热声发动机的格子气模拟

在传统的9-bit格子气模型中引入了温度区的概念,并应用于热声发动机的模拟研究.利用改进后的格子气模型,模拟了热声谐振管中的自激振荡和二维温度场的分布及温度随时间的演化过程,同时,对热声板叠的长度和在声场中的位置对声幅的影响也进行了数值研究,所得结果对板叠的优化设计是有价值的.通过对模拟和实验结果的比较,验证了热声机格子气模型的有效性,表明格子气方法适用于热声模拟.     

EFFECT OF PHONON-ELECTRON INTERACTION ON THE PERSISTENT CURRENT IN ONE-DIMENSIONAL MESOSCOPIC RINGS

本文分别在自由电子和紧束缚近似下研究了电子－声子相互作用对一维介观圆环中持续电流的影响．研究结果表明：在自由电子近似下，圆环中的电流随电－声相互作用的增强而呈倒数关系减小．在紧束缚近似下，环中电流随电－声相互作用的增强而呈指数衰减．且电－声相互作用并不改变环中电流随磁通变化的周期性．     

蒸汽射流压力振荡传播特性实验研究

对蒸汽质量流率298 kg/(m~2·s),水温20~70℃的蒸汽射流凝结压力振荡的传播特性进行了实验研究。发现不同无量纲轴向距离处的压力振荡波峰波谷基本吻合,随无量纲轴向距离的增加压力振荡频率不变,振荡强度衰减,且没有出现反射叠加现象。压力振荡幅值随无量纲轴向距离的增加而减小,随冷却水温的升高先增大后减小.根据实验结果拟合得到了不同工况和空间位置压力振荡幅值的实验关联式,预测误差在-30%~20%以内。     

金属和自散焦周期性介质界面表面间隙孤子的研究

研究了金属和自散焦周期性介质界面表面间隙孤子的形成及其稳定性.这种表面间隙孤子只存在于超过格子深度临界值的有限间隙内,在第一间隙和第二间隙内间隙孤子存在区域包括稳定区域和不稳定区域,第二间隙内格子深度的临界值远大于第一间隙内格子深度的临界值.在第一间隙内,对给定的格子深度,当传播常数增大时,表面间隙孤子的能流变小,格子区域内表面间隙孤子振荡拖尾变短.对给定的传播常数,表面间隙孤子的能流随格子深度的增加而增大,增大格子深度能把表面间隙孤子由不稳定态转为稳定态.不稳定表面间隙孤子在传输中向横轴正向偏转,偏转角度随格子深度的增加而变小,其主瓣传播轨迹是一条锯齿形曲线.在第二间隙内,表面间隙孤子有较多的拖尾振荡,不稳定区域靠近存在区域的上限,并且随着传播常数增加而逐渐减小.     

非对称耦合量子阱中亚毫米波辐射及其带间激子复合发光特性的理论研究

基于V形三能级模型运用密度矩阵方程研究了非对称耦合量子阱退局域化现象，并具体分析了导带中电子作阱间振荡所产生的亚毫米辐射和激子复合所产生的近红外发射物理过程.理论结果表明：阱间电子波包振荡等效为一经典阻尼振子，其寿命由载流子与纵向光学声子散射时间和电子穿过势垒的渡越时间决定，所产生的亚毫米波辐射强度衰减仅与阱间阻尼系数有关，其线性服从洛仑兹分布.由带间激子复合所产生的近红外辐射频率随电子阱间振荡频率增加而减小，表现为量子限域斯塔克红移，即非对称量子阱在近红外发光区特性随反向偏压而变.这一结果预示着耦合量子 关键词： 非对称耦合量子阱 电子波包振荡 亚毫米波辐射 近红外发射     

