管道内均匀与非均匀PEO溶液湍流减阻特性研究

在自制重力式循环水槽中(最高试验Re数可达105),测试分析了管道内均匀与非均匀聚氧化乙烯(PEO)溶液的湍流减阻规律。结果表明,均匀PEO溶液减阻率(DR)随Re数和浓度增加而提高(最大减阻率70%),但浓度大于临界浓度(100p.p.m)后DR不再增加;而相同浓度均匀PEO溶液的减阻效果随时间逐渐减弱,且Re数越大,剪切破坏越严重,DR下降越明显。与均匀PEO溶液不同,非均匀PEO溶液(从管壁狭缝向内喷射PEO溶液所得)不受剪切破坏影响,减阻效果更稳定,且药品使用量小;同时通过增大喷射速率、Re数和喷射液浓度,可提高减阻率(最大减阻率50%)。对比可见,喷射减阻方式仅需消耗少量药剂,就能实现显著、稳定减阻,有望用于水下航行体湍流减阻。     

介绍低雷诺数流体动力强干扰理论的一种新方法--多极子法

引言在化工、大气物理,特别是生物力学等领域中,时常需要讨论低雷诺数下在流体中悬浮粒子的运动和扩散。低雷诺数流动的一个显著特点是流场中的扰动衰减慢,因而传播得远,所以一般不 ...     

明渠中悬移质的弥散-对流方程及悬浮机理

悬移质泥沙通常构成冲积河流总输沙量的主体, 研究悬移质的悬浮机理具有重要的意义. 以双流体模型为基础, 通过引入弥散速度的概念, 建立了悬移质泥沙的输沙方程以及泥沙扩散系数的本构关系. 应用该方程分析了二维明渠均匀流中悬移质泥沙浓度垂向分布规律, 并与Einstein 和Chien 的泥沙浓度实验资料及经典扩散理论进行了对比. 以此为基础, 分析了紊动扩散、颗粒自身的紊动、颗粒碰撞应力对泥沙悬浮的影响在垂向上的变化, 以及浓度、粒径等对这些因素的影响. 结果表明, 泥沙颗粒在明渠紊流中的扩散是浑水的紊动扩散、颗粒自身的紊动、颗粒碰撞应力3 部分不同机制共同作用的结果, 把泥沙颗粒的悬浮简单归因于水流的紊动是不全面的.     

边界拟合坐标系下的差分有限元破开算子法

在边界拟合坐标系下给出耦合了有限差分法和有限单元法的新型的破开算子法.利用该方法,Navier-Stokes方程被分解成对流方程和扩散方程,对流方程采用稳定性好的有限差分法求解,而扩散方程则采用有限单元法求解.由于计算是基于非均匀网格,采用边界加密的51×51网格就达到了前人在计算雷诺数为5000的方腔环流时采用的257×257均匀网格的效果,对于瑞利数Rt=107的竖腔自然对流的计算进一步表明,提出的方法是有效的.对于Re=105的高雷诺数方腔受驱环流,得到了非稳态、非周期的和带有随机特征的流场结构.     

液固流动理论的进展

1.引言研究液固悬浮体的意义在于应用这种研究成果,通过长距离管道输送悬浮体来输送固体颗粒,并使动力消耗合理、单价较低.与气体输送即气力输送相比,在相似的温度以及有效的工作压力下,同样的固体颗粒最小悬浮速度以及压降在液体中比在气体中要低得多。这      

粉尘云激波点火现象实验研究

利用新型两相激波管成功地研究了中等强度入射激波诱导的均匀悬浮粉尘间断面点火现象。平均直径为１０ｍ的玉米粉尘由流化床均匀地分散在氧气或空气中。悬浮粉尘间断面激波点火过程由纹影法连续拍摄。结果表明：颗粒浓度对点火延迟时间影响不大,空气中的玉米粉尘点火延迟大于氧气中的值,低马赫数激波条件下尤为明显。     

