热声压力波放大器的湍流模型及实验验证

热声压力波放大器是一种利用声学特性将压力波幅值进行放大的一种装置,它主要用来连接热声发动机和其驱动的制冷机,增大制冷机的驱动压比.由于声学压力波放大器内的压力幅值通常较大,流动速度也非常大,所以用线性热声理论难以对其进行准确的计算.本文通过对线性热声理论进行修正,获得了热声压力波放大器内湍流流动的修正方法,并对该修正方法进行了实验验证.研究结果表明,通过修正后的理论模型可以对热声压力波放大器进行较为准确的计算.该计算模型预计在其它的一些交变流动系统中也具有一定的适用性.     

弹道扩散机制下热波传递的蒙特卡罗模拟

超短脉冲激光技术在精细加工和微纳米材料热物性测量方面的广泛应用,要求对超快速导热过程有更系统深入的了解。本文利用蒙特卡罗模拟方法研究了声子主导的超快导热过程,并和经典热波模型的结果进行了对比分析。经典热波方程预测了无色散的耗散波;兰贝特边界发射的蒙特卡罗模拟预测了色散的耗散波,而且不同的边界声子发射的模拟结果不同。利用声子玻尔兹曼方程分析了模拟中不同发射方式下热波的色散关系。热波传递过程中的能量均方位移时间关系表明中声子以弹道扩散形式传递。     

热声发动机的CFD数值模拟

采用计算流体动力学(CFD)方法,对高频驻波热声发动机和热声斯特林发动机实验系统分别进行了二维和三维数值模拟。计算模型具有与实验系统相同的几何结构、尺寸和运行工况。对计算模型的有效性进行了研究,表明实现有限换热条件的板叠实物模型适合驻波热声发动机的模拟,而实现局域热平衡的多孔介质模型适合热声斯特林发动机的模拟。计算结果成功观测到了非线性的自激振荡演化过程,捕捉到了两种发动机的不同非线性现象。计算结果分别给出了两种热声发动机内部的声场分布特性和复杂流场。计算结果与实验结果的对比验证了CFD方法对高频驻波热声发动机和热声斯特林发动机模拟的有效性。     

热声制冷机的一种非线性模型研究

为研究热声制冷机内部的非线性过程,发展了一种可考虑管道截面积变化的时域准一维非线性热声模型,其主控方程由可压流基本控制方程经截面平均的数学处理推导得出,数值求解采用具有频散保持特性的高精度计算格式。采用该模型分别针对活塞、扬声器和热声发动机这三种不同驱动源,将驱动源和热声制冷机进行整体求解,模拟了热声制冷板叠两端温度差在气体微团吸放热的循环过程中逐渐增大的整个过程,预测了谐振管内部驻波的非线性畸变。进而研究了平均工作压力、板叠材料、板叠位置以及扬声器形状等参数的选取对于热声制冷机制冷效果的影响。     

热声热机声场的格子气分析

从格子气自动机演化方程恢复了宏观热声方程,采用D2Q9热格子气模型模拟热声热机系统的压力场、温度场和速度场,获得压力驻波的传播和反射过程。模拟结果显示,在所给模拟条件下压力波具有非线性的特征,变截面附近的流场区域会产生引起非线性耗散的涡流。本文也分析了谐振管几何尺寸及格子气粒子密度对系统频率和压力的影响,验证了热声热机格子气模型的有效性。     

Rijke管自激热声振荡的数值模拟

为了对热声不稳定的发生及控制机理进行研究,对Rijke管内的自激热声振荡现象进行了数值模拟。采用具有低频散低耗散特点的计算气动声学方法,对带有非线性热源项的声波方程进行数值求解,并比较了不同的热源模型及边界条件对非线性效应的影响。结果表明,计算气动声学方法可以成功捕捉到Rijke管内压力的起振过程,而且在速度扰动达到平均流速度的1/3时,振荡会由线性增长转为非线性增长,最终达到有限幅值极限循环。相比热源项,考虑管口辐射耗散的非线性边界条件在振荡幅值和频谱方面对结果的影响都比较小。数值模拟得到的结果与实验符合较好,表明计算气动声学方法适合于热声振荡问题的研究。      

