自然循环过冷沸腾流动不稳定性的实验研究

本文以氟里昂作工质，对自然循环过冷沸腾流动不稳定性进行了实验研究．实验过程中发现自然循环系统内可能发生高频脉动和低频脉动二种类型的过冷沸腾流动不稳定性．通过实验研究揭示了这二种类型流动不稳定性的发生机理，证实高频脉动属于声波型脉动，低频脉动属于密度波型脉动．通过实验得出了判断系统稳定性的界限，并使用积分方程无因次分析方法得出了预测密度波型流动不稳定性的经验公式．     

稠密等离子体表面不稳定性的入射光偏振态效应

用二维粒子模拟程序研究了相对论强激光和稠密等离子体相互作用引起的表面不稳定。数值模拟表明,在s偏振光作用下,等离子体表面出现了类瑞利泰勒不稳定性。形成的不稳定结构随时间发展进一步深入到等离子体内部,最终使等离子体密度形成分层泡状结构,并向前传播。这种不稳定的产生与初始等离子体密度有密切关系,在高于20倍临界密度等离子体的表面没有明显观察到这种不稳定。在p偏振激光作用下,等离子体表面不能明显地形成这种结构。因此在三维几何结构下,这种等离子体表面不稳定性将呈现各向异性。这种表面不稳定将直接影响高次谐波产生和离子加速效率。     

Electrostatic instability of a heavily charged conducting liquid jet

The effect of electric charge on the jet surface on the capillary instability of the jet and its disintegration into drops is analyzed. A theoretical explanation is given for the electrostatic mechanism of instability development and jet disintegration that is akin to the mechanisms behind the instability of a heavily charged drop (Rayleigh instability) and flat uniformly charged liquid surface (Tonks-Frenkel instability) but differs qualitatively from the conventional capillary mechanism of instability and disintegration.     

Essence of Inviscid Shear Instability: a Point View of Vortex Dynamics

The essence of shear instability is reviewed both mathematically and physically, which extends the instability theory of a sheet vortex from the viewpoint of vortex dynamics. For this, the Kelvin-Arnol＇d theorem is retrieved in linear context, i.e., the stable flow minimizes the kinetic energy associated with vorticity. Then the mechanism of shear instability is explored by combining the mechanisms of both Kelvin Helmholtz instability （K-H instability） and resonance of waves. The waves, which have the same phase speed with the concentrated vortex, have interactions with the vortex to trigger the instability. The physical explanation of shear instability is also sketched by extending Batchelor＇s theory. These results should lead to a more comprehensive understanding on shear instabilities.     

Parametric resonance of truncated conical shells rotating at periodically varying angular speed

Parametric resonance of a truncated conical shell rotating at periodically varying angular speed is studied in this paper. Based upon the Love?s thin shell theory and generalized differential quadrature (GDQ) method, the equations of motion of a rotating conical shell are derived. The time-dependent rotating speed is assumed to be a small and sinusoidal perturbation superimposed upon a constant speed. Considering the periodically rotating speed, the conical shell system is a parametric excited system of the Mathieu–Hill type. The improved Hill?s method is utilized for parametric instability analysis. Both the primary and combination instability regions for various natural modes and boundary conditions are obtained numerically. The effects of relative amplitude and constant part of periodically rotating speed and cone angle on the instability regions are discussed in detail. It is shown that for the natural mode with lower circumferential wavenumber, only the primary instability regions exist. With the increasing circumferential wavenumber, the instability widths are reduced significantly and the combination instability region might appear. The results for different boundary conditions are substantially similar. Increasing the constant rotating speed (or cone angle) all lead to the movements of instability regions and the appearance of combination instability region. The former will cause the instability width increasing, while the latter will reduce the instability width. The variation of length-to-radius ratio only causes the movements of instability regions.     

Dissipative Instability of Shock Waves

A new condition is obtained for the linear instability of a plane front of an intense shock wave in an arbitrary medium, which is determined by the finiteness of the viscosity. It is shown that the shock front instability occurs due to dissipative instability of the flow behind the front, which is analogous to the flow instability in the boundary layer. It is found that in the low-viscosity limit, one-dimensional longitudinal perturbations increase much faster than two-dimensional (corrugation) perturbations. The results are compared with the available data of experimental observation and numerical simulation of instability of shock waves. The comparison shows a better agreement between the new absolute shock instability as compared to the condition of such instability in the classical D’yakov theory disregarding viscosity.     

Influence of deformations before instability on the parametric instability of conical shells under periodic pressure

On the basis of the dynamic version of linear Donnell type equations and with deformations before instability taken into account, the dynamic instability of clamped, truncated conical shells under periodic pressure is analyzed. The principal instability regions are determined by combining Bolotin's method and a finite difference procedure. Calculations are carried out for two kinds of conical shell. The effect of bending deformations before instability is found to change the width of the principal instability regions in the vicinities of twice the natural frequencies of asymmetric vibration. Other principal instability regions are detected in the neighborhoods of the resonances of symmetrically forced vibrations.     

