一种空间飞行器编队重组的轨道优化方法

现有的空间飞行器编队重组的轨道规划方法在求解能量最优策略时,都预先给定了变轨花费的时间,但没有说明给定的时间是怎么选择的。将空间飞行器主从编队重组的轨道规划视为一个多目标优化问题,提出了一种小生境进化算法。该方法通过使用特定的染色体表示方法和进化算子,能有效的搜索到飞行器编队重组轨道规划问题的时间-能量前沿,并引入等值分享法保证优秀个体具有较大的选中概率和前沿的多样性。该方法能同时提供多种变轨方案,编队飞行的任务制定者从而可以根据实际应用情况选择最合适的方案。仿真结果表明了该方法的正确性。

一种基于进化计算的空间飞行器编队重构轨道规划方法

基于进化算法提出了一种两层结构的空间飞行器编队重构的轨道规划算法,高层算法通过优化构型映射来优化编队的总燃耗,实现全局规划并确保飞行器之间保持一定的安全距离以避免相互碰撞;低层规划算法采用Chebyshev多项式逼近控制变量空间,为每颗飞行器规划满足约束条件的最优轨道。该方法充分利用了编队的分布式结构,由各飞行器并行实现各自的轨道规划,能有效解决大型编队的轨道规划问题。仿真结果表明了该方法的有效性。

带燃料均衡的空间多飞行器编队重构的轨道规划方法

航天任务需求的多样化对空间多飞行器编队重构的轨道规划问题不仅提出了燃料或时间最优的要求,还提出了燃料和时间最优以及燃料均衡的要求。将带燃料均衡的多飞行器编队重构的轨道规划建模为一个多目标优化问题,通过将进化计算与问题领域的知识相结合,提出了一种基于小生境进化算法的最优轨道规划方法。该方法能从变轨时间、燃料消耗和燃料消耗方差三方面分别评价一个变轨方案的最优性,并且一次规划能够提供多个Pareto最优变轨方案。仿真结果证明了该方法的正确性和有效性,还揭示了编队重构轨道规划问题的三个优化目标之间的关系,对于制定任务计划具有重要的参考价值.

空间飞行器编队重构建模与规划方法研究

编队重构是实现空间飞行器编队飞行的一项关键技术，也是自动导航和控制的重要研究领域，无论在理论研究还是实际应用上都具有重大意义。本论文针对空间飞行器编队重构问题展开了深入的研究：（1）编队重构建模；（2）空间飞行器多轨道规划；（3）空间飞行器时间-燃耗的轨道规划；（4）编队重构的协同规划；（5）考虑燃料均衡的编队重构规划。围绕编队重构建模问题，本论文在分析其特点的基础上，完善了传统的单目标优化模型，加入了现有研究通常忽略的发动机推力的精度约束，提高了模型精度以降低控制反馈误差。另外，还根据航天任务的特点，从多目标优化的角度建模了问题，优化的性能指标包括变轨时间、编队的总燃料消耗和飞行器间的燃料均衡。在多轨道规划的研究中，作者基于进化计算和参数优化方法，提出了一种空间飞行器多轨道规划算法。通过将可行轨道按其空间分布分类，并采用一种多轨道保持技术，使新提出的算法能够获得多条在空间中分布较为离散的最优/近优轨道，并可以方便地根据环境和任务要求设定希望获得的轨道之间的差异。该算法充分利用了编队飞行相对动力学方程的解耦性，简化了问题，提高了计算速度，并使最终轨道的选择更加灵活多样。作者还针对空间飞行器的变轨时间与燃料消耗的关系问题展开了研究，基于小生境进化算法，提出了一种时间-燃耗的轨道规划方法。该方法采用一种变长实值染色体编码方式和特定的种群初始化方法，能使生成的轨道满足各种约束条件，并引入等值分享法保证优秀个体具有较大的选中概率和前沿的多样性。实验表明，该算法能有效地搜索到飞行器变轨的时间-燃耗前沿，一次规划生成多个Pareto最优解，为任务制定者选择最合适的变轨方案提供可靠的依据。在编队的协同规划方面，通过分解空间飞行器编队重构问题，并与协同进化的思想相结合，提出了一种两层结构的飞行器编队重构规划算法。高层算法通过优化构型映射来优化编队的总燃料消耗，实现全局规划并确保飞行器之间保持一定的安全距离以避免相互碰撞；低层规划算法采用多轨道规划方法为各飞行器规划满足约束条件的轨道。该算法不仅实现了多颗飞行器的轨道子种群间的协同进化，还实现了高、低层规划结果的协同进化。由于利用编队的分布式结构实现了并行计算，该算法能解决大型编队重构的协同规划问题。虽然给出的最优解是唯一的，但在规划过程中为各飞行器都生成了多条散布在空间中的轨道，提供了可替换解以保证编队重构任务的顺利执行。最后本论文从多目标优化的角度建模了带燃料均衡的多飞行器编队重构规划问题，初步研究了变轨时间、燃料消耗和飞行器间的燃料均衡三个重要指标之间的关系。通过将进化计算与问题领域的知识相结合，提出了一种最优轨道规划方法，能从上述三个指标的角度分别评价一个变轨方案的最优性，并且计算量较小，一次规划能提供多个最优解决方案，能用于实际规划之前进行简单的估算，十分适合方案设计阶段应用。

