全文获取类型
收费全文 | 4560篇 |
免费 | 1056篇 |
国内免费 | 963篇 |
专业分类
航空 | 4925篇 |
航天技术 | 465篇 |
综合类 | 699篇 |
航天 | 490篇 |
出版年
2024年 | 13篇 |
2023年 | 68篇 |
2022年 | 163篇 |
2021年 | 210篇 |
2020年 | 203篇 |
2019年 | 200篇 |
2018年 | 185篇 |
2017年 | 212篇 |
2016年 | 277篇 |
2015年 | 255篇 |
2014年 | 286篇 |
2013年 | 252篇 |
2012年 | 329篇 |
2011年 | 359篇 |
2010年 | 280篇 |
2009年 | 298篇 |
2008年 | 247篇 |
2007年 | 241篇 |
2006年 | 194篇 |
2005年 | 201篇 |
2004年 | 148篇 |
2003年 | 165篇 |
2002年 | 173篇 |
2001年 | 143篇 |
2000年 | 174篇 |
1999年 | 132篇 |
1998年 | 116篇 |
1997年 | 124篇 |
1996年 | 168篇 |
1995年 | 110篇 |
1994年 | 125篇 |
1993年 | 107篇 |
1992年 | 82篇 |
1991年 | 87篇 |
1990年 | 75篇 |
1989年 | 72篇 |
1988年 | 87篇 |
1987年 | 14篇 |
1986年 | 4篇 |
排序方式: 共有6579条查询结果,搜索用时 46 毫秒
151.
《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2020,65(11):2507-2514
The effect of the rocket exhaust products on the D-region of the ionosphere is investigated with the help of Very low frequency (VLF) electromagnetic wave propagation characteristics within the Earth-ionosphere waveguide. The changes in the electron density profile are computed from the observed VLF signal amplitude perturbations about 3 dB during the rocket launch. We find a localized electron depletion in the lower ionosphere at an altitude of around 58 km, that is thought to be originated by the attachment of ionospheric ion and molecular hydrogen along with water molecule in the exhaust product of first stage burn of Geosynchronous Launch Vehicle (GSLV) rocket at the time of GSLV launched from Sriharikota, India, on 27 August 2015 at 11:22 UT (16:52 IST). The ionospheric depletion perturbed the navigational VLF signal (VTX = 17 kHz) 134 s after the launch of the GSLV rocket. 相似文献
152.
对空间碎片检视或抓捕操控中的悬停控制及燃耗问题进行了研究。通过C W方程,建立任务星近距随遇定点悬停控制模型,通过设计状态反馈控制器分析定点悬停的可控性以及推控要求,结果表明近距悬停需要与悬停位置相关的两个正交方向上的常值连续推力控制量,以及用于抵抗扰动的三轴向上的反馈变推力控制量。分析了近距随遇悬停的推控分系统配置,建立了长时近距随遇悬停的燃耗及燃耗速率的数学模型,最后分析了既满足安全距离需求、又满足悬停方位需求的最小悬停燃耗模型。 相似文献
153.
《中国航空学报》2020,33(12):3176-3188
Numerical simulation and theoretical analysis were conducted to study the hysteresis inside scramjet isolator during the reciprocating process of back pressure variation. It is revealed that only a regular reflection is theoretically possible for two leading shocks when the inflow Mach number is greater than 2.0, and no hysteresis can occur in the transition between shock reflection types. Nevertheless, wall suction, gas injection, and background waves cause non-uniformity of the incoming flow and would make hysteresis possible. Besides the classical hysteresis in the transition between shock reflection, new kinds of hysteresis were found in both the deflection angle of separated boundary layer and the location of the shock train. Moreover, the occurrence of hysteresis in the deflection angle of the separated boundary layer is accompanied with the shock reflection hysteresis. In the case with background waves or gas injection, hysteresis in the starting position of leading shock was observed too. As back pressure decreases, the leading shock does not follow the same path as that as the back pressure increases, and it is anchored at the location where the background shock or the injection interacts with the leading shock. It is inferred that, if two strong adverse pressure gradient regions move towards and interact with each other, hysteresis will take place when they start to separate. 相似文献
154.
