阿姆斯特朗电解液液桥的实验现象及断裂机理研究

阿姆斯特朗液桥是电流体力学领域中的一种有趣现象,它悬在空中抵抗重力流动的神奇状态引起了人们的广泛关注．近十几年来,去离子水液桥已经通过理论和实验得到深入研究,但是对于电解液液桥的研究依然十分有限．本文以Na2SO4电解液作为形成液桥的实验液体,利用高速摄相机和红外热像仪研究了电解液液桥的形成过程、焦耳效应、流动中的热气泡产生及其破碎导致的液桥断裂等现象,提出了不同于去离子水液桥的断裂机理,有助于进一步加深人们对液桥这一复杂现象的理解．     

Large eddy simulation of turbulent incompressible flows by a three‐level finite element method

The variational multiscale method provides a methodical framework for large eddy simulation of turbulent flows. In this work, a particular implementation in the form of a three‐level finite element method separating large resolved, small resolved, and unresolved scales is proposed. Residual‐free bubbles are used for the numerical approximation of the small‐scale momentum equation. A stabilizing term is added, in order to take into account the effect of the small‐scale continuity equation. This implementation guarantees the stability of the method without further provisions and offers substantial computational savings on the small‐scale level. Furthermore, it is accounted for the unresolved scales by a specific dynamic modelling procedure. The method is tested for two different turbulent flow situations. Copyright © 2005 John Wiley & Sons, Ltd.     

I. Formation and rise of a bubble stream in a viscous liquid

The continuous emission of gas bubbles from a single ejection orifice immersed in a viscous fluid is considered. We first present a semi empirical model of spherical bubble growth under constant flow conditions to predict the bubble volume at the detachment stage. In a second part, we propose a physical model to describe the rise velocity of in-line interacting bubbles and we derive an expression for the net viscous force acting on the surrounding fluid. Experimental results for air/water-glycerol systems are presented for a wide range of fluid viscosity and compared with theoretical predictions. An imagery technique was used to determine the bubble size and rise velocity. The effects of fluid viscosity, gas flow rate, orifice diameter and liquid depth on the bubble stream dynamic were analyzed. We have further studied the effect of large scale recirculation flow and the influence of a neighbouring bubble stream on the bubble growth and rising velocity. Received: 23 July 1997 / Revised: 16 December 1997 / Accepted: 11 May 1998     

气泡的大振幅振动及其在声致发光和空化核聚变中的应用

评述了气泡大振幅振动方程，特别是R—P方程的来龙去脉，指出了该方程所存在的缺陷并对它进行了修正，将修正方程的数值解和R—P方程的数值解作了比较，在此基础上，对与气泡振动方程有关的应用(如声致发光和空化核聚变)情况作了分析。     

On Causes of Biological Efficiency of Low-Intensive Millimeter Waves

The process of the ions transport through the biological membrane is considered. A role of air microbubbles efficiently mixing a near-membrane liquid layer in the presence of a temperature gradient is pointed out. These circumstances result in lowering an energy barrier height of the transmembrane ion-transport. It is shown that even low-level irradiation by the millimeter waves creates the temperature gradient, which is enough for effective mixing of near-membrane liquid's layer. It is demonstrated that a process of restoration an equilibrium air-distribution in the intercellar medium after ceasing the microwave irradiation proceeds according to the diffusion mechanism, i.e. rather slow.     

Circumventing air bubbles in microfluidic systems and quantitative continuous-flow PCR applications

Polymerase chain reaction (PCR) is an essential part of research based on genomics or cell analysis. The development of a microfluidic device that would be suitable for high-temperature-based reactions therefore becomes an important contribution towards the integration of micro-total analysis systems (μTAS). However, problems associated with the generation of air bubbles in the microchannels before the introduction of the assay liquid, which we call the “initial start-up” in this study, made the flow irregular and unstable. In this report, we have tried to address these problems by adapting a novel liquid-flow method for high-temperature-based reactions. A PDMS-based microfluidic device was fabricated by soft-lithography techniques and placed on a cartridge heater. The generation of the air bubbles was prevented by introducing the fluorinated oil, an inert and highly viscous liquid, as the cap just before the introduction of the sample solutions into the microchannels. The technique was applied for continuous-flow PCR, which could perform PCR on-chip in a microfluidic system. For the evaluation of practical accuracy, plasmid DNA that serves as a reference molecule for the quantification of genetically modified (GM) maize was used as the template DNA for continuous-flow PCR. After PCR, the products were collected in a vial and analyzed by gel electrophoresis to confirm the accuracy of the results. Additionally, quantitative continuous-flow PCR was performed using TaqMan technology on our PCR device. A laser detection system was also used for the quantitative PCR method. We observed a linear relationship between the threshold cycle (Ct) and the initial DNA concentration. These results showed that it would be possible to quantify the initial copies of the template DNA on our microfluidic device. Accurate quantitative DNA analysis in microfluidic systems is required for the integration of PCR with μTAS, thus we anticipate that our device would have promising potential for applications in a wide range of research.     

