Analysis of slip flow heat transfer between two unsymmetrically heated parallel plates with viscous dissipation

This paper presents an analytical investigation to study the heat transfer and fluid flow characteristics in the slip flow region for hydrodynamically and thermally fully developed flow between parallel plates. Both upper and lower plates are subjected to asymmetric heat flux boundary conditions. The effect of first order velocity slip, temperature jump, asymmetric heat flux ratio and viscous dissipation on the heat transfer performance is analyzed. Closed form expressions are obtained for the temperature distribution and Nusselt number. Present predictions are verified for the cases that neglect the viscous heating and microscale effects. The effect of asymmetric heat flux ratio with and without viscous dissipation on Nusselt number for both macroscale and microscale is highlighted. The heat transfer characteristics are found to depend on various modeling parameters, namely, modified Brinkman number, Knudsen number and heat flux ratio.     

Pressure work and viscous dissipation in the equations of thermal convection in a vertical channel

Equations for fully developed flow in a vertical channel have been solved, taking into account viscous dissipation, and using formulations with and without pressure work. Perturbation solutions are used to distinguish the effects of viscous dissipation from pressure work. Viscous dissipation has very little effect on free convection flows driven by temperature differences or heat fluxes at the channel walls, but it may play a major role in forced convection.     

船桥碰撞过程引发的冲击动力学论题

对船桥撞击过程引发的几个冲击动力学论题进行了分析。研究表明:1为降低船撞力,应采用柔性(低的结构动态广义波阻抗)防撞装置;2撞击力所做的功,通过应力波传播转化为内能(变形能)与动能之和;而变形能中的不可逆部分愈高,防撞装置发挥的整体作用愈大,则愈有利于防撞装置发挥缓冲耗能作用。并且如何让船舶尽早结束撞击并带走尽量多的剩余动能,应是防撞装置设计的关键点;3黏性耗能可缓冲撞击过程、延长撞击历时,有利于防撞装置发挥整体作用,进而为船舶在低应力下转向滑离、从而带走尽可能多的剩余动能创造条件。因此,船撞桥防护装置的设计应该建立在如下的科学设计理念上:ⅰ低波阻抗意义上的冲击柔性,ⅱ缓冲撞击过程意义上的粘性耗能,ⅲ防撞装置能及早发挥整体作用,化撞击集中力为分布载荷,以及ⅳ让船尽早滑离而带走尽量多的剩余动能。以钢丝绳防撞圈为主要元件的柔性耗能防撞装置是这一防撞理念的工程应用实例,其有效性已为工程实践和实船撞击试验证实。     

An improved model for droplet solidification on a flat surface

An existing model of the deformation and solidification of a single droplet impinging on a cold surface has been revised and improved. The original model is based on a two-dimensional axisymmetric flow approximation of the velocity field, the Neumann solution to the one-dimensional Stefan solidification problem, and an integral mechanical energy balance. The improved model features a more appropriate velocity field which satisfies the no-shear boundary condition at the free surface, and an accurate derivation of the dissipation term from the mechanical energy equation. This equation has been solved numerically. Comparisons of the original and the improved models have been performed. Results show that the original model over-estimates the final splat size by about 10%. The discrepancy is more pronounced at larger Weber numbers, where viscous effects dominate. The effects of the Weber number, We, the Reynolds numbers, Re, and the solidification parameter have been investigated through detailed numerical calculations. Two regimes of spreading/solidification have been identified. If Re/We is small, the process is one of dissipation of the incident droplet kinetic energy; whereas for large values of Re/We the process can rather be characterized as a transfer between kinetic and potential energy. In the latter case, the variations of the final splat size versus the solidification constant exhibit a non-monotonic behaviour. This indicates that, for a given material, the deposition process can be optimized. Correlations relating the final splat size to the process parameters are given. This revised version was published online in November 2006 with corrections to the Cover Date.     