Rijke管自激热声振荡的数值模拟

为了对热声不稳定的发生及控制机理进行研究,对Rijke管内的自激热声振荡现象进行了数值模拟。采用具有低频散低耗散特点的计算气动声学方法,对带有非线性热源项的声波方程进行数值求解,并比较了不同的热源模型及边界条件对非线性效应的影响。结果表明,计算气动声学方法可以成功捕捉到Rijke管内压力的起振过程,而且在速度扰动达到平均流速度的1/3时,振荡会由线性增长转为非线性增长,最终达到有限幅值极限循环。相比热源项,考虑管口辐射耗散的非线性边界条件在振荡幅值和频谱方面对结果的影响都比较小。数值模拟得到的结果与实验符合较好,表明计算气动声学方法适合于热声振荡问题的研究。     

热声起振现象的模拟研究

热声现象中广泛存在非线性效应,其中最典型的就是热声自激起振.本文建立了一个四分之三波长的Rijke管计算模型,基于并行计算,采用数值模拟对其起振现象进行了模拟.研究了不同初场对起振行为的影响,得出起振后的气体振荡频率,并对管内的流动与换热规律进行了详尽分析.模拟结果与文献结果相吻合.本文工作有助于更多地了解热声起振现象,同时也为我们开展热声问题的介观模拟提供了有力的参考.     

扬声器驱动热声制冷机设计研究

热声制冷机主要由声驱动器、共振管、板叠、高低温端换热器、声容等部件组成。它具有运动部件少、运行可靠、振动小和寿命长等优点 ,在军事、航天、微电子、低温物理等领域有着十分诱人的应用前景。文中运用经典线性热声理论对热声制冷机各个组成部件设计进行了介绍 ,在此基础上设计了一台扬声器驱动热声制冷机 ,并用专业计算软件进行了数值模拟验证 ,为热声制冷机进一步优化与发展提供了一个解决方案。     

基于三端口网络模型的折叠波导行波管注波互作用理论研究

将周期性慢波结构中的单元结构等效为一个三端口网络,利用高频结构模拟软件确定等效模型的参数,由此建立了一种通用的慢波结构等效模型.该方法不需要进行复杂的电路等效,直接由高频软件得到通道内场分布,因此较解析方法简单,又不像等效线路模型那么烦琐.基于该三端口网络模型,建立了适用于折叠波导行波管的一维注波互作用非线性理论模型,编写了数值计算程序,对一支折叠波导行波管进行了模拟.该理论模型模拟的结果与实验结果误差小于10%.该理论模型可以用于指导新型折叠波导行波管的设计及非线性模拟研究.     

Nonlinear saturation of the thermoacoustic instability

A weakly nonlinear theory of the thermoacoustic instability in gas-filled tubes is developed in the time domain by exploiting the difference between the instability time scale and the period of standing waves. By carrying the expansion to fourth order in the perturbation parameter, explicit results for the initial growth, nonlinear evolution, and final saturation are obtained. The dependence of the saturation amplitude upon the temperature difference in the stack, the tube geometry, stack plate spacing, Prandtl number, and other parameters is illustrated.     

电磁迴旋不稳定性的准线性理论

在有弱相对论性电子时,在电子迴旋频率Ω_e附近存在快x模的不稳定性。从这一事实出发,我们用准线性理论分析了具有粒子数倒转的分布函数引起的不稳定性最后达到饱和的物理机制。进行了一些近似的计算,求出了电磁迴旋不稳定性的饱和时间和饱和能量及垂直、平行分布函数随时间的演化过程。发现用此理论算出的饱和能级与观察到的AKR结果是一致的。 关键词：     

Third-harmonic generation in regular domain structures

The process of frequency conversion in regular domain structures is studied using the constant-intensity approximation. The investigations are carried out at values of complex amplitudes of the fundamental radiation and third harmonic at the output of each domain equal to the values of the corresponding complex amplitudes at the input of the subsequent domain. We show that the optimum length of each domain depends on the input pump intensity in the given domain. Thus, it is possible by choosing the optimum lengths of domains, phase mismatch, and pump intensity even at a low number of periods of nonlinear susceptibility modulation of the lattice to reach considerable values of conversion efficiency at the structure output in comparison with the traditional case of homogeneous nonlinear media.     

