变截面微管道内声速点位置及临界压比

等直-收缩-扩张-等直（SCDS）变截面微管道主要应用于微型空间推进器和微型气体涡轮机等微器件中，研究其内流动特性对指导上述两种微器件的设计和性能的提高具有非常重要的意义.采用硅微加工技术在硅片上制作出矩形截面三维SCDS微管道，喉部宽度为16μm，深度为20μm，收缩比为1.625∶1实验测量了不同进出口压比条件下微管道内氮气的流量特性.实验设定进出口压比值范围为1.0—4.0，由此出口体积流量范围为0—0.12mL/s，流动的特征雷诺数小于350在实验研究的基础上，采用有限体积法对其内部流动特性进行了数值模拟，数值模拟结果与实验结果相符合.数值模拟结果发现两点不同于常规流动的特异现象：其一为最早出现声速点不在最小截面喉部附近，而是移到出口截面附近，即声速点的位置发生变化；其二为在进出口压比达到4.0时，SCDS微管道内部流场才出现声速点，即临界压比值（这里将管道内部首次出现声速点时对应的当地点压力与进口压力的比值定义为临界压比）发生变化.将其原因归结为表面效应（表面积与体积比值S/V）的影响，并进一步研究了这两点特异现象与S/V值之间的关系. 关键词： 等直-收缩-扩张-等直变截面微管道 声速点位置 临界压比 表面积与体积比值     

Research on the effective gas diffusion coefficient in dry porous media embedded with a fractal-like tree network

A theoretical model for the relative gas diffusion coefficient in dry porous media embedded with a Y-shaped fractal-like tree network is presented under the combination of bulk diffusion and Knudsen diffusion. The proposed model is expressed as a function of the length ratio, the diameter ratio, the branching level, the branching angle and the relative areal porosity. The effect of the structural parameters of the medium and the tortuosity on gas diffusion is analyzed in detail. Model predictions are compared with available experimental data, and a fair agreement between them is found.     

Theoretical analysis of porous radiant burners under 2-D radiation field using discrete ordinates method

This paper describes a theoretical study to investigate the heat transfer characteristics of porous radiant burners (PRBs). In the present work, a 2-D rectangular model is used to solve the governing equations for porous medium and gas flow before the premixed flame to the exhaust gas. The gas and the solid phases are considered in non-local thermal equilibrium and combustion in the porous medium is modeled by considering a non-uniform heat generation zone. The homogeneous porous media, in addition to its convective heat exchange with the gas, may absorb, emit and scatter thermal radiation. The radiation effect in the gas flow is neglected but the conductive heat transfer is taken into account. In order to analyze the thermal characteristics of porous burners, the coupled energy equations for the gas and porous medium in steady condition are solved numerically and the discrete ordinates method (DOM) is used to obtain the distribution of radiative heat flux in the porous media. Finally, the effects of various parameters on the performance of porous radiant burners are examined. The present results are compared with some reported theoretical and experimental results by other investigators and good agreement is found.     

流化床内超细颗粒的流动

基于气体分子运动论和颗粒动理学,建立超细颗粒气固两相湍流流动模型,模型考虑了气相与颗粒聚团之间以及颗粒聚团之间的动量和能量的传递和耗散。建立超细颗粒固相粘性系数、超细颗粒压力等物性参数计算模型。超细颗粒的聚团改变了单颗粒碰撞动力学以及颗粒相压力、粘性系数等输运特性。模型模拟计算颗粒聚团直径分布与Zhaolin等[1]实测值相吻合。     

The flexural vibration of welded rectangular plates

A theoretical and experimental study of the effect of weld runs on the flexural vibrational characteristics of the common structural element, the rectangular plate, is described. A finite difference technique is utilized for the determination of the in-plane residual stress pattern due to the weld(s) and the Rayleigh-Ritz method, with beam characteristic functions, is used for the out-of-plane vibration analysis. The theoretical approach presented is applicable to rectangular plates of any practical aspect ratio, having any combination of out-of-plane boundary conditions for which beam functions may reasonably be used and subject to one or more weld runs parallel to any edge. Theoretical and experimental results for a number of specific plates are presented, demonstrating the effects of welding on the plate vibration and the capability and accuracy of the analytical approach in predicting these effects. Included is a study of the effect of using the full residual stress pattern as derived from the finite difference analysis, the effect of neglecting certain stress components and the effect of using simplified stress patterns developed primarily for the stress and buckling analysis of long plates.     

