模拟可压缩流的高分辨率有限元法

采用经典的Galerkin方法对N-S方程进行空间离散,再以该半离散格式作为基本格式,参照Kuzmin的思想加入耗散与反耗散项,使所构造的格式具有局部极值不增(LED,Local Extremum Diminishing)性质.对上述半离散格式进行时间离散后所导出的稀疏线性代数方程组,采用了GMRES(Generalized Minimal Residual)迭代法进行求解.为验证所建立的格式及相应的程序,给出了激波管问题和绕圆柱与双椭球超声速流动问题的数值模拟结果.      

微吹减阻技术影响因素的数值模拟

以NASA格伦中心的PN3和PN23型微孔壁板为原形建立"1排微孔"、"4排微孔"、"8排微孔"和"16排微孔"4种计算模型,针对不同的几何、物理参数进行了微吹技术MBT(Micro-Blowing Technique)降低平板摩擦阻力的数值模拟参数研究.结果显示:微量吹气使主流边界层近壁面流动减速,从而改变了壁面局部摩擦力的分布;不同几何、物理参数对MBT技术减阻能力的影响满足一定的规律.参数研究的结果可为该技术的减阻应用提供一定的指导.     

Fast automatic leveling subject to abrupt deterministic input

This investigation applies a modified Kalman filter with a recursive generalized M estimator (GME) of input to a class of leveling problems, that are subject to abrupt environmental disturbances and high noise levels. A least-squares estimator (LSE) based hypothetical testing scheme is also devised to detect the onset and presence of the input. Simulation results demonstrate that the leveling speed of convergence and accuracy is markedly higher than the original unmodified one     

Detecting near collisions for satellites

A highly efficient algorithm is presented for detecting near collisions involving satellites. Rather than examine all possible pairings of a satellite with another orbiting object, the new approach examines trajectories at a sequence of coarse time steps. Objects which are spatially isolated cannot be involved in a collision, and are discarded. The time steps, orbit calculations, and discarding are repeatedly refined until the only remaining objects experience near collisions.     

Effects of output filter on cross regulation in SRC

Cross regulation performance of multioutput series resonant converter (SRC) topologies using capacitive or inductive filters is analyzed. The steady state cross regulation characteristics are derived using the state-plane techniques and are illustrated by the examples of two-output SRCs with capacitive and inductive filters. The characteristic curves show that over some range of output currents, the cross regulation performance of an SRC with an inductive filter is much improved over the SRC with capacitance filter and less dependent on the tolerances in leakage inductances and the loading on unregulated outputs. It is shown that the converter control characteristics of an SRC with an inductive filter are relatively unaffected by the addition of the second output. The theoretical results are verified by simulation     

Relationships Among Classes of Self-Oscillating Transistor Parallel Inverters

It recently has been shown in the literature that many dc-to-squarewave parallel inverters which are widely used in power-conditioning applications can be grouped into one of two families. Each family is characterized by an equivalent RLC network. Based on this approach, a classification procedure is presented for self-oscillating parallel inverters which makes evident natural relationships which exist between various inverter configurations. By utilizing concepts from the basic theory of negative resistance oscillators and the principle of duality as applied to nonlinear networks, a chain of relationships is established which enables a methodical transfer of knowledge gained about one family of inverters to any of the other families in the classification array.     

Space and Maritime Strategy

A globally deployed Navy requires and uses space systems and services. Specific mission support areas of communications, navigation, environmental data, and surveillance are discussed in terms of requirements and program developments. Navy role in pioneering space systems combined with threat assessment validates Navy position of space system acquisition and operation in the future.     

Laser Doppler Velocimeter Development

A new two-channel laser Doppler velocimeter developed for the Ames High Reynolds Channel No. II is described. Design features required for the satisfactory operation of the optical system in the channel environment are discussed. Fiber optics are used to transmit the megahertz Doppler signal to the photodetectors located outside the channel pressure vessel, and provision is made to isolate the optical system from pressure and thermal strain effects. Computer-controlled scanning mirrors are used to psoition the laser beams in the channel flow. Techniques used to seed the flow with 0.5-?-diam polystyrene spheres avoiding deposition on the test-section windows and porous boundary-layer removal panels are described. Preliminary results are presented with a discussion of several of the factors affecting accuracy.     

Coherent spatial filtering for SAR detection of stationary targets

Analysis of UHF synthetic aperture radar (SAR) images has revealed a spatially correlated phase structure over a resolved target such as a long wire or a large truck. This phase variation, approximated by a simple, range-dependent factor 4πr/λ, results in a line-shaped image spatial spectrum. Such image phase and spectral features can be exploited by coherent spatial filtering to improve target-to-clutter ratios of large targets and stationary target detectability in a strong clutter environment