Unsteady two-dimensional and axisymmetric MHD boundary-layer flows

Summary Nonsimilar solution of the unsteady laminar incompressible magneto-hydrodynamic boundary layer flow and heat transfer for an electrically conducting fluid over two-dimensional and axisymmetric bodies in the presence of an applied magnetic field has been obtained. The effects of surface mass transfer, Joule heating and viscous dissipation are included in the analysis. Numerical computation have been carried out for the flow over a circular cylinder and a sphere using an implicit finite difference scheme in combination with a quasi-linearization technique. It is observed that magnetic field and suction cause the location of vanishing skin friction to move downstream while, the effect of injection is just the opposite. The effect of magnetic field on the skin friction is more pronounced as compared to its effect on the heat transfer. On the other hand, the heat transfer is strongly affected by the viscous dissipation and the effect is more for larte times. However, heat transfer responds comparatively less to the fluctuations of the free stream than the skin friction.     

Dynamic pull-in instability of a prestretched viscous dielectric elastomer under electric loading

The dynamic response of a pre-stretched elastomer membrane under electric loading was analysed. Thereby, two cases of voltage application, constant voltage and an incrementally increased voltage were regarded. The equation of motion (EOM) was derived from the Euler–Lagrange equation and the Rayleigh dissipation function. This allowed to include the influence of prestretch into the evolution equation of the viscous stretch. The critical values of pull-in instability under dynamic assumptions were determined at the initial state by an analytical model derived from the classical approaches of stability theory regarding geometric instabilities by using an energy criterion and were used to validate the numerical solution of the EOM. The results showed that both inertia and viscous effects have a notable influence on the pull-in stability behavior. In particular, at applied electric fields below the critical electric field the viscous behavior can still induce failure, which occurs time delayed. A non-monotonic dependence of the failure stretch on the magnitude of the applied electric fields below the critical value could be observed.     

粘滞流体阻尼墙对平面不规则结构的扭转效应控制研究

平面不规则结构由于平扭耦联会使边界处的位移较规则结构增大很多,而给结构配置阻尼器是减小平面不规则结构边界位移的有效途径,该文将耗能效率较高的粘滞流体阻尼墙应用于控制平面不规则结构的扭转响应,研究阻尼墙控制结构扭转效应的机理及控制结构扭转的优化布置。将减震系统刚度矩阵转换到结构的质量中心,将配置阻尼墙后形成阻尼中心的阻尼矩阵转换到质量中心,从而建立可以考虑扭转效应和附加阻尼位置效应的非线性运动控制方程,并根据响应结果推导出结构位移比的表达式;利用运动方程研究阻尼中心的位置、阻尼力的大小、阻尼参数等对结构扭转效应的影响规律,结合结构位移比的表达式提出阻尼墙的平面配置原则和最优阻尼参数的取值方法。以某L型高层建筑为工程背景,根据上述原则配置阻尼墙并选取阻尼参数,并对减震体系在最不利地震动输入方向上进行非线性时程分析,分析结果表明通过合理配置粘滞阻尼墙,L型结构的扭转响应得到了有效的控制,结构最大边界位移和位移比大大降低。该研究结果为采用阻尼墙控制不规则结构扭转效应提供了基本原则和方法,并为不规则结构的减震设计提供有益的借鉴和参考。     

Estimation of magnetohydrodynamic radiative nanofluid flow over a porous non‐linear stretching surface: application in biomedical research

The present study investigates the effects of thermal radiation and chemical reaction on magnetohydrodynamic flow, heat, and mass transfer characteristics of nanofluids such as Cu–water and Ag–water over a non‐linear porous stretching surface in the presence of viscous dissipation and heat generation. Using similarity transformation, the governing boundary layer equations of the problem are transformed into non‐linear ordinary differential equations and solved numerically by the shooting method along with the Runge–Kutta–Fehlberg fourth–fifth‐order integration scheme. The influences of various parameters on velocity, temperature, and concentration profiles of the flow field are analysed and the results are plotted graphically. A backpropagation neural network is applied to predict the skin friction coefficient, Nusselt number, and Sherwood number and these results are presented through graphs. The present numerical results are compared with the existing results and are found to be in good agreement. The results of artificial neural network and the obtained numerical values agree well with an error <5%.Inspec keywords: silver, copper, transforms, nanofluidics, friction, backpropagation, heat radiation, water, external flows, partial differential equations, nonlinear differential equations, boundary layers, Runge‐Kutta methods, mass transfer, flow through porous media, magnetohydrodynamicsOther keywords: magnetohydrodynamic radiative nanofluid flow, nonlinear stretching surface, biomedical research, thermal radiation, chemical reaction, magnetohydrodynamic flow, nonlinear porous stretching surface, viscous dissipation, similarity transformation, governing boundary layer equations, nonlinear ordinary differential equations, shooting method, Runge–Kutta–Fehlberg fourth–fifth‐order integration scheme, flow field, backpropagation neural network, Cu–water nanofluid, Ag–water nanofluid, skin friction coefficient, Nusselt number, Sherwood number, artificial neural network, Ag‐H₂ O, Cu‐H₂ O     