Resonant Response of Rectangular/kFM Cantilever in Liquid

Dynamic characteristics of atomic force microscopy （AFM） cantilevers can be influenced by their working media. We perform an experimental study on the resonant responses of rectangular AFM cantilevers with different sizes immersed in various viscous fluids. The measured resonance frequencies in liquids are used to valldate several theoretical models. Comparison shows the analytical model proposed by Sader [J. Appl. Phys. 84 （1998） 64] can give the best agreement with the experimental results with the maximum relative error nearly 16% for all the cantilevers in different liquids. The ratio between the resonant frequencies in air and water is almost independent of the cantilever length, which is consistent with the theoretical analyses.     

输气管道声波法泄漏检测技术的理论与实验研究

为探究输气管道声波法泄漏检测技术的基本原理和研究方法,建立了输气管道泄漏仿真模型和实验模型。首先,研究输气管道发生泄漏时声波产生机理,从而明确声波法泄漏检测技术的工作原理;其次根据声波法泄漏检测的原理建立Fluent仿真模型,通过CFD软件模拟输气管道泄漏得到流场和声场,得到声波信号特征;再次根据声波法泄漏检测的原理搭建实验管道,设计完成声波信号数据采集终端,并在仿真结果指导下完成输气管道泄漏检测实验,采集得到实验数据并进行分析,得到泄漏时声波信号特征并对仿真结果进行验证;最后对比多工况条件下的实验结果和仿真结果,分析不同工况条件对压力波动值的影响规律,从而总结输气管道声波法泄漏检测的仿真与实验研究方法,为声波法泄漏检测的实际工程应用提供理论基础。研究结果表明:输气管道泄漏产生的声波来源于泄漏时气体不稳定流动产生的偶极子声源和四极子声源;仿真模拟和实验研究的方法都可以完成声波法泄漏检测技术的研究,可以对声波法泄漏检测技术的推广和工程应用提供强大的理论支持和可行性保证。结论是:声波法泄漏检测技术可以很好的检测输气管道泄漏,是一种灵敏度高,检测时间短,值得推广的方法。      

纳米流体对流换热机理分析

考虑在纳米流体中纳米颗粒做布朗运动引起的对流换热, 基于纳米颗粒在纳米流体中遵循分形分布, 本文得到纳米流体对流换热的机理模型. 本解析模型没有增加新的经验常数, 从该模型发现纳米流体池沸腾热流密度是温度、纳米颗粒的平均直径、 纳米颗粒的浓度、纳米颗粒的分形维数、沸腾表面活化穴的分形维数、基本液体的物理特性的函数. 对不同的纳米颗粒浓度和不同的纳米颗粒平均直径与不同的实验数据进行了比较, 模型预测的结果与实验结果相吻合. 所得的解析模型可以更深刻地揭示纳米流体对流换热的物理机理.     

On the acoustic performance of rectangular splitter silencers in the presence of mean flow

Dissipative splitter silencers are often used to reduce the noise emitted in ventilation and gas turbine systems. It is well known that the acoustic performance of a splitter silencer changes under the influence of the convective effects of a mean gas flow and so in this article a theoretical model is developed to include the effects of mean flow. The theoretical model is based on a hybrid finite element method which enables the inclusion of bull nose fairings and a perforated screen separating the mean gas flow from a bulk reacting porous material. Predictions are compared against experimental measurements obtained both with and without mean flow. Good agreement between prediction and measurement is generally observed in the absence of mean flow, although it is seen that for silencers with a low percentage open area the silencer insertion loss is over predicted at higher frequencies. When mean flow is present, problems with the experimental methodology are observed at relatively modest mean flow velocities, and so comparison between prediction and experiment is limited to relatively low face velocities. However, experiment and theory both show that the insertion loss reduces at low frequencies when mean flow is in the direction of sound propagation, and at high frequencies the influence of mean flow is generally much smaller. Following additional theoretical investigations it is concluded that the influence of mean flow on splitter silencer performance should be accounted for at low frequencies when silencer airway velocities are greater than about 20 m/s; however, at higher frequencies one may generally neglect the effect of mean flow, even at higher velocities. Predictions obtained using the hybrid method are also compared to a simplified point collocation approach and it is demonstrated that the computationally efficient point collocation method may be used to investigate the effects of mean flow in a splitter silencer without loss of accuracy.     

