环形腔内双层薄液层热毛细对流的渐近解

为了解水平温度梯度作用下环形腔内双层薄液层热毛细对流的基本特性,采用渐近线方法获得了热毛细对流的近似解. 环形腔外壁被加热,内壁被冷却,上、下壁面绝热. 结果表明,当环形腔宽度与内半径比趋于零时,环形腔退化为矩形腔,所得到的主流区速度场和温度场的表达式演化为Nepomnyashchy 等得到的矩形腔内的结果;与数值模拟结果的比较发现,在主流区渐近解与数值解吻合较好.      

液封液桥内热毛细对流的数值模拟

建立了液封液桥(不相溶混的双层同轴液柱)内热毛细对流的物理数学模型,采用涡量-流 函数法、利用有限差分格式对微重力条件下液封液桥内热毛细对流进行了数值模拟,得到了 双层液柱主流区的温度场和流场,证实了液封技术能削弱液桥主流区的热毛细对流,从而提 高浮区晶体生长质量,找出了液封厚度以及内、外层流体物性参数比对液桥内热毛细对流的 影响规律.     

基于POD-Galerkin降维方法的热毛细对流分岔分析

作为流动与传热相互耦合的非线性过程, 热毛细对流有着复杂的转捩过程, 探究流场和温度场随参数变化而发生的分岔现象, 是热毛细对流研究的一个重要课题. 基于本征正交分解的POD-Galerkin降维方法可以通过提取特征模态, 构建低维模型, 实现流场的快速计算. 数值分岔方法可以通过求解含参数动力系统的分岔方程, 直接计算稳定解和分岔点. 探究了将直接数值模拟方法、POD-Galerkin降维方法、数值分岔方法的优势结合, 以提高热毛细对流转捩过程分析效率的可行性. 利用直接数值模拟得到的流场和温度场数据, 构建了不同体积比下, 二维有限长液层热毛细对流的POD-Galerkin低维模型, 在低维模型上采用数值积分及数值分岔方法计算了分岔点, 得到了低维方程的分岔图. 在一定参数范围内, 在低维模型上模拟热毛细对流, 对雷诺数和体积比进行参数外推, 通过与直接数值模拟的结果对比, 验证了低维模型的准确性与鲁棒性. 说明了低维方程可以定性反映原高维系统的流动特性, 而定量方面, 由低维模型和直接数值模拟计算得到的周期解频率的相对误差大约为5%. 验证了利用POD-Galerkin降维方法研究热毛细对流的可行性.      

磁场对双扩散液层热毛细对流的影响

通过数值模拟的方法对磁场作用下的双扩散液层热毛细对流进行了研究, 模型中同时考虑了热毛细效应和溶质毛细效应的存在. 研究结果显示, 外部磁场能够有效削弱液层内热毛细对流的强度, 改变热毛细对流的对流结构; 随着磁场强度的增大, 液层内热毛细对流的对流强度逐渐减小, 热质传递过程中扩散效应逐渐得到增强; 最终, 溶质浓度沿水平方向呈梯度分布. 因此, 当磁场强度足够大时能够实现晶体生长中所需的纯扩散条件.     

浮区热毛细对流

概述了浮区中平行于自由面的表面张力梯度驱动热毛细对流领域的研究. 研究兴趣集中于振荡热毛细对流的起振, 或者说从定常流动到振荡流动的转捩. 起振依赖于一系列的临界参数, 临界关系可以表示为这些临界参数的复杂函数. 实验结果表明, 振荡流中速度的变化和平均流动的速度有相同的量级, 而其它量的变化, 比如温度和自由面半径的波动, 相比于它们的平均量而言则要小得多. 因此, 起振应是流体中动力学过程的结果, 该问题是强非线性的. 在过去几十年中, 一些理论模型被引入来研究这个问题, 使用的方法包括理论分析方法、 线性不稳定性分析方法、 能量稳定性分析方法以及非定常的三维直接数值模拟. 其中直接数值模拟被认为是对强非线性过程进行深入分析的最适合方法, 通常能得到和实验较符合的结果. 从振荡热毛细对流向湍流的转捩提供了一个研究混沌行为的新系统, 开创了一个非线性科学的新前沿, 是一个集中了大量近期工作的研究热点. 该文对浮区热毛细对流作了一个回顾, 包括理论模型和分析, 以及实验研究.     

