水平流作用下行波对流的成长及周期性重复

数值模拟研究了在极其微弱的水平流动作用下，混合流体Rayleigh-Benard对流系统一 维行波斑图的成长及其时空演化问题.揭示了系统行波对流的周期性现象及其对水平流动强 度的依赖性. 关键词： 时空斑图 混合流体对流 水平流     

弱Soret效应混合流体对流系统的分岔与非线性演化

混合流体Rayleigh-Benard(RB)对流是研究非平衡耗散系统的自组织斑图及非线性动力学特性的典型模型.本文利用高精度数值方法模拟了底部均匀加热的矩形腔体中混合流体RB对流,研究了具有极微弱Soret效应(分离比φ=-0.02)的混合流体对流的分岔特性及斑图的形成和演化,给出了分岔曲线图.获得了行波交替闪动的Blinking状态、局部行波对流和定常对流(SOC)三种稳定状态,讨论了状态之间的过渡.研究发现从Blinking状态到局部行波对流状态的过渡存在迟滞现象,过渡时行波频率、对流振幅和对流传热Nusselt数等均有明显的跳跃.在Blinking状态存在的Rayleigh数区间下界附近,外部施加的不对称初始扰动是形成该状态的诱因.随着Rayleigh数增大,临界SOC状态经过多次分岔并形成多个具有不同波数的SOC状态后过渡为混沌状态.     

水平磁场对双层流体热毛细对流的影响

三维数值研究了零重力时水平温度梯度作用下,B2O3封闭液与InP熔液组成的双层流体系统在水平磁场作用下的热毛细对流.结果显示,当磁场强度较小时,上层流体中对流涡的结构变化其微,而下层流体中的逆对流涡得到了抑制;随着磁场的增强,温度场分布逐渐趋于均匀,涡量强度逐渐减小,表明热毛细对流强度逐步地得到削弱.当磁场增强列Bx=0.4 T时,上层流体中的对流涡较均匀地充满上层流体区,而下层流体中对流涡紧贴着交界面.     

简化Lorenz多涡卷混沌吸引子的设计与应用

将简化Lorenz系统线性化成两个线性系统,采用控制方法得到两涡卷混沌系统,通过扩展两涡卷混沌系统的指标2鞍焦点,设计了多涡卷混沌吸引子.利用相图、分岔图、Poincaré截面和最大Lyapunov指数等方法,分析了该多涡卷混沌系统的动力学特性.设计了多涡卷混沌吸引子的模拟电路,并进行了仿真,数值仿真与电路仿真相一致.将多涡卷混沌系统应用于图像加密,设计了多涡卷混沌与高级加密标准(AES)的改进混合加密算法,并分析了其加密性能.结果表明,基于多涡卷混沌系统的改进混合加密算法具有更高的安全性.     

基于电流传输器的网格多涡卷混沌吸引子在混合图像加密中的研究

该文提出了一种新的基于第二代电流传输器（CCII）的网格多涡卷混沌吸引子产生器,用于物理混沌加密和高级加密标准（AES）加密的混合图像加密算法．因CCII比普通运放有更好的频率特性和更大的动态范围,能产生频率更高,动力学特性更复杂的多涡卷物理混沌信号．基于CCII的多涡卷物理混沌加密和AES加密的混合加密系统。不存在确定的明文密文映射关系,密文统计特性也应优于其他加密系统．基于该算法研究了混合加密和单级加密的抗统计分析能力,以及涡卷数目不同的混沌信号在该算法中应用时密文统计特性的不同．完成了基于CCII的混沌电路设计与硬件实现,对加密系统进行了数值仿真,仿真结果与理论分析一致,同时表明涡卷数目越多的混沌系统其加密产生的密文相关性越弱．     

基于微流控光学可调谐的渐变折射率特性研究

本文基于层流液体的对流扩散效应,提出了一种新型的具有渐变折射率分布特性并且可以实现调节的微腔流体透镜.利用有限元法分析微腔中芯层与包层流体在对流扩散过程达到稳定后的浓度分度,即混合流体的折射率分布,结果表明,分别调节芯层与包层流体的流速,微腔中的流体可以实现不同的渐变折射率分布.同时,本文还分析了即使考虑腔壁与流体之间的黏滞阻力因素,芯层与包层流体的流速组合依然是混合流体折射率分布的决定因素.应用微腔流体透镜的这种调节效果,可以实现不同要求的光束聚焦、分束、偏转等功能,在微流控光学检测系统以及微成像系统中有重要的应用.     