Subexponential instability in one-dimensional maps implies infinite invariant measure

We characterize dynamical instability of weak chaos as subexponential instability. We show that a one-dimensional, conservative, ergodic measure preserving map with subexponential instability has an infinite invariant measure, and then we present a generalized Lyapunov exponent to characterize subexponential instability.     

Rayleigh-Taylor instability in variable density swirling flows

Using linear instability theory and nonlinear dynamics, the Rayleigh-Taylor instability of variable density swirling flows is studied. It is found that the flow topology could be predicted, when the instability sets in, using a function χ dependent on density and axial and azimuthal velocities. It is shown that even when the inner axial-flow is heavier than the outer one (a favorable case for the development of the Rayleigh-Taylor instability thanks to the centrifugal force) the instability is not necessarily Rayleigh-Taylor-dominated. It is also shown that when the Rayleigh-Taylor instability develops, it is helical.     

最相邻后车综合信息对交通流不稳定性的影响分析

基于抑制交通流不稳定性所需的条件,分析司机后视获得的最相邻后车综合信息对交通流不稳定性的影响. 在司机关注前车信息概率大于关注后车信息概率与司机敏感系数大于0的现实条件下,解析分析以及仿真分析得到了以下结论:1)最相邻后车车头距信息减小了交通流的不稳定性,且关注概率越大,减小作用越大;跟驰车与最相邻后车的速度差信息增加了交通流的不稳定性,且关注概率越大,增加作用越大. 2)最相邻后车综合信息对交通流的不稳定性的减小作用大于增加作用,即最相邻后车综合信息减小了交通流的不稳定性. 3)司机距离差敏感系数越大,最相邻后车综合信息对交通流的不稳定性减小作用越大. 4)司机速度差敏感系数越大,最相邻后车综合信息对交通流的不稳定性增加作用越大. 关键词： 交通堵塞 交通流不稳定性 最相邻后车综合信息 跟驰模型     

预混气体燃烧火焰闪烁现象分析

在低速射流的预混火焰和扩散火焰中都存在火焰闪烁现象。对扩散火焰,其机理已比较明确,是由于浮力诱导引起的一种水力学不稳定性。而对预混火焰闪烁现象则存在水力学不稳定性和热驱动不稳定性两种观点。本文根据水力学不不稳定性观点,把预混火焰的闪烁现象看成是包围火焰锋面的已燃混气层中内、外区间在垂直方向上的相对脉动,应用Ｋｅｌｖｉｎ－Ｈｅｌｍｈｏｌｔｚ不稳定性机理进行了分析,获得了火焰闪烁频率与重力和压力的关系式,并与已有的结果作了对比。     

离子束-等离子体系统的电磁不稳定性

本文研究了离子束-等离子体系统的电磁不稳定性,考虑了离子-电子和离子-离子碰撞对维泊耳(Weibel)型电磁不稳定性增长率的影响。结果表明,在离子束-等离子体系统中可以激发维泊耳型电磁不稳定性,离子和电子之间的碰撞将使这类不稳定性的增长率增加。     

Selkov模型不同扩散和流速下非Turing不稳定化学反应机制

建立了Selkov模型中间反应物具有不同扩散和不同流速条件下的反应 扩散 流动方程 ,理论分析了非Turing不稳定形成的条件 ,求得其参数区间 ,对Andresen的结论作了拓展 .研究还发现 ,在振荡Hopf区域之外 ,静止波动 (空间周期结构FDS)仍然可以存在 .因而 ,此结构存在的参数空间大于Andresen的结果 .同时 ,还将此种不稳定参数区间与Turing不稳定和差速流动引起不稳定 (DIFI)的结果进行了比较 ,结果发现静态FDS值总是处于DIFI临界曲线相应的最小值之上 ,这表明动力学机制是由DIFI不稳定造成的 ,DIFI不稳定区是产生静止波FDS不稳定结构的必要条件     

Upstream ionization instability associated with a current-free double layer

A low frequency instability has been observed using various electrostatic probes in a low-pressure expanding helicon plasma. The instability is associated with the presence of a current-free double layer (DL). The frequency of the instability increases linearly with the potential drop of the DL, and simultaneous measurements show their coexistence. A theory for an upstream ionization instability has been developed, which shows that electrons accelerated through the DL increase the ionization upstream and are responsible for the observed instability. The theory is in good agreement with the experimental results.     

An Experimental Study on Cyclotron–Cherenkov Radiation

We searched numerically dielectric-loaded cylindrical waveguide configurations with an injected electron beam in which the growth rate of the cyclotron-Cherenkov instability surpassed that of the Cherenkov instability, and found such a configuration. This configuration consists of a metallic core and an outer metallic cylinder with a dielectric liner on the inner surface. In order to investigate experimentally radiation due to the cyclotron-Cherenkov instability, we designed and assembled an experimental device using the computational results. We studied beam propagation in the dielectric-loaded coaxial waveguide and microwave radiation due to the cyclotron-Cherenkov instability and the Cherenkov instability.     

