磁流变液在圆筒间的粘塑性流动

磁流变液具有粘性和粘塑性流体特性.基于Navier-Stokes方程,分析了外加磁场对在两圆筒间隙间的磁流变液粘塑性流动的影响,得到了速度和流量表达式,为圆筒式磁流变器件的设计提供了理论依据.研究结果表明:速度沿磁场方向呈抛物线和直线分布;流量可由外加磁场连续调控.     

A hybrid finite element/method of moment formulation for singlefrequency eddy current inversion

Eddy-current inverse techniques using single-frequency currents have been applied with limited success to the reconstruction of crack width and thickness profiles primarily for one-dimensional and axisymmetric geometries. Because of the diffusive nature of the induced low-frequency eddy currents, the reconstruction process differs from high-frequency wave propagation methods. On the physical basis that both diffusive and wave phenomena can be described by the same Green's function with either a complex or real wave number, an integral formulation for the low-frequency magnetic vector potential is presented. By employing an iterative Born approximation algorithm and the method of moments, a reconstruction method for the conductivity profile in a metallic specimen is developed. To make this formulation amenable to complex geometries, finite-element analysis techniques are utilized to compute the integral kernel. The inversion process is tested with synthetic data generated by the numerical solution of a generic embedded flaw in a full-space and a surface-breaking defect     

聚合物气辅共挤成型中挤出胀大的数值模拟

以一矩形截面共挤型材为例,采用Giesekus本构方程和Navier滑移模型建立数值模型,使用EVSS、SU等有限元方法对气辅共挤和传统共挤时两种聚合物熔体在口模内外的等温粘弹流动做了三维数值模拟,得到了气辅共挤和传统共挤时的挤出胀大率、速度场、应力场及剪切速率分布。对模拟结果进行了分析和对比,结果表明,气辅共挤能消除挤出胀大和模外熔体偏转流动现象;气辅共挤时两相熔体的速度场均匀一致,熔体流动稳定,呈柱塞状挤出;熔体表面的切向和法向应力为零,因而可有效提高挤出速率,并防止制品表面"鲨鱼皮"现象的出现。     

磁流变液的流体动力学理论

在外加磁场作用下,磁流变液从牛顿流体变成了Bingham体,超过屈服应力开始流动,其的流变性(弹性、塑性、粘性)、磁化性、导电性、传热性以及其它的机械性质和物理学性质皆发生显著的改变.研究磁流变液在外加磁场作用下,流场分布规律随磁场强度变化的动态特性,建立磁流变液的流体动力学理论,对开发和设计磁流变器械至关重要.本文运用物理学和流体力学的基本理论,结合本构方程,考虑磁场对磁流变液的流动的影响,建立了磁流变液力磁耦合的流体动力学模型,给出了描述磁流变液流动的基本方程组.     

关于磁性液体研究中的一些问题

一种性能优良的磁流体,应当是超微磁性粒子分散在基液中,有着一定浓度的稳定的悬浮体。但是就这类材料本身的一些问题,相对来看,却很少引起人们的关注。表面活性剂通常为这种胶体提供稳定性。即使采用非磁性超微粒子,也很难制成长期稳定的悬浮体,因为,在不同环境中,存在着聚集作用、蒸发和化学变化。磁流体中的超微磁性粒子因氧化问题,使它不可能具有较高的磁化强度;一些最有意义的基液正期待着开发相应的表面活性剂,而那些最常用的液体,却应当选择不易被氧化和不易被水解的表面活性剂。最理想的表面活性剂,则是那些永久地附着在粒子界面上的表面活性剂。关于聚集作用,主要问题是往往被人们忽视了。一些简便易行的定量检测和清除聚集体的方法,将是最受欢迎的。     

Design of a magnetic flowmeter for conductive fluids

The operation of a magnetic flowmeter for conductive fluids is analyzed and the magnetic, and electric fields in it are described as solutions of boundary value problems. Finite-element methods are utilized to solve the problems, and solutions are presented for a variety of flow profiles and inner tube conductivities. The method gives an insight into both the phenomenon involved and the effects of the geometry and the material characteristics on the accuracy and sensitivity of the flowmeter     

基于磁流体独有特性的各种潜在传感器

详细分析磁流体所具有的磁通门原理、粘度智能性、液体流动性、可浸泡性、磁光效应等各种独有特性,探讨基于上述特性的潜在传感机理及应用方向,将上述特性单独或组合使用,将可以开发出各类新型磁流体传感器,具有用于倾斜、速度、加速度、体积、流量、非磁性体或磁流体密度、磁场以及磁流体磁化强度等传感的潜在可能性,对磁流体传感器研究具有指导意义.     

