微通道液体流动双电层阻力效应

采用数值方法求解双电层的Poisson-Boltzmann方程和液体运动的Navier-Stokes方程,研究微通道双电层对压强梯度液体流动的阻力效应． 量纲分析表明,双电层阻力大小可以用一个无量纲的电阻力数表示．它与液体的介电系数、固体表面的zeta电位平方成正比,与液体的动力粘性系数、电导率以及微通道的宽度平方成反比．在计算流动诱导的流动电位势和电阻力时,提出电流密度平衡条件,可以消除传统电流平衡条件导致的固壁附近产生局部回流的不合理物理现象．还给出不同电阻力数的微通道流量、流量损失率、速度剖面的数值结果,合理解释了双电层对微通道液体流动的阻力效应．     

微通道周期流动电位势及电粘性效应

求解了双电层的Poisson-Boltzmann方程和流体运动的Navier-Stokes方程,得到在周期压差作用下,二维微通道的周期流动电位势,流动诱导电场和液体流动速度的解析解．量纲分析表明,流体电粘性力与以下3个参数有关:1) 电粘性数,它表示定常流动时,通道最大电粘性力与压力梯度的比;2) 形状函数,它表示电粘性力在通道横截面的分布形态; 3) 耦合系数,它表示电粘性力的振幅衰减特征和相位差．分析结果表明,微通道周期流动诱导电场、流动速度与频率Reynolds数有关．在频率Reynolds数小于1时,流动诱导电场随频率Reynolds数变化很慢．在频率Reynolds数大于1时,流动诱导电场随频率Reynolds数的增加快速衰减．在通道宽度与双电层厚度比值较小情况下,电粘性效应对周期流动速度和流动诱导电场有重要影响．     

充液弹性毛细管低温相变的力学分析

充液弹性毛细管广泛存在于生物体（如毛细血管、植物导管等）和工程领域（如微流控冰阀门、制冷系统热管、MEMS微通道谐振器等）.低温工作环境中,充液弹性毛细管内部的液柱会发生相变并引发冻胀效应,从而导致管壁的变形、损伤乃至断裂.该文建立并求解了考虑温度梯度、界面张力及液体冻胀作用的弹性毛细管平衡方程,分析了液柱低温相变过程中毛细管壁的径向和环向应力,发现管壁应力分布受热毛细弹性数和冻毛细弹性数的影响,且影响大小跟壁厚相关.该研究不仅有助于理解生物体内充液弹性毛细管冻胀失效机制,还可为MEMS微流控芯片的抗冻胀失效设计提供理论指导.     

异质材料有限长微通道电渗流热效应

采用数值方法,分析有限长PDMS/玻璃微通道电渗流热效应．数值求解双电层的Poisson-Boltzmann方程,液体流动的Navier-Stokes方程和流-固耦合的热输运方程,分析二维微通道电渗流的温度特性．考虑温度变化对流体特性（介电系数、粘度、热和电传导率）的反馈效应．数值结果表明,在通道进口附近有一段热发展长度,这里的流动速度、温度、压强和电场快速变化,然后趋向到一个稳定状态．在高电场和厚芯片的情况下,热发展长度可以占据相当一部分的微通道．电渗流稳定态温度随外加电场和芯片厚度的增加而升高．由于壁面材料的热特性差异,在稳定态时的PDMS壁面温度比玻璃壁面温度高．研究还发现在微通道的纵向和横向截面有温度变化．壁面温升降低双电层电荷密度．微通道纵向温度变化诱发流体压强梯度和改变微通道电场特性．微通道进流温度不改变热稳定态的温度和热发展长度．     

矩形微通道热沉内单相稳态层流流体的流动与传热分析

采用解析方法分析了矩形微通道热沉内单相稳态层流流体的流动与传热．基于y方向流速和导热不变的假设,建立流体在矩形微通道内流动的流速方程和传热的温度方程,进而推导出Nusselt数和Poiseuille数的理论表达式．通过计算结果可以看出,推导的Nusselt数和Poiseuille数的解析解与其他文献的结果吻合较好,而且当宽高比趋于无穷大时,Nusselt数和Poiseuille数分别趋近于8.235和96,这与其他文献结果完全相同．在Reynolds数相同时,摩擦因数随着宽高比的增加而增加,而在相同宽高比时,摩擦因数随Reynolds数的增加而减小．     

基于GERT网络的产学研知识流动效应度量

产学研合作中的知识流动效应度量一直是一个难题,本文以GERT网络与协同学为理论基础,寻求知识流动效应度量的方法。为此,根据知识流动特性,利用GERT网络中的实现概率及矩母函数参量来解析知识流动效应。为了得到矩母函数的分布类型,利用协同学,分析了影响产学研合作的内外因素,建立起随时间变化的知识流动方程,并以此方程作为矩母函数。同时,以大型飞机起落架系统研制为案例,利用专家调查法获取初始数据,并运用前述方法进行知识流动效应分析。通过GERT网络模型对知识流动进行测度,提供了产学研项目研制过程中知识增值大小、流动的顺畅性等重要信息,以此帮助决策者为保证研制项目的按时按质完成采取积极对应措施。     

