二维射流的稳定性分析与流动控制

本文在绝对／对流不稳定性的理论框架下，对二维射流的稳定性进行了数值分析。研究表明射流流场各局部剖面是对流不稳定的，不会产生整体失稳，射流频率由流场中最大增长的不稳定频率确定。将一ＮＡＣＡ０１２翼型放置于射流势流核心区中，改变翼型的偏转角度θ，可使射流剪切层频率与翼型尾流频率发生共振，整个流场产生自激振荡，加强了射流与外界流场和混合，并使射流发生矢量偏转。     

钝体尾迹的稳定性研究及流动控制探讨

系统地回顾了研究钝尾尾迹稳定性的主要方法，并利用局部绝对／对流稳定性分析及整体稳定性分析两种方法，研究了圆柱绕流定常和时均尾迹流场的稳定性。采用低维Ｇａｌｅｒｋｉｎ方法研究了尾迹对圆柱流向振动的响应问题，揭示了频率锁定，涡街的激发和抑制等现象。     

横向不稳定性的试飞评估

在研发F／A-18E／F“超级大黄蜂”的一次飞行试验过程中，当进行跨音速机动时飞机遇到了非指令滚转运动。被称为“翼下冲”的急剧大幅度横向不稳定现象削弱了飞行员在此飞行条件下有效执行空一空跟踪任务的能力。对前缘襟翼的一系列修改和在机翼折叠区域增加透气量解决了该问题。然而，由于缺乏与计算流体力学、风洞和试飞分析有关的确认品质因数，获得该答案是缓慢的。目前已经开发了一种利用试飞数据评估横向不稳定性的方法。横向不稳定性评估包括定性和定量两个方面的评价。飞行员进行定性评价，而定量评价是基于对滚转速度、滚转加速度、横向驾驶杆位置和倾斜角的分析。这项技术提供一种根据飞行试验可靠评估横向不稳定性程度的途径。在飞行员定性评价和定量的试飞数据评估之间取得了极好的相关性。     

基于前缘平行射流的缝翼噪声控制研究

为了降低缝翼噪声,提出了一种基于前缘平行射流的缝翼噪声控制技术,利用数值计算验证了该方法的有效性,并分析了其噪声控制机理。采用FREQUENZ两段翼型,利用DDES混合方法进行二维非定常数值计算,获取声源分布,采用FW-H积分获得远场噪声特性。计算结果表明平行射流基本不影响翼型的气动力特性,同时缝翼的中低频噪声得到有效的抑制,宽频噪声强度也有所减弱。通过对时均流场和瞬态流场的分析,初步阐述了两种降噪机理:(1)平行射流与缝翼尖端分离流相互作用,改变了剪切层中的大尺度拟序结构,展向涡的尺度及其不稳定性均得到有效抑制,从而减弱了剪切层与缝翼压力面撞击而产生的压力脉动,达到降低声源强度的目的;(2)射流减小了回流速度,削弱了声反馈机制。     

超声速对流推力矢量喷管的数值模拟

利用二次流对流剪切激励主射流方式来产生射流对流推力矢量方法是一种效率很高的矢量化喷流新方法,又称为对流推力矢量法。本文运用三阶精度MUSCL TVD有限体积格式求解二维非定常、可压缩、雷诺平均N-S方程和Spalart-Allmaras湍流模型,计算了对流推力矢量喷管的超声速射流问题,获得几种不同二次流吸气压强下对狭缝和喷管肩部壁面压强分布、剪切层诱导旋涡的结构特征和主射流矢量角等结果,计算结果与实验数据对比取得良好的一致性,并对计算结果进行了详细分析和讨论。     

POD和DMD方法分析不同间隙压气机旋转不稳定性特性

对一台单级低速轴流压气机旋转不稳定性(RI)进行分析。在前期的旋转不稳定性特性、流场成因和不同间隙影响的研究基础上,进一步基于全通道数值模拟,采用两种流场降阶方法分析不同叶顶间隙条件下旋转不稳定性各阶模态对应的流场成分、模态能量占比和稳定性等特性。计算结果表明,前几阶模态能量占比大,对流场起主要贡献,并且稳定性相对较强。随着间隙的减小,旋转不稳定性的能量占比和稳定性下降,各阶的叶片转动成为流场的主要成分。      

高压涡轮工作叶片结构设计

高压涡轮工作叶片采用高强度镍基高温合金DZ－22材料，无余量定向精密铸造成型，再加盖板经高温真空钎焊，构成整体。为了提高涡轮性能，降低应力水平，提高绝对温降，采用了高效内冷结构和有效的封严，减振措施，同时以变厚度，大刚度和变形协调的指导思想进行结构设计，用对流－冲击－气膜保护的复合冷却结构形式来满足绝对温降400K以上的要求。     

