高超声速飞行器表面气动热和粘性摩擦力计算

本文给出了高超声速飞行器表面摩阻和传热系数 (斯坦顿数 )的计算结果。采用两种方法平面切面法亦即二维边界层近似法和工程方法计算了飞行器高超声速绕流的粘性效应 ,并对两种方法的计算结果作了仔细的比较。由文可见 ,对于在稠密大气层内 ,沿轨道运行头速度恒定的高超声速有翼飞行器 ,能够用本文所采用的两种方法计算其表面摩阻和热载荷。此二法可成功地应用于绕复杂形状物体的流动参数计算。     

Multisensor covariance control strategies for reducing bias effects in interacting target scenarios

Algorithms are presented for managing sensor information to reduce the effects of bias when tracking interacting targets. When targets are close enough together that their measurement validation gates overlap, the measurement from one target can be confused with another. Data association algorithms such as the joint probabilistic data association (JPDA) algorithm can effectively continue to track targets under these conditions, but the target estimates may become biased. A modification of the covariance control approach for sensor management can reduce this effect. Sensors are chosen based on their ability to reduce the extent of measurement gate overlap as judged by a set of heuristic parameters derived in this work. Monte Carlo simulation results show that these are effective methods of reducing target estimate bias in the JPDA algorithm when targets are close together. An analysis of the computational demands of these algorithms shows that while they are computationally demanding, they are not prohibitively so.     

Total solar and spectral irradiance variations from solar cycles 21 to 23

Total solar and UV irradiances have been measured from various space platforms for more than two decades. More recently, observations of the “Variability of solar IRradiance and Gravity Oscillations” (VIRGO) experiment on SOHO provided information about spectral irradiance variations in the near-UV at 402 nm, visible at 500 nm, and near-IR at 862 nm. Analyses based on these space-borne irradiance measurements have convinced the skeptics that solar irradiance at various wavelengths and in the entire spectrum is changing with the waxing and waning solar activity. The main goal of this paper is to review the short- and long-term variations in total solar and spectral irradiances and their relation to the evolution of magnetic fields from solar cycles 21 to 23.     

On the possible detection of solid O2 and O3 interstellar grains with ISO

In various models of interstellar grain chemistry, solid O₂ is formed by accretion as well as by surface reactions on grains. In dense molecular cloud models, at a later stage of the evolution, the O₂ molecule may become a substantial grain mantle constituent. Since IR dipole vibrational transitions for the homonuclear diatomic molecule O₂ are forbidden, the abundance of this potentially important grain mantle component can not be determined. However, embedded in a dirty ice matrix, the fundamental vibration of O₂ at 1550 cm⁻¹ becomes observable at 10 K, due to interactions with surrounding molecules, which break the symmetry of molecular oxygen. This process might be applicable for the dust mantle environment of interstellar grains. We have studied the role of solid O₂ and O₃ in astrophysically relevant ice mixtures and discuss the possible detection of solid O₂ and its major photolysis product O₃ in interstellar grains, in dense molecular clouds. Both molecules represent a specific target to be observed by the ISO satellite in the near future.     

SiO2-水纳米流体在波壁管内流动特性的实验研究

实验研究了不同质量分数的SiO2-水纳米流体在波壁管内的流动特性,由于波壁管自身的结构特点,使流体在较小雷诺数下达到湍流状态,可以方便测出流体在层流、过渡流、湍流区的流动特性.研究发现:相同温度条件下,纳米流体的粘度随着质量分数的提高而增大;流动可视化照片显示纳米流体中由于内部纳米粒子的微运动促使流体均匀性更好;沿程阻力测试表明在层流区内摩擦系数随纳米流体质量分数的增加而增大,在过渡流和湍流区内摩擦系数随质量分数增加变化不大.     

Motion of three viscoelastic planets in the field of their mutual attraction forces

Translational-rotational motion of three planets modeled by viscoelastic balls in the gravitational field of mutual attraction is studied in this paper. The system of equations of motion for the mechanical system under consideration is deduced from the d’Alembert-Lagrange variational principle. Using the method of separation of motions, an approximate system of ordinary differential equations, describing the translational-rotational motion of the planets, is obtained with taking into account perturbations caused by elasticity and dissipation. The found steady-state motion of the system is an analog to triangular libration points in the classical three-body problem.     

Quality management for space systems in ISRO

In a little over four decades, the Indian Space Program has carved a niche for itself with the unique application driven program oriented towards National development. The end-to-end capability approach of the space projects in the country call for innovative practices and procedures in assuring the quality and reliability of space systems. The System Reliability (SR) efforts initiated at the start of the projects continue during the entire life cycle of the project encompassing design, development, realisation, assembly, testing and integration and during launch. Even after the launch, SR groups participate in the on-orbit evaluation of transponders in communication satellites and camera systems in remote sensing satellites. SR groups play a major role in identification, evaluation and inculcating quality practices in work centres involved in the fabrication of mechanical, electronics and propulsion systems required for Indian Space Research Organization's (ISRO's) launch vehicle and spacecraft projects. Also the reliability analysis activities like prediction, assessment and demonstration as well as de-rating analysis, Failure Mode Effects and Criticality Analysis (FMECA) and worst-case analysis are carried out by SR groups during various stages of project realisation. These activities provide the basis for project management to take appropriate techno-managerial decisions to ensure that the required reliability goals are met. Extensive test facilities catering to the needs of the space program has been set up. A system for consolidating the experience and expertise gained for issue of standards called product assurance specifications to be used in all ISRO centres has also been established.     

On an analytical model of the aerothermodynamics of fluid–structure interaction

An analytical model of the aerothermodynamics of fluid–structure interaction through an ablative wall is developed, involving: (i) the solution to the Navier–Stokes equations in the fluid; (ii) the solution to the equations of thermoelasticity in the solid wall; (iii) the matching of the two solutions across the fluid–wall interface by continuity of normal and tangential stresses and jump of the heat flux equal to the heat of ablation. These three matching conditions determine the three constants of integration, besides the boundary conditions: (i) in the free stream far above the wall, the fluid velocity and temperature are given; (ii) in the backing material below the wall the temperature is given and displacement is assumed to be zero. In order to arrive at a simple analytical solution the simplest geometry is chosen: two-dimensional, with a flat fluid–wall interface, and all quantities in the fluid and structure depending only on distance from the wall. Extensions to unsteady and three-dimensional cases would be more complex. The present theory specifies: (i) the velocity, pressure and density in the fluid; (ii) the displacement vector, and strain and stress tensors in the structure; (iii) the temperature and heat flux in the fluid and structure, including discontinuities across the fluid–wall interface. The model is applied to examples of (a) hypersonic flight and (b) atmospheric re-entry.     

LMI approach to the mixed H2/H∞filtering design for discrete-time uncertain systems

The problem of full-order robust filtering design for discrete-time uncertain linear systems is addressed. The uncertain parameters are assumed to belong to convex bounded domains (polytope type uncertainty). The main purpose is to design a stable linear filter such that the filtering error output signal remains bounded. For that, the parameterization of all linear filters assuring quadratic stability with an H_∞ attenuation constraint to the filtering error system is provided in terms of linear matrix inequalities (LMIs). Then, through the definition of an auxiliary cost, an upper bound to the filtering error variance is minimized, providing a mixed H₂/H _∞ guaranteed cost filtering design. Standard optimization procedures with global convergence assured can be used to solve the problem, as illustrated by an example