全文获取类型
收费全文 | 1041篇 |
免费 | 91篇 |
国内免费 | 348篇 |
专业分类
航空 | 835篇 |
航天技术 | 399篇 |
综合类 | 165篇 |
航天 | 81篇 |
出版年
2024年 | 1篇 |
2023年 | 20篇 |
2022年 | 15篇 |
2021年 | 38篇 |
2020年 | 38篇 |
2019年 | 44篇 |
2018年 | 54篇 |
2017年 | 43篇 |
2016年 | 54篇 |
2015年 | 58篇 |
2014年 | 56篇 |
2013年 | 49篇 |
2012年 | 75篇 |
2011年 | 89篇 |
2010年 | 57篇 |
2009年 | 92篇 |
2008年 | 95篇 |
2007年 | 77篇 |
2006年 | 67篇 |
2005年 | 58篇 |
2004年 | 34篇 |
2003年 | 53篇 |
2002年 | 31篇 |
2001年 | 42篇 |
2000年 | 27篇 |
1999年 | 20篇 |
1998年 | 28篇 |
1997年 | 29篇 |
1996年 | 25篇 |
1995年 | 20篇 |
1994年 | 28篇 |
1993年 | 15篇 |
1992年 | 14篇 |
1991年 | 6篇 |
1990年 | 11篇 |
1989年 | 8篇 |
1988年 | 6篇 |
1986年 | 2篇 |
1984年 | 1篇 |
排序方式: 共有1480条查询结果,搜索用时 0 毫秒
51.
Nikolai M. Gavrilov Andrey V. Koval Alexander I. Pogoreltsev Elena N. Savenkova 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2018,61(7):1819-1836
Parameterization schemes of atmospheric normal modes (NMs) and orographic gravity waves (OGWs) have been implemented into the mechanistic Middle and Upper Atmosphere Model (MUAM) simulating atmospheric general circulation. Based on the 12-members ensemble of runs with the MUAM, a composite of the stratospheric warming (SW) has been constructed using the UK Met Office data as the lower boundary conditions. The simulation results show that OGW amplitudes increase at altitudes above 30 km in the Northern Hemisphere after the SW event. At altitudes of about 50 km, OGWs have largest amplitudes over North American and European mountain systems before and during the composite SW, and over Himalayas after the SW. Simulations demonstrate substantial (up to 50–70%) variations of amplitudes of stationary planetary waves (PWs) during and after the SW in the mesosphere-lower thermosphere of the Northern Hemisphere. Westward travelling NMs have amplitude maxima not only in the Northern, but also in the Southern Hemisphere, where these modes have waveguides in the middle and upper atmosphere. Simulated variations of PW and NM amplitudes correspond to changes in the mean zonal wind, EP-fluxes and wave refractive index at different phases of the composite SW events. Inclusion of the parameterization of OGW effects leads to decreases in amplitudes (up to 15%) of almost all SPWs before and after the SW event and their increase (up to 40–60%) after the SW in the stratosphere and mesosphere at middle and high northern latitudes. It is suggested that observed changes in NM amplitudes in the Southern Hemisphere during SW could be caused by divergence of increased southward EP-flux. This EP-flux increases due to OGW drag before SW and extends into the Southern Hemisphere. 相似文献
52.
一类非线性系统的变结构控制问题 总被引:1,自引:0,他引:1
本文研究了简约型非线性控制系统的变结构控制问题,针对变结构控制切换函数的确定问题进行了深入研究,给出了一类非线性系统切换函数的确定方法。研究结果表明,该方法适用于一类难于或不能可控正则化和线性化的非线性控制系统的综合。最后通过仿真证实了该方法的有效性。 相似文献
53.
54.
55.
56.
雷达目标跟踪中的非线性滤波技术 总被引:6,自引:6,他引:0
对雷达目标跟踪中的9种非线性滤波技术进行了综述,介绍了其基本思想,分析了各方法的优缺点并进行了比较。 相似文献
57.
从弹性梁的非线性应变位移关系出发,结合Hamilton变分原理推导出了作大挠度振动的空间梁的动力学有限元方程。方程中考虑了与变形相耦合的非线性惯性项。计算表明,作大挠度振动的悬臂梁的响应是由不同的谐波成分组成;在相同结构阻尼影响下,大挠度梁的稳态振幅比线性情况下的稳态振幅要低;非线性惯性项与弯扭耦合项对系统的动力学特性有比较明显的影响。 相似文献
58.
为研究燃气轮机周向分布式拉杆转子轮盘间存在的连接刚度对轴系转子振动特性产生的影响,本文提出一种基于六自由度的弹簧单元等效方法;首先将轮盘接触刚度与拉杆弯曲刚度同时等效进连接界面,便于使用一维梁单元求解其临界转速与振动响应变化,验证其与全三维有限元法吻合度在3%以内;然后改写了非线性动力学方程,采用谐波平衡法对其进行求解,使用经典Newmark方法验证其准确性;最后使用该等效方法计算了某模拟转子连接刚度对其临界转速、响应特性、非线性特性等的影响。计算结果表明:当连接面剪切刚度与弯曲刚度减小3个数量级,均造成了临界转速的降低和振动响应的提高,而其影响程度可由各阶振型大致推测,且响应幅值受剪切刚度影响较大;使用临界转速趋于收敛时的连接刚度与试验值的对比误差小于2%,较连续模型提升显著;在连接刚度各向异性时,每一阶临界转速均出现两个响应峰值,轴心轨迹变成椭圆;考虑刚度随位移变化(非线性刚度)后,转子的响应峰值发生“歪扭”特性,且该峰值特征随不平衡量、无量纲阻尼等因素变化而呈现“波浪形”变化。 相似文献
59.
The problem of aeroelasticity and maneuvering of command surface and gust wing interaction involves a starting flow period which can be seen as the flow of an airfoil attaining suddenly an angle of attack. In the linear or nonlinear case, compressive Mach or shock waves are generated on the windward side and expansive Mach or rarefaction waves are generated on the leeward side. On each side, these waves are composed of an oblique steady state wave, a vertically-moving one-dimensional unsteady wave, and a secondary wave resulting from the interaction between the steady and unsteady ones. An analytical solution in the secondary wave has been obtained by Heaslet and Lomax in the linear case, and this linear solution has been borrowed to give an approximate solution by Bai and Wu for the nonlinear case. The structure of the secondary shock wave and the appearance of various force stages are two issues not yet considered in previous studies and has been studied in the present paper. A self-similar solution is obtained for the secondary shock wave, and the reason to have an initial force plateau as observed numerically is identified. Moreover, six theoretical characteristic time scales for pressure load variation are determined which explain the slope changes of the time-dependent force curve. 相似文献
60.
O.K. Cheremnykh A.N. Kryshtal A.A. Tkachenko 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2018,61(2):603-610
In this paper, we study conditions of realization and stability of kink modes with azimuthal wave numbers in cylindrical plasma flex with twisted magnetic field and homogeneous current along its axis. We assume permanent axial magnetic field both inside and outside the flex, surrounded by currentless plasma. Azimuthal magnetic field decreases inversely proportional to the distance from the boundary beyond the flex. We derived dispersion equations for stable and unstable modes in approximation of “thin” plasma flex. The analysis of equations has been provided for the case of discontinuous axial magnetic field on flex’s boundary. Conditions of propagation of wave modes have been defined. It was shown, that unstable modes can be implemented in certain interval of longitudinal wavenumbers. Results can be applied for the interpretation of solar magnetic tubes behavior, using measurements, provided by spacecrafts. 相似文献