半浮区热毛细对流的不稳定性与转捩

近二十年来,微重力流体开展了半浮区液桥热毛细对流的不稳定性与转捩的研究．文中给出了热毛细振荡对流发生的临界参数,分析了液桥几何位形（尺度比,体积比）、物理参数及传热参数对临界Ｍａｘａｎｇｏｎｉ的影响．报导了有关的地面模拟实验,微重力实验以及本问题的线性稳定性分析、能量分析和数值模拟结果,并介绍了定常轴对称热毛细对流通过非定常振荡热毛细对流到湍流的转捩过程和三种热毛细振荡对流的产生机理．      

半浮区热毛细对流的不稳定性与转捩

近二十年来,微重力流体开展了半浮区液桥热毛细对流的不稳定性与转捩的研究．文中给出了热毛细振荡对流发生的临界参数,分析了液桥几何位形（尺度比,体积比）、物理参数及传热参数对临界Ｍａｘａｎｇｏｎｉ的影响．报导了有关的地面模拟实验,微重力实验以及本问题的线性稳定性分析、能量分析和数值模拟结果,并介绍了定常轴对称热毛细对流通过非定常振荡热毛细对流到湍流的转捩过程和三种热毛细振荡对流的产生机理．     

浮力-热毛细对流表面位型研究

将栅线结构光源系统与图像处理技术相结合,形成一种实时诊断浮力-热毛细对流表面形貌的光学测量系统.研究两端带有温差的矩形池内薄层流体的表面变形.应用图像互相关方法对实验结果进行计算和分析,得到了流体表面变形定量的实验结果.实验结果表明了在浮力-热毛细对流的发展过程中,出现流体表面变形,温度梯度、表面张力、浮力、液固亲润问题、以及壁面温度决定了该表面变形大小和变形方向.     

液层厚度对浮力-热毛细对流面型的影响

将Michelson光学干涉测量系统与图像处理技术相结合，发展形成一种实时诊断热毛细对流和浮力对流流体表面形貌的实验测量系统. 采用光学干涉测量方法研究了两端带有温差的矩形池内薄层流体的对流、表面变形、以及表面波的基本问题. 应用Fourier变换方法对实验结果进行计算和分析，得到了流体表面变形和表面波的定量的实验结果. 实验结果表明了在浮力-热毛细对流的发展过程中，首先出现流体的表面变形，之后在该变形的基础上，叠加了一个表面波的信息，该表面变形和表面波与流体的温度梯度、表面张力、以及浮力有直接的关系；表面波隐藏在表面变形内.     

三维数值模拟内环加热液池内热毛细对流

为了深入理解热毛细对流不稳定性的特征,数值模拟了内环壁加热、外环壁被冷却的硅油环形浅液池内热毛细对流.发现在内外壁问温差相同的情况下,在内壁加热的液层里,流体表面的温度梯度高于外壁加热情况下10%左右;相应地,发生热流体波的临界Marangoni数Mac=3.06×105(或临界温差△Tc=4.58K)低于外壁加热情况下的Mac=3.34×105(或△Tc=5.0 K).超过临界条件后,内壁加热液池内形成振荡的多涡胞热流体波,它由共存的两组波数相同、频率和振幅相同、沿周向传播方向相反、沿径向凹向高温壁传播的、弯曲螺旋状的热流体波组成,并相互干涉形成点状波纹.     