Energy separation mechanism in unconfined laminar compressible vortex

A theoretical investigation from the view point of gas-dynamics and thermodynamics was carried out, in order to clarify the energy separation mechanism in an unconfined laminar compressible vortex, as a primary flow element of a vortex tube. The mathematical solutions of density and temperature in a viscous compressible vortical flow, with tangential velocity, were examined using an evaluation equation of total temperature. It is found from the results that a hotter gas in the peripheral region of the vortex is mainly generated by heat caused by viscous dissipation. A colder gas in the vortex center is mainly generated by viscous shear work done by the fluid element onto the surface of the surrounding gas. In addition, it is also found that the larger the representative Mach number of a vortex is, the lower the total temperature at the center of the vortex is, and at the same time, the higher the maximum total temperature in the peripheral region is. The increase in specific heat ratio of the working gas has the same effect, as increasing the representative Mach number of the vortex, on the total temperature in the vortex.     

Viscous flux motion in anisotropic magnetic superconductors

Magnetic field penetrates in the form of flux lines or vortex thread into type-II magnetic superconductors (MSC) and induces magnetization of magnetic subsystem over a distance of an order of the London penetration depth surrounding the normal cores. When a flux line moves by, surrounding magnetization moves as a whole through the sample and a free motion of vortices is subjected to magnetic viscous drag, giving rise to dissipation. The flux flow resistance in the mixed state of anisotropic MSC has been studied on the basis of the London theory. Expressions for the dissipation and viscosity coefficient associated with the change of the magnetic subsystem as a vortex moves about are derived.     

Optimizing the operation of a direct-flow filtration device

A mathematical model of an oscillating wave surge converter is developed to study the effect that viscous dissipation has on the behaviour of the device. Recent theoretical and experimental testing have suggested that the standard treatment of viscous drag (e.g. Morison’s equation) may not be suitable when the effects of diffraction dominate the wave torque on the device. In this paper, a new model of viscous dissipation is presented and explored within the framework of linear potential flow theory, and application of Green’s theorem yields a hypersingular integral equation for the velocity potential in the fluid domain. The hydrodynamic coefficients in the device’s equation of motion are then calculated, and used to examine the effect of dissipation on the device’s performance. A Haskind relationship, expressing the link between the scattering- and radiation-potential problems is derived, and its connection to existing Haskind relations is explored. A sensitivity study of the device’s power capture to the magnitude of the dissipation present in the system is carried out for a selection of device widths. The results of the sensitivity study are explained with reference to existing experimental and numerical data. A special focus is given to the effects of dissipation on the performance of a device whose pitching motion is tuned to resonate with the incoming waves.     

Energy Dissipation and Apparent Viscosity of Semi-solid Metal during Rheological Processes Part Ⅰ： Energy Dissipation

The energy dissipation caused by the viscous force has great effects on the flow property of semi-solid metal during rheological processes such as slurry preparing, delivering and cavity filling. Experimental results in this paper indicate that the viscous friction between semi-solid metal and pipe wall, the collisions among the solid particles, and the liquid flow around particles are the three main types of energy dissipation. On the basis of the hydromechanics, the energy dissipation calculation model is built. It is demonstrated that the micro-structural parameters such as effective solid fraction, particle size and shape, and flow parameters such as the mean velocity, the fluctuant velocity of particles and the relative velocity between the fluid and solid phase, affect the energy dissipation of semi-solid metal.     

