电流变体粘弹性能的实验研究

电流变体是智能材料与结构中一种重要的致动器材料。本文对电流变体在电场作用下的粘弹性特性进行了实验研究。使用旋转式流变仪对表征流体粘性的蠕变特性进行了测试。同时,对电流变体进行了强制振荡试验,测试了流体在不同应力幅值、应力频率下的剪切模量主变化,得到了电流变体的复剪切模量以及剪切存储模量、损耗模量承承中电场变化的规律。     

章动角对旋转章动充液腔体运动稳定性的影响

旋转章动充液腔体的运动稳定性，主要分两大类。当Ｒｅｙｎｏｌｄｓ数很小时，出现转速衰减不稳定性，Ｒｅｙｎｏｌｄｓ数增大到一定程度，又出现共振不稳定性。本文考虑Ｒｅｙｎｏｌｄｓ数较大时的情况，此时腔内液体的惯性波振动与腔体的章动频率耦合即可发生共振不稳定性。当章动角增大时，共振频带又发生转移。采用变形参数法讨论章动角增大时弱非线影响，很简明地得出特征值修正公式，并计算了液体力矩的变化曲线。     

Inertia effects of the fluid and cylinder in coaxial cylindrical rheometer

The inertias of the fluid and the inner cylinder in coaxial cylinder rheometer (CACR) have great influence on the unsteady flow of non-Newtonian fluid. Even for the Newtonian fluid there exist the so called “stress overshoot” phenomenon. In the present article this phenomenon was studied in detail and a method correcting the measured results for an unsteady flow in the rheometer was proposed. It is found that the inertia effect of the fluid can be ignored when the gap between cylinders is small. The project was supported by the Youth Foundation of the Fourth Military Medical University     

Slip and no-slip squeezing flow of liquid food in a wedge

Squeezing flow in a wedge simulates a number of practical processes, e.g. lubrication, coating and the sensory evaluation of liquid foods. This paper reports analytical approximate solutions for both slip (or lubricated) and no-slip squeezing flow of liquid food in a wedge, in which the power law fluid model was used. The solutions do not seem to be more complex than that for squeezing flow between two parallel disks and may be used as a variation of the psycho-physical models of in-mouth viscosity and food spreadability. Alternatively, a rheometer for testing liquid foods may be developed according to the equations described in this paper.     

A domain perturbation study of steady flow in a cone-and-plate rheometer of non-ideal geometry

The method of domain perturbation developed by Joseph is used to calculate velocity and stress profiles in a slightly misaligned cone-and-plate rheometer where the cone is spinning and the plate is stationary. Results for a Newtonian fluid, a Criminale-Ericksen-Filbey fluid, an upper-convected Maxwell fluid, and a White-Metzner fluid are presented and compared with earlier results in which the cone is stationary and the plate is spinning (Dudgeon and Wedgewood, 1993). Streamlines calculated for the Newtonian fluid show a very small recirculation region near the stationary plate. Velocity and stress contours are symmetric around the plane of largest gap width. For the elastic fluids studied, streamlines are asymmetric. The fluid response lags where the fluid is dominated by memory effects. Much larger recirculation regions are calculated for fluids dominated by shear thinning. These recirculation regions contain a large fraction of the fluid in the apparatus and have the effect of changing the shape of the flow domain for the remaining fluid that rotates around the cone's axis. Elasticity also has a pronounced effect on the stress profile, indicating that the accuracy of the cone and plate may be compromised even for small mis-alignments.     

An experimental investigation of upstream flow characteristics of viscoelastic fluids in an annular die entry

Flow behaviour of viscoelastic polymer solutions on the upstream side of an annular die entry has been experimentally investigated and compared with published results on entry flow in circular die. Stable and unstable flow patterns were observed depending on the magnitudes of Reynolds and elasticity numbers. The latter number represents the relative effects of elastic and inertial forces. The stable flow region consists of an elasticity-controlled vortex growth regime and an inertia-controlled divergent flow regime. These two flow regimes have also been observed in circular entry flow. The criteria for the onset of flow instability in an annulus, based on the maximum vortex size, agree qualitatively with various criteria proposed for polymer metls and solutions in circular entry flow. The unstable flow patterns revealed a two-stage instability with a metastable region in between. The first-stage instability is characterised by low frequency disruption of the stationary vortex; while the second-stage instability, which occurs at high Reynolds numbers, is characterised by high frequency random distortion of the flow field.     

