Two-dimensional viscous vortex flow around a circular cylinder

A code based on a vortex method for simulating viscous vortex flow around a circular cylinder has been developed. The original method to obtain the no-through and no-slip condition introduced here is described in detail. The diffusive part of the vorticity transport equation is treated using a deterministic method based on the solution of the heat equation. Results have been obtained and are presented concerning the vorticity field, the velocity field, the evolution of drag and lift coefficients and the Strouhal number.     

Measurements of water vapour in the mesosphere with the spectral absorption line imager (SALI)

Water vapour concentration is one of the most important, yet one of the least known quantities of the mesosphere. Knowledge of water vapour concentration is the key to understanding many mesospheric processes, including the one that is primary focus of our investigation, Polar Mesospheric Clouds (PMC). The processes of formation and occurrence parameters of PMC constitute an interesting problem in their own right, but recent evidence had been provided which suggests that PMC are a critical indicator of climate change. In this context the feasibility of a low cost method of water vapour measurements using an instrument carried aloft by a sounding rocket has been examined and some of the results discussed. It is proposed to measure the strength of the 936nm water absorption line in a solar occultation configuration employing a CCD detector. This leads to the design of a small, low cost and low-mass instrument, which can be flown on a small rocket, of the type of the Orbital Sciences Corporation Viper 5. Alternatively the instrument can be flown as a “passenger” on larger rocket carrying other experiments. In either case flight costs are relatively low. Some performance simulations are presented showing that the instrument we have designed will be sufficiently sensitive to measure water vapor in concentrations that are expected at the summer mesopause, about 85 km height. Sufficient payload design work was carried out showing that the structural, thermal and electrical requirements for a flight on the Viper 5 rocket can be met and thus making the experiment feasible for a flight.     

聚焦透镜天线焦斑宽度估算

根据惠更斯-菲涅耳原理,通过简单的推导,得出聚焦透镜天线焦平面上焦斑宽度的近似计算公式,并讨论了与焦平面平行的平面上的焦斑场.为证明该方法的计算精度及效率,设计了一个Ka波段聚焦透镜天线,并加工了两个天线样品进行比较.对两天线进行了平面近场扫描测试,采用近似公式计算得到的Ka波段聚焦透镜天线半功率点宽度与实验数据接近,表明文中给出的公式有一定工程参考作用.      

Characterization of the optimal mass problem for aerobraking tethers

In the aerobraking tether concept, a probe, connected to an orbiter by a long, thin tether, passes through the atmosphere of a target planet to provide a desired velocity change, while keeping the orbiter above the sensible atmosphere. In earlier work, simple analytic models have been developed which accurately describe the characteristics of the mass-optimal tether. In this paper these models are generalized so that design of the spacecraft and the aerobraking maneuver can be completely characterized by four independent parameters. By comparing the tether mass (e.g. for aerocapture) with the propellant mass required to capture the orbiter, we show that aerobraking tethers have a clear advantage for a wide range of maneuvers.     

Satellite pitch and roll attitude maneuvers through very short tethers

Explored here is the feasibility of achieving satellite pitch and roll attitude maneuvers through tethers. The proposed tethered satellite system (TSS) comprises of four identical tethers connecting the auxiliary mass to the satellite at its four distinct off-centered and equiangularly spaced points. The open-loop tether length control laws have been developed in order to achieve arbitrary pitch and roll attitude slewing maneuvers. Numerical simulation of the nonlinear governing equations of motion for these tether length variations establishes the feasibility of executing fixed as well as chase-slewing maneuvers. Nearly passive nature of the proposed mechanism using very short tethers along with small auxiliary mass needed makes the concept particularly attractive for future space missions.     

运载火箭推力故障不确定性下轨迹可达包络分析

针对运载火箭突发推力故障且故障存在不确定性的轨迹设计问题,提出了一种基于状态转移张量法(STT)的故障火箭轨迹可达包络分析方法。首先,建立了推力故障火箭的运动模型,通过构建故障火箭轨迹重规划问题,基于序列二次规划法获得了不考虑故障不确定性的重规划轨迹; 其次,基于线性协方差对非线性系统的不确定性传播进行建模,并对模型进行线性化,在此基础上提出了基于SST的运载火箭轨迹可达包络分析方法; 最后,通过与多次蒙特卡洛仿真对比验证所提方法在运载火箭轨迹可达包络定量分析中的有效性。     

Numerical investigation of dynamic stall suppression of rotor airfoil via improved co-flow jet

The decrease in aerodynamic performance caused by the shock-induced dynamic stall of an advancing blade and the dynamic stall of a retreating blade at low speed and high angles of attack limits the flight speed of a helicopter. However, little research has been carried on the flow control methods employed to suppress both the dynamic stall induced by a shock wave and the dynamic stall occurring at high angles of attack. The dynamic stall suppression of a rotor airfoil by Co-Flow Jet(CFJ) is nume...     

基于上升轨迹可达范围的目标拦截发射窗口计算

针对低轨目标拦截任务,提出一种利用上升轨迹可达范围分析的发射窗口计算方法。首先,建立以上升时长和航程为性能指标的上升轨迹和优化模型,确定拦截器上升轨迹可达范围。然后,根据目标星下点与上升轨迹可达范围外包络的穿越关系以及拦截器的上升时长范围,对发射窗口进行初筛,得到准发射窗口。最后,针对每一段筛选出的准发射窗口,通过精确判定目标星下点与每一上升时长可达范围子环的位置关系,得到每段准窗口中能够实现目标拦截的发射窗口,将其取并集得到针对目标拦截的精确发射窗口。仿真表明本文提出的计算方法能够快速准确得到目标拦截的发射窗口。     

Comparison analyses on the 150 × 150 lunar gravity field models by gravity/topography admittance,correlation and precision orbit determination

We analyzed the 150 × 150 lunar gravity field models, LP150Q, GLGM-3 and SGM150, using the power spectrum on the lunar nearside and farside, the lunar global and localized gravity/topography admittance and correlation, and Chang’E-2 precision orbit determination to investigate which model is a more effective tool to estimate geophysical parameters and determine the lunar satellite precision orbit. Results indicate that all gravity field models can be used to estimate the lunar geophysical parameters of the nearside of the Moon. However, SGM150 is better in such computation of the farside. Additionally, SGM150 is shown to be the most useful model for determining the lunar satellite orbit.     

基于递推自适应权重的快速稠密立体匹配

针对经典自适应权重稠密立体匹配算法计算量大的问题,提出了一种递推自适应权重算法.重新定义相邻像素的权重为距离衰减因子和色彩差异函数的乘积,不相邻像素权重为相邻像素权重的累乘,色彩差异越小、距离越近的像素权重越大;证明了在新的权重定义下,一维空间的匹配代价融合可以通过两次递推完成,真实图像的匹配代价融合可以通过4次递推完成,同时给出相应递推公式;递推匹配代价融合时每个像素每一视差只做4次乘法和8次加法,计算量比窗口大小为35×35的经典自适应权重算法小约两个数量级;基于递推匹配代价融合实现了一种快速稠密立体匹配算法.使用Middlebury大学的测评集测试该算法,证明了递推自适应权重算法的速度和精度均优于经典自适应权重算法.