输入电功率在大范围内变动时声致发光与电功率之间的关系

采用频率为１．１ＭＨｚ，声辐射面积为３ｃｍ＾２换能器，在输入电功率０－１２０Ｗ范围内研究了被空气自然饱和纯水中声致发光强度和输入电功率在大范围内变化时的关系，结果表明，声致发光在输入电功率为７０－９０Ｗ之间，不再随输入电功率的增加而增加，呈现出饱和现象，在饱和值的出现前，发光强度随输入电功率的增加呈不规则的阶状增加，在饱和值结束后，随输入电功率的增加，发光强度迅速减小，本文对实验结果进行了些定性的     

磁台阶势垒结构中二维电子气的自旋极化输运

运用散射矩阵方法,研究了台阶磁势垒量子结构中二维电子气的隧穿输运性质.结果表明：在零偏压下,电子传输概率的自旋极化曲线随入射能量的增加而振荡衰减;随着磁台阶数的增加,电子传输概率的自旋极化度最大值减小,同时电子传输概率的自旋极化度振荡衰减也越来越慢;随着磁台阶的总宽度增加,电子传输概率的自旋极化曲线出现更明显的振荡,电子隧穿磁台阶势垒表现出明显的量子尺寸效应;在偏置电压的作用下,电子传输概率的自旋极化度在宽广的入射能量区出现明显的振荡增大,电子隧穿磁台阶势垒表现出更明显的自旋过滤效应. 关键词： 磁台阶势垒 自旋极化 自旋过滤     

柱形量子线中体纵光学声子平均数

考虑电子与体纵光学声子相互作用时,采用LLP变分方法,研究柱形量子线中极化子性质,导出了柱形量子线中极化子光学声子平均数随量子线截面半径和电子-LO声子耦合强度的变化关系.结果表明柱形量子线中极化子的光学声子平均数随量子线截面半径减小而减小,随电子-LO声子耦合强度增强而增加.     

太赫兹波在非磁化等离子体中的传输特性研究

本文对太赫兹波在非磁化等离子体中的传输特性进行了理论和实验研究,得到了非磁化等离子体中太赫兹波传输特性随太赫兹波频率、等离子体密度、碰撞频率和厚度的变化规律．发现了一些新的现象：随着太赫兹波频率增加,反射率曲线出现周期性振荡,振荡周期为0．03THz．随着太赫兹波频率增加,振荡幅度增加：随着等离子体密度增加,振荡幅度减小;随着等离子体碰撞频率增加,振荡幅度增加．反射率曲线出现振荡的原因是电磁波在z=0和z=-d界面处的多次反射所致．以激波管为实验平台进行了0．22THz太赫兹波在等离子体中传输特性的实验研究,实验结果和理论结果符合较好．理论和实验结果均表明,采用太赫兹来实现地面与飞行器之间的通信互联是解决黑障问题的可选途径之一．     

Rashba自旋-轨道相互作用影响下量子盘中强耦合磁极化子性质的研究

本文基于Lee-Low-Pines幺正变换法,采用Tokuda改进的线性组合算符法研究了Rashba自旋-轨道相互作用效应下量子盘中强耦合磁极化子的性质.结果表明,磁极化子的相互作用能Eint的取值随量子盘横向受限强度ω0、外磁场的回旋频率ωc、电子-LO声子耦合强度α和量子盘厚度L的变化均与磁极化子的状态性质密切相关;磁极化子的平均声子数N随ωc,ω0和α的增加而增大,随L的增加而振荡减小;在Rashba自旋-轨道相互作用效应影响下磁极化子的有效质量将劈裂为m*+,m*-两种,它们随ωc,ω0和α的增加而增大,随L的增加而振荡减小;在研究量子盘中磁极化子问题时,电子-LO声子耦合和Rashba自旋-轨道相互作用效应的影响不可忽略,但Rashba自旋-轨道相互作用和极化子效应对磁极化子的影响只有在电子运动的速率较慢时显著.     