Semi-analytical model for the Seebeck coefficient in semiconductors with isotropic DOS given by a power function

The relations for the Seebeck coefficient in a semiconductor with the isotropic density of states given by a power function are introduced within the scope of a semi-analytical model, which is based on the theoretical relations given by the foundations of the semiconductor physics as well as on experimentally defined temperature dependences of various semiconductor characteristics, but does not include any adjustable parameters. Between those characteristics the major role plays the intrinsic carrier concentration. It was demonstrated that although the introduced model is based on the simplified Maxwell-Boltzmann statistic, it is not compromised by this choice. A comparison with experimental data for five different semiconductors proves its ability to provide reliable predictions over a wide range of parameters (temperature, dopant type and concentration) not only for non-degenerated wide bandgap semiconductors (Si, Ge) but also for InAs, which represents partly degenerated narrow bandgap semiconductors with a non-parabolic density of states. Even in the case of a HgCdTe, with its extremely narrow bandgap and complex temperature dependence of the carrier concentration, the model is in good agreement with experimental data. The semi-analytical nature of the introduced model and its dependence on the abundance and reliability of the used experimental data were discussed on the example of Bi₂Te₃. Although the relative deficiency and controversy of the experimental results in this case significantly impede the model’s applicability, it is still able to give at least qualitative predictions, which are nevertheless better than the results of the calculation of the thermopower from first principles. Being primarily addressed to the experimental community, the model provides simple relations in the case of the parabolic non-intrinsic semiconductor for thermoelectric voltage and for optimal dopant concentration for the thermogenerator within the known working temperature range, which can be useful in real-life ‘energy harvesting’ applications.     

长微直管内气体低速流动的亚堵塞现象

长微直管道内多流态并存的流动特性研究不仅有理论意义，而且在太空飞行器的控制系统中有着重要的应用价值.采用实验研究和理论模型近似分析相结合的方法研究了长微直管道内的气体流动特性.实验中，以空气为工作介质，进口压力分别设定为130，250，320kPa，出口压力变化范围为9—100?kPa.沿程分布有五个测压点，进出口设有温度传感器，测量出口流量的同时可以得到沿程压力分布.近似理论模型采用二维平板近似模型.研究发现，在保持进口压力不变、不断降低出口压力的条件下，当进出口压比低于5.3左右时，质量流量随压比增加 关键词： 长微直管道 亚堵塞 表面积与体积比 多流态并存 临界压比     

II. Recirculation flow induced by a bubble stream rising in a viscous liquid

The recirculation flow induced by the rising motion of a bubble stream in a viscous fluid within an open-top rectangular enclosure is studied. The three-dimensional volume averaged conservation equations are solved by a control-volume method using a hybrid finite differencing scheme to describe the liquid phase hydrodynamics. The momentum exhange between the bubbles and the liquid phase is modeled with a source term equals to the volumetric buoyancy force acting on the gas in the bubble stream. The volumetric buoyancy force accounts for in line interactions between bubbles through the average gas volume fraction in the gas liquid column which depends on the size and the rising velocity of bubbles. The fluid flow within an open-top rectangular enclosure is further investigated by particle image velocimetry for a bubble stream rising in a water-glycerol solution. The measured fluid velocities in a vertical plane are compared with the predictions of the numerical model over a wide range of fluid viscosity (43 mPa s-800 mPa s) and gas flow rates. Finally, the recirculation flows resulting from the interaction of two neighbouring vertical bubble streams are studied. Received: 23 July 1997 / Revised: 19 December 1997 / Accepted: 11 May 1998     

Bulk reaction modeling of ducts with and without mean flow

A general formulation for analysis of sound field in a uniform flow duct lined with bulk-reacting sound-absorbing material is presented here. Presented theoretical model predicts the rate of attenuation for symmetric as well as asymmetric modes in rectangular duct lined with loosely bound (bulk-reacting) sound-absorbing material, which allows acoustic propagation through the lining. The nature of attenuation in rectangular ducts lined on two and four sides with and without mean flow is discussed. Computed results are compared with published theoretical and experimental results. The presented model can be used as guidelines for the acoustic design of silencers, air-conditioning ducts, industrial fans, and other similar applications.     

