Fully distributed time-varying formation tracking control for multiple quadrotor vehicles via finite-time convergent extended state observer

This paper investigates a time-varying anti-disturbance formation problem for a group of quadrotor aircrafts with time-varying uncertainties and a directed interaction topology. A novel Finite-Time Convergent Extended State Observer (FTCESO) based fully-distributed formation control scheme is proposed to enhance the disturbance rejection and the formation tracking performances for networked quadrotors. By adopting the hierarchical control strategy, the multi-quadrotor system is separated into two subsystems: the outer-loop cooperative subsystem and the inner-loop attitude subsystem. In the outer-loop subsystem, with the estimation of disturbing forces and uncertain dynamics from FTCESOs, an adaptive consensus theory based cooperative controller is exploited to ensure the multiple quadrotors form and maintain a time-varying pattern relying only on the positions of the neighboring aircrafts. In the inner-loop subsystem, the desired attitude generated by the cooperative control law is stably tracked under a FTCESO-based attitude controller in a finite time. Based on a detailed algorithm to specify the cooperative control protocol, the feasibility condition to achieve the time-varying anti-disturbance formation tracking is derived and the rigorous analysis of the whole closed-loop multi-quadrotor system is given. Some numerical examples are conducted to intuitively demonstrate the effectiveness and the improvements of the proposed control framework.     

前向数据接入方式对导航星座网络通信性能的影响分析

对采用时分多址轮询建链体制的导航星座星间链路网络，为了指导星间链路网络地面前向数据接入的应用，分析了不同地面前向数据接入方式的星-星-地联合数据交互机制及其对网络通信性能的影响，并结合应用代价给出了综合评价和使用建议。以前向数据最快路径接入方式和固定节点接入方式为代表，分析了星间链路网络与地面间星 星 地联合数据交互机制的特点，明确了两种方式下星间链路网络拓扑路由规划策略的制定原则。通过构建Walker 24/3/1导航星座星间链路网络数据处理模型及典型星地运行场景，对两种地面前向数据接入方式仿真分析了星间链路通信容量和传输时延等网络通信性能，比较分析了通信容量及数据拥塞规律，得出前向数据最快路径接入方式相比于固定节点接入方式在通信性能上有较明显优势的结论。最后，综合给出了两种星 星 地联合数据交互机制的应用性评价,同时给出了在通信容量较低且通信时延要求不高、但可靠性要求较高的应用条件下，可适度采用固定节点接入方式的建议。     

大气环境对直升机旋翼桨-涡干扰噪声辐射特性的影响

建立了一个适用于旋翼桨-涡干扰气动载荷计算的计算流体力学(Computational fluid dynamics,CFD)/自由尾迹耦合模型,为提高计算效率,提出了一种高效的耦合策略进行不同计算域间的信息交换策略。在此基础上,结合基于声类比法的FW-H方程构建了旋翼桨-涡干扰噪声的计算方法。应用所建立的方法,以OLS(Operational loads survey)旋翼为研究对象,深入分析了大气环境对旋翼噪声辐射特性的影响。研究发现:随着飞行高度的增加,旋翼噪声辐射特性发生了明显的改变,逐渐由桨盘前行侧转变为指向桨盘前方,噪声幅值先增大后减小。文中从桨-涡干扰距离、干扰位置变化角度计算分析了大气压力、音速及空气密度等环境参数对旋翼桨-涡干扰噪声辐射特性的影响,并得出了一些有实际意义的影响规律。     

共轴刚性旋翼气动干扰数值计算方法

建立了一个适用于共轴刚性旋翼气动特性分析的数值模拟方法。该方法采用任意拉格朗日欧拉方法(Arbitrary Lagrange Euler,ALE)描述的可压缩Navier-Stokes(N-S)方程求解流场,采用低数值耗散的Roe格式进行空间离散;使用多重嵌套网格方法以模拟双旋翼的运动。针对共轴刚性旋翼配平,引入"差量修正"策略解决了传统配平中雅克比矩阵计算复杂的问题。首先,对Harrington-2共轴双旋翼的悬停气动性能进行了计算,然后,对某2 m直径共轴双旋翼的悬停及前飞状态进行了计算,并与试验值进行了对比。结果表明:在典型状态下拉力系数的计算结果与试验值误差在3%以内,扭矩系数的计算结果与试验值误差基本在5%以内;所采用的数值计算方法对旋翼涡尾迹特征具有较高的捕捉精度,可以有效模拟共轴刚性旋翼悬停和小速度前飞下的复杂流场及其细节特征。     

