排序方式: 共有37条查询结果,搜索用时 46 毫秒
21.
Direct numerical simulations of the geodynamo and other planetary dynamos have been successful in reproducing the observed magnetic fields. We first give an overview on the fundamental properties of planetary magnetism. We review the concepts and main results of planetary dynamo modeling, contrasting them with the solar dynamo. In planetary dynamos the density stratification plays no major role and the magnetic Reynolds number is low enough to allow a direct simulation of the magnetic induction process using microscopic values of the magnetic diffusivity. The small-scale turbulence of the flow cannot be resolved and is suppressed by assuming a viscosity far in excess of the microscopic value. Systematic parameter studies lead to scaling laws for the magnetic field strength or the flow velocity that are independent of viscosity, indicating that the models are in the same dynamical regime as the flow in planetary cores. Helical flow in convection columns that are aligned with the rotation axis play an important role for magnetic field generation and forms the basis for a macroscopic α-effect. Depending on the importance of inertial forces relative to rotational forces, either dynamos with a dominant axial dipole or with a small-scale multipolar magnetic field are found. Earth is predicted to lie close to the transition point between both classes, which may explain why the dipole undergoes reversals. Some models fit the properties of the geomagnetic field in terms of spatial power spectra, magnetic field morphology and details of the reversal behavior remarkably well. Magnetic field strength in the dipolar dynamo regime is controlled by the available power and found to be independent of rotation rate. Predictions for the dipole moment agree well with the observed field strength of Earth and Jupiter and moderately well for other planets. Dedicated dynamo models for Mercury, Saturn, Uranus and Neptune, which assume stably stratified layers above or below the dynamo region, can explain some of the unusual field properties of these planets. 相似文献
22.
This paper presents a status of the ARIANE 5 launch vehicle dynamic environments after the 5 first flights. It concerns mainly the low frequency vibrations, the acoustic and the shocks that the payloads experience and the related external excitations. The flight measurement plan is described. The results show that the frequencies are well characterized and the levels are low pointing out margins with regard to the specifications Furthermore, one describe the efficiency of attenuation solutions implemented to reduce the payload environments. 相似文献
23.
G Sonnenfeld L Schaffar D A Schmitt C Peres E S Miller 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1994,14(8):395-397
The Rhesus monkey has been proposed as a model for the effects of space flight on immunity. In order to determine the feasibility of the use of the Rhesus monkey as a model, we studied the use of Rhesus monkey cells for immunological procedures that have been shown to be affected by space flight in both rodents and humans. We have shown that both lymph node cells and peripheral blood leukocytes can be stained with monoclonal antibodies to detect the following surface markers: CD4, CD-8, Ia and surface immunoglobulin. Also, the level of Ia antigen expression was increased by treatment of the cells with human interferon-gamma. In addition, cells were induced to produce interferons and interleukins. Isolated neutrophils also demonstrated increased oxidative burst. These data indicate that the Rhesus monkey will be a useful model for space flight studies of immunity. 相似文献
24.
25.
26.
钝头体窄条翼布局导弹在大攻角下拥有极为优异的纵向气动特性,但横向容易失稳,做快速机动时容易诱发非指令的横向不稳定运动。通过开展高速风洞自由摇滚试验和数值模拟,研究了窄条翼导弹自由摇滚特性和流动机理,试验与计算吻合较好。研究发现:较大迎角时,窄条翼面积中心距离尾舵前缘根部5~6倍直径时,模型会进入极限环摇滚,窄条翼位置对模型稳定性有显著的影响,去掉窄条翼或尾舵时,模型均不会进入摇滚;模型空间流场特性表明,气流经过窄条翼时形成的片涡,对背风舵产生强烈的干扰,抑制了尾舵涡的形成和发展,使背风舵动态失稳,导致模型进入极限环摇滚。 相似文献
27.
We give a progress report of our EXOSAT observations of active M dwarfs. The possibilities of filter spectroscopy of coronal X-ray sources using the available CMA filters are discussed, and we confirm that M dwarfs are rather hot coronal sources with X-ray temperatures in excess of 107 K, a result previously obtained with the Einstein Observatory. 相似文献
28.
29.
针对无人机(UAV)通信网络中频谱资源紧缺的问题,构建基于认知无线电的多无人机通信网络,通过多机协作频谱感知有效探索授权频谱。提出一种基于Bisection算法的迭代算法,通过联合优化感知时间和判决门限对构建的复杂非凸问题求解,显著提高了无人机次级认知网络的能量效率(EE)。分析了无人机飞行过程中能效的变化情况,仿真结果表明,存在最优感知时间使能效获得最大值,且判决门限的选择会影响该能效最优值;提出的高能效迭代算法具有较好收敛性,有效提高了认知无人机网络的能量利用率。 相似文献
30.