非均匀圆柱壳中传播孤立波的动力学稳定性

利用积分因子法对非均匀圆柱壳中传播孤立波的动力学稳定性进行详细的数值研究.以高斯波扰动、简谐波扰动和随机扰动作为初始扰动,考察受到这三种不同扰动的孤立波能否较长时间保持波形结构和传播速度而稳定传播的问题.结果表明：孤立波的稳定传播与扰动幅度、宽度和波长有关,只在受到幅度、宽度和波长都足够小的扰动下非均匀圆柱壳中传播的孤立波才能够表现出较强的抗干扰性,具有良好的动力学稳定性.     

石墨烯中热脉冲传播的分子动力学研究

基于非平衡分子动力学方法,模拟研究了梯形、三角形和矩形热脉冲在石墨烯中的传播过程和规律。三种类型的温度脉冲采用直接速度修正法,通过调整对控温区原子施加的温度增量来实现。结果表明,热扰动在石墨烯中以有限的速度传播,并表现出明显的波动传递特征。热扰动在传播过程中激发出两道以不同速度传递的行波,一道波具有宏观动量,波速等于石墨烯声子群速度,为声波;另一道波是无宏观动量的热量的传播,波速是声速的1/3~(1/2),为热波。研究还发现,热脉冲形状对热波现象的产生以及声波、热波的传播速度没有影响,但对其峰值温度影响显著。     

一维爆轰波在扰动来流中传播的数值研究

爆轰波在静止气体或定常来流中的传播得到了广泛研究, 然而在扰动来流中的传播研究较少。这方面的研究不仅是爆轰传播机制的重要组成部分, 还可为爆轰发动机的应用提供参考。文章基于两步诱导-放热总包反应模型, 开展了一维爆轰波在正弦密度扰动来流中的传播数值模拟。通过对数值结果分析, 获得了放热反应控制参数与爆轰波内在不稳定性的关系, 并在此基础上研究了扰动波长和幅值对一维爆轰波动力学过程的影响。研究发现, 在波前施加连续扰动会诱导爆轰波表现出更复杂的动力学行为, 且影响过程与爆轰波的内在不稳定性相关。对于稳定爆轰波, 扰动只在特定波长范围内引起前导激波后的压力振荡。对于不稳定爆轰波, 扰动会进一步强化其内在不稳定性。扰动幅值越大, 对爆轰波动力学过程的影响越显著。      

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采用流体模型理论推导了等熵平衡条件下环向转动托卡马克等离子体中带状流的色散关系。从理论上分析了环向转动对测地声模、低频带状流和声波的频率、压力和密度扰动量的影响。结果表明,环向转动对低频带状流的频率没有影响,但会使测地声模的频率逐渐增大。此外,存在环向转动时,低频带状流会具有驻波形式的压力和密度扰动量,且测地声模和声波可以沿着极向传播。而且还发现,等熵平衡可以看成是等温平衡的一种特殊情况。     

托卡马克等熵平衡条件下等离子体环向转动对带状流的影响

采用流体模型理论推导了等熵平衡条件下环向转动托卡马克等离子体中带状流的色散关系。从理论上分析了环向转动对测地声模、低频带状流和声波的频率、压力和密度扰动量的影响。结果表明,环向转动对低频带状流的频率没有影响,但会使测地声模的频率逐渐增大。此外,存在环向转动时,低频带状流会具有驻波形式的压力和密度扰动量,且测地声模和声波可以沿着极向传播。而且还发现,等熵平衡可以看成是等温平衡的一种特殊情况。     

高超声速飞行器气动热环境的化学非平衡效应数值研究（英文）

针对高超声速飞行器气动热环境的化学非平衡效应进行数值模拟.首先以二维圆柱钝体为模型,采用三种气体模型比较和分析化学非平衡效应对气动特性和气动加热特性的影响;接着以三维球头钝体为模型,进一步分析化学非平衡效应及壁面催化特性对气动加热特性的影响.分析表明,化学非平衡效应对压力分布的影响不大;但它能显著缩短激波脱体距离,更重要的是,能大幅降低波后温度,对飞行器气动热环境产生深远影响.另外,壁面催化特性对传热也有较大的影响,对壁面材料催化进程的有效控制可以大幅降低壁面传热.     

