Standing wave drift of hemispherical resonator with quality factor non-uniformity under a ring electrode excitation

Hemispherical Resonator Gyroscope(HRG) is a classical high precision Coriolis Vibration Gyroscope(CVG), which performs attitude estimation of carrier by detecting the precession of standing wave of resonator, thus, the drift of standing wave of resonator has a great influence on the output accuracy of gyroscope, where the quality factor nonuniformity of resonator is one of main error sources. Ring electrode is a classical excitation structure of HRG because the standing wave can precess freely u...     

突加高能载荷作用下航空发动机结构动态响应及安全性综述

航空发动机在服役期间可能遭受鸟撞、叶片丢失等突加高能载荷的作用,造成发动机整机/部件动力学特性恶化和关键构件的损伤,危及发动机的结构安全性。本文从突加高能载荷复现方法与传递规律、突加高能载荷作用下转子/整机结构响应研究、突加高能载荷作用下关键构件损伤机理三个方面综述了现有研究工作,并针对近年来发展的抗突加高能载荷的安全性设计方法进行了探讨,最后分析了突加高能载荷问题的科学本质及发展趋势,为突加高能载荷作用下航空发动机安全性设计提供了重要参考。     

自由阻尼梁高频能量流响应的解析模型

针对自由阻尼梁的高频振动问题,基于波动理论提出了大阻尼复合结构的能量流解析模型。利用等效复刚度方法确定了完全自由阻尼梁结构的等效弯曲刚度和损耗因子,基于能量流分析方法构建了结构的能量密度控制方程,求解了结构的高频能量流响应。分析了弯曲波在阻尼结构耦合处的能量传递特性,构建了局部自由阻尼梁的高频能量流解析模型,预测了大阻尼耦合结构的高频振动特性。数值结果表明,提出的能量流解析解与经典的时空平均波动解一致逼近,因而能够精确地预测自由阻尼梁等大阻尼复合结构的高频能量流响应。     

压气机转子叶片的抑颤设计(“两机”专刊)

以一高压压气机转子叶片为对象开展了叶片抑颤工程设计方法研究,采用基于相位延迟边界条件的能量法和特征值法对原转子叶片模型的气动弹性稳定性进行评估,通过分析近失速工况下的非定常气动功密度分布,对叶片安装角沿径向分布、弦长和叶尖间隙等设计参数进行调整,以明确各参数对气动弹性稳定性的影响,最终达到提高气动阻尼的目的。研究结果表明：叶尖间隙对气动阻尼的影响较大,安装角次之,弦长影响相对较小。叶片气动阻尼随叶尖间隙的变化并非单调,而是存在一个叶尖间隙使其气动阻尼最小,即叶片气动弹性稳定性最差。减小进口气流攻角和增加折合频率,能够提高气动阻尼,设计中可以通过调节安装角来减小气流攻角,增加弦长来增大折合频率。     

The menagerie of geospace plasma waves

Sounding rockets and satellites have discovered a large variety of plasma waves within the Earth's magnetosphere—geospace. These waves are found over a frequency range of millihertz to megahertz. The frequency ranges are generally associated with characteristic frequencies such as the plasma frequency and gyrofrequency. Most waves are generated by hot or streaming magnetospheric plasma; some waves are due to lightning discharges, to intentional man-made transmitters or to incidental radiation from power transmission systems. Propagation of waves from the observation region back to a probable source region can be modelled using ray tracing techniques in a model magnetosphere where the electron number density, ion composition and magnetic field vector is specified. Information in addition to the common amplitude-frequency-time spectrograms can be obtained from the received waves using multiple antennas and receivers. Cross-correlation of the wave electric and magnetic components can provide information on the wave polarization and direction of propagation and on the wave distribution function.     

Microphysics of Quasi-parallel Shocks in Collisionless Plasmas

Shocks in collisionless plasmas require dissipation mechanisms which couple fields and particles at scales much less than the conventional collisional mean free path. For quasi-parallel geometries, where the upstream magnetic field makes a small angle to the shock normal direction, wave-particle coupling produces a broad transition zone with large amplitude, nonlinear magnetic pulsations playing an important role. At high Mach numbers, ion reflection and acceleration are dominant processes which control the structure and dissipation at the shock. Accelerated particles produce a precursor, or foreshock, characterized by low frequency magnetic waves which are convected by the plasma flow into the shock transition zone. The interplay between energetic particles, waves, ion reflection and acceleration leads to a complicated interdependent system. This review discusses the spacecraft observations which have motivated the current view of the high Mach number quasi-parallel shock, and the theories and simulation studies which have led to a better understanding of the microphysics on which the quasi-parallel shock depends.     