用CSCM格式模拟二维收缩管道内的流动

 采用Lombard等人提出的CSCM(Conservative Supra-Characteristics Method)差分方法数值模拟二维收缩管道内波系干扰及激波边界层干扰等问题。所得结果同已往计算结果进行了比较。通过压力场和速度场的计算结果可看到Ma数对流场的影响。在Ma数较小时激波角较大,可看到激波经两次相交一次反射逐渐变弱的情况。Re数改变对流场也有明显影响,且给出由于激波干扰引起边界层分离的计算结果。最后还给出收缩管道内激波由正激波逐步形成斜激波的非定常过程。     

Falkner-Skan流空间演化的二次稳定性研究

依据Floquet理论建立三维亚谐扰动的控制方程，研究Falkner-Skan流的二次稳定性问题。采用高精度的谱方法进行数值模拟，结合边界层特性有效地配置边界条件，引入不同类型压力梯度，研究在压力梯度下的二维有限振幅的Tollmien-Schlichting（TS）波对三维亚谐扰动的产生、发展和演化的作用和影响。结果表明，顺压梯度的存在可以减缓二次不稳定的发生和演化，而逆压梯度却加速了二次不稳定性。     

自由剪切层中的三维不稳定性

本文是在文[1]的基础上研究自由剪切层中由Kelvin-Helmholtz不稳定波发展而形成的展向大涡结构的三维不稳定性。以大涡结构为基本流动,将稳定性分析归结为二维特征值问题,用pseudo spectral(伪谱)方法数值求解。研究发现:在没有亚谐波存在的情况下,大涡结构的最不稳定的扰动波是流向波长和其相同。展向波数较高,有对流特性的三维扰动波。它在剪切层中的发展与展向涡量的分布有关,大涡结构的涡核不稳定性和辫子不稳定性是流向涡形成的主要力学机制。本文还给出了不同雷诺数下三维扰动波增长率与展向波数的关系,这些结论与实验及数值模拟结果基本一致。     

Absolute and Convective Instabilities of Circularly Polarized Alfvén Waves

We discuss the recent progress in studying the absolute and convective instabilities of circularly polarized Alfvén waves (pump waves) propagating along an ambient magnetic field in the approximation of ideal magnetohydrodynamics (MHD). We present analytical results obtained for pump waves with small dimensionless amplitude a, and compare them with numerical results valid for arbitrary a. The type of instability, absolute or convective, depends on the velocity U of the reference frame where the pump wave is observed with respect to the rest plasma. One of the main results of our analysis is that the instability is absolute when U _l < U < U _r and convective otherwise. We study the dependences of U _l and U _r on a and the ratio of the sound speed to the Alfvén speed b. We also present the results of calculation of the increment of the absolute instability on U for different values of a and b. When the instability is convective (U < U _l or U > U _r) we consider the signalling problem, and show that spatially amplifying waves exist only when the signalling frequency is in two symmetric frequency bands. Then, we write down the analytical expressions determining the boundaries of these frequency bands and discuss how they agree with numerically calculated values. We also present the dependences of the maximum spatial amplification rate on U calculated both analytically and numerically. The implication of the obtained results on the interpretation of observational data from space missions is discussed. In particular, it is shown that circularly polarized Alfvén waves propagating in the solar wind are convectively unstable in a reference frame of any realistic spacecraft.     

On the spatial structure of global linear instabilities and their experimental identification

The purpose of the present paper is to discuss the spatial structure of global instabilities, solutions of the partial derivative eigenvalue problem resulting from a nonparallel linear instability analysis of the incompressible Navier–Stokes and continuity equations, as developing upon four prototype essentially two-dimensional steady laminar flows. Theoretical knowledge of these eigendisturbances is instrumental to devising measurement techniques appropriate for their experimental recovery and ultimate control of laminar-turbulent transition mechanisms. Raising the awareness of the global linear flow eigenmodes contributes to redefining the boundaries between experimental observations which may be attributed to linear global as opposed to nonlinear mechanisms.     

对流传热场协同原理在高速可压缩边界层流动中的推广

应用理论分析方法对适用于不可压缩层流与湍流流动的对流传热场协同原理进行了可压缩层流与湍流流动上的推广。分析结果表明,与不可压缩流动的对流传热场协同原理不同,可压缩层流与湍流的对流传热取决于流动当地单位体积的动量与总焓梯度的协同。用当地单位体积的动量与总焓梯度的协同研究可压缩流动的壁面传热问题,对层流热流,不但计及了高速流动的密度变化对热流的作用,而且包括了静焓梯度、压力梯度、壁面分子黏性剪切效应对热流的影响;对湍流问题,除了高速流动的密度变化、压力梯度、壁面分子黏性剪切效应对热流的影响外,还计及了雷诺剪切应力对热流的作用。另外,对黏性影响不能忽略的不可压缩流动的对流传热问题,用速度向量与总温（总焓）梯度协同更精确。     

