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201.
202.
In this paper we review the possible mechanisms for production of non-thermal electrons which are responsible for the observed
non-thermal radiation in clusters of galaxies. Our primary focus is on non-thermal Bremsstrahlung and inverse Compton scattering,
that produce hard X-ray emission. We first give a brief review of acceleration mechanisms and point out that in most astrophysical
situations, and in particular for the intracluster medium, shocks, turbulence and plasma waves play a crucial role. We also
outline how the effects of the turbulence can be accounted for. Using a generic model for turbulence and acceleration, we
then consider two scenarios for production of non-thermal radiation. The first is motivated by the possibility that hard X-ray
emission is due to non-thermal Bremsstrahlung by nonrelativistic particles and attempts to produce non-thermal tails by accelerating
the electrons from the background plasma with an initial Maxwellian distribution. For acceleration rates smaller than the
Coulomb energy loss rate, the effect of energising the plasma is to primarily heat the plasma with little sign of a distinct
non-thermal tail. Such tails are discernible only for acceleration rates comparable or larger than the Coulomb loss rate.
However, these tails are accompanied by significant heating and they are present for a short time of <106 years, which is also the time that the tail will be thermalised. A longer period of acceleration at such rates will result
in a runaway situation with most particles being accelerated to very high energies. These more exact treatments confirm the
difficulty with this model, first pointed out by Petrosian (Astrophys. J. 557:560, 2001). Such non-thermal tails, even if possible, can only explain the hard X-ray but not the radio emission which needs GeV or
higher energy electrons. For these and for production of hard X-rays by the inverse Compton model, we need the second scenario
where there is injection and subsequent acceleration of relativistic electrons. It is shown that a steady state situation,
for example arising from secondary electrons produced from cosmic ray proton scattering by background protons, will most likely
lead to flatter than required electron spectra or it requires a short escape time of the electrons from the cluster. An episodic
injection of relativistic electrons, presumably from galaxies or AGN, and/or episodic generation of turbulence and shocks
by mergers can result in an electron spectrum consistent with observations but for only a short period of less than one billion
years. 相似文献
203.
基于Kriging模型插值的孔位修正策略 总被引:2,自引:0,他引:2
飞机装配时的孔位精度直接影响其最终装配质量,而待制孔零件易发生变形和整体偏移,因此一般通过基准孔信息对待制孔位进行补偿。提出一种基于Kriging插值的孔位修正方法,通过基准孔建立孔位理论坐标与实际偏差值的Kriging模型,进而预测待制孔的孔位偏差,并给出每个位置的预测标准误差,以指导基准孔的增添和布置。最后通过有限元和数值实验分别验证了零件在极限变形和整体平移旋转情况下该方法的有效性,实现了孔位估计误差小于0.3 mm。 相似文献
204.
C919飞机全机静力试验技术 总被引:2,自引:2,他引:2
介绍了C919大型客机全机静力试验的目的及任务需求,分析了试验特点和难点。从试验飞机支持、试验加载及控制、试验测量及监控、损伤检测及状态监测等方面制定了总体技术实现方案,验证结果表明试验系统可靠,技术实现方案满足任务要求。试验中采用了多项创新技术:试验综合加载平台设计技术优化了多系统集成,加快了试验准备速度,降低了试验准备风险;约束点误差转移控制技术将约束部位误差转移至非重点考核部位,提高了关键考核区域试验精度;机身双层地板双向加载技术优化了机身加载及扣重设计,改善了局部载荷的加载精度。技术成果为后续型号试验提供了较高参考价值。 相似文献
205.
Robust adaptive fault-tolerant control of a tandem coaxial ducted fan aircraft with actuator saturation 总被引:1,自引:0,他引:1
This paper is concerned with the robust adaptive fault-tolerant control of a tandem coaxial ducted fan aircraft under system uncertainty, mismatched disturbance, and actuator saturation. For the proposed aircraft, comprehensive controllability analysis is performed to evaluate the controllability of each state as well as the margin to reject mismatched disturbance without any knowledge of the controller. Mismatched disturbance attenuation is ensured through a structured H-infinity controller tuned by a non-smooth optimization algorithm. Embedded with the H-infinity controller, an adaptive control law is proposed in order to mitigate matched system uncertainty and actuator fault. Input saturation is also considered by the modified reference model. Numerical simulation of the novel ducted fan aircraft is provided to illustrate the effectiveness of the proposed method. The simulation results reveal that the proposed adaptive controller achieves better transient response and more robust performance than classic Model Reference Adaptive Control (MRAC) method, even with serious actuator saturation. 相似文献
206.
A method combining rotor actuator disk model and embedded grid technique is presented in this paper, aimed at predicting the flow fields and aerodynamic characteristics of tilt rotor aircraft in conversion mode more efficiently and effectively. In this method, rotor's influence is considered in terms of the momentum it impacts to the fluid around it; transformation matrixes among different coordinate systems are deduced to extend actuator method's utility to conversion mode flow fields' calculation. Meanwhile, an embedded grid system is designed, in which grids generated around fuselage and actuator disk are regarded as background grid and minor grid respectively, and a new method is presented for ‘donor searching' and ‘hole cutting' during grid assembling. Based on the above methods, flow fields of tilt rotor aircraft in conversion mode are simulated, with threedimensional Navier–Stokes equations discretized by a second-order upwind finite-volume scheme and an implicit lower–upper symmetric Gauss–Seidel(LU-SGS) time-stepping scheme. Numerical results demonstrate that the proposed CFD method is very effective in simulating the conversion mode flow fields of tilt rotor aircraft. 相似文献
207.
针对飞机导管实际装配过程中因装配应力大导致故障频发的问题,提出了一种基于导管装配特征的误差补偿方法.首先采用几何建模法进行了导管装配过程的几何建模,并提出了一种基于轴线矢量的导管装配模型建模方法.在此基础上,采用罗德里格旋转公式建立了基于实际装配工况下的导管空间位姿计算模型.然后基于导管装配要求进行了装配约束分析并识别... 相似文献
208.
针对高超声速飞行器纵向模型的高度非线性特点,在考虑模型不确定性的情况下,提出了高超声速飞行器动态神经网络调节函数反推自适应控制方法.对给定的速度指令,引入积分型Lyapunov函数设计跟踪控制器,取消了控制增益一阶导数上界的限制,且避免了控制器的奇异性;对给定的高度指令,引入调节函数技术,设计了反推控制器,避免了将模型化为严反馈形式;采用动态神经网络对未知系统动态进行自适应在线逼近.根据Lyapunov理论证明了设计的控制律保证了闭环系统的稳定性与指令跟踪的精确性.仿真结果验证了该方法的可行性及有效性. 相似文献
209.
210.