Ag2S纳米粒子原位合成以增强PAI涂层机械和摩擦学性能

借助单源前驱体热分解在聚酰胺酰亚胺(PAI)涂层中原位合成了硫化银(Ag₂S)纳米粒子,并通过调节单源前驱体的含量进一步调控纳米粒子尺寸. 采用X射线衍射仪和高分辨场发射扫描电镜对原位合成Ag₂S纳米粒子的物相结构、形貌、尺寸和尺寸分布进行了表征和分析;详细研究了Ag₂S纳米粒子对PAI涂层机械性能和摩擦学性能的影响;对其增强机制进行了探讨. 结果表明:PAI涂层中原位合成的Ag₂S纳米粒子粒径较小而且分散均匀,且调节单源前驱体能有效调控Ag₂S纳米粒子的尺寸和尺寸分布. Ag₂S纳米粒子的原位引入(优化质量分数为5.0%)有效改善了PAI涂层的机械性能和摩擦学性能,其摩擦学性能的增强归因于机械强度的提高和诱导转移膜的形成.      

多自由度静电悬浮系统几何对称性迭代调节方法

静电悬浮系统在重力卫星和天基减振领域有重要的应用,当悬浮质量块或悬浮平台被调节至系统电极笼的对称中心时能使悬浮系统的线性度达到最优,进而获得优良的传感或控制特性。首先,建立了多自由度静电悬浮系统模型,介绍了单自由度静电悬浮系统的对称性调节方法,并分析了将其应用于多自由度静电悬浮系统时由于力耦合造成的失准问题。然后,提出一种基于迭代调节的多自由度静电悬浮系统几何对称性逼近方法,通过迭代逼近克服了多自由度耦合所引起的误差,并给出了迭代调节方法的收敛性分析。最后,基于Matlab建立了多自由度静电悬浮系统仿真模型,对调节方法进行了仿真验证,并采用实际静电悬浮系统进行了实验验证。仿真与实验结果均表明,所提方法可以在有限步迭代下将悬浮体调节至电极笼的中心位置,误差小于标称间隙的0.1%。     

Reynolds数对大攻角细长旋成体绕流滚转角效应影响的数值研究

使用低耗散的Roe格式,数值模拟了Reynolds数(Re)对大攻角细长旋成体绕流滚转角效应的影响.模型头部加了几何小扰动块以引发流场的不对称.在较大的Re数(Re=10 5)下,本文的计算结果与实验是相符的,此时细长体的滚转会导致双稳态、双周期现象,即侧向力随滚转角呈现类似方波形式的双周期变化,方波中侧向力基本保持不变的状态对应于流场的正则态,且两个正则态的侧向力方向相反,方波中侧向力基本保持不变的状态对应于流场的正则态,且两个正则态的侧向力方向相反;而在较小的Re数(Re=4 000)下,如果扰动足够大,细长体的滚转将导致不同的双稳态现象,此时两个正则态的侧向力方向相同,而在较小扰动下双稳态现象不再出现;Re数更小时(Re=1 000),即使在较大的扰动下,双稳态现象也不再出现,侧向力随滚动角仍是连续变化的.本文的计算结果表明,Re数越小,流场对头部扰动的感受性越弱.     

小型可调Re数弹道靶上的跨声速实验

本文介绍了一种可调 Re 数小型弹道靶设备;讨论了该设备实验中的一些问题.由此开展了两种尖头细长旋成体不同 Re 数下的跨声速实验,并给出了部分流场显示结果.     

Überlegungen zur Scherviskosität von Suspensionen aufgrund einer Dimensionsanalyse

Dimensional analysis of the motion of solid particles suspended in a fluid phase shows that the macroscopic relative shear viscosity of suspensions generally depends not only on the volume concentration and particle shape but also on two Reynolds numbers and a dimensionless sedimentation number. These dimensionless numbers are formed using parameters characterizing the structure and motion of the suspension at the microscopic level. The analysis was based on the assumptions that the dispersed particles are rigid and sufficiently large that Brownian motion may be neglected, that the continuous fluid phase is Newtonian and that the interactions between particles and between particles and fluid phase are only hydrodynamic. The Reynolds numbers describe the influence of the inertial forces at the microscopic level, and the sedimentation number the influence of gravity. The dimensionless numbers can be neglected if their values are much smaller than one. For each of the dimensionless numbers both the shear rate and the particle size influence the shear viscosity. Thus sedimentation number is large for low shear rates, whereas the Reynolds numbers are large for high shear rates. The viscosity function for one suspension can be transformed into the viscosity function for another suspension with geometrically similar particles but of a different size. The scale-up rules are derived from the requirement that the relevant dimensionless numbers must be constant. The influence of non-hydrodynamic effects at the microscopic level on the shear viscosity can be detected by deviations from the derived scale-up rules.     

