Universality in crossover function for convection in fluids with a free surface

We show that in the onset of convection in a thin fluid layer with a free surface, the passage from surface tension driven to buoyancy driven convection with changing thickness of the fluid layer follows a universal curve and can be calculated very accurately using a variational method. We have shown that the balance between surface tension traction to buoyancy force determines the crossover length scale of the fluid which is independent of viscosity or thermal diffusivity. We suggest a scenario near critical point of fluids in which this crossover can be observed.     

Instabilities of thermocapillary-buoyancy convection in open rectangular liquid layers

This article presents the experimental investigation on instabilities of thermocapillary-buoyancy convection in the transition process in an open rectangular liquid layer subject to a horizontal temperature gradient. In the experimental run,an infrared thermal imaging system was constructed to observe and record the surface wave of the rectangular liquid layer. It was found that there are distinct convection longitudinal rolls in the flow field in the thermocapillary-buoyancy convection transition process. There are different wave characterizations for liquid layers with different thicknesses. For sufficiently thin layers, oblique hydrothermal waves are observed, which was predicted by the linear-stability analysis of Smith Davis in 1983. For thicker layers, the surface flow is distinct and intensified, which is because the buoyancy convection plays a dominant role and bulk fluid flow from hot wall to cold wall in the free surface of liquid layers. In addition, the spatiotemporal evolution analysis has been carried out to conclude the rule of the temperature field destabilization in the transition process.     

Experimental Investigation of Thermocapillary Convection in a Liquid Layer with Evaporating Interface

Thermocapillary convection coupling with the evaporation effect of evaporating liquids is studied experimentally. This study focused on an evaporation liquid layer in a rectangular cavity subjected to a horizontal temperature gradient when the top evaporating surface is open to air, while most previous works only studied pure thermocapillary convection without evaporation. Two liquids with different evaporating rates are used to study the coupling of evaporation and thermocapillary convection, and the interfacial temperature profiles for different temperature gradients are measured. The experimental results indicate evidently the influence of evaporation effect on the thermocapillary convection and interfacial temperature profiles. The steady multicellular flow and the oscillatory multicellular flow in the evaporation liquid layer are observed by using the particle-image-velocimetry method.     

Influence of the surface deformability and variable viscosity on buoyant - thermocapillary instability in a liquid layer

The primary stationary and oscillatory Bénard-Marangoni instability is investigated in a fluid layer of infinite horizontal extent, bounded below by a rigid plane and above by a deformable upper surface, subjected to a vertical temperature gradient. Since the viscosity is temperature-dependent the consequences of relaxing Oberbeck-Boussinesq approximation and free surface deformability are theoretically examined by means of small disturbance analysis. The problem has been solved numerically by the Taylor series expansion method. The results obtained confirm that when the free surface is undeformable, stationary convection develops in the form of polygonal cells, and oscillatory motion cannot be detected. When the surface deformability is considered, stationary convection sets in, either as a short-wavelength hexagonal instability or as a long-wavelengh mode or as both, and oscillatory convection is also possible. The stability threshold for the short-wavelength mode depends mainly on the viscosity variation while the long-wavelength mode is determined by the surface deformation. Numerically, it is found that the neutral oscillatory Marangoni numbers are only negative. When a variable-viscosity model is used the theoretical and experimental results are in better agreement. Received 15 May 1997     

稳定自然对流下的温度梯度液相外延

介绍了稳定自然对流下的温度梯度液相外延,稳定自然对流由加在溶液上的水平温差产生,用简化模式计算了该生长的生长速率,计算给出:该生长的生长速率与水平温差的平方根成比例;具体对Ga_0.85Al_0.15AS生长,在本工作给定的条件下,生长速率为稳态扩散理论预示生长速率的1124倍,在大部份生长面积内,外延层的厚度变化小于平均厚度的±10％,设计了稳定自然对流下的温度梯度液相外延装置,用该装置生长了厚Ga_1-xAl_xAs层,实 关键词：     

Convective flows in a layer of a viscous liquid with the uniformly charged free surface

It is found theoretically that the critical conditions under which a charged liquid surface becomes unstable against the electric charge relax as a result of interaction between capillary-gravitational and convective flows in the liquid. As the surface charge density approaches a value critical in terms of development of Tonks-Frenkel instability, convection in the liquid arises at a temperature gradient however small, this effect depending on the liquid layer thickness.     

