多喷管燃气降噪方案可行性数值研究

根据燃气射流噪声的发声机理及辐射特性,提出了采用多喷管代替单喷管进行降噪的方案。以四喷管为例,构建相应的物理及计算模型,综合运用三维大涡模拟得到的近场瞬态流场数据与FW-H面积分相结合的计算气动声学方法,对射流流场及声学特性进行数值研究。通过与单喷管对比,得出多喷管结构在保证发动机质量流率及推力性能的前提下,降低了整个观测区域的噪声,尤其是射流下游,并且还改变了噪声辐射的指向性,起到了明显的降噪效果,验证了该方案的可行性。     

Launch cost reduction by the reuse of the core stage of Ariane 5

The high cost of launching payloads into Earth orbit is a main limiting factor on the development of space. In order to reduce the high cost of launch, reuse of (parts of) the launch vehicle is needed. This study analyses the possibilities of recovering and reusing the core stage of Ariane 5. Recovery of the core stage sets demands on re-entry trajectory, attitude, stability, thermal protection, structural strength, terminal deceleration, salt water protection, recovery and refurbishment. All these subject areas require solutions to their individual problems. Added subsystems to the stage are defined and their mass is determined. These masses are used to determine the financial feasibility of the recovery concept, by weighing the payload demise and operational cost against the gains of reduced production cost. It is concluded that the recovery is technologically feasible, using a detachable ablative heat shield on the nose of the stage and a stabilisation device (an inflatable drag cone), a parachute system and an engine enclosure device. Total mass of these systems is 1320 kg, with financial savings amounting to $8.5 million per flight.     

Decision program on asteroid threat mitigation

The Association of Space Explorers Committee on Near-Earth Objects (NEOs) and its Panel on Asteroid Threat Mitigation have prepared a decision program to aid the international community in organizing a coordinated response to asteroid impact threats. The program is described in the ASE's report, Asteroid Threats: A Call for Global Response, which will be considered by the United Nations Committee on the Peaceful Uses of Outer Space in its 2009 sessions. The findings and recommendations of this report are presented here as well as some of the major implications of the complex decision-making involved in developing a coordinated international response to the challenge of protecting the Earth from NEO impacts.     

上单翼布局飞机翼根整流罩设计分析

根据翼身干扰形成的位移效应、升力效应、不对称效应、粘性效应等流动现象和原理,从流动机理上分析讨论了上单翼布局飞机整流罩的减阻原理;对上单翼布局飞机,提出一种整流罩设计形式,并对该整流罩各部分对气动性能的影响进行详细分析,采用RANS方程求解流场,以翼身组合体为基准,对整流罩的6个主要参数逐一变化,将每个参数的不同取值的计算结果各自对比,从流动原理上对数值模拟计算结果进行了分析,得出了上单翼布局飞机整流罩设计的几点思路。     

后缘锯齿襟翼对翼型气动特性的影响研究

针对在翼型后缘增加锯齿襟翼与格尼襟翼的差异性,以及二者对翼型的空气动力特性有何改善等问题,在某民用运输机机翼翼型下表面后缘处增加锯齿襟翼,对其进行了空气动力特性试验和计算。结果表明,增加一定高度的后缘锯齿襟翼,能够减弱和改变翼型下表面后缘处气流的流动,从而减少和改变翼型下表面尾涡的涡量和涡量变化率,达到增加翼型升力、减小翼型阻力的目的。     

不对称导致的土卫六尾迹的偏转

旅行者1号和卡两尼飞船都与土卫六近距离相遇过,它们探测到了离子在土卫六尾迹的向土星侧和背土星侧分布不对称.本文将通过一个简单的模型来模拟在上卫六外逸层中产生的离子轨迹.结果说明了重离子有与士卫六半径相当的回旋半径,但是轻离子的回旋半径并没有这么大.由于有限回旋半径效应,向上星侧的大部分重离子被土卫六大气层吸收,所以在土卫六尾迹两边的离子密度是不对称的.这个不对称使尾迹处于不平衡的状态,并偏离共转方向.通过力学平衡,可以发现尾迹偏离共转方向朝土星偏转约19°.这与旅行者1号的观测是一致的.     

机载武器传递对准的可观测性分析

采用"速度+姿态"量测匹配法设计了机载武器传递对准系统,建立了系统的动态误差模型和量测模型,并在这些模型的基础上设计了卡尔曼滤波器.介绍了基于可观测性矩阵奇异值分解的分段定常系统(PWCS, Piece-Wise Constant System)可观测性分析法,并通过该方法分析了机载武器传递对准系统各个状态变量的可观测性.在可观测性分析的基础上忽略不可观测状态变量,实现系统滤波器降维.设计了摇摆机动,在该机动基础上对降维前后滤波器性能和传递对准精度进行了对比,证明了可观测性分析的必要性和正确性.     

逆向卸荷式气体减压阀的动态特性仿真

考虑到摩擦力、阻尼力及流体流动作用力等主要非线性因素对减压阀动态特性的影响,建立了逆向卸荷式气体减压阀的动态特性数学模型,并对其启动过程与负载变化下的动态响应特性进行了仿真计算,研究了动态过程的压力、温度、流量以及阀芯位移等参数变化情况,分析了不同的参数对减压阀启动特性的影响,提出了改善减压阀动态响应特性的措施。     

Reducing mission operations costs through spacecraft autonomy: the near earth asteroid rendezvous (near) experience

With a maximum time of 12 days out of ground contact and a round-trip light time as high as 56 minutes, The Near Earth Asteroid Rendezvous (NEAR) spacecraft requires a moderate degree of onboard autonomy to react to faults and safe the spacecraft. Beyond the basic safing requirements, additional functions are carried out onboard. For example, on-board calculation of the Sun, Earth, asteroid, and spacecraft positions allow the spacecraft to autonomously orient itself for science and downlink operations. On-board autonomous momentum management during cruise relieves Mission Operations from planning, scheduling, and carrying out many manual momentum dumps. During development, additional operations, such as center-of-mass management during propulsive maneuvers and optical navigation were also considered for onboard autonomy on the NEAR spacecraft, but were not selected. The allocation of functions to onboard software or to ground operations involved tradeoffs such as development time for onboard software versus ground software, resource management, life cycle costs, and spacecraft safety.After two years of cruise operations, considerable experience with the NEAR autonomy system has accrued. The utility of some autonomous capabilities is greater than expected, others less so. Software uploads increased spacecraft autonomy in some cases, and the impact on Mission Operations can be assessed. Allocation of functions between spacecraft autonomy and ground operation during development of future missions can be improved by applying the lessons learned from the NEAR experience.     

Dynamic modeling and simulation of a pressurized system used in flight vehicle

For a typical pressurized system with a novel dual-stage gas pressure reducing regulator, a system model is established with modular models of various typical components. The simulation study on the whole working period shows that the general trends and magnitudes of simulation curves are in agreement with experimental measured curves. As the key component in the pressurized system, the regulator is studied by a series of numerical simulations to reveal the influences of various structure parameters on its stability. Furthermore, the variable ranges which can guarantee the stability of regulator and system are obtained to provide guidance for design. The modeling and analysis approach can be applied to other systems and components.