Pipe flow of suspensions in turbulent fluid

The general case of a fully developed pipe flow of a suspension in a turbulent fluid with electrically charged particles or with significant gravity effect, or both, and for any inclination of the pipe with the direction of gravity, is formulated. Parameters defining the state of motion are: pipe flow Reynolds number, Froude number, electro diffusion number, diffusion response number, momentum transfer number and particle Knudsen number. Comparison with experimental results is made for both gas-solid and liquid-solid suspensions. It is shown that the gravity effect becomes significant in the case of large pipe diameters and large particle concentrations.     

A review of research on nanoparticulate flow undergoing coagulation

Nanoparticulate flows occur in a wide range of natural phenomena and engineering applications and, hence,have attracted much attention. The purpose of the present paper is to provide a review of the research conducted over the last decade. The research covered relates to the Brownian coagulation of monodisperse and polydisperse particles, the Taylor-series expansion method of moment, and nanoparticle distributions due to coagulation in pipe and channel flow,jet flow, and the mixing layer and in the process of flame synthesis and deposition.     

Pipe flow of suspensions

This study shows that fully developed pipe flow of a particulate suspension is defined by four dimensionless parameters of particle-fluid interactions in addition to the Reynolds number. Effects accounted for include the Magnus effect due to fluid shear, electrostatic repulsion due to electric charges on the particles, and Brownian or turbulent diffusion. In the case of a laminar liquid-solid suspension, electrostatic effect is negligible but shear effect is prominent. Solution of the basic equations gives the density distribution of particles with a peak at the center (Einstein, Jeffery) or at other radii between the center and the pipe wall (Segré et al) depending on the magnitudes of the various flow parameters. In the case of a turbulent gas-solid suspension, the Magnus effect is significant only within the thickness of the laminar sublayer. However, charges induced on the particles by the impact of particles at the wall produce a higher density at the wall than at the center of the pipe. The velocity distribution of particles is characterized by a slip velocity at the wall and a lag in velocity in the core from the fluid phase. These results are verified by earlier measurements.     

Pipe flow of suspensions

This study shows that fully developed pipe flow of a particulate suspension is defined by four dimensionless parameters of particle-fluid interactions in addition to the Reynolds number. Effects accounted for include the Magnus effect due to fluid shear, electrostatic repulsion due to electric charges on the particles, and Brownian or turbulent diffusion. In the case of a laminar liquid-solid suspension, electrostatic effect is negligible but shear effect is prominent. Solution of the basic equations gives the density distribution of particles with a peak at the center (Einstein, Jeffery) or at other radii between the center and the pipe wall (Segré et al) depending on the magnitudes of the various flow parameters. In the case of a turbulent gas-solid suspension, the Magnus effect is significant only within the thickness of the laminar sublayer. However, charges induced on the particles by the impact of particles at the wall produce a higher density at the wall than at the center of the pipe. The velocity distribution of particles is characterized by a slip velocity at the wall and a lag in velocity in the core from the fluid phase. These results are verified by earlier measurements.     

Influence of induced magnetic field on the peristaltic flow of nanofluid

This article investigates the effects of an induced magnetic field on the mixed convection peristaltic motion of nanofluid in a vertical channel. Transport equations involve the combined effects of Brownian motion and thermophoretic diffusion of nanoparticles. Analysis has been addressed subject to long wavelength and low Reynolds number assumptions. Explicit expressions of stream function, magnetic force function, temperature and nanoparticles concentration are developed. Analytic expressions are validated with the obtained numerical solutions. Peristaltic pumping rate is found to increase upon increasing the strengths of electric and magnetic fields and the buoyancy force due to temperature gradient. Moreover temperature rises and nanoparticles concentration decreases with an intensification in the Brownian motion effect.     