《中国航空学报》2020,33(8):2162-2175
The rim seal is used to prevent mainstream ingestion to the gap between the vane of a turbine and its blade. In this article, the dolphin lip with a hook configuration and a large seal cavity with hook structures are designed based on the high-pressure turbine datum single shark lip rim seal configuration. The sealing effect and parameters of the flow field are measured by an experiment method and a numerical simulation is used to explain the mechanism. For three configurations, the effect of the leakage slot vortex on the efficiency of the seal and the influence of leakage vortex, generated by the interaction between purge flow and mainstream flow, are discussed in depth. The result shows that the reverse vortex formed by the dolphin lip rim seal with hook structure will increase the sealing efficiency. The seal configuration with a large cavity improves sealing efficiency to a greater extent than the datum structure. At different purge flow rates and with unequal seal structures, the purge flow produces three types of leakage vortices in the passage. Besides, the seal configuration with dolphin lip produces a Kelvin-Helmholtz instability at the interface of the purge and the mainstream flows at a low purge flow rate to induce new leakage vortex branches in the passage of the blade. 相似文献
155.
Mukul Kumar Wang Chi 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2019,63(1):640-652
We study the Propagation and damping properties of Alfvén gravity waves in the presence of the vertical magnetic field in the viscous solar plasma under influence of the background flow by deriving a fourth order dispersion relation in terms of the Doppler shifted frequency. We derive the dispersion relation under WKB and Boussinesq approximation. We study the damping of Alfvén gravity waves for the wave frequencies less than and greater than the Brunt-Väisälä frequency. We find that the Brunt-Väisälä frequency divides the frequency ranges where the weakly or strongly damped oscillations occur. The background flow exhibits a strong effect on weakly damped oscillations and a weak effect on the strongly damped oscillations. We also notice that the damping of both the strong and weakly damped oscillations depend on the Brunt-Väisälä frequency and wave number. The effect of the background flow is also being governed by the Brunt-Väisälä frequency and wave number. We also study the properties of gravity wave mode after filtering the Alfvén wave mode by minimizing the magnetic field and noticed that the background flow shows a very strong effect on the gravity wave mode. 相似文献
156.
157.
喷射油道是一种高精度旋转雾化喷嘴,其非连续窄深端槽、窄深槽、3/4圆弧窄端槽为行业内少见结构.通过分析喷射油道特有结构的加工工艺,摸索油孔尺寸与燃油流量的匹配关系,解决了非连续深窄端槽、窄深槽、细微孔电火花成形加工、3/4圆弧窄端槽高效数控加工、高精度喷嘴孔流量调试等关键技术瓶颈,具有一定的指导意义和广阔的应用前景. 相似文献
158.
159.
This paper presents a brief review of activities in laminar flow control being performed at the Central Aerohydrodynamic Institute named after Prof. N.E. Zhukovsky (TsAGI). These efforts are focused on the improvement of the existing laminar flow control methods and on the development of new ones. The investigations have demonstrated the effectiveness of aircraft surface laminarization applications with the aim of friction drag reduction. The opportunity of considerable delaying of laminar-turbulent transition due to special wing profile geometry and using boundary layer suction and surface cooling has been verified at sub- and supersonic speeds through various wind tunnel testing at TsAGI and during flying laboratory experiments at the Flight Research Institute (LII). The investigations on using hybrid laminar flow control systems for friction drag reduction were also carried out. New techniques of laminar flow control were proposed, in particular, the method of local heating of the wing leading edge, boundary layer laminarization by means of receptivity control, and electrohydrodynamic methods of boundary layer stability control. 相似文献
160.
为了提高燃料使用效率、降低污染物排放,在已有分级燃烧燃油分配器基础上,针对航空发动机双环腔燃烧室改进设计了1种高性能燃油分配器。简述了分级燃烧燃油分配器的工作原理,建立了其AMESim模型。根据设计要求,通过仿真验证了燃油分配器的稳定性、分配比与输入信号的稳态关系,计算了燃油分配器输出与燃油分配计划表之间的误差,分析了燃油分配器的动态特性以及对外部参数变化的鲁棒性,并且针对该分配器的非线性特性提出了解决办法。仿真结果表明:与文献[4]所设计的燃油分配器相比,改进设计的燃油分配器鲁棒性更好,稳态和动态性能更优。 相似文献