Cutting path-dependent machinability of SiCp/Al composite under multi-step ultra-precision diamond cutting

Particle-tool interactions, which govern the synergetic deformation of SiC particle reinforced Al matrix composites under mechanical machining, strongly depend on the geometry of particle position residing on cutting path. In the present work, we investigate the influence of cutting path on the machinability of a SiCp/Al composite in multi-step ultra-precision diamond cutting by combining finite element simulations with experimental observations and characterization. Be consistent with experimentally characterized microstructures, the simulated SiCp/Al composite is considered to be composed of randomly distributed polygonally-shaped SiC particles with a volume fraction of 25vol%. A multi-step cutting strategy with depths of cut ranging from 2 to 10 μm is adopted to achieve an ultimate depth of cut of 10 μm. Intrinsic material parameters and extrinsic cutting conditions utilized in finite element simulations of SiCp/Al cutting are consistent with those used in corresponding experiments. Simulation results reveal different particle-tool interactions and failure modes of SiC particles, as well as their correlations with machining force evolution, residual stress distribution and machined surface topography. A detailed comparison between numerical simulation results and experimental data of multi-step diamond cutting of SiCp/Al composite reveals a substantial impact of the number of cutting steps on particle-tool interactions and machined surface quality. These findings provide guidelines for achieving high surface finish of SiCp/Al composites by ultra-precision diamond cutting.     

Injection of 40-kHz-modulated electron beam from the satellite: II. Excitation of electrostatic and whistler waves

Beam-plasma interaction effects are studied during the active space experiment with electron and Xe-ion beam injections in an ionospheric plasma. Permanent 40-kHz-modulated electron beam injection occurs simultaneously with a xenon-ion beam injected by the Hall-type plasma thruster operating in a square-pulse mode (100/50 s for a job/pause duration). The unusual behavior of the background charged particle fluxes and wave activity stimulated during the beam-plasma interaction have been registered by the scientific instruments onboard Intercosmos-25 station (IK-25) and Magion-3 subsatellite. The longitudinal and electromagnetic wave instabilities and their mutual relationship are considered in order to explain the observed effects. The excitation of electrostatic waves by the electron injection has been considered for different resonance conditions near the linear stability boundary. Beam-driven electromagnetic instability is responsible for the backward-propagating whistler waves excited via cyclotron resonance. Competition of these two beam instabilities is one of the subjects of the present study.     

Variation of dayside chorus waves associated with solar wind dynamic pressure based on MMS observations

The whistler-mode chorus waves are one of the most important plasma waves in the Earth’s magnetosphere. Generally, the amplitude of whistler-mode chorus waves prefers to strengthen when the energetic fluxes of anisotropic electrons increase outside the plasmapause. This characteristic is commonly associated with the geomagnetic storms or substorms. However, the relationship between the solar wind dynamic pressure (P_sw) and the long-time variation of chorus waves during the quiet period of the geomagnetic activity still needs more detailed investigations. In this paper, based on MMS observations, we present a chorus event just observed in the inner side of magnetopause without obvious geomagnetic storms or substroms. Interestingly, during this time interval, some P_sw fluctuations were recorded. Both the amplitudes and frequencies of chorus waves changed as a response to the variation in P_sw. It proved that the enhancement of P_sw increases the energetic electrons fluxes, which provides free energies for the chorus amplification. Furthermore, the wave growth rates calculated using linear theory increases and the central frequency of the chorus waves shifts to a higher frequency when the P_sw enhancement is greater, which are also consistent well with the observations. The results provide a direct evidence that the P_sw play an important role in the long-time variation of whistler-mode chorus waves inside the magnetopause.     