Non-linear radiation effects on magnetic/non-magnetic nanoparticles with different base fluids over a flat plate

This work investigates the two-dimensional steady convective boundary layer flow and heat transfer of Newtonian/non-Newtonian base fluids with magnetic/non-magnetic nanoparticles over a flat plate which incorporates non-linear thermal radiation and slip effects. We considered magnetite and aluminium oxide as magnetic and non-magnetic nanoparticles suspending inside the two sorts of base fluids specifically Water and Sodium Alginate. For physical significance we analyzed the behavior on non-Newtonian profiles by employing Casson model individually. The particular intrigue lies in looking the impacts of non-linear thermal radiation on the behavior of the flow. The solution of wide class of boundary value problems are facilitated by the change of the partial differential equations administering the flow utilizing similarity transformations into ordinary differential equations. The ODE’s are numerically handled by applying fourth order Runge-Kutta integration scheme in association with shooting procedure. The novel results for the dimensionless velocity and temperature inside the boundary layer are exhibited graphically for various parameters that describe the flow. A graphical demonstration is given for the skin friction coefficient and the local Nusselt number.     

加工流场中高聚物复杂流体内部结构的唯象定律

讨论了高聚物复杂流体的内部结构张量,热力学状态空间,自由能函数和非平衡热力学唯象定律,综述了唯象定律在处于流场力作用下的高聚物熔体的大分子构象结构,两相共混物的相结构,纤维充高聚物复合体系的纤维取向结构的流变方程的应用,指出了一条建立流场中高聚物复杂结构流变方程的途径。     

Transversely Oscillating MEMS Viscometer: The “Spider”

The analysis of a new viscometer that takes the form of an oscillating plate, fabricated from silicon using the methods of micro-electro-mechanical-systems (MEMS) is considered. The instrument is designed principally for experimental use in the oil industry. The plate is 1.6 mm wide, 2.4 mm long, and 20μm thick. It is suspended from a 0.4 mm thick support by 48 square cross-section legs, each of length 0.5 mm width and depth of 20μm. The process of lithography is used to deposit layers atop the silicon. These layers can then be formed into resistors and metallic tracks. The tracks traverse the supporting legs to provide connections between the plate and external electronics. The oscillating plate is a mechanical element that can be set in motion by the force produced by the interaction between an electric current flowing in the plate and an externally applied magnetic field. The viscometer can be operated either in forced or transient mode and is intended for use in both Newtonian and non-Newtonian fluids. The motion of the viscometer is analyzed for incompressible fluids, using the Navier–Stokes equations to model the flow for both a Newtonian viscous fluid and a viscoelastic fluid where the stress is modeled by a reduced form of Maxwell’s equations.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Nanocomposite magneto-rheological fluids with uniformly dispersed Fe nanoparticles

A systematic study of the magnetic and rheological properties of magneto-rheological (MR) fluids containing micron-size and nano-size iron particles is presented. The MR fluids were prepared with hydraulic oil as the carrier liquid and lecithin as an effective surfactant medium that promotes uniform particle dispersion. Magnetic measurements on micron-, hybrid-(nano + micron), and nano-MR fluids clearly indicate that the partial replacement of the micro-size particles by nanoparticles results in a better suspension and robust chain formation under applied external magnetic fields. For nano-MR fluids, the measured yield stress was found to be lower than micron-MR fluids. However, better flow properties and sharper magnetic switching make nanoparticle-based MR fluids appealing for microfluidics device applications where higher yield stress is not required.     