Surface tension driven jet break up of strain-hardening polymer solutions

Experimental studies attempting to ascertain the influence of viscoelasticity on the atomization of polymer solution are often hindered by the inability to decouple the effect of shear thinning from the effect of extensional hardening. Here, the influence of viscoelasticity on the jet break up of a series of non-shear-thinning viscoelastic fluids is quantified. Previous characterization using an opposed-nozzle rheometer identified the critical extensional rates for strain hardening of these model fluids. The strain hardening fluids exhibit a beads-on-string structure with reduction or elimination of satellite drops. Capillary instabilities grow on the filaments connecting the spheres and eventually break the filaments up into a string of very small drops about one order of magnitude smaller than the satellite drops formed by a Newtonian fluid with the same shear viscosity, surface tension, and density. These results confirm that strain hardening is the key rheological property in jet break up and that the critical extensional rate of a fluid is pertinent in determining the final characteristics of break up. Results suggest that the opposed-nozzle rheometer does probe extensional behavior in the range of extensional rates that are relevant to jet break up, providing a tool to roughly predict jet break up.     

Stability analysis in spatial mode for channel flow of fiber suspensions

Different from previous temporal evolution assumption, the spatially growing mode was employed to analyze the linear stability for the channel flow of fiber suspensions. The stability equation applicable to fiber suspensions was established and solutions for a wide range of Reynolds number and angular frequency were given numerically . The results show that, the flow instability is governed by a parameter H which represents a ratio between the axial stretching resistance of fiber and the inertial force of the fluid. An increase of H leads to a raise of the critical Reynolds number, a decrease of corresponding wave number, a slowdown of the decreasing of phase velocity , a growth of the spatial attenuation rate and a diminishment of the peak value of disturbance velocity. Although the unstable region is reduced on the whole, long wave disturbances are susceptible to fibers.     

A new method for direct simulations of flexible filament suspensions in non‐zero Reynolds number flows

A new method for direct simulations of flexible filament suspensions in a non‐zero Reynolds number flow is developed. For fluid domain, simulations are based on a lattice Boltzmann equation. For solid domain, a slender solid body is discretized into a chain of consecutive spherical segments contacting each other. A constraint force algorithm is proposed to warrant constant bonding distance between two neighbouring segments and non‐slip velocity conditions at the contacting points so that the flexible filament moves and rotates as a whole body. The fibre could be bent and twisted in the model. Non‐linear inertial interactions between fluid and flexible filament can be naturally studied by using this model embedded in the lattice Boltzmann scheme. The present flexible fibre method is tested by using a rigid particle method when the fibre stiffness is very large and by comparing the present results with theoretical and experimental results. It is demonstrated that the present results have a reasonable accuracy and that the computational results are consistent with the existed experimental results at non‐zero Reynolds number flows. Copyright © 2006 John Wiley & Sons, Ltd.     

弹性湍流及其对微混合效率影响研究的若干进展

介绍和评述弹性湍流的产生及其对于微混合效率的影响等问题上的若干研究进展. 弹性 湍流和惯性湍流具有类似的流场特征,但引发机理有所不同. 惯性湍流产生的原因 是惯性引起的Reynolds应力,而弹性湍流则是由弹性应力所引起的. 鉴于在微流动 中,惯性力可忽略不计,因此牛顿流体的混合变得十分困难. 此时可在流体中加入 微量高分子聚合物以生成黏弹性流体. 由黏弹性流体所引发的弹性湍流在提高流体 微混合的效率上可发挥重要作用.     