Formation and structure of composite coating of HDA and micro-plasma oxidation on A3 steel

Composite coatings were obtained on A3 steel by hot dipping aluminum(HDA) at 720 ℃ for 6 min and micro-plasma oxidation(MPO) in alkali electrolyte. The surface morphology, element distribution and interface structure of composite coatings were studied by means of XRD, SEM and EDS. The results show that the composite coatings obtained through HDA/MPO on A3 steel consist of four layers. From the surface to the substrate, the layer is loose Al2O3 ceramic, compact Al2O3 ceramic, Al and FeAl intermetallic compound layer in turn. The adhesions among all the layers are strengthened because the ceramic layer formed at the Al surface originally, FeAl intermetallic compound layer and substrate are combined in metallurgical form through mutual diffusion during HDA process.Initial experiment results disclose that the anti-corrosion performance and wear resistance of composite coating are obviously improved through HDA/MPO treatment.

Formation of Fivefold Deformation Twins in Nanocrystalline Face-Centered-Cubic Copper Based on Molecular Dynamics Simulations

Fivefold deformation twins were reported recently to be observed in the experiment of the nanocrystalline face-centered-cubic metals and alloys. However, they were not predicted previously based on the molecular dynamics (MD) simulations and the reason was thought to be a uniaxial tension considered in the simulations. In the present investigation, through introducing pretwins in grain regions, using the MD simulations, the authors predict out the fivefold deformation twins in the grain regions of the nanocrystal grain cell, which undergoes a uniaxial tension. It is shown in their simulation results that series of Shockley partial dislocations emitted from grain boundaries provide sequential twining mechanism, which results in fivefold deformation twins. (c) 2006 American Institute of Physics.

Numerical Simulation of Nox Formation in Coal Combustion with Inlet Natural Gas Burning

A full two-fluid model of reacting gas-particle flows and coal combustion is used to simulate coal combustion with and without inlet natural gas added in the inlet. The simulation results for the case without natural gas burning is in fair agreement with the experimental results reported in references. The simulation results of different natural gas adding positions indicate that the natural gas burning can form lean oxygen combustion enviroment at the combustor inlet region and the NOz concentration is reduced. The same result can be obtained from chemical equilibrium analysis.

Twinning and Stacking Fault Formation during Tensile Deformation of Nanocrystalline Ni

Deformation twins and stacking faults have been observed in nanocrystal line Ni, for the first time under uniaxial tensile test conditions. These partial dislocation mediated deformation mechanisms are enhanced at cryogenic test temperatures. Our observations highlight the effects of deformation conditions, temperature in particular, on deformation mechanisms in nanograins.

Formation, Thermal Stability and Deformation Behavior of Graphite-Flakes Reinforced Cu-based Bulk Metallic Glass Matrix Composites

Graphite-flake reinforced Cu47Ti34Zr11 Ni-8 bulk metallic glass matrix composite was fabricated by water-cooled copper mould cast. Most of the graphite flakes still keep unreacted and distribute uniformly in the amorphous matrix except that some reactive wetting occurs by the formation of TiC particles around the flakes. It reveals that the presence of graphite flakes does not affect the onset of the glass transition temperature, crystallization reaction and liquidus of the metallic glass. The resulting material shows obvious serrated flow and higher fracture strength under room temperature compressive load, comparing with the monolithic bulk metallic glass (BMG). Three types of interaction between the shear bands and graphite flakes, namely, shear band termination, shear bands branching and new shear bands formation near the graphite flakes can be observed by quasi-static uniaxial compression test and bonded interface technique through Vickers indentation.

