全文获取类型
收费全文 | 1044篇 |
免费 | 231篇 |
国内免费 | 194篇 |
专业分类
航空 | 898篇 |
航天技术 | 156篇 |
综合类 | 140篇 |
航天 | 275篇 |
出版年
2024年 | 8篇 |
2023年 | 20篇 |
2022年 | 54篇 |
2021年 | 43篇 |
2020年 | 63篇 |
2019年 | 56篇 |
2018年 | 46篇 |
2017年 | 56篇 |
2016年 | 84篇 |
2015年 | 58篇 |
2014年 | 66篇 |
2013年 | 63篇 |
2012年 | 76篇 |
2011年 | 89篇 |
2010年 | 68篇 |
2009年 | 70篇 |
2008年 | 54篇 |
2007年 | 64篇 |
2006年 | 76篇 |
2005年 | 56篇 |
2004年 | 56篇 |
2003年 | 30篇 |
2002年 | 29篇 |
2001年 | 29篇 |
2000年 | 28篇 |
1999年 | 16篇 |
1998年 | 25篇 |
1997年 | 20篇 |
1996年 | 11篇 |
1995年 | 12篇 |
1994年 | 7篇 |
1993年 | 12篇 |
1992年 | 4篇 |
1991年 | 7篇 |
1990年 | 6篇 |
1989年 | 5篇 |
1987年 | 1篇 |
1986年 | 1篇 |
排序方式: 共有1469条查询结果,搜索用时 342 毫秒
861.
脉冲式燃烧风洞起动特性数值研究 总被引:4,自引:2,他引:2
利用计算流体力学软件Fluent,采用隐式双时间步方法模拟非定常过程,控制方程的离散采用二阶迎风格式,湍流模型选择剪切应力输运(SST)k-ω模型,并求解组分扩散方程,对脉冲式燃烧风洞的起动过程进行了数值模拟,分析了喷管起动时流场结构的变化情况和自由射流区超声速流场的建立过程.计算结果为改进风洞起动特性提供了一定的参考依据. 相似文献
862.
863.
《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2020,65(1):392-406
The separation process of multi-spheres in hypersonic flow has been experimentally investigated. The experiments were conducted in a shock tunnel at a nominal freestream Mach number of 6. Iron or acetal small light spheres with different sizes, varying from 2.38 to 6 mm, were considered. They were mounted by a thin wire. The trajectory of the spheres was analyzed using optical images. By varying the radius and the number of spheres from a single to multiple spheres, the lateral velocity and the separation behavior including the phenomenon known as ‘shock wave surfing’ were measured and analyzed. A new equation to account for the lateral velocity of the multi-spheres was proposed by extending the well-known Passey and Melosh’s theory based on two bodies. The theoretical results were compared with the presented experimental data and a good agreement was found. Using the derived equation, the re-entry trajectory analysis of multi-spheres, that is regarded as the hypothetical break-up, was performed. The ground footprint and downrange due to the separation of the multi-spheres were compared with that of single- and two-spheres. The results showed that as the number of spheres increased, the lateral velocity increased while the ballistic coefficient decreased. This led to a large discrepancy in the ground footprint as well as the downrange when compared to the single sphere. Caution should therefore be exercised in the trajectory analysis when the effect of separation induced due to fragmentation is not considered. 相似文献
864.
针对相控阵雷达多目标跟踪波束调度和波形参数优化控制的问题,本文提出了一种基于马尔可夫决策过程(MDP)的相控阵雷达跟踪波束调度与波形参数优化策略,该方法以无迹卡尔曼滤波(UKF)算法为基础来估计目标的状态。首先将本文的序列决策问题建模为马尔可夫决策过程,定义了资源的效费比和长期回报率,然后与当前实际跟踪误差综合考虑作为MDP的回报函数,进而给出了调度的优化模型,最后将长时决策问题转化为动态规划算法结构进行求解,并且提出了一种并行混合遗传粒子群优化算法来求解各决策时刻的最优策略。仿真结果表明了长时策略的先进性以及寻优算法的优越性,与传统的短时策略相比,跟踪精度可提高11.17%。 相似文献
865.
基于位姿测量不确定度的飞机对接质量评估 总被引:2,自引:0,他引:2
针对基于位姿的数字化测量辅助飞机大部件对接技术的发展与应用,对位姿测量不确定度以及基于不确定度的质量评价方法进行了研究.给出了数字化对接环境下大部件位姿的数学表达形式及意义.提出了基于协方差矩阵的位姿测量不确定度解析算法,并通过仿真算例与蒙特卡洛仿真法进行了对比,验证了该算法的有效性.给出了飞机大部件对接过程数字化测量工艺能力指数的概念,用于对测量结果的可信性进行评价;通过构建位姿测量不确定度与对接质量评估指标间的映射关系,提出了一种基于位姿测量不确定度的大部件对接质量评估方法,并以机翼-机身对接过程为案例,对方法的可行性、算法的有效性进行了验证. 相似文献
866.
L1门区域改装是波音757-200客改货工作的核心内容,本文简要介绍了进行波音757-200客改货(15个货盘改装方案)L1门区域改装中所采用的的工作方法。 相似文献
867.
868.
869.
《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2020,65(1):677-683
Solar cycle prediction is a key activity in space weather research. Several techniques have been employed in recent decades in order to try to forecast the next sunspot-cycle maxima and time. In this work, the Gaussian process, a machine-learning technique, is used to make a prediction for the solar cycle 25 based on the annual sunspot number 2.0 data from 1700 to 2018. A variation known as Warped Gaussian process is employed in order to deal with the non-negativity constraint and asymmetrical data distribution. Tests using holdout data yielded a root mean square error of 10.0 within 5 years and 25.0–35.0 within 10 years. Simulations using the predictive distribution were performed to account for the uncertainty in the prediction. Cycle 25 is expected to last from 2019 to 2029, with a peak sunspot number about 117 (110 by the median) occurring most likely in 2024. Thus our method predicts that solar Cycle 25 will be weaker than previous ones, implying a continuing trend of declining solar activity as observed in the past two cycles. 相似文献
870.
为了研发具有自主知识产权的气源起动装备,立足于大型飞机较高压力、超大流量的特殊气源起动保障需求,设计了以双级无油螺杆压缩为核心技术的供气流程,对其中涉及的级间压力、散热量、除水量等关键参数进行了分析与计算,为解决超大流量气源的二次升压、风冷散热、高温除水等核心技术难题奠定了基础。 相似文献