只测量 kx 项,这对低频情况是足够的。为了精确地记录推力的变化,从而要提高装置的固有频率,使之相对于推力变化的频率来说足够高,有时这是不现实的。为克服这一困难有两种可采用的途径。第一种方法是把运动方程的所有项合并到远远超过装置的固有频率所给定外力上。第二种方法是一个质量块在沿轴向作用的轴承上被发动机自由加速,加速度计给出推力/时间曲线。通过使质量块停止所需要的力对所预定时间内的积分而得到总冲。     

碳/碳复合材料

引言近年来,欧洲动力公司(S.E.P.)对固体发动机喷管部件所用的碳材料,特别是碳/碳材料进行了研制。喷管材料具有耐烧蚀的特性,要经受3000℃以上的高温。由于喷管性能要求非常高,材料必须具有良好的耐烧蚀性能和较低的比重。考虑到在大多数喷管里,化学反应发生在还原介质中,我们认为所能使用的耐热材料中,碳是最可取的。在此文中,我们不评述构成喷管部件的碳结构材料,只简要地论述多晶石墨、热解石墨、碳增强酚醛复合材料和碳/碳材料,侧重介绍后两种,这是欧洲动力公司重点研制项目。