绝热电荷扰动对非均匀热尘埃等离子体中三维孤波的影响

尘埃颗粒上不断有电子流和离子流的出入以及二次电离、光电离等因素,所以尘埃颗粒上的带电量不是一个常量而是随着时间和空间变化的,因此尘埃电荷成为尘埃等离子体中的一个新的动力学变量,研究其对等离子体中各类非线性过程的作用成为尘埃等离子体物理中的一个重要课题.当今对于非均匀尘埃等离子中非线性波的研究大多数都集中于一维,对于三维非线性波的研究非常少.基于这种情况,在考虑非均匀性、尘埃颗粒绝热电荷扰动以及外部磁场等物理因素的情况下,运用约化摄动方法得出描述三维孤波的变系数的Korteweg-de Vries(KdV)方程.由结果可以看出,非均匀性、电荷扰动、外部磁场、斜向传播、尘埃温度对三维非线性波的传播有着极大的影响.利用适当的变换,得到了变系数方程的近似解.     

热电子等离子体低频不稳定性的非线性理论

本文分析了热电子等离子体中低频漂移不稳定性和交换不稳定性的非线性性质,导出了扰动幅度随时间变化的非线性耦合方程,得到了等离子体密度扰动的饱和幅度,讨论了热电子成分对饱和幅度的影响。漂移波引起的等离子体密度扰动相对幅度约20％,交换模引起的等离子体密度相对扰动幅度约5％。当热电子成分达到线性稳定条件所要求的阈值时,漂移波和交换模的饱和幅度降为零。 关键词：     

非热离子对非均匀碰撞热尘埃等离子体中三维孤波的影响

从理论上研究了非热离子、外部磁场、碰撞对非均匀热尘埃等离子体中三维非线性尘埃声孤波的影响。运用约化摄动法得到描述三维非线性尘埃声孤波的非标准的变系数Korteweg-de Vries(KdV)方程。然后把非标准KdV方程变为标准的变系数KdV方程,并且得到了标准的变系数KdV方程的近似解析解。由此解析解可以看出,非热离子的数目、碰撞、非均匀性、波的斜向传播、尘埃颗粒和非热离子的温度对三维非线性尘埃声孤波的振幅和宽度有很大的影响。外部磁场对三维非线性尘埃声孤波的宽度有影响,而对其振幅没有影响。此外,波的相速度与非热离子、波的斜向传播、尘埃颗粒的温度和非均匀性有关。     

爆震波传播速度同阻力和热损失的一般关系

本文采用－维稳态ＺＮＤ模型，从理论分析和数值计算上详细研究了爆震波的结构和非绝热粗糙管中摩擦阻力和热损失对爆震波传播的影响．推导了爆震波的传播方程，揭示了多种爆震机制和爆震极限的存在及其机理。对低速爆震的发生给出了理论解释．     

Longitudinal electro-kinetic waves in ion-implanted semiconductor plasmas

A comprehensive investigation of propagation of new longitudinal electro-kinetic modes and novel properties introduced due to presence of negatively charged colloids in semiconductor plasma is presented. By employing the multi-fluid balance equations, a compact dispersion relation for the cases in which wave phase velocity is either larger or smaller than electron thermal velocity is derived. This dispersion relation is used to study wave phenomena and electro-kinetic mode instability numerically. We find important modifications in electro-kinetic branch as well as the existence of new modes of propagation in colloids laden semiconductor plasma. The results of this investigation should be useful in understanding the characteristics of longitudinal electro-kinetic wave in colloids laden semiconductor plasmas whose main constituents are electrons, holes and negatively charged colloids.Received: 15 June 2004, Published online: 7 September 2004PACS: 52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves) - 72.30. + q High-frequency effects; plasma effects - 61.72.Ww Doping and impurity implantation in other materials - 82.70.Dd Colloids     

