Alfvén waves as a driving mechanism in stellar winds

Alfvén waves have been invoked as an important mechanism of particle acceleration in stellar winds of cool stars. After their identification in the solar wind they started to be studied in winds of stars located in different regions of the HR diagram. We discuss here some characteristics of these waves and we present a direct application in the acceleration of late-type stellar winds.     

Arbitrary amplitude ion-acoustic waves in a plasma with nonthermal electrons

We investigated the effect of the presence of a nonthermal electron population on the electrostatic nonlinear waves. We considered positively charged ions with two electron populations. Using the fluid equations for the unmagnetized case and the Sagdeev pseudo-potential approach the nonlinear ion-acoustic waves are studied. The cold electrons are in thermal equilibrium while the hot electron population follows a nonthermal distribution. Numerical investigation shows the importance of the presence of small amount of cold electrons that make it possible for the plasma to support nonlinear waves. We obtained the minimum cold electron density necessary to sustain these nonlinear waves. The relevant situation corresponds to the upper ionosphere where energetic electrons have been observed.     

横流转捩模型参数不确定度量化分析与应用研究

为获得横流转捩模型重要参数在不同工况下对转捩结果的定量影响规律并尝试获得更加准确可靠的转捩模型预测结果,开展了基于非嵌入式多项式混沌(NIPC)方法的横流转捩模型参数不确定度量化分析、参数敏感性分析及其应用研究。不确定度量化分析方面,对基于γ-Re_(θt)模型的完全当地化横流转捩模型的重要模型参数进行了拉丁超立方抽样,通过有限参数样本的S-K低速平板和无限展长后掠翼的数值计算,运用非嵌入式多项式混沌方法,定量分析了模型参数在不同类型转捩流场中的影响效能。在应用研究方面,以不确定度与参数敏感性定量分析结果为指导,对模型参数进行了横流诱导转捩条件下的筛选与标定,在NLF(2)-0415后掠翼、6:1标准椭球体以及DLR-F4翼身组合体算例中进行了验证,标定后的模型对多个算例的三维边界层横流转捩进行了较好的预测。     

MHD oscillations in solar and stellar coronae: Current results and perspectives

Wave and oscillatory activity is observed with modern imaging and spectral instruments in the visible light, EUV, X-ray and radio bands in all parts of the solar corona. Magnetohydrodynamic (MHD) wave theory gives satisfactory interpretation of these phenomena in terms of MHD modes of coronal structures. The paper reviews the current trends in the observational study of coronal oscillations, recent development of theoretical modelling of MHD wave interaction with plasma structures, and implementation of the theoretical results for the mode identification. Also the use of MHD waves for remote diagnostics of coronal plasmas is discussed. In particular, the applicability of this method to the estimation of the coronal magnetic field is demonstrated.     

Furthering our understanding of electrostatic solitary waves through Cluster multispacecraft observations and theory

Nonlinear isolated electrostatic solitary waves (ESWs) are observed routinely at many of Earth’s major boundaries by the Wideband Data (WBD) plasma wave receivers that are mounted on the four Cluster satellites. The current study discusses two aspects of ESWs: their characteristics in the magnetosheath, and their propagation in the magnetosheath and in the auroral acceleration (upward current) region. The characteristics (amplitude and time duration) of ESWs detected in the magnetosheath are presented for one case in which special mutual impedance tests were conducted allowing for the determination of the density and temperature of the hot and cold electrons. These electron parameters, together with those from the ion experiment, were used as inputs to an electron acoustic soliton model as a consideration for the generation of the observed ESWs. The results from this model showed that negative potential ESWs of a few Debye lengths (10–50 m) could be generated in this plasma. Other models of ESW generation are discussed, including beam instabilities and spontaneous generation out of turbulence. The results of two types of ESW propagation (in situ and remote sensing) studies are also presented. The first involves the propagation of bipolar type ESWs from one Cluster spacecraft to another in the magnetosheath, thus obtaining the velocity and size of the solitary structures. The structures were found to be very flat, with large scale perpendicular to the magnetic field (>40 km) and small scale parallel to the field (<1 km). These results were then discussed in terms of various models which predict such flat structures to be generated. The second type of propagation study uses striated Auroral Kilometric Radiation (SAKR) bursts, observed on multiple Cluster satellites, as tracers of ion solitary waves in the upward current region. The results of all studies discussed here (pulse characteristics and ESW velocity, lifetime, and size) are compared to in situ measurements previously made on one spacecraft and to theoretical predictions for these quantities, where available. The primary conclusion drawn from the propagation studies is that the multiple spacecraft technique allows us to better assess the stability (lifetime) of ESWs, which can be as large as a few seconds, than can be achieved with single satellites.     