正交梯度下双扩散对流的数值模拟

本文采用有限单元法求解涡量、流函数、能量和组分控制方程,数值模拟了在温度梯度和浓度梯度正交情况下竖直环形容器内的双扩散对流结构。通过改变浮升力比N=GrS／GrT和Le数的大小,分析了溶质浮升力方向及其大小和Le数对容器内双扩散对流的影响。结果表明:当溶质浮升力改变时,壁面处的流体流动状况、边界层厚度、Nu数和Sh数都发生了变化;随着Le数的增大,竖直壁面处Nu数逐渐降低而水平壁面处的Sh数逐渐增大。     

浮力对环形液池内热毛细对流的影响

为了了解浮力的影响,对水平温度梯度作用时环形液池内的热毛细对流进行了非稳态三维数值模拟,环形液池外壁被加热,半径为40 mm,内壁被冷却,半径为20 mm,液池深度为(1-17)mm,流体为0．65cSt的硅油,其Pτ数为6．7．模拟结果表明,当水平温度梯度较小时,流动为轴对称稳态流动,随着温度梯度的增加,流动将会失去其稳定性,在浅液池内,转化为热流体波,浮力对失稳后的流型无影响,但会使热流体波的振幅下降;在深液池内,在常重力条件下,转化成三维稳定流动,在微重力和小重力条件下,转化为三维振荡流动．     

旋转环形浅液池内双组分溶液耦合热-溶质毛细对流渐近解

为了了解水平温度梯度作用下旋转环形浅液池内耦合热-溶质毛细对流基本特征, 采用匹配渐近展开法对旋转环形浅液池内耦合热-溶质毛细对流过程进行了求解, 获得了中心区域的速度、温度和浓度分布,分析了旋转、Soret效应、浮力、溶质扩散 系数和液池的几何尺寸对流动结构的影响.将所得到的渐近解和文献中的已有结果进行对比,证明了所求结果的正确性;在浅液池内,耦合热-溶质毛细力对流体流动起主导作用, 旋转和浮力对流动的影响较小,溶质扩散系数和几何尺寸有较明显影响;当各种耦合的 驱动力作用方向相同时,流动增强;否则, 流动减弱. 关键词： 旋转 环形浅液池 耦合热-溶质毛细对流 渐近解     

The principle of non-mechanical transport of a liquid crystal in thin capillaries

The non-mechanical principle of transport of a liquid crystal (LC) encapsulated in a narrow cavity between two coaxially arranged cylinders is introduced based on the interaction of the temperature and director field gradients. The temperature gradient is created due to a heat flow from the inner cylinder surface, whereas the temperature on the outer cylinder surface is maintained constant. The director field gradient is caused by the deformation of the planar-oriented LC cavity upon exposure to a double electrostatic layer, which naturally appears at the LC phase-solid interface. The size of the gap between the bounding surfaces, cylinder curvatures, and thermal conditions are determined, which allow initiation of the LC phase flow in the horizontal direction.     

光学系统透镜热形变特性仿真研究

光学系统透镜组件在环境温度变化时发生的形变,特别是透镜的厚度、面形以及透镜间空间位置的变化,对系统成像质量有着不可忽视的影响。分析计算了某型导引头光学系统透镜组件在温度载荷下的形变特性。计算结果表明,当环境温度从20℃降低到-40℃时,透镜表面位移呈现出显著的梯度变化,7个透镜中表面的最大位移达0.05mm,透镜厚度的变化率超过0.05%,透镜间距离的最大变化率为3.46%。利用数据重构技术,计算结果可以作为透镜镜面形变分析的参考数据。     

A parallel computer implementation of the Asymptotic Numerical Method to study thermal convection instabilities

We have developed a numerical model to efficiently compute steady-state combined buoyancy and thermocapillary convection solutions. It features a parallel computer implementation of an Asymptotic Numerical Method to perform steady-state path-following and locate bifurcation points in problems involving large size algebraic systems, up to few million degrees of freedom. The model has been first validated on a problem for which a reference solution exists and then is used to analyse the influence of the container size and shape on Rayleigh–Bénard–Marangoni convection and its related cellular pattern.     

Linear Stability of Taylor-Couette Flows with Axial Heat Buoyancy

The linear stability is studied of flows confined between two concentric cylinders, in which the radial temperature gradient and axial gravity are considered for an incompressible Newtonian fluid. Numerical method based on the Petrov-Galerkin scheme is developed to deal with the buoyancy term in momentum equations and an additional temperature perturbation equation. Computations of the neutral stability curves are performed for different rotation cases. It is found that the flow instability is influenced by both centrifugal and axial shear instabilities, and the two instability mechanisms interact with each other. The outer cylinder rotation plays dual roles of stabilizer and destabilizer under different rotating stages with the inner cylinder at rest. For the heat buoyancyinduced axial flow, spiral structures are found in the instability modes.     

Performance of Non-Contact Linear Actuators Driven by Surface Acoustic Waves*

A new kind of non-contact linear actuator （motor） driven by surface acoustic waves （SAWs） is presented, in which the stators are made from SAW delay lines using 128° YX-LiNbO3 substrates. A fluid layer is introduced between the slider and the stator of the actuator, and the slider is a circular aluminum disk suspended on the surface of the liquid （water） layer. As the SAW is excited on the stator, the SAW is converted to a leaky wave in the interface of the stator and the liquid, and then propagates into the liquid. Owing to the nonlinear effect of wave propagation, acoustic streaming is generated, which pushes the slider to move. By the experiments, the relations between the slider velocity and the experimental parameters, such as the exciting voltage of the SAWs, the thickness and the kinematic viscosity of the liquid layer, are obtained.     