方腔内Cu/Al2O3水混合纳米流体自然对流的格子Boltzmann模拟

纳米流体作为一种较高的导热介质,广泛应用于各个传热领域.鉴于纳米颗粒导热系数和成本之间的矛盾,本文提出了一种混合纳米流体.为了研究混合纳米流体颗粒间相互作用机理和自然对流换热特性,在考虑颗粒间相互作用力的基础上,利用多尺度技术推导了纳米流体流场和温度场的格子Boltzmann方程,通过耦合流动和温度场的演化方程,建立了Cu/Al2O3水混合纳米流体的格子Boltzmann模型,研究了混合纳米流体颗粒间的相互作用机理和纳米颗粒在腔体内的分布.发现在颗粒间相互作用力中,布朗力远远大于其他作用力,温差驱动力和布朗力对纳米颗粒的分布影响最大.分析了纳米颗粒组分、瑞利数对自然对流换热的影响,对比了混合纳米流体(Cu/Al2O3-水)与单一金属颗粒纳米流体(Al2O3-水)的自然对流换热特性,发现混合纳米流体具有更强的换热特性.     

分离比对混合流体Rayleigh-Bénard对流解的影响

混合流体Rayleigh-Bénard对流是研究非平衡对流的非线性动力学特性的典型模型之一.基于流体力学方程组的数值模拟,首先探讨了矩形腔体中具有强Soret效应(分离比Ψ=-0.60)的混合流体行波对流的分叉特性及斑图演化,沿着分叉曲线的上部分支,随着相对瑞利数的增加,此系统依次出现了局部行波对流、具有缺陷的行波对流、行波对流、摆动行波对流及定常对流5种行波对流解.然后,研究了分离比Ψ对对流解的影响,与弱Soret效应(Ψ=-0.11)时的对流解相比较,强Soret效应(Ψ=-0.60)时出现的对流解更丰富.由于有强Soret效应的对流的复杂性,Ψ=-0.60时的对流解与Ψ=-0.20,-0.4时的对流解不同.     

微通道内近临界流体瞬态混合与换热研究

近年来,微尺度条件下功能性流体换热与流动已经成为极具潜力和挑战性的课题,在化工、医药、传热与能源利用等系统中获得广泛应用。超临界CO_2流体作为一种天然替代性环保工质,在相关细微尺度下已证明具有良好的热力学性能。本研究采用了数值计算的方法对近临界CO_2流体在微通道内流动稳定性和换热特性进行了系列的探索。研究发现,在靠近临界点的相对较宽泛的区域内,流体具有强膨胀特性和低热扩散特性,从而在微尺度条件下产生局部旋涡流动,大大促进了微尺度的混合和对流换热效率。进一步,研究针对这种微尺度局部涡动进行了机理分析,获得了微通道内近临界流体瞬态换热和超临界热膨胀效应特性。     

方腔内Cu/Al2O3水混合纳米流体自然对流的格子Boltzmann模拟

纳米流体作为一种较高的导热介质, 广泛应用于各个传热领域. 鉴于纳米颗粒导热系数和成本之间的矛盾, 本文提出了一种混合纳米流体. 为了研究混合纳米流体颗粒间相互作用机理和自然对流换热特性, 在考虑颗粒间相互作用力的基础上, 利用多尺度技术推导了纳米流体流场和温度场的格子Boltzmann方程, 通过耦合流动和温度场的演化方程, 建立了Cu/Al₂O₃水混合纳米流体的格子Boltzmann模型, 研究了混合纳米流体颗粒间的相互作用机理和纳米颗粒在腔体内的分布. 发现在颗粒间相互作用力中, 布朗力远远大于其他作用力, 温差驱动力和布朗力对纳米颗粒的分布影响最大. 分析了纳米颗粒组分、瑞利数对自然对流换热的影响, 对比了混合纳米流体(Cu/Al₂O₃-水)与单一金属颗粒纳米流体(Al₂O₃-水)的自然对流换热特性, 发现混合纳米流体具有更强的换热特性.     

Localized waves without the existence of extended waves: oscillatory convection of binary mixtures with strong soret effect

Spatially confined solutions of traveling convection rolls are determined numerically for binary mixtures such as ethanol-water. The appropriate field equations are solved in a vertical cross section of the rolls perpendicular to their axes subject to realistic horizontal boundary conditions. The localized convective states are stably and robustly sustained by strongly nonlinear mixing and complex flow-induced concentration redistribution. We elucidate how this enables their existence for strongly negative separation ratios at small subcritical heating rates below the saddle node of extended traveling convection rolls where the quiescent fluid is strongly stable.     