流场可压缩性对涡相互作用影响的数值研究

涡相互作用作为冲压发动机喷注装置的典型抽象流动现象,研究其可压缩性影响对于认识包含化学反应的真实燃料喷射场具有一定基础理论价值。基于经过数值验证的可压缩Navier-Stokes算法与压力泊松方程初始条件设置方法研究流场可压缩性对涡相互作用演化过程的影响。结果表明,以较高涡旋马赫数表征的可压缩性在改变涡对形态的同时,具有延缓涡旋相互靠近,迟滞融合进程的作用。关于涡对系统开始融合的临界条件,可压缩相互作用开始的临界展弦比与无量纲时间相比于低速涡对明显提高。为在无量纲时间意义下统一不同马赫数涡对相互作用的进程,从涡心密度随时间变化规律出发在不可压涡对特征时间的基础上,初步构建了考虑可压缩性的时间尺度修正关系。     

超声速膨胀角入射激波/湍流边界层干扰直接数值模拟

为了揭示膨胀效应对激波/湍流边界层干扰区内复杂流动现象的影响规律,采用直接数值模拟方法对来流马赫数2.9、30°激波角的入射激波与10°膨胀角湍流边界层相互作用问题进行了数值研究。系统地探讨了激波入射点分别位于膨胀角上游、膨胀角角点和膨胀角下游3种工况下膨胀角干扰区内若干基本流动现象,如分离泡、物面压力脉动及激波非定常运动、湍流边界层统计特性和相干结构动力学过程等。结果表明,激波入射点流向位置改变对分离区流向和法向尺度的影响显著,尤其是当激波入射点位于角点及其下游区域。研究发现,膨胀角干扰区内物面压力脉动强度急剧减小,分离区内压力波向下游传播速度将降低而在膨胀区内将升高,膨胀效应极大地抑制了分离激波的低频振荡运动。相较于入射激波与平板湍流边界层干扰,入射激波流向位置改变对膨胀角再附区速度剖面对数区及尾迹区影响显著,将导致其内层结构参数升高而外层降低,近壁区内将呈现远离一组元湍流状态的趋势。此外,流向速度脉动场本征正交分解分析指出,主模态空间结构集中在分离激波及剪切层根部附近而高阶模态以边界层内小尺度正负交替脉动结构为主。低阶重构流场结果表明,前者对应为分离泡低频膨胀/收缩过程而后者表征为分离泡高频脉动。     

膨胀效应对激波/湍流边界层干扰的影响

采用直接数值模拟方法对来流马赫数2.9,30°激波角的入射激波与膨胀角湍流边界层干扰问题进行了数值研究。入射激波在壁面上的名义入射点固定在膨胀角角点,膨胀角角度分别取为0°、2°、5°和10°。通过改变膨胀角角度,考察了膨胀效应对干扰区内复杂流动现象的影响规律和作用机制,如分离泡、物面压力脉动特性、膨胀区湍流边界层和物面剪切应力脉动场等。研究发现,膨胀角角度的增大使得分离区流向长度和法向高度急剧降低,尤其是在强膨胀效应下分离泡形态呈现整体往下游偏移的双峰结构。物面压力脉动功率谱结果表明,膨胀角为2°和5°时,分离激波的非定常运动仍表征为大尺度低频振荡,而膨胀角为10°,强膨胀效应极大地抑制了分离激波的低频振荡,加速了下游再附边界层物面压力脉动的恢复过程。膨胀区湍流边界层雷诺剪切应力各象限事件贡献和出现概率呈现逐步恢复到上游湍流边界层的趋势,Görtler-like流向涡结构展向和法向尺度变化剧烈,同时在近壁区将诱导生成大量小尺度流向涡。此外,物面剪切应力脉动场的本征正交分解分析指出,膨胀效应的影响体现在低阶模态能量的急剧降低从而使得高阶模态的总体贡献相对升高。