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931.
通过模型方程分析看到了一个重要的现象, 如果源项涉及的时间尺度远小于对流项的时间尺度, 那么基于对流时间尺度作为步长的传统数值方法, 即使源项相对于墩流项是个小量, 也会导致平均尺度上错误的结果。为了克服这种困难, 采用时间分裂方法, 把方程分裂成含对流项部分的偏微分方程( PDE) 和包含源项的常微分方程(ODE) 基础上, PDE 使用传统的数值方法, ODE 用解析的方法求解。该混合方法在数值格式时间步长小于平均流动时间尺度时, 得到正确解, 而与点源所隐含的时间尺度无关。把这个方法应用在含源流动的Euler 方程的计算中, 计算了翼型振荡问题, 取得了理想的结果。 相似文献
932.
933.
934.
采用二维N-S方程/结构振动方程组耦合数值方法,分析计算了二维非定常气动力强迫振动条件下振动能量及气动力对叶栅所做的功,分别根据振动能量和气动力做功分析了叶栅的颤振特性。NACA0012和PROF叶栅在不同折合频率下气动参数随时间的变化表明,折合频率是影响叶栅颤振的重要因素。气固耦合方法得到的NA-CA0012叶栅颤振折合频率比非耦合的结果高,而对PROF叶栅则相反。这表明叶栅结构动力参数对其发生颤振时的折合频率影响很大且很复杂,要准确预测颤振应该考虑叶栅结构动力与气动力的耦合因素。 相似文献
935.
王丽灵 《中国民航学院学报》2002,20(Z1):42-45
客户/服务器(Client/Server)采用并发服务器和socket编程方法,巧妙地解决了网络通信的同步问题。基于Client/Server模式,根据动态联盟的组织原则,提出了开放性的敏捷制造信息系统结构。这种结构实现了动态联盟中分散的制造资源的共享;维护了联盟单元在解决问题时的自主性;通过联盟单元间的双向委托协议,可以保证信息数据的一致性;保证了联盟单元组织不同结构的兼容性。解决了在组织动态联盟时的几个关键问题,为进一步开展敏捷制造方法奠定了基础。 相似文献
936.
C. M. Lisse M. F. A’Hearn T. L. Farnham O. Groussin K. J. Meech U. Fink D. G. Schleicher 《Space Science Reviews》2005,117(1-2):161-192
As comet 9P/Tempel 1 approaches the Sun in 2004–2005, a temporary atmosphere, or “coma,” will form, composed of molecules
and dust expelled from the nucleus as its component icy volatiles sublimate. Driven mainly by water ice sublimation at surface
temperatures T > 200 K, this coma is a gravitationally unbound atmosphere in free adiabatic expansion. Near the nucleus (≤ 102 km), it is in collisional equilibrium, at larger distances (≥104 km) it is in free molecular flow. Ultimately the coma components are swept into the comet’s plasma and dust tails or simply
dissipate into interplanetary space. Clues to the nature of the cometary nucleus are contained in the chemistry and physics
of the coma, as well as with its variability with time, orbital position, and heliocentric distance.
The DI instrument payload includes CCD cameras with broadband filters covering the optical spectrum, allowing for sensitive
measurement of dust in the comet’s coma, and a number of narrowband filters for studying the spatial distribution of several
gas species. DI also carries the first near-infrared spectrometer to a comet flyby since the VEGA mission to Halley in 1986.
This spectrograph will allow detection of gas emission lines from the coma in unprecedented detail. Here we discuss the current
state of understanding of the 9P/Tempel 1 coma, our expectations for the measurements DI will obtain, and the predicted hazards
that the coma presents for the spacecraft.
An erratum to this article is available at . 相似文献
937.
Deep Impact Mission Design 总被引:1,自引:0,他引:1
William H. Blume 《Space Science Reviews》2005,117(1-2):23-42
The Deep Impact mission is designed to provide the first opportunity to probe below the surface of a comet nucleus by a high-speed
impact. This requires finding a suitable comet with launch and encounter conditions that allow a meaningful scientific experiment.
The overall design requires the consideration of many factors ranging from environmental characteristics of the comet (nucleus
size, dust levels, etc.), to launch dates fitting within the NASA Discovery program opportunities, to launch vehicle capability
for a large impactor, to the observational conditions for the two approaching spacecraft and for telescopes on Earth. 相似文献
938.
Since its discovery in 1867, periodic comet 9P/Tempel 1 has been observed at 10 returns to perihelion, including all its returns
since 1967. The observations for the seven apparitions beginning in 1967 have been fit with an orbit that includes only radial
and transverse nongravitational accelerations that model the rocket-like thrusting introduced by the outgassing of the cometary
nucleus. The successful nongravitational acceleration model did not assume any change in the comet’s ability to outgas from
one apparition to the next and the outgassing was assumed to reach a maximum at perihelion. The success of this model over
the 1967–2003 interval suggests that the comet’s spin axis is currently stable. Rough calculations suggest that the collision
of the impactor released by the Deep Impact spacecraft will not provide a noticeable perturbation on the comet’s orbit nor
will any new vent that is opened as a result of the impact provide a noticeable change in the comet’s nongravitational acceleration
history. The observing geometries prior to, and during, the impact will allow extensive Earth based observations to complement
the in situ observations from the impactor and flyby spacecraft. 相似文献
939.
940.
王香%于德志%连晓明%李庆芬 《宇航材料工艺》2006,36(4):50-53
采用自蔓延高温合成(SHS)法制备高TiC颗粒含量的TiC/Al复合材料。利用自制的实验装置研究复合材料中TiC颗粒在静止锌液中的均匀化过程。结果表明:当锌液温度低于铝的熔点时,TiC/Al复合材料置于锌液后,锌向其内部扩散,引起复合材料表层内液相线温度降低,当表层内Al-Zn合金的液相线温度等于或低于锌液温度时,Al-Zn合金便处于熔融状态,TiC颗粒随其一起从TiC/Al复合材料块上脱落,并不断地向锌液内部传输,最终均匀分布在锌液中;而当锌液温度高于铝熔点时,TiC/Al复合材料置入后,锌和铝同时进行扩散,但是当复合材料表面温度达到铝熔点时,铝开始熔化,铝的熔化导致TiC颗粒的脱落,脱落下来的TiC颗粒不断向锌液内部传输,最终均匀分布在锌液中。 相似文献