The fate of self-aligned rolls in gravity modulated magnetoconvection

We explore the modifications brought about by gravity modulation in self-aligned two-dimensional (2D) stationary rolls and higher-order convective instabilities with horizontal magnetic field. The Chandrasekhar number plays no role in determining critical values of the forcing parameters for convection to begin. The onset is in the form of 2D rolls with periodically varying intensity. The external magnetic field is solely responsible for aligning the rolls while the induced magnetic field comes into play after three-dimensional convection sets in. Gravity modulation destabilizes the system and results in chaotic flow much earlier above onset.     

Density ratio effect on particle sedimentation in a vertical channel

A circular particle settling in a confined vertical channel with static fluid is investigated by lattice Boltzmann method, the effect of density ratio on the particle motion and flow pattern is discussed. It shows that the particle starting from an initial position off-center displays various moving modes accompanied by different flow patterns for 1.003 ≤ γ ≤ 5.0: when γ < 1.1, the particle is finally in a stable equilibrium near the centerline and settles uniformly with a symmetrical flow, but there are different modes before reaching the centerline at different density ratios, a monotonic approach appears at γ ≤ 1.005 and an transient overshoot appears at 1.005 < γ < 1.1; When γ = 1.1, a weak oscillation appears in the particle moving but the flow is almost symmetrical, it is in a critical state of the particle moving from stable equilibrium with a transient overshoot to oscillation; When 1.1 < γ ≤ 3.0, the particle oscillates regularly around the centerline with the uniform amplitude and frequency, and it turns into irregularity for γ > 3.0. Meanwhile the rotation of the particle depends on the distance from a wall; it rolls up the closer wall and stops at centerline.     

竖壁传热对方腔内Benard对流的影响

本文采用具有QUICK差分格式的SIMPLE算法对边界竖壁传热的方腔内空气Benard对流进行了数值计算,根据计算结果探讨了竖壁传热对Benard对流的影响.计算表明,在所考虑的几何和物理条件下,所有竖壁绝热时,腔内流体形成平行与短轴的多个涡卷;竖壁存在传热时,腔内流体形成平行于长轴的两个涡卷,并且平行于长轴竖壁的传热热流方向相反时,涡卷的旋转方向也相反;垂直于长轴蛏壁的传热对近壁附近的流动有一定影响.     

Stochastic elastohydrodynamics of a microcantilever oscillating near a wall

We consider the thermally driven motion of a microcantilever in a fluid environment near a wall, a configuration characteristic of the atomic force microscope. A theoretical model is presented which accounts for hydrodynamic interactions between the cantilever and wall over a wide range of frequencies and which exploits the fluctuation-dissipation theorem to capture the Brownian dynamics of the coupled fluid-cantilever system. Model predictions are tested against experimental thermal spectra for a cantilever in air and water. The model shows how, in a liquid environment, the effects of non-delta-correlated Brownian forcing appear in the power spectrum, particularly at low frequencies. The model also predicts accurately changes in the spectrum in liquid arising through hydrodynamic wall effects, which we show are strongly mediated by the angle at which the cantilever is tilted relative to the wall.     

Analytical evaluation of the acoustic insulation provided by double infinite walls

The acoustic insulation provided by infinite double panel walls, when subjected to spatially sinusoidal line pressure loads, is computed analytically. The methodology used extends earlier work by the authors on the definition of the acoustic insulation conferred by a single panel wall. It does not entail any simplification other than the assumption that the panels are of infinite extent. The full interaction between the fluid (air) and the solid layers is thus taken into account and the calculation does not involve limiting the thickness of any layer, as the Kirchhoff or Mindlin theories require. The problem is first formulated in the frequency domain. Time domain solutions are then obtained by means of inverse Fourier transforms using complex frequencies.The model is first used to compute the sound reduction provided by a double homogeneous brick wall, with identical panels, when illuminated by plane sound waves. The results are then compared with those provided by the simplified method proposed by London, which was later extended by Beranek (London-Beranek method). The limitations of the simplified London-Beranek model, namely, its applicability only to double walls with identical mass, subjected to plane waves, and its failure to account for the coincidence effect, are overcome by the method proposed.Time signatures are produced to illustrate the different sound transmission mechanisms. Several types of body and guided waves are originated, giving rise to a complex dynamic system with multiple reflections within the solid and fluid layers and the global resonance of the system. The effect of the cavity absorption is considered by attributing a complex density to the air filling the space between the two wall panels. Absorption attenuates the dips of insulation controlled by the cavity resonances. Several simulations are then performed for different combinations of wall and air layer thickness to assess the influence of this variable on the final acoustic insulation. The influence of the air cavity on sound reduction was found to be dependent on the frequency. At low frequencies a better performance was achieved for thicker air layers, while at higher frequencies a thinner air layer is preferable. The use of wall panels with different mass resulted in the wall performing better, particularly for high frequencies.     