Energy Dissipation and Apparent Viscosity of Semi-solid Metal during Rheological Processes Part Ⅰ: Energy Dissipation

The energy dissipation caused by the viscous force has great effects on the flow property of semi-solid metal during rheological processes such as slurry preparing, delivering and cavity filling. Experimental results in this paper indicate that the viscous friction between semi-solid metal and pipe wall, the collisions among the solid particles, and the liquid flow around particles are the three main types of energy dissipation. On the basis of the hydromechanics, the energy dissipation calculation model is built. It is demonstrated that the micro-structural parameters such as effective solid fraction, particle size and shape, and flow parameters such as the mean velocity, the fluctuant velocity of particles and the relative velocity between the fluid and solid phase, affect the energy dissipation of semi-solid metal.     

Unsteady MHD forced flow due to a point sink

Summary An analysis is performed to study the unsteady laminar incompressible boundary-layer flow of an electrically conducting fluid in a cone due to a point sink with an applied magnetic field. The unsteadiness in the flow is considered for two types of motion, viz. the motion arising due to the free stream velocity varying continuously with time and the transient motion occurring due to an impulsive change either in the strength of the point sink or in the wall temperature. The partial differential equations governing the flow have been solved numerically using an implicit finite-difference scheme in combination with the quasilinearization technique. The magnetic field increases the skin friction but reduces heat transfer. The heat transfer and temperature field are strongly influenced by the viscous dissipation and Prandtl number. The velocity field is more affected at the early stage of the transient motion, caused by an impulsive change in the strength of the point sink, as compared to the temperature field. When the transient motion is caused by a sudden change in the wall temperature, both skin friction and heat transfer take more time to reach a new steady state. The transient nature of the flow and heat transfer is active for a short time in the case of suction and for a long time in the case of injection. The viscous dissipation prolongs the transient behavior of the flow.     

A Through-Thickness Arrayed Carbon Fibers Elastomer with Horizontal Segregated Magnetic Network for Highly Efficient Thermal Management and Electromagnetic Wave Absorption

Multifunctional thermal management materials with highly efficient electromagnetic wave (EMW) absorption performance are urgently required to tackle the heat dissipation and electromagnetic interference issues of high integrated electronics. However, the high thermal conductivity (λ) and outstanding EMW absorption performance are often incompatible with each other in a single material. Herein, a through-thickness arrayed NiCo₂O₄/graphene oxide/carbon fibers (NiCO@CFs) elastomer with integrated functionalities of high thermal conductivity, highly efficient EMW absorption, and excellent compressibility is reported. The NiCO@CFs elastomer realizes a high out-of-plane thermal conductivity of 15.55 W m⁻¹ K⁻¹, due to the through-thickness vertically aligned CFs framework. Moreover, the unique horizontal segregated magnetic network effectively reduces the electrical contact between the CFs, which significantly enhances impedance matching of NiCO@CFs elastomer. As a result, the vertically arrayed NiCO@CFs elastomer synchronously exhibits ultrabroad effective absorption bandwidth of 8.25 GHz (9.75–18 GHz) at a thickness of 2.4 mm, good impedance matching, and a minimum reflection loss (RL_min) of −55.15 dB. Given these outstanding findings, the multifunctional arrayed NiCO@CFs elastomer opens an avenue for applications in EMW absorption and thermal management. This strategy of constructing thermal/electrical/mechanical pathways provides a promising way for the high-performance multifunctional materials in electronic devices.     

Experiments on metal-silicate plumes and core formation

Short-lived isotope systematics, mantle siderophile abundances and the power requirements of the geodynamo favour an early and high-temperature core-formation process, in which metals concentrate and partially equilibrate with silicates in a deep magma ocean before descending to the core. We report results of laboratory experiments on liquid metal dynamics in a two-layer stratified viscous fluid, using sucrose solutions to represent the magma ocean and the crystalline, more primitive mantle and liquid gallium to represent the core-forming metals. Single gallium drop experiments and experiments on Rayleigh-Taylor instabilities with gallium layers and gallium mixtures produce metal diapirs that entrain the less viscous upper layer fluid and produce trailing plume conduits in the high-viscosity lower layer. Calculations indicate that viscous dissipation in metal-silicate plumes in the early Earth would result in a large initial core superheat. Our experiments suggest that metal-silicate mantle plumes facilitate high-pressure metal-silicate interaction and may later evolve into buoyant thermal plumes, connecting core formation to ancient hotspot activity on the Earth and possibly on other terrestrial planets.     