基于改进磁密计算的开关磁阻电机径向力解析建模

现有的麦克斯韦应力法计算开关磁阻电机(SRM)径向力的过程中,气隙磁通密度的求取过于简化。由于麦克斯韦应力法的积分路径较简单,气隙磁通密度的准确度直接影响了径向力求取的精度。综合等效磁路法和麦克斯韦应力法,精确计算了SRM气隙磁通密度,给出了单相激励下定子径向电磁力的表达式。在角度位置控制 (APC) 方式下,推导了转子运转过程中定子径向电磁力的解析表达式。在有限元软件上进行了仿真,对比分析了解析计算得到的径向力在时域和频域的分布,解析结果和有限元仿真结果相比表明,即使在2倍额定电流下误差也能维持在10%以内。     

Upstream Ion Cyclotron Waves at Venus and Mars

The occurrence of waves generated by pick-up of planetary neutrals by the solar wind around unmagnetized planets is an important indicator for the composition and evolution of planetary atmospheres. For Venus and Mars, long-term observations of the upstream magnetic field are now available and proton cyclotron waves have been reported by several spacecraft. Observations of these left-hand polarized waves at the local proton cyclotron frequency in the spacecraft frame are reviewed for their specific properties, generation mechanisms and consequences for the planetary exosphere. Comparison of the reported observations leads to a similar general wave occurrence at both planets, at comparable locations with respect to the planet. However, the waves at Mars are observed more frequently and for long durations of several hours; the cyclotron wave properties are more pronounced, with larger amplitudes, stronger left-hand polarization and higher coherence than at Venus. The geometrical configuration of the interplanetary magnetic field with respect to the solar wind velocity and the relative density of upstream pick-up protons to the background plasma are important parameters for wave generation. At Venus, where the relative exospheric pick-up ion density is low, wave generation was found to mainly take place under stable and quasi-parallel conditions of the magnetic field and the solar wind velocity. This is in agreement with theory, which predicts fast wave growth from the ion/ion beam instability under quasi-parallel conditions already for low relative pick-up ion density. At Mars, where the relative exospheric pick-up ion density is higher, upstream wave generation may also take place under stable conditions when the solar wind velocity and magnetic field are quasi-perpendicular. At both planets, the altitudes where upstream proton cyclotron waves were observed (8 Venus and 11 Mars radii) are comparable in terms of the bow shock nose distance of the planet, i.e. in terms of the size of the solar wind-planetary atmosphere interaction region. In summary, the upstream proton cyclotron wave observations demonstrate the strong similarity in the interaction of the outer exosphere of these unmagnetized planets with the solar wind upstream of the planetary bow shock.     

利用互相关函数进行环境激励下的模态分析

对互相关函数理论的进一步发展作了详细推导 ,探索出一条将互相关函数理论同多种经典模态分析方法相结合进行环境激励下模态分析的方法。以一悬臂梁为试验模型 ,在白噪声激励下将互相关函数代替脉冲响应函数用于多参考点复指数法和特征实现算法进行模态识别 ,并同功率谱峰值法结果进行了对比。研究表明 ,利用互相关函数理论同某些经典模态识别方法相结合能够有效地识别出环境激励下的系统模态参数     

Experimental Aspects of Mid-Frequency Pulsations (f≈10–100 mHz) in the Southern Polar Cap

We review the results obtained in the frequency range of Pc3 (22-100 mHz) and Pc4 (7-22 mHz) pulsations at Italian Antarctic stations in the southern polar cap (“Mario Zucchelli”, at Terra Nova Bay, TNB, 80^˚.S; “Concordia”, the Italian/French base at Dome C, DMC, 89^˚.S). The absence of a midnight enhancement in the pulsation power suggests a negligible substorm influence at extreme latitudes, while the sharp noon enhancement, which appears only at TNB, is determined by the closer proximity of the station to cusp related phenomena. The relationship between the frequency of the band-limited signals and the interplanetary magnetic field strength, the cone angle influence, and the higher correlation of the Pc3 power with the solar wind speed in the morning hours suggest a global scenario in which upstream waves would be mainly responsible for the mid-frequency activity in the polar cap. However, the polarization pattern is odd with respect to the predictions for tailward propagating modes.     