Thermal-relaxation dissipation in thermoacoustic systems

Pressure oscillations in a sound wave are accompanied by temperature oscillations. In the presence of a solid boundary, the heat transfer from the oscillating gas to the solid boundary causes dissipation of the acoustic energy. This results in the attenuation of the sound wave. This thermal-relaxation dissipation process has a negative effect on the performance of thermoacoustic heat pumps and engines. A simple analytical model describing the interaction between an acoustic wave and a solid boundary is presented. The effect of the solid material and gas type on thermal-relaxation dissipation is analysed. The main result of this model is that the choice of a solid material with the smallest possible heat capacity per unit area in combination with a gas with the largest possible heat capacity per unit area minimises the thermal-relaxation dissipation. From the different combinations solid-gas used in the calculations, the combination cork-helium leads to the lowest thermal attenuation of the sound wave. In this case, the heat transfer from the gas to the wall less damps the temperature oscillations. However, because of the porosity of cork that may cause some problems, it is suggested that the combination polyester-helium can be used in practice to minimise the thermal-relaxation losses.     

Effect of viscosity on stability and accuracy of the two-component lattice Boltzmann method with a multiple-relaxation-time collision operator investigated by the acoustic attenuation model

A two-component lattice Boltzmann method (LBM) with a multiple-relaxation-time (MRT) collision operator is presented to improve the numerical stability of the single relaxation time (SRT) model. The macroscopic and the momentum conservation equations can be retrieved through the Chapman—Enskog (C-E) expansion analysis. The equilibrium moment with the diffusion term is calculated, a diffusion phenomenon is simulated by utilizing the developed model, and the numerical stability is verified. Furthermore, the binary mixture channel model is designed to simulate the sound attenuation phenomenon, and the obtained simulation results are found to be consistent with the analytical solutions. The sound attenuation model is used to study the numerical stability and calculation accuracy of the LBM model. The simulation results show the stability and accuracy of the MRT model and the SRT model under different viscosity conditions. Finally, we study the influence of the error between the macroscopic equation of the MRT model and the standard incompressible Navier—Stokes equation on the calculation accuracy of the model to demonstrate the general applicability of the conclusions drawn by the sound attenuation model in the present study.     

Sound waves generated due to the absorption of a pulsed electron beam in gas

The results of an experimental investigation of acoustic vibrations (their frequency, amplitude, and attenuation coefficient) generated in a gas mixture as a result of the injection of a high-current pulsed electron beam into a closed reactor are presented. It is shown that the change in the phase composition of the initial mixture under the action of the electron beam leads to a change in the frequency of the sound waves and to an increase in the attenuation coefficient. By measuring the change in frequency, it is possible to evaluate with sufficient accuracy (about 2%) the degree of conversion of the initial products in the plasmochemical process. Relations describing the dependence of the sound energy attenuation coefficient on the size of the reactor and on the thermal and physical properties of the gases under study are derived. It is shown that a simple experimental setup measuring the parameters of acoustic waves can be used for monitoring the plasmochemical processes initiated by a pulsed excitation of a gas mixture.     

Sound wave attenuation in foam

Experimental measurements are presented for sound wave attenuation in foam without additives (standing wave method) and in foam with added particles (pulse method). A setup is developed that makes it possible to obtain a standing sound wave in stable foam and estimate the attenuation coefficient. A comparison is made of the coefficients of sound attenuation in foam in the sonic and ultrasonic frequency ranges, which have been published in a number of works. It is shown that the introduction of particles into foam leads to an increase in sound wave attenuation and may be the result of the viscous mechanism of sound wave energy loss.     

Sound velocity and absorption measurements under high pressure using picosecond ultrasonics in a diamond anvil cell: application to the stability study of AlPdMn

We report an innovative high pressure method combining the diamond anvil cell device with the technique of picosecond ultrasonics. Such an approach allows us to measure sound velocity and attenuation of solids and liquids under pressure of tens of GPa, overcoming all the drawbacks of traditional techniques. The power of this experimental technique is demonstrated in studies of lattice dynamics and relaxation processes in a metallic single grain of AlPdMn quasicrystal, and in rare gas solids neon and argon.     