Comparison of performance between bipolar and unipolar double-frequency sinusoidal pulse width modulation in a digitally controlled H-bridge inverter system

By deriving the discrete-time models of a digitally controlled H-bridge inverter system modulated by bipolar sinu- soidal pulse width modulation （BSPWM） and unipolar double-frequency sinusoidal pulse width modulation （UDFSPWM） respectively, the performances of the two modulation strategies are analyzed in detail. The circuit parameters, used in this paper, are fixed. When the systems, modulated by BSPWM and UDFSPWM, have the same switching frequency, the stabil- ity boundaries of the two systems are the same. However, when the equivalent switching frequencies of the two systems are the same, the BSPWM modulated system is more stable than the UDFSPWM modulated system. In addition, a convenient method of establishing the discrete-time model of piecewise smooth system is presented. Finally, the analytical results are confirmed by circuit simulations and experimental measurements.     

FORCED AND FREE VIBRATIONS OF RECTANGULAR SANDWICH PLATES WITH PARAMETRIC STIFFNESS MODULATION

An active control of the resonant vibrations of a rectangular sandwich plate performed by the parametric stiffness modulation is analyzed. The controlled vibrations are those of the dominantly flexural type excited by the transverse force acting at the first resonant frequency of dominantly flexural vibrations. The stiffness modulation is performed at a comparatively high frequency identified by the resonance of a mode of the dominantly shear type. The method of direct partition of motions is used that predicts an existence of the modal interaction between these two modes of vibrations due to the parametric stiffness modulation. It is shown that such a parametric control can provide a significant shift of the first eigenfrequency of a controlled plate (the one subjected to the stiffness modulation) from its nominal value for an uncontrolled plate. Heavy fluid loading conditions are accounted for as well as the energy dissipation in the material of a plate. It is demonstrated that although heavy fluid loading reduces resonant frequencies of forced vibrations, the suggested mechanism of control remains valid in these cases. Dynamics of an elementary two-degree-of-freedom model mechanical system is considered to illustrate the mechanism of modal interaction, which is involved in the suggested way of an active control of vibrations of sandwich plates.     

Some effects of auditory grouping factors on modulation detection interference (MDI).

The ability to detect the existence of amplitude modulation at a target frequency is reduced when amplitude modulation exists at a flanking frequency. This effect has been termed modulation detection interference (MDI) [Yost and Sheft, J. Acoust. Soc. Am. 85, 848-857 (1989)]. One explanation for MDI holds that the masking and target frequencies are grouped together by the auditory system such that it is difficult to analyze the modulation at each frequency separately. The present study investigated conditions where the asynchrony of temporal gating of the target and flanking frequencies was manipulated in order to make the frequencies more or less likely to be grouped together by the auditory system and perceived as originating from a single putative source. A second experimental manipulation attempted to perceptually segregate the masking and target frequencies on the basis of harmonicity or spectral proximity. The results of the experiments indicated that manipulations that were intended to enhance the segregation of the masking and target frequencies reduced the magnitude of MDI effects. This generally supported an interpretation that MDI is related in some way to auditory grouping. A final experiment was performed in which the subject had to detect the presence of amplitude modulation, but also had to identify which of two frequency components carried the modulation. Subjects were often poor in discriminating which of two frequencies was amplitude modulated, even when the modulation itself was clearly audible. It was concluded that part of the MDI effect might be due to the poor ability of the auditory system to associate modulation with the carrier of the modulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

On the realization of the Hunt-Ott strange nonchaotic attractor in a physical system

The object of investigation is a system consisting of two coupled nonautonomous van der Pol oscillators the characteristics frequencies of which differ by a factor of 2. The system is subjected to an external action in the form of slow periodic modulation of an oscillation-controlling parameter and also to an additional action at a frequency that is in an irrational relation with the modulation frequency. It is shown that the variation of the oscillation phase over a modulation period can be approximated by a 2D map on a torus that has a robust (structurally stable) Hunt-Ott strange nonchaotic attractor. Calculations of the quantitative characteristics of the attractor corresponding to the initial set of nonautonomous coupled oscillators (such as phase sensitivity exponent, structures and scaling of rational approximations, as well as Lyapunov exponents and their parameter dependence) confirm the presence of the Hunt-Ott strange nonchaotic attractor.