纳米通道内气体剪切流动的分子动力学模拟

采用分子动力学模拟方法研究了表面力场对纳米通道内气体剪切流动的影响规律.结果显示通道内的气体流动分为两个区域:受壁面力场影响的近壁区域和不受壁面力场影响的主流区域.近壁区域内,气体流动特性和气体动力学理论预测差别很大,密度和速度急剧增大并出现峰值,正应力变化剧烈且各向异性,剪切应力在距壁面一个分子直径处出现突变.主流区域的气体流动特性与气体动力学理论预测相符合,该区域内的密度、正应力与剪切应力均为恒定值,速度分布亦符合应力-应变的线性响应关系.不同通道高度及密度下,近壁区域的归一化密度、速度及应力分布一致,表明近壁区域的气体流动特性仅由壁面力场所决定.随着壁面对气体分子势能作用的增强,气体分子在近壁区域的密度和速度随之增大,直至形成吸附层,导致速度滑移消失.通过剪切应力与切向动量适应系数(TMAC)的关系,得到不同壁面势能作用下的TMAC值,结果表明壁面对气体分子的势能作用越强,气体分子越容易在壁面发生漫反射.     

EXPERIMENTAL AND THEORETICAL INVESTIGATION OF THE LINEAR AND NON-LINEAR DYNAMIC BEHAVIOUR OF A GLARE 3 HYBRID COMPOSITE PANEL

A theoretical model based on Hamilton's principle and spectral analysis is used to study the non-linear free vibration of hybrid composite plates made of Glare 3, a new aircraft structural material. It consists of alternating layers of metal- and fibre-reinforced composites. In previous work, the theoretical model has been used to calculate the first non-linear mode of fully clamped rectangular composite fibre-reinforced plastic (CFRP) laminated plates. This study concerns determination of the linear dynamic properties of the Glare 3 hybrid composite rectangular panel (G3HCRP) such as natural frequencies and mode shapes. The theoretical model is used to calculate the fundamental non-linear mode shape and associated flexural behaviour of the fully clamped G3HCRP. A series of experimental investigations have been conducted using a G3HCRP in order to determine linear dynamic properties. The response due to random excitation was investigated and the experimental measurements are analyzed and discussed. Comparisons are made with finite element predictions and response estimates given by the ESDU method, the latter being a “design guide” approach used by industry. Concerning the non-linear analysis, the results are given for various plate aspect ratios and vibration amplitudes, showing a higher increase of the induced bending stress near the clamps at large deflections. Comparisons between the dynamic behaviour of an isotropic plate and G3HCRP at large vibration amplitudes are presented and good results are obtained.     

Mixing layer resonance under high-speed stream forcing

In the majority of fluid–structure interaction problems, the biggest challenge lies in the fundamental understanding of the flow physics. Forced mixing layers is an important phenomenon found in many cases of flow-induced vibrations and acoustics. The response of a mixing layer to high-speed stream acoustic forcing is investigated with a theoretical and experimental approach. Two different experiments demonstrating the fluid mechanic phenomenon are presented. The first experiment consists of a circular jet impinging on a vibrating plate. The second experiment demonstrates the mixing layer resonance in the context of a fluidelastic instability causing high-amplitude vibrations in gas turbine high-pressure compressor rotor blades. Both the plate and the adjacent blade vibration induce an acoustic feedback that propagates within the jet and blade tip clearance flow, respectively. The resonance was found to occur when the feedback wavelength matched either the jet-to-plate or the inter-blade distance. In both experimental cases, the resonance condition has been simply modeled by the coincidence of a 1D feedback wave, which propagates upstream at reduced velocity by the high-speed flow. The coupling between the jet induced mixing layer and the feedback wave is assumed to naturally occur when one of the wave crests reaches the separation edge. The objective of this study is to improve the understanding of the coupling mechanism between an emanating shear layer and the acoustic forcing originating within a fast flow stream. The study is based on a simplified analytical model in order to enlarge the current understanding of the mixing layer receptivity to the more specific case of its response to high-speed stream forcing. To identify the mixing layer resonant modes, an analytical resonance condition is proposed. It is found that the mixing layer response becomes spatially resonant for specific source locations downstream in the high-speed flow. The study also provides an analytical mean to capture the critical source location periodicity that has been experimentally observed. The resulting theoretical prediction of the resonant source locations is in good agreement with the experimental data. Therefore, it supports the stream forced mixing layer analytical model and the proposed spatial resonance condition. The simple 1D reduced speed feedback wave model, which has been used to identify the experimental resonance conditions, is also in good agreement, and thus validated, with the results of this study.     