低速叶型气动反问题设计方法

 低马赫数不可压流动中声速与流速大小差别巨大,采用基于可压缩流动控制方程的计算格式求解流场时,由于数值黏性的污染,解的精度低且收敛性差,通常可使用时间预处理技术来解决这一问题。在基于控制理论的优化方法中,共轭方程的Jacobian矩阵和流动方程的系数矩阵相似,因而在低流动马赫数下,求解共轭方程存在着与求解流动方程相同的数值污染和数值刚性问题。首先推导了带有预处理的Roe格式,然后发展了适合全速度流动的共轭方程求解方法,最后选取翼型和叶栅两个典型算例进行了验证。计算结果表明所发展的方法可很好地用于低马赫数时的气动反问题设计。     

Order out of Randomness: Self-Organization Processes in Astrophysics

Self-organization is a property of dissipative nonlinear processes that are governed by a global driving force and a local positive feedback mechanism, which creates regular geometric and/or temporal patterns, and decreases the entropy locally, in contrast to random processes. Here we investigate for the first time a comprehensive number of (17) self-organization processes that operate in planetary physics, solar physics, stellar physics, galactic physics, and cosmology. Self-organizing systems create spontaneous “order out of randomness”, during the evolution from an initially disordered system to an ordered quasi-stationary system, mostly by quasi-periodic limit-cycle dynamics, but also by harmonic (mechanical or gyromagnetic) resonances. The global driving force can be due to gravity, electromagnetic forces, mechanical forces (e.g., rotation or differential rotation), thermal pressure, or acceleration of nonthermal particles, while the positive feedback mechanism is often an instability, such as the magneto-rotational (Balbus-Hawley) instability, the convective (Rayleigh-Bénard) instability, turbulence, vortex attraction, magnetic reconnection, plasma condensation, or a loss-cone instability. Physical models of astrophysical self-organization processes require hydrodynamic, magneto-hydrodynamic (MHD), plasma, or N-body simulations. Analytical formulations of self-organizing systems generally involve coupled differential equations with limit-cycle solutions of the Lotka-Volterra or Hopf-bifurcation type.     

Acquisition in phase demodulation: application to ranging inradar/sonar systems

Ranging in radar/sonar systems is cast as a phase/frequency detection problem. In phase/frequency demodulation, there are two stages: acquisition, which resolves the global prior uncertainty before phase/frequency locking; and tracking, which follows the phase/frequency variations. In this work, ranging corresponds to absolute phase acquisition formulated as a global nonlinear filtering problem. The proposed solution propagates the associated multimodal density and generates a process relating global covariance with the single modes common covariance. Acquisition is then defined as the first passage of this process across a given threshold. Statistics such as acquisition time histograms and acquisition performance, characterizing the behavior of the developed estimator/detector, are obtained by Monte Carlo simulations     

MHD Waves and Instabilities in Space Plasma Flows

Shear flow instabilities are an important aspect of hydrodynamic studies. The present review article discusses the role of an ambient magnetic field which both modifies the Kelvin-Helmholtz instability and may introduce new types of magnetohydrodynamic waves and instabilities. A brief overview of magnetospheric pulsations is presented with an emphasis on the long-period resonant Alfv??n waves associated with the high speed solar wind. The spatio-temporal evolution of magnetically modified shear flow instabilities in various space plasma structures is addressed. A distinction between convective and absolute instabilities is necessary for proper understanding of theory and correct interpretation of the observations. Finally, it is shown how incompressible Alfv??nic disturbances may become unstable in a compressible flow in the absence of any shear. An application to coronal loops is presented.     

Atomization mechanism of a charged viscoelastic liquid sheet

In order to study atomization mechanism of a viscoelastic liquid sheet in an electric field,the spatial–temporal stability analysis of a viscoelastic liquid sheet injected into a dielectric stationary ambient gas in the presence of a vertical electric field is conducted. The dispersion relations of both sinuous and varicose disturbance modes are solved to explore the spatial–temporal instability of a charged viscoelastic sheet, by setting both the wave number and frequency complex. A parametric study is performed to test the influence of the dimensionless parameters on the absolute instability of the sheet. The results show that the increase of liquid Weber number and time constant ratio, or decrease of gas to liquid density ratio and Reynolds number, can damp the absolute instability. The effect of the liquid elasticity depends on the value of time constant ratio: when time constant ratio is small, the increase of liquid elasticity could amplify absolute growth rate, but the effect is weak when the elasticity number is relatively large; when time constant ratio is large, the increase of liquid elasticity cannot affect the absolute growth rate. Moreover, the variation of electrical Euler number can hardly influence the absolute instability of a charged viscoelastic sheet.