二维矩形域内Stokes流问题的辛解析和数值方法

给出了一种新的解析求解二维矩形域中的Stokes流动问题的方法——辛体系方法（Hamilton体系方法）．在辛体系下,基本问题归结为本征值和本征解的问题．由于辛本征解之间存在辛正交共轭关系,问题的解和边界条件均可以由本征解描述和表示．利用辛本征解空间的完备性,建立一套封闭的求解问题方法．研究结果表明零本征值本征解描述了基本流动,而非零本征值本征解则表示问题的局部效应．数值结果给出了几种有代表性的流动情况,显示了该求解方法对求解许多问题的有效性．同时,这种方法也为研究其他问题提供了一条思路．     

混合微通道中不稳定的广义Couette流

在一个平行板通道中,部分充满了均匀的多孔介质,部分为纯流体的流动区,对其微通道中完全发展的不稳定层流进行了数值分析,流动由其中一块板的运动和压力梯度所引起.多孔介质区域的流动,采用扩展的Brinkman模型,即Darcy模型,纯净流动区域的流动,采用Stokes方程.还对稳定的完全发展流进行了理论分析,给出了分界面速度、边界板处的速度和表面摩擦的闭式解.通过数值计算发现,稳定完全发展流的闭式解,和不稳定流动的数值解,在所有时间点上得到很好地吻合.     

带有球凹/球凸的旋转通道内流动结构和强化换热的数值研究

对带有凹坑和凸包的内流通道在不同旋转数下的对流换热特性进行了数值分析,探讨了Coriolis力对通道中流场和换热特征的影响．研究发现,随着旋转数增加,通道前缘呈现出较弱的流动冲击,但存在较大的尾迹和延迟的流动再附着,后缘凹坑内部有一较小旋涡和较强射流使得后缘传热得到强化,最高可达60%．总体Nusselt数随着旋转数的增加先减小而后增大.     

近临界流体微通道内流动稳定性和换热特性研究

微尺度条件下的化工、医药、传热与能源利用等系统的研究已经成为极具潜力和挑战性的课题.相应条件下流体流动和换热的分析必须考虑尺度效应所带来的系列问题.该研究采用了数值模拟方法对近临界二氧化碳流体在微尺度通道内的流动稳定性和换热特性进行了探索.研究发现,在近临界区域内由于流体较强的膨胀特性和较低的热扩散特性,在微尺度几何条件下会产生瞬态不稳定的漩涡流动.该种条件下微尺度对流换热和混合效率都得到了大幅提高.进一步,研究针对微尺度局部稳定性演化进行了机理分析并应用了参数估计,总结获得了微通道内近临界流体瞬态换热和混合的基本特性.     

Peristatcally induced MHD slip flow in a porous medium due to a surface acoustic wavy wall

The influences of Hall current and slip condition on the MHD flow induced by sinusoidal peristaltic wavy wall in two dimensional viscous fluid through a porous medium for moderately large Reynolds number is considered on the basis of boundary layer theory in the case where the thickness of the boundary layer is larger than the amplitude of the wavy wall. Solutions are obtained in terms of a series expansion with respect to small amplitude by a regular perturbation method. Graphs of velocity components, both for the outer and inner flows for various values of the Reynolds number, slip parameter, Hall and magnetic parameters are drawn. The inner and outer solutions are matched by the matching process. An interesting application of the present results to mechanical engineering may be the possibility of the fluid transportation without an external pressure.     

Effective models for reactive flow under a dominant Péclet number and order one Damköhler number: Numerical simulations

The paper is devoted to the longitudinal dispersion of a soluble substance released in a steady laminar flow through a slit channel with heterogeneous reaction at the outer wall. The reactive transport happens in the presence of a dominant Péclet number and order one Damköhler number. In particular, these Péclet numbers correspond to Taylor’s dispersion regime. An effective model for the enhanced diffusion in this context was derived recently. It contains memory effects and contributions to the effective diffusion and effective advection velocity, due to the flow and chemistry reaction regime. In the present paper, we show through numerical simulations the efficiency of this new model. In particular, using Taylor’s ‘historical’ parameters, we illustrate that our derived contributions are important and that using them is necessary in order to simulate correctly the reactive flows. We emphasize three main points. First, we show how the effective diffusion is enhanced by chemical effects at dispersive times. Second, our model captures an intermediate regime where the diffusion is anomalous and the distribution is asymmetric. Third, we show how the chemical effects also slow down the average speed of the front.     

