全文获取类型
收费全文 | 593篇 |
免费 | 29篇 |
国内免费 | 19篇 |
专业分类
航空 | 259篇 |
航天技术 | 295篇 |
综合类 | 25篇 |
航天 | 62篇 |
出版年
2023年 | 20篇 |
2021年 | 3篇 |
2020年 | 26篇 |
2019年 | 17篇 |
2018年 | 44篇 |
2017年 | 26篇 |
2016年 | 2篇 |
2015年 | 3篇 |
2014年 | 42篇 |
2013年 | 12篇 |
2012年 | 13篇 |
2011年 | 19篇 |
2010年 | 12篇 |
2009年 | 24篇 |
2008年 | 50篇 |
2007年 | 51篇 |
2006年 | 30篇 |
2005年 | 29篇 |
2004年 | 10篇 |
2003年 | 13篇 |
2002年 | 6篇 |
2001年 | 40篇 |
2000年 | 8篇 |
1999年 | 7篇 |
1998年 | 19篇 |
1997年 | 2篇 |
1996年 | 14篇 |
1995年 | 14篇 |
1994年 | 62篇 |
1993年 | 15篇 |
1992年 | 1篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1984年 | 4篇 |
排序方式: 共有641条查询结果,搜索用时 640 毫秒
541.
G. Nath 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
The propagation of a strong cylindrical shock wave in an ideal gas with azimuthal magnetic field, and with or without axisymmetric rotational effects, is investigated. The shock wave is driven out by a piston moving with time according to power law. The ambient medium is assumed to have radial, axial and azimuthal component of fluid velocities. The fluid velocities, the initial density and the initial magnetic field of the ambient medium are assumed to be varying and obey power laws. Solutions are obtained, when the flow between the shock and the piston is isothermal. The gas is assumed to have infinite electrical conductivity and the angular velocity of the ambient medium is assumed to be decreasing as the distance from the axis increases. It is expected that such an angular velocity may occur in the atmospheres of rotating planets and stars. The shock wave moves with variable velocity and the total energy of the wave is non-constant. The effects of variation of the initial density and the Alfven-Mach number on the flow-field are obtained. A comparison is also made between rotating and non-rotating cases. 相似文献
542.
靳凤山 《华北航天工业学院学报》2001,(1)
债转股作为经济体制改革的一个重要探索,已经成为当今法学界和经济学界讨论的主题。作为一个新事物的出现,债转股必然会遇到一系列来自不同方向的阻碍,本文就这些困惑进行论述,阐明了债转股面临的主要问题,并提出相应的解决方案和建议。国民经济体制改革需要大胆的探索和创新,债转股就是一个很好的范例,尽管其实施过程中难免遇到一些难题,但必将取得骄人的成果。 相似文献
543.
A design study of wing tip devices at high and low speeds is described. The basis of the design study is an equivalent drag approach containing both aerodynamic drag gain and structural weight penalty. A comprehensive parameter study is carried out using a rapid aerodynamic prediction tool named Lift and Drag Component Analysis (LIDCA). Adding to an available lifting-line method a databased module for airfoil data is employed that uses results of two-dimensional flow simulations by multidimensional interpolation. Detailed validation studies of the method at high lift and high speed have demonstrated good accuracy. RANS computations of the selected wing tip designs confirm the predicted benefits at cruise condition. The results of the most effective wing tip designs are analysed at both flight conditions. Finally, options for improving the performance at take-off are suggested. 相似文献
544.
V. V. Izmodenov 《Space Science Reviews》2007,130(1-4):377-387
Interstellar atoms penetrate deep into the heliosphere after passing through the heliospheric interface—the region of the
interaction of the solar wind with the interstellar medium. The heliospheric interface serves as a filter for the interstellar
atoms of hydrogen and oxygen, and, to a lesser extent, nitrogen, due to their coupling with interstellar and heliospheric
plasmas by charge exchange and electron impact ionization. The filtration has great importance for the determination of local
interstellar abundances of these elements, which becomes now possible due to measurements of interstellar pickup by Ulysses
and ACE, and anomalous cosmic rays by Voyagers, Ulysses, ACE, SAMPEX and Wind. The filtration of the different elements depends
on the level of their coupling with the plasma in the interaction region. The recent studies of the filtration of the interstellar
atoms in the heliospheric interface region is reviewed in this paper. The dependence of the filtration on the local interstellar
proton and H atom number densities is discussed and the roles of the charge exchange and electron impact ionization on the
filtration are evaluated. The influence of electron temperature in the inner heliosheath on the filtration process is discussed
as well. Using the filtration coefficients obtained from the modeling and SWICS/Ulysses pickup ion measurements, the local
interstellar abundances of the considered elements are determined. 相似文献
545.