Effect of Particle Size on the Formation of Adiabatic Shear Band In Particle Reinforced Metal Matrix Composites

In this paper, the effect of particle size on the formation of adiabatic shear band in 2024 All matrix composites reinforced with 15% volume fraction of 3.5, 10 and 20 mum SiC particles was investigated by making use of split Hopkinson pressure bar (SHPB). The results have demonstrated that the onset of adiabatic shear banding in the composites strongly depends on the particle size and adiabatic shear banding is more readily observed in the composite reinforced with small particles than that in the composite with large particles. This size dependency phenomenon can be characterized by the strain gradient effect. Instability analysis reveals that high strain gradient is a strong driving force for the formation of adiabatic shear banding in particle reinforced metal matrix composites (MMCp).

Evaluation of the Cumulative Formation of High-Speed Liquid Jets

This paper describes the generation of pulsed, high-speed liquid jets using the cumulation method. This work mainly includes (1) the design of the nozzle assembly, (2) the measurement of the jet velocity and (3) flow visualization of the injection sequences. The cumulation method can be briefly described as the liquid being accelerated first by the impact of a moving projectile and then further after it enters a converging section. The experimental results show that the cumulation method is useful in obtaining a liquid jet with high velocity. The flow visualization shows the roles of the Rayleigh-Taylor and Kelvin-Helmholtz instabilities in the breakup of the liquid depend on the jet diameter and the downstream distance. When the liquid jet front is far downstream from the nozzle exit, the jet is decelerated by air drag. Meanwhile, large coherent vortex structures are formed surrounding the jet. The liquid will break up totally by the action of these vortices. Experimental results showing the effect of the liquid volume on the jet velocity are also included in this paper. Finally, a method for measuring the jet velocity by cutting two carbon rods is examined.

Formation Mechanism of Cracks in Saturated Sand

The formation mechanism of “water film” (or crack) in saturated sand is analyzed theoretically and numerically. The theoretical analysis shows that there will be no stable “water film” in the saturated sand if the strength of the skeleton is zero and no positions are choked. It is shown by numerical simulation that stable water films initiate and grow if the choking state keeps unchanged once the fluid velocities decrease to zero in the liquefied sand column. The developments of “water film” based on the model presented in this paper are compared with experimental results.

Electrochemical, spectroscopic and SPM evidence for the controlled formation of self-assembled monolayers and organised multilayers of ferrocenyl alkyl thiols on Au(111)

Organised multilayers were formed from the controlled self-assembly of ferrocene alkyl thiols on Au(111) surfaces. The control was accomplished by increasing the concentration of the thiol solutions used for the assembly. Cyclic voltammetry, ellipsometry, scanning probe microscopy (STM and AFM) and in situ FTIR spectroscopy were used to probe the differences between mono- and multilayers of the same compounds. Electrochemical desorption studies confirmed that the multilayer structure is attached to the surface via one monolayer. The electrochemical behaviour of the multilayers indicated the presence of more than one controlling factor during the oxidation step, whereas the reduction was kinetically controlled which contrasts with the behaviour of monolayers, which exhibit kinetic control for the oxidation and reduction steps. Conventional and imaging ellipsometry confirmed that multilayers with well-defined increments in thickness could be produced. However, STM indicated that at the monolayer stage, the thiols used promote the mobility of Au atoms on the surface. It is very likely that the multilayer structure is held together through hydrogen bonding. To the best of out knowledge, this is the first example of a controlled one-step growth of multilayers of ferrocenyl alkyl thiols using self-assembly techniques.

On the mechanism of the formation of horizontal cracks in a vertical column of saturated sand

Extended horizontal cracks have! been observed experimentally in a vertical column of saturated sand when a flow of water is forced to percolate upward through it. This paper provides a theory for this phenomenon. It will be shown that the presence of inhomogeneity in permeability along the length of the column is essential for such cracks to develop. It will also be shown that small initial inhomogeneity may be magnified through the transport of the finer component of the sand by percolation. Under certain conditions liquefaction takes place at a section of the sand column causing a crack to initiate and grow there. This theory is found to be in good qualitative agreement with the experimental findings.