一种新的用于反射面天线辐射远场分析的快速积分方法

对一种新的用于反射面天线辐射远场的快速积分方法进行了研究,采用物理光学法分析,基于辐射场积分计算,考虑积分网格内的相位影响并用闭合表达式表示,显著减少了积分所需网格数,加快了赋形反射面天线辐射远场的计算速度,并能与其他方法组合以进一步提高辐射远场的计算速度。模拟计算结果与直接积分法比较结果表明方法可靠且有效。     

高超声速边界层转捩高速纹影显示

为建立适用于激波风洞边界层转捩测量的高速纹影显示技术，在Ma10条件下，采用高速纹影显示技术，研究了半锥角7°钝锥边界层中第二模态不稳定波的演变特性。试验在中国空气动力研究与发展中心超高速空气动力研究所(HAI)的FD-14激波风洞上进行。试验来流单位雷诺数为4.9×10 6 m -1 、1.6×10 7m -1 ，模型攻角0°，头部钝度有0.2 mm、0.5 mm和2 mm三种。通过对纹影图像灰度进行功率谱密度(PSD)分析得到了第二模态不稳定波的波长，对不稳定波纹影图像进行互相关分析计算了不稳定波的传播速度。基于纹影显示结果计算的第二模态波主频与PCB压力传感器测量结果符合较好，证明了纹影测量的可靠性。     

加强双师型教师队伍建设

职业院校“双师型”教师，既是文化理论知识的合成者，又是生产实践技能的指导者。加强“双师型”教师队伍建设，是顺应职业教育目标任务和职业院校办学定位，提高办学实力和教学质量，满足人力资源能力建设的内在要求。     

接收模式下加罩天线系统特性的快速电磁仿真

针对接收模式下电大尺寸的天线-天线罩系统电磁特性分析问题,提出了一种全波法和高频法结合的混合算法:积分方程/修正型表面积分+多层快速多极子(IE/MSI+MLFMA)方法.该方法将由高频方法MSI得到的天线罩内场分布作为天线阵列的激励场,再利用基于体-面混合积分方程(VSIE)和MLFMA的快速全波分析方法精确计算天线阵;同时应用球谐函数展开、预处理技术、混合并行等技术进一步改进算法的计算效率.同传统的全波数值法相比,该方法在保证计算精度、满足工程需要的前提下,解决了求解计算时间长、计算效率低的问题,实现了对电大尺寸天线-天线罩系统的快速、高效仿真.      

Feedback control of boundary layer Tollmien-Schlichting waves using a simple model-based controller

The attenuation of spatially evolving instability Tollmien-Schlichting (T-S) waves in the boundary layer of a flat plate with zero pressure gradients using an active feedback control scheme is theoretically and numerically investigated. The boundary layer is excited artificially by various perturbations to create a three-dimensional field of instability waves. Arrays of actuators and sensors are distributed locally at the wall surface and connected together via a feedback controller. The key elements of this feedback control are the determination of the dynamic model of the flat plate boundary layer between the actuators and the sensors, and the design of the model-based feedback controller. The dynamic model is established based on the linear stability calculation which simulates the three-dimensional input-output behaviour of the boundary layer. To simplify the control problem, an uncoupled control mode of the dynamic model is made to capture only those dynamics that have greatest influences on the input-output behaviour. A Proportional-Integral-Derivative (PID) controller, i.e. a lead-lag compensator, combining with a standard Smith predictor is designed based on the system stability criterion and the specifications using frequency-response methods. Good performance of the feedback control with the uncoupled control mode is demonstrated by the large reduction of the three-dimensional disturbances in the boundary layer. This simple feedback control is realistic and competitive in a practical implementation of T-S wave cancellation using a limited number of localised sensors and actuators.