多离散源驱动的双扩散自然对流传输结构

本文通过"矢量线,,可视化方法,解析了一个方腔内由双离散热源和污染源驱动的双扩散自然对流系统的传输结构.发现在不同浮升力比下,该系统存在着三种明显不同的宏观传输结构,即由热浮升力主导的传输结构,由热质浮升力共同支配的传输结构以及由质浮升力主导的传输结构.     

A phase analysis of vorticity vectors associated with tropical convection

Three new vorticity vectors have been proposed by Gao et al to study the two-dimensional tropical convection. In the present paper, phase relations between surface rain rate and the vorticity vectors are analysed with the calculations of lag correlation coefficients based on hourly zonally-averaged mass-integrated cloud-resolving simulation data. The cloud-resolving model is integrated with the vertical velocity, zonal wind, horizontal thermal and moisture advections, and sea surface temperature observed and derived from tropical ocean global atmosphere - coupled ocean atmosphere response experiment （TOGA-COARE） for 10 days. Maximum local increase of the vertical component of the convective vorticity vector leads maximum surface rain rate by 2 hours mainly due to the interaction between vorticity and zonal gradient of ice heating. While maximum local increase of the vertical component of the moist vorticity vector leads maxfinum surface rain rate by 2 hours mainly because of the interaction between zonal specific humidity gradient and zonal buoyancy gradient. And the maximum local decrease of the zonal component of the dynamic vorticity vector leads maximum surface rain rate by 2 hours mainly due to the interactions between vorticity and vertical pressure gradient as well as vorticity and buoyancy.     

磁场对水平温度梯度下双层流体热对流的影响

在水平温度梯度下,双层流体交界面的表面张力会出现梯度,驱动热毛细对流运动,造成热剪切层内的扰动.本文数值模拟了不同重力条件下,双层流体内的对流现象,得出了在微重力时,对流运动将引起热剪切层内强烈的扰动.为了减弱这种扰动,我们利用磁场对流体的运动进行控制.为此,又对微重力条件下,不同方向应用磁场下的热剪切层内扰动行为进行了数值研究,结果显示,磁场对热剪切层稳定性有促进作用,加入法向的应用磁场最为有效.     

Coupling of Evaporation and Thermocapillary Convection in a Liquid Layer with Mass and Heat Exchanging Interface

We propose and analyse a new model of thermocapillary convection with evaporation in a cavity subjected to horizontal temperature gradient, rather than the previously studied model without evaporation. The pure liquid layer with a top free surface in contact with its own vapour is considered in microgravity condition. The computing programme developed for simulating this model integrates the two-dimensional, time-dependent Navier-Stokes equations and energy equation by a second-order accurate projection method. We focus on the coupling of evaporation and thermocapillary convection by investigating the influence of evaporation Biot number and Marangoni number on the interracial mass and heat transfer. Three different regimes of the coupling mechanisms are found and explained from our numerical results.     

Thermocapillary deformation of a thin locally heated horizontal liquid layer

In this paper, steady thermocapillary flow in a thin horizontal layer of a viscous incompressible liquid with a free surface is considered. An axially symmetric steady problem with a localized thermal action on a horizontal liquid layer with a deformable free surface is solved in a thin-layer approximation. In addition to the thermocapillary effect, the model takes into account the capillary pressure caused by the free surface variable curvature and the convective mechanism of heat transfer in the liquid. Analytical expressions for the velocity vector components as functions of the liquid layer thickness and surface temperature are obtained. The free surface and velocity profiles caused by various kinds of heating are calculated. The influence of convective heat transfer on the flow pattern is analyzed.     

Crackless linear through-wafer etching of Pyrex glass using <Emphasis Type="Italic">liquid-assisted</Emphasis> CO<Subscript>2</Subscript> laser processing

Pyrex glass etching is an important technology for the microfluid application to lab-on-a-chip devices, but suffers from very low etching rate and mask-requiring process in conventional HF/BOE wet or plasma dry etching as well as thermal induced crack surface by CO₂ laser processing. In this paper, we applied the liquid-assisted laser processing (LALP) method for linear through-wafer deep etching of Pyrex glass without mask materials to obtain a crackless surface at very fast etching rates up to 25 μm/s for a 20 mm long trench. The effect of laser scanning rate and water depth on the etching of the 500 μm thick Pyrex glass immersed in liquid water was investigated. The smooth surface without cracks can be achieved together with the much reduced height of bulge via an appropriate parameter control. A mechanism of thermal stress reduction in water and shear-force-enhanced debris removal is discussed. The quality improvement of glass etching using LALP is due to the cooling effect of the water to reduce the temperature gradient for a crackless surface and natural convection during etching to carry away the debris for diminishing bulge formation. An erratum to this article can be found at