The ionic charge of solar energetic particles (SEP) as observed in interplanetary space is an important parameter for the
diagnostic of the plasma conditions at the source region and provides fundamental information about the acceleration and propagation
processes at the Sun and in interplanetary space. In this paper we review the new measurements of ionic charge states with
advanced instrumentation onboard the SAMPEX, SOHO, and ACE spacecraft that provide for the first time ionic charge measurements
over the wide energy range of ∼0.01 to 70 MeV/nuc (for Fe), and for many individual SEP events. These new measurements show
a strong energy dependence of the mean ionic charge of heavy ions, most pronounced for iron, indicating that the previous
interpretation of the mean ionic charge being solely related to the ambient plasma temperature was too simplistic. This energy
dependence, in combination with models on acceleration, charge stripping, and solar and interplanetary propagation, provides
constraints for the temperature, density, and acceleration time scales in the acceleration region. The comparison of the measurements
with model calculations shows that for impulsive events with a large increase of Q
Fe(E) at energies ≤1 MeV/nuc the acceleration occurs low in the corona, typically at altitudes ≤0.2 R
S
. 相似文献
546.
英语课堂提问策略调研 总被引:7,自引:0,他引:7
提问是课堂教学中最常见的教学活动,提问的质量将直接影响课堂教学活动的开展。提问的有效进行依赖于策略的使用,那么课堂提问中有哪些策略可以利用,目前大学英语课堂提问策略使用状况如何,本文结合在北京、河北两地的部分高校中所做的调研,揭示了目前大学英语课堂提问中存在的问题,也为更好地开展课堂教学提供的必要的启示。 相似文献
547.
548.
Michael Frank Michael Mederer Brigitte Stolz Thomas Hanschke 《Aerospace Science and Technology》2005,9(8):738-744
More than half of all flights in and out of Frankfurt International Airport are conducted by Lufthansa airlines, as it is one of their two major hubs. The so called hub and spokes systems allow airlines to bundle major passenger flows via connecting flights across the in- and outbound traffic. The reliability of these transfer connections, and specifically the arrival punctuality at the hub airports are crucial to the economics of the daily operation.Since Frankfurt International Airport has lately been one of the most congested airports in Europe, delays (in particular arrival delays) have increased quite significantly throughout the last years. To compensate for additional queueing time in the arrival processes, the scheduled block times inbound Frankfurt were continuously adjusted upwardly keeping the arrival punctuality (and thereby the connection reliability) close to stable. The two disadvantages of this approach have been the decrease in aircraft productivity and over-deliveries in peak hours, which in turn induce either additional block time delay or ground delay programs inbound Frankfurt.This paper introduces an iterative stochastic-simulation approach that models the vicious circle of lengthening block times and increased over-deliveries. In a second step it quantifies the operational and economical effects of depeaking the schedule and illustrates the overall growth potentials for the depeaked operation of the airport. The schedule was successfully implemented in Frankfurt in summer 2004. 相似文献
549.
Following on from IRAS, ISO has provided a huge advancement in our knowledge of the phenomenology of the infrared (IR) emission
of normal galaxies and the underlying physical processes. Highlights include the discovery of an extended cold dust emission
component, present in all types of gas-rich galaxies and carrying the bulk of the dust luminosity; the definitive characterisation
of the spectral energy distribution in the IR, revealing the channels through which stars power the IR light; the derivation
of realistic geometries for stars and dust from ISO imaging; the discovery of cold dust associated with H I extending beyond the optical body of galaxies; the remarkable similarity of the near-IR (NIR)/mid-IR (MIR) SEDs for spiral
galaxies, revealing the importance of the photo-dissociation regions in the energy budget for that wavelength range; the importance
of the emission from the central regions in shaping up the intensity and the colour of the global MIR luminosity; the discovery
of the “hot” NIR continuum emission component of interstellar dust; the predominance of the diffuse cold neutral medium as
the origin for the main interstellar cooling line, [C II] 158 μm, in normal galaxies.
Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries:
France, Germany, The Netherlands, and the United Kingdom), and with the participation of ISAS and NASA. 相似文献
550.
M. E. Wiedenbeck W. R. Binns A. C. Cummings A. J. Davis G. A. de Nolfo M. H. Israel R. A. Leske R. A. Mewaldt E. C. Stone T. T. von Rosenvinge 《Space Science Reviews》2007,130(1-4):415-429
The galactic cosmic rays arriving near Earth, which include both stable and long-lived nuclides from throughout the periodic
table, consist of a mix of stellar nucleosynthesis products accelerated by shocks in the interstellar medium (ISM) and fragmentation
products made by high-energy collisions during propagation through the ISM. Through the study of the composition and spectra
of a variety of elements and isotopes in this diverse sample, models have been developed for the origin, acceleration, and
transport of galactic cosmic rays. We present an overview of the current understanding of these topics emphasizing the insights
that have been gained through investigations in the charge and energy ranges Z≲30 and E/M≲1 GeV/nuc, and particularly those using data obtained from the Cosmic Ray Isotope Spectrometer on NASA’s Advanced Composition
Explorer mission. 相似文献