Non-Darcian flow and heat transfer along a permeable vertical surface with nonlinear density temperature variation

The non-Darcy free convection flow on a vertical flat plate embedded in a fluid-saturated porous medium in the presence of the lateral mass flux with prescribed constant surface temperature is considered. The coupled nonlinearities generated by the density variation with temperature, inertia, and viscous dissipation are included in the present study. In particular, we analyze a system of nonlinear ODEs describing self-similar solutions to the flow and heat transfer problem. These transformed equations are integrated numerically by a second-order finite difference scheme known as the Keller box method. Furthermore, some analytical results are provided to establish relationships between the physical invariants in the problem, and also to validate the numerical method. One of the important findings of our study is that an increase in the Rayleigh number increases the velocity boundary layer thickness, while the opposite is true for the thermal boundary layer thickness.     

A variational constitutive framework for the nonlinear viscoelastic response of a dielectric elastomer

We formulate a variational constitutive framework that accounts for nonlinear viscous behavior of electrically sensitive polymers, specifically Dielectric Elastomers (DEs), under large deformation. DEs are highly viscoelastic and their actuation response is greatly affected in dynamic applications. We used the generalized Maxwell model to represent the viscoelastic response of DE allowing the material to relax with multiple mechanisms. The constitutive updates at each load increment are obtained by minimizing an objective function formulated using the free energy and electrostatic energy of the elastomer, in addition to the viscous dissipation potential of the dashpots in each Maxwell branch. The model is then used to predict the electromechanical instability (EMI) of DE. The electro-elastic response of the DE is verified with available analytical solutions in the literature and then the material parameters are calibrated using experimental data. The model is integrated with finite element software to perform a variety of simulations on different types of electrically driven actuators under various electromechanical loadings. The electromechanical response of the DE and the critical conditions at which EMI occurs were found to be greatly affected by the viscoelasticity. Our model predicts that under a dead load EMI can be avoided if the DE operates at a high voltage rate. Subjected to constant, ramp and cyclic voltage, our model qualitatively predicts responses similar to the ones obtained from the analytical solutions and experimental data available in the literature.     

亚格子尺度湍流特性研究

采用大涡模拟方法模拟了雷诺数Re_H=18,400的后台阶湍流流动,研究了亚格子尺度湍流动能和湍流耗散的特性。给出了后台阶湍流流动的流场结构以及亚格子湍动能和亚格子湍耗散的空间分布结果,比较了大涡模拟预报湍流粘性以及等效计算粘性。研究表明,亚格子尺度湍动能和亚格子湍耗散随着流动在空间的发展而呈现减弱趋势,回流区内亚格子湍动能和耗散较弱;在台阶截面(y/H=1处)亚格子湍动能和耗散最大。亚格子湍动能小于脉动动能统计量,亚格子粘性小于等效湍流模型粘性预报结果。     

基于分数阶黏弹性模型的温拌改性沥青低温性能

基于分数阶黏弹性模型,通过低温弯曲梁流变试验（BBR）分析RH和Evotherm温拌剂对苯乙烯-丁二烯-苯乙烯嵌段共聚物（SBS）改性沥青低温性能的影响。结果表明：利用BBR试验数据拟合出的分数阶黏弹性模型参数可较好地预估温拌改性沥青的蠕变柔量和蠕变速率; RH温拌剂可以提高SBS改性沥青的阻尼比和耗散能比,但Evotherm温拌剂在低于-18℃时不能够提高;因7种沥青按照阻尼比和耗散能比排序相同,故也可以将阻尼比作为温拌改性沥青低温性能的一种评价指标; RH可以改善沥青的蠕变柔量,提高黏性流动柔量,且掺量越高改善效果越好,但不能提高黏弹性比,故不能用黏弹性比评价温拌SBS改性沥青的低温性能,而Evotherm温拌剂的规律性不强;两种温拌剂都在低于-24℃时对SBS改性沥青的低温性能改善不佳。