Flow Characteristics of a “Multiconfigurational”, Shear Thinning Viscoelastic Fluid with Particular Reference to the Orthogonal Rheometer

This paper deals with the flow characteristics of a class of nonsimple viscoelastic fluid models developed by Rajagopal and Srinivasa (1999). The central feature of these models is that the stress response is lastic from a changing natural configuration with the viscous dissipation occurring due to changes in the natural state. The class of models considered are characterized by three independent parameters that represent respectively the elasticity, the viscosity and the shear thinning index. The stress relaxation response of the material is compared with experimental data reported by Bower et al. (1987) for polyisobutelene in cetane, and parameters that fit the data are calculated. The flow of such a fluid between parallel disks rotating about noncoincident axes (the orthogonal rheometer) is then studied. It is shown that the assumed velocity field leads to a system of second-order nonlinear ordinary differential equations (Rajagopal, 1982). A parametric study is then undertaken to see the effect of the various material, geometrical, and flow parameters on the flow characteristics. It is observed that inertial effects and shear thinning effects are roughly complementary in the range of parameters considered. While it is well known that boundary layers occur in these flows due to inertial effects, it is demonstrated that these boundary effects are insensitive to the Reynolds number but rather are determined by the absorption number. Finally, in the range of parameters that are commonly observed in such rheometers, it is shown that neglect of inertia causes significant discrepancies in the calculation of the boundary shear rates. Received 3 June 1999 and accepted 2 October 1999     

A Semi-empirical Correlation for Pressure Drop in Packed Beds of Spherical Particles

The effect of a confining wall on the pressure drop of fluid flow through packed beds of spherical particles with small bed-to-particle diameter ratios was investigated to develop an improved pressure drop correlation. The dependency of pressure loss on both wall friction and increased porosity near the wall was accounted for by using a theoretical approach. A semi-empirical model was created based upon the capillary-orifice model, which included a wall correction factor for the inertial pressure loss. In this model, packed beds were treated as a bundle of capillary tubes whose orifice diameter in the core region was different from that of the wall region. Using this model, a new pressure drop correlation was obtained, based on the Ergun equation and applicable for a wide range of Reynolds numbers (10⁻²–10³). The proposed correlation was compared with previous correlations, as well as with experimental data. This correlation showed close agreement with the experimental data for both low- and high-Reynolds number regimes and for a wide range of bed-to-particle diameter ratios. The ratio of the pressure drop in finite packing to that in homogeneous packing was then calculated. This ratio clearly shows how the wall effect depends on the Reynolds number and the bed-to-particle diameter ratio.     

Flow of viscoelastic fluids between plates rotating about distinct axes

We discuss the flow of BKZ fluids in an orthogonal rheometer. Some analytical results are proved, and numerical solutions are obtained for the Currie model. These solutions show a boundary layer behavior at high Reynolds numbers and the possibility of discontinuous solutions or nonexistence at high Weissenberg numbers.     

黏弹性流体纯弹性不稳定现象研究综述

近年来粘弹性流体流动的弹性不稳定性现象引起了越来越多学者的关注与研究,与牛顿流体惯性不稳定现象不同,这种现象是由粘弹性流体流动中的弹性应力和粘性力之间相互作用,使得在较低的雷诺数下即可产生复杂的流动分岔不稳定现象。当流动中的弹性数（表现为 Deborah 数 De 与Reynolds 数Re 的比值,其中 De 数定义为粘弹性流体的松弛时间和流动的特征时间的比值,Re 数表征流动中惯性力与粘性力之比）较大时,在 Re<相似文献   

Numerical simulation of non-Darcian flow through a porous medium

This work reports on fluid flow in a fluid-saturated porous medium, accounting for the boundary and inertial effects in the momentum equation. The flow is simulated by Brinkman-Forchheimer-extended Darcy formulation (DFB), using MAC (Marker And Cell) and Chorin pressure iteration method. The method is validated by comparison with analytic results. The effect of Reynolds number, Darcy number, porosity and viscosity ratio on velocity is investigated. As a result, it is found that Darcy number has a decisive influence on pressure as well as velocity, and the effect of viscosity ratio on velocity is very strong given the Darcy number. Additional key findings include unreasonable choice of effective viscosity can involve loss of important physical information.     