Influence of induced magnetic field on peristaltic flow in an annulus

This paper looks at the influence of the induced magnetic field on peristaltic transport through a uniform infinite annulus filled with an incompressible viscous and Newtonian fluid. The present theoretical model may be considered as mathematical representation to the movement of conductive physiological fluids in the presence of the endoscope tube (or catheter tube). The inner tube is uniform, rigid, while the outer tube has a sinusoidal wave traveling down its wall. The flow analysis has been developed for low Reynolds number and long wave length approximation. Exact solutions have been established for the axial velocity, stream function, axial induced magnetic field, current distribution and the magnetic force function. The effects of pertinent parameters on the pressure rise and frictional forces on the inner and outer tubes are investigated by means of numerical integrations, also we study the effect of these parameters on the pressure gradient, axial induced magnetic field and current distribution. The phenomena of trapping is further discussed.     

A three-layer asymptotic analysis of turbulent channel flow

In the classical theory for large-Reynolds number fully developed channel flow, the solutions obtained by asymptotic-expansion techniques for the outer Karman defect layer and the inner Prandtl wall layer are demonstrated to match through the introduction of an intermediate layer, based on a general intermediate limit. From an examination of the results for this general intermediate layer, the distinguished intermediate limit and the corresponding intermediate layer for which the turbulent and laminar contributions to the difference of the Reynolds stress from the wall stress are of the same order of magnitude are identified. The thickness of this distinguished intermediate layer is found to be of the order of the geometric mean of the thicknesses of the outer and inner layers     

On the Influence of Thermal Convection in Classical Taylor‐Couette Flows

The presented work deals with the instabilities that occur in the flow of a viscous fluid between axisymmetric cylinders with a rotating inner and stationary outer cylinder. The results of a numerical study of convective flows are presented. The inner cylinder is rotating and heated from within, while the outer cylinder is stationary and cooled outside. Stationary horizontal endplates are used to seal the annulus, forming an enclosure. The working fluid is silikon oil M3. The flow of oil was rendered visible by injecting aluminium powder. By increasing the Reynolds number with angular velocity of the driving inner cylinder, the flow bifurcates into different types of instabilities. Investigation was aimed to find the values of critical Reynolds and Rayleigh numbers corresponding to the critical speeds and temperature differences at which these instabilities set in. The three‐dimensional problem was modelled numerically using software package FLUENT in which discretization is performed by means of finite volume techniques. Computational grid was created in preprocessor Gambit. Numerical experiments are conducted to determine the interdependence between the heat transfer mechanism and the structure of secondary flows     

Viscous flow and temperature distribution in rotating annulus with suction and injection

Part A.—Steady viscous incompressible flow in a rotating coaxial cylindrical annulus with suction and injection is studied. The unsteady flow is also considered. Part B.—An exact solution for temperature distribution at different constant wall temperatures is obtained. It is assumed that the rate of injection at the inner wall equals the rate of suction at the outer wall.     

浅水亚临界航速舰船水压场计算方法研究

基于浅水波动势流理论和薄船假定,建立了浅水亚临界航速舰船水压场理论数学模型．采用有限差分方法,对浅水亚临界航速舰船水压场分布特征进行了数值计算．分析了航道岸壁、Froude数、色散效应对舰船水压场的影响,利用虚拟长度法改善了计算结果．通过与源汇分布法、Fourier积分变换法以及实验结果进行比对,验证了所建立的舰船水压场数学模型和计算方法的正确性．     

Effects of pulsatile flow and straight length on low-density lipoprotein transport in a curved arterial wall

Abnormal accumulation of macromolecules such as low-density lipoproteins (LDLs) in the arterial wall causes narrowing and blockage of vessels, which leads to atherosclerosis. Effects of pulsatile nature of blood flows as well as the initial length on transport of the LDL species in the arterial boundary layer region are analyzed numerically in the present work. The set of governing equations consisting of continuity, Navier-Stokes, and species transport is solved using a projection method based on the second-order central difference discretization. The obtained results are in excellent agreement with the pertinent data. The computational results imply that the flow field and concentration distribution are time dependent but the variation of the filtration velocity can be ignored. The LDL concentration boundary layer thickness decreases in the outer part and increases in the inner part for both with or without straight length. Presence of initial straight length generates about 26% growth in the boundary layer thickness, although its effect on the LDL surface concentration (LSC) is negligible. The maximum LSC is related to the regions with minimum wall shear stress in the inner part of the curved artery, which have more potential for formation of atherosclerosis. A new numerical correlation between the LSC and boundary layer thickness is proposed and examined.     

纤维悬浮剪切湍流中纤维旋转扩散系数的理论研究

对纤维悬浮剪切湍流中纤维旋转扩散系数进行了理论研究．首先建立了流场不同脉动速度梯度间的相关矩函数,然后推导出了纤维旋转扩散系数的表达式,该表达式依赖于特征长度、时间、速度和一个与壁面作用相关的无量纲参数．得到的纤维旋转扩散系数可以应用于非均匀和非各向同性的湍流场,此外还可以推广到三维湍流场,因而为纤维悬浮湍流场的研究提供了理论基础．