Nonaxisymmetric unsteady motion over a rotating disk in the presence of free convection and magnetic field

The nonaxisymmetric unsteady motion produced by a buoyancy-induced cross-flow of an electrically conducting fluid over an infinite rotating disk in a vertical plane and in the presence of an applied magnetic field normal to the disk has been studied. Both constant wall and constant heat flux conditions have been considered. It has been found that if the angular velocity of the disk and the applied magnetic field squared vary inversely as a linear function of time (i.e. as (1−λt*)⁻¹, the governing Navier-Stokes equation and the energy equation admit a locally self-similar solution. The resulting set of ordinary differential equations has been solved using a shooting method with a generalized Newton's correction procedure for guessed boundary conditions. It is observed that in a certain region near the disk the buoyancy induced cross-flow dominates the primary von Karman flow. The shear stresses induced by the cross-flow are found to be more than these of the primary flow and they increase with magnetic parameter or the parameter λ characterizing the unsteadiness. The velocity profiles in the x- and y-directions for the primary flow at any two values of the unsteady parameter λ cross each other towards the edge of the boundary layer. The heat transfer increases with the Prandtl number but reduces with the magnetic parameter.     

Finite element method in magnetohydrodynamic channel flow problems

The finite element method has been applied to the steady-state fully developed magnetohydrodynamic channel flow of a conducting fluid in the presence of transverse magnetic field. Simple elements have been used to obtain the numerical values of velocity and induced magnetic field. To test the efficiency of the method, three different geometries, viz., rectangle, circle and triangle, are taken as the section of the pipe whose walls are non-conducting. Comparison is made with those cases in which exact solutions are available. Apart from giving good results, the FEM makes it possible to solve the problem for a pipe with arbitrary cross-section which was not possible by the other methods.     

MHD stagnation-point flow of an electrically conducting fluid on the surface of another quiescent fluid

An analysis is made of the orthogonal stagnation-point flow of an incompressible viscous electrically conducting fluid on the surface of another incompressible viscous electrically conducting quiescent fluid in the presence of a uniform magnetic field normal to the interface between the fluids. It is found that for small magnetic Reynolds number, the velocity at a point in the upper fluid increases and that at a point in the lower fluid decreases with increase in the magnetic field. It is also observed from the temperature distributions in the two given fluids that the interface temperature increases with increase in the magnetic field.     

The elastic properties, elastic models and elastic perspectives of metallic glasses

Bulk metallic glass (BMG) provides plentiful precise knowledge of fundamental parameters of elastic moduli, which offer a benchmark reference point for understanding and applications of the glassy materials. This paper comprehensively reviews the current state of the art of the study of elastic properties, the establishments of correlations between elastic moduli and properties/features, and the elastic models and elastic perspectives of metallic glasses. The goal is to show the key roles of elastic moduli in study, formation, and understanding of metallic glasses, and to present a comprehensive elastic perspectives on the major fundamental issues from processing to structure to properties in the rapidly moving field.A plentiful of data and results involving in acoustic velocities, elastic constants and their response to aging, relaxation, applied press, pressure and temperature of the metallic glasses have been compiled. The thermodynamic and kinetic parameters, stability, mechanical and physical properties of various available metallic glasses especially BMGs have also been collected. A survey based on the plentiful experimental data reveals that the linear elastic constants have striking systematic correlations with the microstructural features, glass transition temperature, melting temperature, relaxation behavior, boson peak, strength, hardness, plastic yielding of the glass, and even rheological properties of the glass forming liquids. The elastic constants of BMGs also show a correlation with a weighted average of the elastic constants of the constituent elements. We show that the elastic moduli correlations can assist in selecting alloying components with suitable elastic moduli for controlling the elastic properties and glass-forming ability of the metallic glasses, and thus the results would enable the design, control and tuning of the formation and properties of metallic glasses.We demonstrate that the glass transition, the primary and secondary relaxations, plastic deformation and yield can be attributed to the free volume increase induced flow, and the flow can be modeled as the activated hopping between the inherent states in the potential energy landscape. We then propose an extended elastic model to understand flow in metallic glass and glass-forming supercooled liquid, and the model presents a simple and quantitative mathematic expression for flow activation energy of various glasses. The elastic perspectives, which consider all metallic glasses exhibit universal behavior based on a small number of readily measurable parameters of elastic moduli, are presented for understanding the nature and diverse properties of the metallic glasses.     

Lattice Boltzmann simulations of binary fluid flow through porous media

The lattice Boltzmann equation is often advocated as a simulation tool that is particularly effective for complex fluids such as multiphase and multicomponent flows through porous media. We construct a three-dimensional 19 velocity lattice Boltzmann model for immiscible binary fluids with variable viscosities and density ratio based on the model proposed by Gunstensen. The model is tested for the following binary fluid flow problems: a stationary planar interface among two fluids; channel flow of immiscible binary fluids; the Laplace problem; and a rising bubble. The results agree well with semi-analytic results in a range of the E?tv?s, Morton and Reynolds number. We also present preliminary simulation results for two large-scale realistic applications: the flow of an air-water mixture in a waste-water batch reactor and the saturation hysteresis effect in soil flow. We discuss some limitations of the lattice Boltzmann method in the simulation of realistic and difficult multiphase problems.     