Numerical simulation of nonlinear acoustic attenuation of multi-component gas mixture

The direct simulation Monte Carlo （DSMC） method was introduced to model the acoustic propagation in multi-component gas mixtures. And a theoretical predictive model of acoustic attenuation was proposed, which does not rely on experiential parameters. The acoustic attenuation spectra of various multi-component gas mixtures, consisting of nitrogen, oxygen, carbon dioxide, methane and water, were estimated by the DSMC method. The sound frequency range of interest is from 8 MHz to 232 MHz. Compared with the result of the relaxation attenuation based on the DL model plus that of the classical attenuation calculated by the Stokes-Kirchhoff formula, the estimations of acoustic attenuation of our model agreed with them. The precision of the model depends upon the understanding of the physical mechanism of molecule collision from which the attenuation arises. In addition, the result of our model shows that the characters of the frequency-dependent acoustic attenuation rely on the composition of the gas mixtures. And this could lead to the development of smart acoustic gas sensors capable of quantitatively determining gas composition in various environments and processes.     

多原子分子气体中声波弛豫衰减谱的重建算法

提出了一种宽频率范围的弛豫衰减谱重建算法,并采用基于SSH理论的方法和基于实验数据的方法估计气体的有效弛豫时间.通过该算法得到了包括氮气、甲烷、氧气、二氧化碳和水蒸气在内的多种多原子分子混合气体的声衰减谱,研究的声波频率范围从1Hz到10GHz.与预测弛豫衰减的DL模型的结果比较表明,该算法获得的弛豫衰减谱结果与之相符,其预测精度取决于对分子弛豫过程的正确认识.另外该算法还被用于几种混合气体中水蒸气和二氧化碳含量的分析,其结果表明弛豫衰减谱可被用于定量分析多原子分子气体的成分组成,这使得实现高灵敏度地检测气体成分的智能声气体传感技术成为可能. 关键词： 声弛豫衰减 有效弛豫时间 重建算法 声气体传感器     

Sound attenuation by multiple barriers

A method (tentatively called ‘the method of equivalent sound source’) of predicting the sound attenuation achieved by multiple barriers with knife edges and/or right-angled wedges is proposed. This paper shows that the sound pressure level in the shadow region behind the multiple barriers can be obtained by successively setting imaginary sound sources for respective edges and/or wedges. The locations of the imaginary sound sources are determined by means of the traditional solution for sound attenuation by a single edge. The calculated results for several kinds of small-scale model, obtained by ‘the method of equivalent sound source’, are compared with experimental results.     

二维卡门涡街的格子Boltzmann仿真

格子气自动机和格子Boltzmann方法的迅速发展提供了一类求解流体力学问题的新的方法。本文中，我们介绍了Boltzmann方法，解决了格子气方法中的缺点，通过选择适当平衡分布及其参数，导出了Navier－Stokers方程，并得到了声速和粘性系数。最后在微机上模拟了在无限长平板流动问题及绕单分离板的流动问题，得到了卡门涡街。结果表该模型有格子气方法及其它的数值方法所没有的优点，计算更精确、更直观、更有效。     

Sound attenuation study on the bose-Einstein condensation of magnons in TlCuCl3

We investigate experimentally and theoretically sound attenuation in the quantum spin system TlCuCl3 in magnetic fields at low temperatures. Near the point of Bose-Einstein condensation of magnons a sharp peak in the sound attenuation is observed. The peak demonstrates a hysteresis as a function of the magnetic field pointing to a first-order contribution to the transition. The sound damping has a Drude-like form arising as a result of hard-core magnon-magnon collisions. The strength of the coupling between lattice and magnons is estimated from the experimental data. The puzzling relationship between the transition temperature and the concentration of magnons is explained by their "relativistic" dispersion.