排序方式: 共有61条查询结果,搜索用时 15 毫秒
21.
Tomokatsu Morota Jun’ichi Haruyama Chikatoshi Honda Yasuhiro Yokota Makiko Ohtake Muneyoshi Furumoto 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008
The degree of apex–antapex cratering asymmetry of a synchronously rotating satellite primarily depends on the mean encounter velocity of impactors with respect to the planetary system and the orbital velocity of the satellite. This means that we can estimate the mean encounter velocity of impactors by observing the apex–antapex cratering asymmetry, if the relationship between these is known. To apply this technique to the Moon, we attempt to derive the relationship between the mean encounter velocity of impactors and the degree of the lunar cratering asymmetry as a function of time, considering the temporal variation in the lunar orbital velocity during the last 4.0 Gyr. We used the cratering asymmetry of Zahnle et al. [Zahnle, K., Schenk, P., Sobieszczyk, S. et al. Differential cratering of synchronously rotating satellites by ecliptic comets. Icarus 153, 111–129, 2001] to obtain the relationship. Applying this relationship enables us to estimate the impactor’s velocity of the Earth–Moon system from an investigation of the spatial distribution of lunar craters. Furthermore, we re-evaluate the cratering asymmetry’s influence on lunar cratering chronology. 相似文献
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气动非谐对叶片非定常气动激振力影响 总被引:5,自引:3,他引:2
为降低转子叶片在前排静子叶片尾流激振情况下的共振应力, 提高转子叶片疲劳寿命, 对静子叶片的气动非谐设计进行研究.对静子叶片的气动非谐设计考虑了两种方案, 一种为非均匀叶栅设计, 另一种为不对称叶栅设计.通过对静子-转子模型流场进行非定常计算, 比较了两种非谐设计思想下转子叶片表面气动激振力频谱成分和幅值大小.分析结果标明:非均匀叶栅非谐设计并不能降低转子叶片表面激振力, 同时会引起频率成分和幅值都较大的倍频成分;不对称叶栅非谐设计虽然会引起与转速同频的较大幅值激振力成分, 但从整体考虑可有效降低叶片振动应力. 相似文献
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Geetashree Kakoti Bitap Raj Kalita Rumajyoti Hazarika Pradip Kumar Bhuyan Sanjay Sharma Ramesh Chandra Tiwari 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2018,61(11):2837-2853
The total electron content (TEC) derived from GNSS measurements at a trans-hemispheric meridional chain of ground stations around 95°E longitude are used to study the quiet time inter-hemispheric structure and dynamics of the equatorial ionization anomaly (EIA) during the period March 2015 to February 2016. The stations are Dibrugarh (27.5°N, 95°E, 43° dip), Kohima (25.6°N, 94.1°E, 39° dip), Aizawl (23.7°N, 92.8°E, 36° dip), Port Blair (11.63°N, 92.71°E, 9° dip) and Cocos Islands (12.2°S, 96.8°E, 43° dip). The observation shows that the northern crest of the EIA lies in the south of 23°N (Aizawl) in all seasons but recedes further south towards the equator during December solstice. The largest poleward expansion of the northern (southern) EIA is observed in the March equinox (December solstice). The equinoctial and hemispherical asymmetry of TEC is noted. The winter anomaly is observed in the northern hemisphere but not in the southern hemisphere. The highest midday TEC over any station is observed in the March equinox. The TEC in southern summer (December solstice) is significantly higher than that in the northern summer (June solstice). The observed northern EIA contracts equatorward in the postsunset period of solstice but the southern EIA persists late into the midnight in the December solstice. The asymmetry may be attributed to the different geographic location of the magnetically conjugate stations. The SAMI3 simulations broadly capture the EIA structure and the inter-hemispheric asymmetry during solstices. The difference between observations and the SAMI3 is higher in March equinox and December solstice. The higher E?×?B vertical drift in the 90–100°E sector and the large geographic-geomagnetic offset in observing stations may have contributed to the observed differences. 相似文献
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M.M. Shaikh R. Notarpietro B. Nava 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2014
‘Onion-peeling’ is a very common technique used to invert Radio Occultation (RO) data in the ionosphere. Because of the implicit assumption of spherical symmetry for the electron density (N(e)) distribution in the ionosphere, the standard Onion-peeling algorithm could give erroneous concentration values in the retrieved electron density profile. In particular, this happens when strong horizontal ionospheric electron density gradients are present, like for example in the Equatorial Ionization Anomaly (EIA) region during high solar activity periods. In this work, using simulated RO Total Electron Content (TEC) data computed by means of the NeQuick2 ionospheric electron density model and ideal RO geometries, we tried to formulate and evaluate an asymmetry level index for quasi-horizontal TEC observations. The asymmetry index is based on the electron density variation that a signal may experience along its path (satellite to satellite link) in a RO event and is strictly dependent on the occultation geometry (e.g. azimuth of the occultation plane). A very good correlation has been found between the asymmetry index and errors related to the inversion products, in particular those concerning the peak electron density NmF2 estimate and the Vertical TEC (VTEC) evaluation. 相似文献
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单通道控制的旋转弹锥形运动稳定性研究 总被引:1,自引:0,他引:1
锥形运动是旋转弹角运动的基本形式,其稳定性问题一直是旋转弹领域的研究热点。以一对鸭舵作用下的单通道控制旋转弹为研究对象,给出了弹体坐标系下的线性化角运动模型,通过数值仿真,揭示了一对鸭舵带来的气动不对称对其角运动特性的影响规律。利用劳斯判据,给出了解析形式的锥形运动稳定条件,该条件可等效为锥形运动稳定条件下的转速范围,不同转速下的仿真结果证明了该稳定条件的正确性。在相同的转速条件下,正弦式鸭舵的控制频率在慢模态衰减频率附近时,可诱发强烈的共振不稳定。研究结果可为一对鸭舵作用下的旋转弹总体设计及制导控制系统设计提供参考。 相似文献
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V. V. Lyubimov 《Russian Aeronautics (Iz VUZ)》2009,52(2):172-175
A disturbed rotational orbital motion of an asymmetrical satellite with a strong magnet in the geomagnetic field is considered. It is shown that operating disturbances result in appearing of non-resonant phenomena in the satellite rotation caused by resonance effects. 相似文献
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多相电机在定子开路故障下具有良好的容错性能,与此相关的设备和控制策略改进方面的研究理论也不断被提出。然而,却少有基于统一模型的适用于多相开路故障的容错控制策略。提出了一种应用于双三相永磁同步电机(PMSM)系统三相或三相以下开路故障的容错控制策略,并获得了良好的电机电力和动态性能。针对发生单相到三相开路故障的PMSM提出了统一数学模型,提取出每种故障下对应电机不对称性的矩阵。通过电压方程,推导出每种故障对应的解耦变换矩阵。为了对相电流中的每种谐波进行无差别抑制,建立了谐波子空间。考虑开路故障相数的增加,提出了用于三相开路的有中性点电流的容错控制策略,达到了降低相电流幅值和提升系统冗余度的目的。最后通过试验和仿真结果,验证了理论分析的正确性。 相似文献
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介绍了A300—600飞机前缘缝翼系统的工作原理,对于航线维护中出现的典型故障进行了详细的分析,简述了故障的排除方法,给出了提高该系统可靠性的维护建议。 相似文献