Physical properties of some iron based alloys in liquid and amorphous states

The atomic structure and the physical properties of amorphous ribbons depend strongly on the state of the melt before quench. It is known that slightly above liquidus metallic melts can preserve a non-equilibrium metastable state for a long time. Moreover some structural transformations in liquid metallic alloys, similar to phase transitions in solids, may take place with an increase of the temperature. In this paper we report measurements of the viscosity, magnetic susceptibility and surface tension in some Fe-based melts. Amorphous ribbons of the same alloys were prepared by standard planar method from different states of the melt. The electrical resistivity, the kinetics of crystallization and the magnetic properties of the ribbons were investigated. It was found that the properties depending upon nanoscale inhomogeneities are different for ribbons produced after different heat treatments of the melt before quench.     

Theoretical design of vascular imaging based on hall effect

The conceptual technique of vascular imaging and blood flow functional imaging based on Hall Effect is presented in this article. With this non‐invasive approach, both 3D anatomical imaging of the vasculature and functional imaging of blood flow in the deep structure of the human body can be obtained without radiation risk. The technique is based on the fact that the induced charges can be generated when the blood flows through the magnetic field. The induced electric field strength is measured by two groups of detector arrays, which captures not only the position of vasculature in each section, but also the velocity of blood flow and vessel size. The captured images can also be used for 3D reconstruction of the anatomical models. The designed system architecture including both hardware and software is described. © 2013 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 23, 85–96, 2013     

Magnetohydrodynamic (MHD) flows of viscoelastic fluids in converging/diverging channels

The present work is a theoretical investigation of the applicability of magnetic fields for controlling hydrodynamic separation in Jeffrey-Hamel flows of viscoelastic fluids. To achieve this goal, a local similarity solution was found for laminar, two-dimensional flow of a viscoelastic fluid obeying second-order/second-grade model as its constitutive equation with the assumption being made that the flow is symmetric and purely radial. These assumptions enabled a third-order nonlinear ODE to be obtained as the single equation governing the MHD flow of this particular fluid in flow through converging/diverging channels. With three physical boundary conditions available, Chebyshev collocation-point method was used to solve this ODE numerically. Results are presented in terms of parameters such as Reynolds number, Weissenberg number, channel half-angle, and the magnetic number. It was found that these parameters all have a profound effect on the velocity profiles in Jeffrey-Hamel flows. The effect of magnetic field was found to be more striking in that it is predicted to force fluid elements near the wall to exceed centerline velocity in converging channels and to suppress separation in diverging channels. Interestingly, the effect of the magnetic field in delaying flow separation is predicted to become more pronounced the higher the fluid’s elasticity.     

Unsteady mixed convection flow at the stagnation point

The unsteady mixed convection flow of an electrically conducting fluid at the stagnation point of a two-dimensional body and an axisymmetric body in the presence of an applied magnetic field has been studied. The effect of induced magnetic field has been included in the analysis. Both prescribed wall temperature and prescribed heat flux conditions have been considered. It is found that if the free stream velocity, applied magnetic field and square root of the wall temperature vary inversely as a linear function of time, i.e. as (1 − λt′)⁻¹, the governing boundary layer equations admit a locally self-similar solution. If surface heat flux is prescribed, it should vary as (1 − λt^*)^−5/2 for the existence of a local self-similar solution. The resulting ordinary differential equations have been solved using a finite element method as well as a shooting method with Newton's corrections for missing initial conditions. The skin friction and heat transfer coefficients and x-component of the induced magnetic field on the surface increase with the applied magnetic field or buoyancy force. Also they are found to change more for decelerating free stream velocity than for accelerating free stream velocity. Furthermore, they change little with the reciprocal of the magnetic Prandtl number. The buoyancy parameter causes overshoot in the velocity profile. For a given Prandtl number, beyond a certain critical value of the dissipation parameter, the hot wall ceases to be cooled due to the “heat cushion” provided by frictional heat.