Review of chaos in the dynamics and rheology of suspensions of orientable particles in simple shear flow subject to an external periodic force

We review results obtained over a period of about a decade on a class of technologically and fundamentally important problems in suspension rheology viz., the dynamics and rheology of dipolar suspensions of orientable particles in simple shear flow. The areas explored in this review include effects such as the fluid flow field, external forcing, Brownian diffusion, hydrodynamic interactions and their impact on the rheological properties of the suspension. The main feature of the presentation is the use of a uniform framework in which one or more of the above effects can be studied, based on Langevin type equations for particle orientations combined with a brute-force technique for computing orientational averages. These models are capable of capturing complex dynamical behaviour in the system such as the presence of subharmonics or chaos, both in the dynamics and rheology. The tools developed allow for investigating how chaos in the system is affected by Brownian diffusion and hydrodynamic interactions. The presence of chaos opens up a number of novel possibilities for dynamical and rheological behaviour of the system, which can be put to efficient use in many ways, e.g. in separating particles by aspect ratio and possibly developing computer controlled intelligent rheology. The results also have implications for certain areas of chaos theory, such as a new intermittency route to chaos and the possibility of non-trivial collective behaviour in spatially extended systems. These studies highlight certain deficiencies in current techniques in the literature for handling the rheology of dilute and semi-dilute suspensions. In the presence of Brownian motion the proposed method computes the averages by simulating a set of deterministic ordinary differential equations rather than stochastic differential equations. The systems considered may also serve as a paradigm for analysing how microscopic chaotic fluctuations in spatially extended systems affect macroscopic averages. We also attempt to put our results into context with respect to recent work on rheochaos in complex fluids such as liquid crystals and nematic polymers.     

Asymptotics for filtration of polydisperse suspension with small impurities

A model for deep bed filtration of a polydisperse suspension with small impurities in a porous medium is considered.Different suspended particles move with the same velocity as the carrier water and get blocked in the pore throats due to the size-exclusion mechanism of particle retention.A solution of the model in the form of a traveling wave is obtained.The global exact solution for a multiparticle filtration with one high concentration and several low concentrations of suspended particles is obtained in an explicit form.The analytic solutions for a bidisperse suspension with large and small particles are constructed.The profiles of the retained small particles change monotony with time.The global asymptotics for the filtration of a polydisperse suspension with small kinetic rates is constructed in the whole filtration zone.     

Spectrum of density fluctuations in a particlefluid system—II. Polydisperse spheres

This paper presents an extension of the analysis shown in Part I to a polydisperse particle-fluid system. The density autocorrelation is shown to be a function of two quantities, a generalized Overlap function for which an analytical expression is derived, and the radial distribution function (RDF). In Fourier transform space, the density spectrum again appears to be a strong function of the mean particle size, and secondarily the mean particle separation distance. One unusual result is previously observed oscillations in the density spectrum of a monodisperse system of particles are severely dampened or even eliminated in the polydisperse case, depending on the width of the particle size distribution. Apparently contributions from different particle correlations interfere with each other, thereby reducing the coherent oscillations seen in the monodisperse particle-fluid system. Furthermore at large wavenumbers, the spectrum decays with a −2 power-law, independent of the shape of the particle size distribution. This behavior can be traced to the Overlap function which controls the behavior of the spectrum beyond the first peak. Remarkably the −2 power-law spectrum is determined by the shape of the particles (i.e. spheres) rather than their spatial distribution (RDF).

The effect of an asymptotically large pressure gradient on the correlation of several important higher-order moments is revisited for the polydisperse system. The relatively simple relationships developed for the monodisperse system are lost in the polydisperse case because particles of different sizes will be influenced differently by an applied pressure gradient. The result is moments that are of different order in velocity can no longer be related to each other (as they were in the monodisperse system), even in this idealized flow. A more comprehensive understanding of this phenomenon can only be achieved through direct numerical simulation or experiment.     

Effect of particle polydispersity on micromechanical properties and energy dissipation in granular mixtures

A series of numerical tests was conducted to study the micromechanical properties and energy dissipation in polydisperse assemblies of spherical particles subjected to uniaxial compression. In general, distributed particle size assemblies with standard deviations ranging from 0% to 80% of the particle mean diameter were examined. The microscale analyses included the trace of the fabric tensor, magnitude and orien- tation of the contact forces, trace of stress, number of contacts and degree of mobilization of friction in contacts between particles. In polydisperse samples, the average coordination numbers were lower than in monodisperse assemblies, and the mobilization of friction was higher than in monodisperse assemblies due to the non-uniform spatial rearrangement of spheres in the samples and the smaller displacements of the particles. The effect of particle size heterogeneity on both the energy density and energy dissipation in systems was also investigated.