Rectangular Lattice Boltzmann Equation for Gaseous Microscale Flow

The lattice Boltzmann equation (LBE) is considered as a promising approach for simulating flows of liquid and gas. Most of LBE studies have been devoted to regular square LBE and few works have focused on the rectangular LBE in the simulation of gaseous microscale flows. In fact, the rectangular LBE, as an alternative and efficient method, has some advantages over the square LBE in simulating flows with certain computational domains of large aspect ratio (e.g., long micro channels). Therefore, in this paper we expand the application scopes of the rectangular LBE to gaseous microscale flow. The kinetic boundary conditions for the rectangular LBE with a multiple-relaxation-time (MRT) collision operator, i.e., the combined bounce-back/specular-reflection (CBBSR) boundary condition and the discrete Maxwell's diffuse-reflection (DMDR) boundary condition, are studied in detail. We observe some discrete effects in both the CBBSR and DMDR boundary conditions for the rectangular LBE and present a reasonable approach to overcome these discrete effects in the two boundary conditions. It is found that the DMDR boundary condition for the square MRT-LBE can not realize the real fully diffusive boundary condition, while the DMDR boundary condition for the rectangular MRT-LBE with the grid aspect ratio a≠1 can do it well. Some numerical tests are implemented to validate the presented theoretical analysis. In addition, the computational efficiency and relative difference between the rectangular LBE and the square LBE are analyzed in detail. The rectangular LBE is found to be an efficient method for simulating the gaseous microscale flows in domains with large aspect ratios.     

一种基于矩形腔的MIM波导滤波器的滤波特性研究（英文）

理论提出并研究了一种基于矩形腔的窄带金属-介质-金属波导滤波器.建立滤波器内电场的传递矩阵模型,研究了矩形微腔与直通波导间耦合特性对器件滤波特性的影响.同时,研究了耦合长度、矩形微腔腔长、传输损耗等因素对滤波带宽的影响.研究结果表明,对于不同的矩形微腔腔长,存在一个可使器件滤波带宽达到最窄的耦合系数.此外,当微腔腔长越长且传输损耗越小时,滤波带宽也将越窄.该研究为表面等离子体波导的研究与设计提供了一定的参考.     

Band-structure determination for finite 3-D photonic crystals

The partial band structure from a finite photonic crystal is determined using a model based on light diffraction and the transfer-matrix formalism. The predictions from such a model are compared to an experimental measurement of the bands in the LU direction of a face centered cubic colloidal crystal. Then, both the theoretical predictions and the experimental measurements are compared with the usual band-structure calculation based on a plane-wave expansion with perfectly periodic boundary conditions. As in measurements performed in the past, discrepancies between the predictions of this later model and the experimentally determined bands are observed. On the contrary, using the model presented based on light propagation through a finite crystal, where no periodicity is imposed in the direction perpendicular to any of the set of planes considered to determine a specific branch of the band structure, we found a very good agreement between the experimentally determined and the predicted bandwidths.     

Theoretical and experimental investigation of onset characteristics of standing-wave thermoacoustic engines based on thermodynamic analysis

A simplified physical model for calculating the onset temperature ratio and the frequency of a standing wave thermoacoustic engine (SWTE) in the time domain is built based on thermodynamic analysis. Coefficients of transient pressure drop and heat transfer are first deduced from linear thermoacoustic theory. By numerical computation, the evolutions of the pressure amplitude and the spectrum characteristics during the onset process are presented. Furthermore, the effects of stack spacing, charge pressure, and resonator length on the onset temperature ratio and the frequency are calculated. Relatively good agreement between the computational and the experimental results has been achieved, which validates the model for calculating the onset characteristics.