On the instability of electrostatic waves in a nonuniform electron–positron magnetoplasma

The dispersion properties of three-dimensional electrostatic waves in a nonuniform electron–positron (EP) magnetoplasma are analyzed. A new dispersion relation is derived by use of the electron and positron density responses arising from the electron and positron continuity and Poisson equations. In the local approximation, the dispersion relation admits two wave modes with different velocities. The growth rates of various modes are illustrated both analytically and numerically. Considering the temperature gradients produces a linearly stable transverse mode. The growth rate of the slow mode instability due to the density inhomogeneity only is the highest one, though it appears at higher thermal energy. The angle of the wave propagation affects drastically on the instability features in each case. The applications of the present analysis are briefly discussed.     

Wave motion and dispersion phenomena: veering, locking and strong coupling effects

The dispersion curves describe wave propagation in a structure, each branch representing a wave mode. As frequency varies the wavenumbers change and a number of dispersion phenomena may occur. This paper characterizes, analyzes, and quantifies these phenomena in general terms and illustrates them with examples. Two classes of phenomena occur. Weak coupling phenomena-veering and locking-arise when branches of the dispersion curves interact. These occur in the vicinity of the frequency at which, for undamped waveguides, the dispersion curves in the uncoupled waveguides would cross: if two dispersion curves (representing either propagating or evanescent waves) come close together as frequency increases then the curves either veer apart or lock together, forming a pair of attenuating oscillatory waves, which may later unlock into a pair of either propagating or evanescent waves. Which phenomenon occurs depends on the product of the gradients of the dispersion curves. The wave mode shapes which describe the deformation of the structure under the passage of a wave change rapidly around this critical frequency. These phenomena also occur in damped systems unless the levels of damping of the uncoupled waveguides are sufficiently different. Other phenomena can be attributed to strong coupling effects, where arbitrarily light stiffness or gyroscopic coupling changes the qualitative nature of the dispersion curves.     

Ionenschallinstabilitäten in Edelgas-Gleichstromentladungen bei niedrigen Drücken. II. Vergleich der experimentellen Ergebnisse mit einer eindimensionalen kinetischen Dispersionstheorie

The experimental measurements of the propagation properties of self-exicted ion acoustic waves in a de-low pressure column are compared with a one dimensional dispersion theory. The corresponding dispersion relation is developed starting from the linearised Vlasov equation with a Davydov-type collision term. It is shown, that the mean features of the propagation behaviour of the ion acoustic waves can be explained by the theory. The experimental results concerning the dependence of the upper limit frequency of the waves upon the discharge current should be analysed on the base of new results from plasma diagnostics, taking also into account nonlinear and geometrical effects.     

Linear and nonlinear excitations in complex plasmas with nonadiabatic dust charge fluctuation and dust size distribution

Both linear and nonlinear excitation in dusty plasmas have been investigated including the nonadiabatic dust charge fluctuation and Gaussian size distribution dust particles. A linear dispersion relation and a Korteweg-de Vries-Burgers equation governing the dust acoustic shock waves are obtained. The relevance of the instability of wave and the wave evolution to the dust size distribution and nonadiabatic dust charge fluctuation is illustrated both analytically and numerically. The numerical results show that the Gaussian size distribution of dust particles and the nonadiabatic dust charge fluctuation have strong common influence on the propagation of both linear and nonlinear excitations.     

Acoustic properties of wood in tridimensional representation of slowness surfaces

Mechanical behaviour of wood considered as an orthotropic solid can be determined with ultrasonic technique. The propagation phenomena in wood are complex and theoretically are regulated by Christoffel's equation. Three type of waves can propagate in wood. During the propagation phenomena three slowness sheets are observed, corresponding to a fast longitudinal wave (inner sheet) and two shear waves, one fast and one slow (outer sheet). These waves are submitted continuously to mode conversion phenomena. The polarization angle changes when the propagation direction is out of the principal directions of symmetry of the material. In this article an analysis of the propagation phenomena in tridimensional representation is performed for different wood species. This approach contributes to the understanding of dynamic aspects of particle displacement associated with the wave fronts propagation. Globally, the anisotropy of each species, expressed by their acoustical behaviour is well represented.     