贷款定价中的低估行为与房地产价格泡沫——扩展的PW模型与中国经验

Pavlov和Wachter清晰地阐述了商业银行对抵押贷款看跌期权内在价值的低估导致资产价格膨胀,使其脱离基础价值,即产生泡沫。但是其假设条件缺乏一般性,并且没有考虑开放条件下的汇率风险。本文在对其假设条件进行修正,并添加汇率因素的基础上,对PW模型进行扩展,从而得出开放条件下商业银行贷款定价行为与房地产价格泡沫的相关分析模型,得出的结论是,房地产价格与利差和汇率负相关,与存款利率正相关,而提高借款人的自付权益成本可削弱贷款定价中的低估行为对房价的影响。最后,采用我国2005年7月至2008年9月的月度数据进行计量分析,基本证实了模型的结论,同时,也说明了房地产需求对样本期间内的房价飚涨影响不大,货币政策的调控效果也非常微弱。     

Collapse mechanisms and extreme deformation of particle-laden interfaces

Particle-laden interfaces are at the basis of many advanced materials, such as bijels and dry water. While the final properties of these materials can generally be controlled, their response to deformation during processing and use is still poorly understood. In particular, the dynamics of particle-laden interfaces in relevant flow conditions is receiving increasing attention. These conditions are typically highly dynamic and can involve unsteady flow or large deformations. This article gives an overview of the remarkable phenomena of particle-laden interfaces undergoing deformations of large amplitude and at high strain rate, in other words extreme deformation. Upon large-amplitude compression, a monolayer of particles can collapse by buckling or by expelling particles in the liquid. The criteria for buckling or expulsions, as well as recent experiments in highly dynamic conditions, are discussed, showing that these criteria can depend also on the rate of deformation. The emerging use of ultrasound-driven bubbles as an experimental platform for controlled deformation of particle-laden interfaces at high strain rate is also discussed. The ability to control the fate of particles at interfaces during dynamic deformation of droplets or bubbles ultimately underpins a variety of applications from controlled release to catalysis.     

A residual‐based Allen–Cahn phase field model for the mixture of incompressible fluid flows

The hydrodynamics of fluid mixtures is receiving more and more attention in many science and engineering applications. Within the techniques for dealing with front displacements and moving boundaries between different density and/or viscosity fluids, phase fields are a class of models in which a diffusive transition region is taken into account instead of a steep interface. Although these models have a physical motivation, they require the definition of extra parameters. In order to make it less parameter dependent, the classic Allen–Cahn phase field model is modified, exploring its similarities with residual‐based discontinuity‐capturing schemes, making the phase field equation dependent on its own residual. We solve the coupling between incompressible viscous fluid flow and the phase field advective–diffusive–reactive transport to simulate the main processes in interface tension and/or buoyancy driven problems. For the solution of the Navier–Stokes and transport equations, we use a stabilized finite element formulation. The implementation has been performed using the libMesh finite element library, written in C++ , which provides support for adaptive mesh refinement and coarsening. A chemical convection benchmark problem is used to validate the proposed model, and then we solve two bubble interaction problems. Copyright © 2014 John Wiley & Sons, Ltd.     

Oscillation and breakup of a bubble under forced vibration

Coupled shape oscillations and translational motion of an incompressible gas bubble in a vibrating liquid container is studied numerically. The bubble oscillation characteristics are mapped based on the bubble Bond number (Bo) and the ratio of the vibration amplitude of the container to the bubble diameter (A/D). At small Bo and A/D, the bubble oscillation is found to be linear with small amplitudes, and at large Bo and A/D, it is nonlinear and chaotic. This chaotic bubble oscillation is similar to those observed in two coupled nonlinear systems, here being the gas inside the bubble and its surrounding liquid. Further increases in the forcing, results in the bubble breakup due to large liquid inertia.