Thermal cyclotron radiation from a hot coronal loop with helical magnetic field

The spectral and polarization properties of thermal cyclotron radio emission from a hot coronal loop with a current along the axis are computed. The magnetic field is supposed to have a component along the loop axis as well as a poloidal part due to the current, both components being of comparable magnitude. In this specific configuration a helical magnetic field is present with a remarkable minimum of its absolute value along the loop axis and a maximum at its periphery. The presence of one or two maxima of magnetic field value along the line of sight results in increasing optical thickness of the gyroresonance layers at appropriate frequencies in the microwave band and, therefore, in enhanced radio emission at those harmonics which are optically thin (for example,s=4). These cannot be observed in models with the commonly employed magnetic field configuration (longitudinal along the loop axis).We show that the frequency spectrum of thermal cyclotron radiation from a hot coronal loop with a helical magnetic field differs from that of the standards-component source (with smooth frequency characteristics and polarization corresponding toe-mode) in that plenty of fine structures (line-like features and cut-offs) are present and theo-mode is prevalent in some frequency intervals. The enhanced radio emission at high harmonics and the complicated form of frequency spectrum in the model considered imply that some microwave sources, which are poorly explained in traditional models of solar active regions, may be associated with helical magnetic fields in hot coronal loops. Computations allow one to indicate spectral and polarizational peculiarities of local sources testifying to the presence of a helical magnetic field.     

Hybrid Simulations of Wave Propagation and Ion Cyclotron Heating in the Expanding Solar Wind

We present results from hybrid (particle ions, fluid electrons) simulations of the evolution of Alfvén waves close to the ion cyclotron frequency in the solar wind, which take into account the basic properties of the background solar wind flow, i.e., the spherical expansion and the consequent decrease in magnetic field and cyclotron frequency with increasing distance from the Sun. We follow the evolution of a plasma parcel in a frame of reference moving with the solar wind using a 1D expanding box hybrid model; use of the hybrid model yields a fully self-consistent treatment of the resonant cyclotron wave-particle interaction. This model is related to a previous MHD model (Velli et al. 1992), which allows the use of a simple Cartesian geometry with periodic boundary conditions. The use of stretched expanding coordinates in directions transverse to the mean radial solar wind flow naturally introduces an anisotropic damping effect on velocity and magnetic field. We present results for the case of a single circularly polarized Alfvén wave propagating radially outward. Initially, the wave is below the cyclotron frequency for both the alpha partcles and protons. As the wind expands, the wave frequency (as seen in the solar wind frame) decreases more slowly than the cyclotron frequencies and the wave comes into resonance. With only protons, heating occurs as the wave frequency approaches the proton cyclotron frequency. With both alphas and protons, the alphas, which come into resonance first, are observed to be preferentially heated and accelerated. This revised version was published online in June 2006 with corrections to the Cover Date.     

跨声速风洞颤振试验模型激振与数据处理方法研究

简述了几种主要激励方法在颤振实验中的优缺点,分别用瞬态激励、有限带宽白噪声激励和单频激励进行地面模态实验,测量机翼模型的振动响应,并用STD、ARMA和复指数模态参数识别等时域方法识别固有频率和模态阻尼,通过与软件计算结果对比分析得知,实测的固有频率与计算结果吻合很好。     

基于鬃毛摩擦理论的环形橡胶减振器动态迟滞特性模型

对环形橡胶减振器的动态特性进行了理论建模。综合考虑材料的高弹特性、频率相关动态特性以及摩擦相关动态特性,并将形状因素的影响考虑在内,提出了一种基于鬃毛摩擦理论的描述减振器动态迟滞特性模型。比较以往的基于库仑摩擦理论的模型,该模型具有对摩擦速度依赖特性的描述能力,且能够预测摩擦应力在零速附近的非线性特征,使得拟合出的动态迟滞特性曲线具有很多微小的波浪特征。开展了测量减振器径向动态迟滞特性的试验,试验结果表明:环形橡胶减振器的实际动态迟滞曲线并非光滑的椭圆环,而是具有很多细微波浪的近似椭圆环,且在激振频率偏高时,波浪特征相对明显,验证了基于鬃毛摩擦理论的理论模型与实际情况的规律和趋势一致、特征相似。      

带中心锥航空发动机腔体电磁散射特性数值研究

为了研究中心锥顶角和电磁波入射方位改变对航空发动机腔体的电磁散射特性的影响,采用物理光学(PO)法和等效棱边电磁流(EEC)法,对带中心锥发动机腔体在C波段入射频率f=6 GHz下进行电磁散射计算。计算结果表明:在水平极化下入射角为4°~28°范围内,中心锥顶角30°的发动机腔体的雷达散射截面(RCS)值较小;由等效电流图上得到特定角度下发动机腔体散射强弱分布,为发动机腔体关键散射区域采取隐身措施以提高隐身性能提供参考。     