Shock-wave mach-reflection slip-stream instability: a secondary small-scale turbulent mixing phenomenon

Theoretical and experimental research, on the previously unresolved instability occurring along the slip stream of a shock-wave Mach reflection, is presented. Growth rates of the large-scale Kelvin-Helmholtz shear flow instability are used to model the evolution of the slip-stream instability in ideal gas, thus indicating secondary small-scale growth of the Kelvin-Helmholtz instability as the cause for the slip-stream thickening. The model is validated through experiments measuring the instability growth rates for a range of Mach numbers and reflection wedge angles. Good agreement is found for Reynolds numbers of Re 2 x 10(4). This work demonstrates, for the first time, the use of large-scale models of the Kelvin-Helmholtz instability in modeling secondary turbulent mixing in hydrodynamic flows, a methodology which could be further implemented in many important secondary mixing processes.     

Dynamical parametric instability of carbon nanotubes under axial harmonic excitation by nonlocal continuum theory

Structures under parametric load can be induced to the parametric instability in which the excitation frequency is located the instability region. In the present work, the parametric instability of double-walled carbon nanotubes is studied. The axial harmonic excitation is considered and the nonlocal continuum theory is applied. The critical equation is derived as the Mathieu form by the Galerkin's theory and the instability condition is presented with the Bolotin's method. Numerical calculations are performed and it can be seen that the van der Waals interaction can enhance the stability of double-walled nanotubes under the parametric excitation. The parametric instability becomes more obvious with the matrix stiffness decreasing and small scale coefficient increasing. The parametric instability is going to be more significant for higher mode numbers. For the nanosystem with the soft matrix and higher mode number, the small scale coefficient and the ratio of the length to the diameter have obvious influences on the starting point of the instability region.     

Speedup of doping fronts in organic semiconductors through plasma instability

The dynamics of doping transformation fronts in organic semiconductor plasma is studied for application in light-emitting electrochemical cells. We show that new fundamental effects of the plasma dynamics can significantly improve the device performance. We obtain an electrodynamic instability, which distorts the doping fronts and increases the transformation rate considerably. We explain the physical mechanism of the instability, develop theory, provide experimental evidence, perform numerical simulations, and demonstrate how the instability strength may be amplified technologically. The electrodynamic plasma instability obtained also shows interesting similarity to the hydrodynamic Darrieus-Landau instability in combustion, laser ablation, and astrophysics.     

Flame fingers and interactions of hydrodynamic and thermodiffusive instabilities in laminar lean hydrogen flames

Lean hydrogen/air flames are prone to hydrodynamic and thermodiffusive instabilities. In this work, the contribution of each instability mechanism is quantified separately by performing detailed simulations of laminar planar lean hydrogen/air flames with different diffusivity models and equations of state to selectively suppress the hydrodynamic or thermodiffusive instability mechanism.From the analysis of the initial phase of the simulations, the thermodiffusive instability is shown to dominate the flame dynamics. If differential diffusion and, hence, the thermodiffusive instability is suppressed, the flame features a strong reduction of the instability growth rates, whereas if present, a wide range of unstable wave numbers is observed due to the strong destabilizing nature of differential diffusion. When instabilities are fully developed, lean hydrogen/air flames feature the formation of small-scale cellular structures and large-scale flame fingers. While the size of the former is known to be close to the most unstable wave length of a linear stability analysis, this work shows that flame fingers also originate from the thermodiffusive instability and most noteworthy, are not linked to an interaction of the two instability mechanisms. They are stable with respect to external perturbations and feature an enhanced flame propagation as the formation of a central cusp at their tip enables the co-existence of two strongly curved leading edges with high reactivity. The thermodiffusive instability is shown to significantly affect the flames’ consumption speed, while the consumption speed enhancement caused by the hydrodynamic instability is significantly smaller. Further, the surface area increase due to wrinkling is strongly diminished if one of the two instability mechanisms is missing. This is linked to a synergistic interaction between the two mechanisms, as the propagation of flame fingers is enhanced by the presence of the hydrodynamic instability due to a widening of the streamlines ahead of the flame fingers.     

Resistive drift instability and Rayleigh-Taylor instability in a dusty plasma

The resistive drift instability and the Rayleigh-Taylor instability are studied self-consistently in a magnetized inhomogeneous dusty plasma. The effect of grain charge fluctuations is taken into consideration. It is found that the presence of the dust grains in the plasma can significantly affect the resistive drift instability but less significantly the Rayleigh-Taylor instability. Further, the grain charge fluctuation has a tendency to stabilize both instabilities.