Numerical and experimental analysis of strongly nonlinear standing acoustic waves in axisymmetric cavities

In this paper the behaviour of strongly nonlinear waves in axisymmetric resonators is experimentally and numerically studied. Experiments are carried out in a cylindrical cavity, which transversal dimension is bigger than the longitudinal one, excited by a narrow band transducer. The quality factor and displacement amplitudes are experimentally quantified. A finite difference numerical model is developed to solve, in the time domain, a proposed set of full nonlinear differential equations written in Lagrangian coordinates. Pressure field is obtained for complicated modes. Good agreement between numerical and experimental results is found. New nonlinear properties of quasi-standing waves in axisymmetric resonators are described. Results are compared to linear approximation and show the importance of three-dimensional analysis.     

Resonant excitation of rossby waves in the equatorial waveguide and their nonlinear evolution

Nonlinear interactions between the baroclinic Rossby waves trapped in the equatorial waveguide and the barotropic Rossby waves freely propagating across the equator are studied within the two-layer model of the atmosphere, or the ocean. It is shown that a barotropic wave can resonantly excite a pair of baroclinic waves with amplitudes much greater than its proper amplitude. The envelopes of the baroclinic waves obey Ginzburg-Landau-type equations and exhibit nonlinear saturation and formation of characteristic "domain-wall" and "dark-soliton" defects.     

Wheel/rail noise generation due to nonlinear effects and parametric excitation

Two models are developed, one in the time domain and another in the frequency domain, to explain when a wheel/rail noise generation model requires the inclusion of discrete supports, parametric excitation, and the nonlinear contact spring. Numerical simulations indicate the inclusion of discrete supports to describe low frequency response, and also at higher frequencies, especially where the rail is very smooth or has a corrugation/wavelength corresponding to the pinned-pinned frequency. With a corrugation, it may become essential to include the nonlinear contact spring, as contact loss occurs at high corrugation amplitudes. As nonlinearity causes force generation over a broad frequency range, some contributions excite wheel resonances, resulting in high radiation levels, that require the inclusion of wheel/rail nonlinear effects and parametric excitation for accurate prediction.     

Finding thermoacoustic limit cycles for a ducted Burke-Schumann flame

This paper examines nonlinear thermoacoustic oscillations of a ducted Burke-Schumann diffusion flame. The nonlinear dynamics of the thermoacoustic system are studied using two distinct approaches. In the first approach, a continuation analysis is performed to find limit cycle amplitudes over a range of operating conditions. The strength of this approach is that one can characterize the coupled system’s nonlinear behaviour over a large parameter space with relative ease. It is not able to give physical insight into that behaviour, however. The second approach uses a Flame Describing Function (FDF) to characterize the flame’s response to harmonic velocity fluctuations over a range of forcing frequencies and forcing amplitudes, from which limit cycle amplitudes can be found. A strength of the FDF approach is that it reveals the physical mechanisms responsible for the behaviour observed. However, the calculation of the FDF is time consuming, and it must be recalculated if the flame’s operating conditions change. With the strengths and shortcomings of the two approaches in mind, this paper advocates combining the two to provide the dynamics over a large parameter space and, furthermore, physical insight into that behaviour at judiciously-chosen points in the parameter space. Further physical insight concerning the flame’s near-linear response at all forcing amplitudes is given by studying the forced flame in the time domain. It is shown that, for this flame model, the limit cycles arise because of the flame’s nonlinear behaviour when it is close to the inlet.     

Weakly nonlinear model for oscillatory instability in Saturn's dense rings

Viscous overstability (oscillatory instability) may play an important role in the formation of small scale structure in dense planetary rings such as Saturn's B ring. We investigate the growth and saturation of such modes in local particle simulations. Starting from a hydrodynamic model, we develop a set of ordinary differential equations to model the evolution of the amplitudes of the linearly overstable modes in the nonlinear regime. The NASA/ESA space probe Cassini can make direct observations of these modes in Saturn's rings, including their sizes and temporal development.     

Nonlinear stage of development of resistive hose instability of a high-current relativistic electron beam in a plasma

The nonlinear stage in the development of a resistive hose instability of a highcurrent relativistic electron beam in a finite-conductivity plasma has been studied in the rigid-beam model. The attenuation of the force of the interaction of the beam with the magnetic field of the total current for large beam displacements is shown to result in the stabilization of the instability. The stabilization time and the amplitudes of the oscillations in the saturation regime are determined as functions of the parameters of the beam in the plasma.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fiz., No. 10, pp. 78–81, October, 1987.