On the propagation of linear transverse acoustic waves in isotropic media with mechanical relaxation phenomena due to viscosity and a tensorial internal variable: II. Some cases of special interest (Poynting-Thomson,Jeffreys, Maxwell,Kelvin-Voigt,Hooke and Newton media)

The propagation of linear transverse acoustic waves in isotropic media in which mechanical relaxation phenomena occur was considered in a previous paper. In particular expressions for the velocity and attenuation of the waves were obtained and the limiting cases of waves with high and low frequencies were discussed. In the present paper we investigate the propagation of linear transverse acoustic waves in Poynting-Thomson, Jeffreys, Maxwell, Kelvin-Voigt, Hooke and Newton media. We show that the dispersion relations for these waves may be considered as degeneracies of the dispersion relation which we derived in the general case of a viscoanelastic medium with memory. In particular we investigate the explicit dependence of the dispersion relations on the thermodynamic parameters and the phenomenological coefficients.     

Elastic metamaterials with simultaneously negative effective shear modulus and mass density

We propose a type of elastic metamaterial comprising fluid-solid composite inclusions which can possess a negative shear modulus and negative mass density over a large frequency region. Such a material has the unique property that only transverse waves can propagate with a negative dispersion while longitudinal waves are forbidden. This leads to many interesting phenomena such as negative refraction, which is demonstrated by using a wedge sample and a significant amount of mode conversion from transverse waves to longitudinal waves that cannot occur on the interface of two natural solids.     

Material property estimation for tubes and arteries using ultrasound radiation force and analysis of propagating modes

Arterial elasticity has been proposed as an independent predictor of cardiovascular diseases and mortality. Identification of the different propagating modes in thin shells can be used to characterize the elastic properties. Ultrasound radiation force was used to generate local mechanical waves in the wall of a urethane tube or an excised pig carotid artery. The waves were tracked using pulse-echo ultrasound. A modal analysis using two-dimensional discrete fast Fourier transform was performed on the time-space signal. This allowed the visualization of different modes of propagation and characterization of dispersion curves for both structures. The urethane tube/artery was mounted in a metallic frame, embedded in tissue-mimicking gelatin, cannulated, and pressurized over a range of 10-100 mmHg. The k-space and the dispersion curves of the urethane tube showed one mode of propagation, with no effect of transmural pressure. Fitting of a Lamb wave model estimated Young's modulus in the urethane tube around 560 kPa. Young's modulus of the artery ranged from 72 to 134 kPa at 10 and 100 mmHg, respectively. The changes observed in the artery dispersion curves suggest that this methodology of exciting mechanical waves and characterizing the modes of propagation has potential for studying arterial elasticity.     

Laser-generated thermoelastic acoustic sources and Lamb waves in anisotropic plates

The effect of anisotropy and temperature on the dispersive Lamb wave generation and propagation in a transversely isotropic thin plate has been investigated. A quantitative numerical model for the laser-generated transient ultrasonic Lamb waves propagating along arbitrary directions is presented by using a finite-element method. All factors, such as spatial and time distributions of the incident laser beam, optical penetration, thermal diffusivity, thickness of the plate, and source–receiver distance, can be taken into account. The effects on the ultrasound waveform of the size of the optoacoustic source are investigated; in the limit of strong optical absorption, a subsurface thermal source gives rise to both vertical and lateral shear tensions. The lateral shear tension is equivalent to applying a shear dipole at the top face; the amplitude of the dipole is a function of material symmetry, contrary to the isotropic case, and the character and strength of the equivalent surface stress are a function of propagation direction. The specific results for the lower anti-symmetric and symmetric mode propagation in all planar directions are presented in the thermoelastic regime; the spatial dispersion (variation of the velocity with the direction of propagation) as well as the frequency dispersion is analyzed. PACS 43.35.+d; 02.70.Dh; 42.62.-b; 78.20.Nv; 81.70.Cv