Problems of non-uniqueness when interpreting the effect of fluid inertia on the complex viscosity function

A theoretical investigation is carried out into the interpretation of the effect of fluid inertia on the complex viscosity function as measured on a controlled stress rheometer. The problem of non-unique solutions to the governing equations is considered for the parallel plate geometry. The locations of these solutions are investigated by considering the critical points of the complex mapping associated with the linear viscoelastic equations of motion. It is shown that these critical points play an important role in determining where convergence problems are likely to occur when applying numerical methods of solution to the governing equations. Analytical approximations based on a series expansion about a critical point are developed as an alternative approach to a numerical solution in the neighbourhood of a critical point. In order to verify the theoretical predictions a numerical simulation of the behaviour of a single element Maxwell fluid on a controlled stress rheometer is carried out for a parallel plate geometry. Received: 27 July 1998 Accepted: 9 April 1999     

Numerical investigations on vortical structures of viscous film flows along periodic rectangular corrugations

Two-dimensional gravity-driven film flows along a substrate with rectangular corrugations are studied numerically by using Finite Volume Method. The volume of fluid (VOF) method is utilized to capture the evolution of free surfaces. The film flows down an inclined plate are simulated to validate the numerical implementation of the present study. Results obtained indicate that the phase shift between the surface wave and the wall corrugation increases as the Reynolds number. The parametric studies on the interesting resonant phenomenon indicate that the peak Reynolds numbers increase as the raise of the wall depth or the decline of the inclination angle. The dependence of the flow fields is analyzed on the Reynolds numbers and wall depth in details. It is found that the vortical structures in the steady flows, either produced by the interaction between capillary wrinkling and inertia, or by the rectangular geometry, are closely related to the remarkable deformation of the free surfaces. This conclusion is also confirmed by the transient flow development of two typical simulations, i.e., flows in capillary–inertial regime and in inertial regime.     

Very large‐scale computation of very small‐scale motions in a fully developed pipe flow

Local isotropy theory is examined using direct numerical simulation in a fully developed pipe flow at two Reynolds numbers Re_τ=1285.6 and 684.8. The approach to local isotropy is assessed with reference to the two Kolmogorov classical equations for longitudinal and transverse velocity structure functions. The results for the second‐order longitudinal structure functions in both the dissipative and inertial ranges indicate an improved agreement with the local isotropy hypothesis as the centreline is approached. However, the transverse structure functions satisfy isotropy neither in the dissipative range or in the inertial range. The distribution of the longitudinal and transverse structure functions also shows a substantial Reynolds number dependance in the logarithmic region of the flow and beyond. The results for the third‐order longitudinal structure function demonstrate an increased Reynolds number influence, and a deteriorating tendency to local isotropy for large separations. Contour images of axial velocity differences in the dissipative and inertial ranges have exhibited interesting patterns in relation to those of the instantaneous axial velocity. Finally, the results obtained in this investigation are in very good agreement with other published experimental and numerical data on channel and duct flows. Copyright © 2010 John Wiley & Sons, Ltd.     

串列布置三圆柱涡激振动频谱特性研究

对串列三圆柱体双自由度涡激振动问题进行了数值计算, 并分析了雷诺数、固有频率比和约化速度对串列三圆柱体结构动力响应及频谱特性的影响. 研究发现: 雷诺数、频率比对上游圆柱的振幅和流体力系数的影响较小. 中游圆柱频率锁定区域随着雷诺数的增大而增大, 其动力响应受上游圆柱尾流的影响较大, 但频率比的影响较小. 同时, 流体力系数在约化速度较小时受雷诺数和频率比的影响较大. 另外, 下游圆柱的振幅和流体力系数受雷诺数及频率比的影响较大. 雷诺数、频率比和约化速度对圆柱流体力系数能量谱密度(PSD)曲线中主峰幅值、频谱成分及波动性的影响较大. 流体力系数PSD曲线波动性的增强, 导致圆柱运动轨迹会从"8"字形转变成不规则形状. 当频率比为2.0时, 上游圆柱尾流出现P$+$S模式, 导致其发生非对称运动, 且升、阻力系数PSD曲线主峰重合. 最后, 激励荷载平均功率值随约化速度的变化趋势与对应的结构动力响应的变化类似. 在同一约化速度区间内, 结构振动响应的强弱与位移的平均功率值成正比. 对不同约化速度区间内的升力系数功率谱密度分析时, 振动频率比($f_{s}/f_{n, y})$对结构振动响应的影响更大.