某超高层建筑刚性模型测力试验研究

在4m×3m边界层风洞中对某高295m的超高层建筑分别进行了有、无周边建筑的刚性模型测力天平试验,得到建筑3个主轴方向基底力矩均值和均方根值随风向角的变化情况,通过比较表明周边超高层建筑对其具有较严重的干扰影响;分析了功率谱和相干函数所表现出的力矩频域特性,横-扭方向基底弯矩相关性明显较顺-横、顺-扭两向大;通过敲击试验得到基底弯矩的自由衰减信号,采用HHT变换方法,识别了模型-天平系统在两个主轴方向的一阶频率和阻尼比,并根据频率和阻尼比消除了基底弯矩信号中由于模型非绝对刚性引起的共振响应分量,由此修正得到较为精确的基底力谱和各角度均方根值,为计算风振响应提供了依据。     

Transverse Oscillations of Coronal Loops

On 14 July 1998 TRACE observed transverse oscillations of a coronal loop generated by an external disturbance most probably caused by a solar flare. These oscillations were interpreted as standing fast kink waves in a magnetic flux tube. Firstly, in this review we embark on the discussion of the theory of waves and oscillations in a homogeneous straight magnetic cylinder with the particular emphasis on fast kink waves. Next, we consider the effects of stratification, loop expansion, loop curvature, non-circular cross-section, loop shape and magnetic twist. An important property of observed transverse coronal loop oscillations is their fast damping. We briefly review the different mechanisms suggested for explaining the rapid damping phenomenon. After that we concentrate on damping due to resonant absorption. We describe the latest analytical results obtained with the use of thin transition layer approximation, and then compare these results with numerical findings obtained for arbitrary density variation inside the flux tube. Very often collective oscillations of an array of coronal magnetic loops are observed. It is natural to start studying this phenomenon from the system of two coronal loops. We describe very recent analytical and numerical results of studying collective oscillations of two parallel homogeneous coronal loops. The implication of the theoretical results for coronal seismology is briefly discussed. We describe the estimates of magnetic field magnitude obtained from the observed fundamental frequency of oscillations, and the estimates of the coronal scale height obtained using the simultaneous observations of the fundamental frequency and the frequency of the first overtone of kink oscillations. In the last part of the review we summarise the most outstanding and acute problems in the theory of the coronal loop transverse oscillations.     

槽间垫片功用分析及减振机理试验

针对国内高涵道比涡扇发动机风扇叶片的设计亟需,分析得到槽间垫片功用及设计要求,并通过机理试验揭示了槽间垫片减振机理。研究结果表明:在常规简谐激励作用于风扇叶片时,槽间垫片并不能提供有效阻尼,无法起到减振作用。在高能冲击载荷作用于风扇叶片时,槽间垫片可以起到较好的阻尼作用,快速地衰减自由振动,降低疲劳损伤积累。金属橡胶或橡胶槽间垫片能使叶片的冲击响应时间减少50%以上。槽间垫片应设计足够大的预紧力以实现叶片稳定的径向定位,降低风扇叶片在风车状态时的晃动幅值,减小榫头榫槽接触磨损;应具备足够大的柔度,确保高能冲击载荷下允许叶片偏摆一定的角度,通过垫片变形吸收冲击能量,从而降低叶片根部应力峰值。      

转子倾斜对交错式迷宫密封动静特性的影响

建立了交错式迷宫密封数值分析模型,并通过基于微元理论的密封动力特性系数理论识别方法研究了转子倾斜对密封动静特性的影响。结果表明：转子倾斜可降低交错式迷宫密封泄漏量,倾斜角为0.6°时泄漏量降低约2.5%,且进出口压比越大,效果越显著;转子倾斜带来的交错式密封腔室几何形状与齿径向间隙变化使得腔室在周向存在压力分布不均,且随倾斜角增加而增大;密封各腔室对系统稳定性影响规律不同,与交错式迷宫密封腔室的几何特性有关,密封腔室进口靠近转子、出口远离转子的腔室局部出现逆转动方向的速度场,能提高密封系统稳定性,而密封腔室进口远离转子、出口靠近转子的腔室则降低系统稳定性;整个密封段的有效阻尼随倾斜角增大而增大,转子在密封段中心倾斜不会降低交错式迷宫系统的稳定性。