地球物理流体动力学的发展和室内实验研究的影响

系统介绍了大气、海洋与新兴的地质系统中的流体力学,尤其是通过室内实验的研究进展．详细描述了诸如羽流、湍流、卷挟、异重流、海洋环流、混合层、双扩散、盐指、锋面、岩浆库、地幔对流、板块运动、热斑等有趣的地球物理现象,它们对于了解因浮力引起的对流的机理是十分重要的．同时在气候、环境、灾害、成矿等领域有广泛的应用．论文反映了在 Ｇ．Ｉ． Ｔａｙｌｏｒ, Ｇ．Ｋ． Ｂａｔｃｈｅｌｏｒ领导下剑桥研究组的风格──通过小型室内实验了解机理,再用应用数学方法求解问题,他们对流体力学的发展作出了重大贡献．     

Convection in a two-layer system due to the combined action of the Rayleigh and thermocapillary instability mechanisms

In a two-layer system loss of stability may be monotonic or oscillatory in character. Increasing oscillatory perturbations have been detected in the case of both Rayleigh [1, 2] and thermocapillary convection [3–5]; however, for many systems the minimum of the neutral curve corresponds to monotonic perturbations. In [5] an example was given of a system for which oscillatory instability is most dangerous when the thermogravitational and thermocapillary instability mechanisms are simultaneously operative. In this paper the occurrence of convection in a two-layer system due to the combined action of the Rayleigh (volume) and thermocapillary (surface) instability mechanisms is systematically investigated. It is shown that when the Rayleigh mechanism operates primarily in the upper layer of fluid, in the presence of a thermocapillary effect oscillatory instability may be the more dangerous. If thermogravitational convection is excited in the lower layer of fluid, the instability will be monotonic.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 166–170, January–February, 1987.     

Air entrapment during a high-speed liquid jet entry in a deep water surface

The fluid dynamic phenomena of a high speed liquid jet impact on a deep water surface have been studied using Imacon high-speed photography. Both framing and streak techniques are applied to investigate the initial impact stage and penetration velocity. The cavitation caused by air entrapment between two colliding liquid surfaces has been found. The bubble collapse experiences different stages in relation to the contact area, liquid shock wave, release wave and fluid convection.     

Layered Thermohaline Convection in Hypersaline Geothermal Systems

Thermohaline convection occurs in hypersaline geothermal systems due to thermal and salinity effects on liquid density. Because of its importance in oceanography, thermohaline convection in viscous liquids has received more attention than thermohaline convection in porous media. The fingered and layered convection patterns observed in viscous liquid thermohaline convection have been hypothesized to occur also in porous media. However, the extension of convective dynamics from viscous liquid systems to porous media systems is complicated by the presence of the solid matrix in porous media. The solid grains cause thermal retardation, hydrodynamic dispersion, and permeability effects. We present simulations of thermohaline convection in model systems based on the Salton Sea Geothermal System, California, that serve to point out the general dynamics of porous media thermohaline convection in the diffusive regime, and the effects of porosity and permeability, in particular. We use the TOUGH2 simulator with residual formulation and fully coupled solution technique for solving the strongly coupled equations governing thermohaline convection in porous media. We incorporate a model for brine density that takes into account the effects of NaCl and CaCl2. Simulations show that in forced convection, the increased pore velocity and thermal retardation in low-porosity regions enhances brine transport relative to heat transport. In thermohaline convection, the heat and brine transport are strongly coupled and enhanced transport of brine over heat cannot occur because buoyancy caused by heat and brine together drive the flow. Random permeability heterogeneity has a limited effect if the scale of flow is much larger than the scale of permeability heterogeneity. For the system studied here, layered thermohaline convection persists for more than one million years for a variety of initial conditions. Our simulations suggest that layered thermohaline convection is possible in hypersaline geothermal systems provided the vertical permeability is smaller than the horizontal permeability, as is likely in sedimentary basins such as the Salton Trough. Layered thermohaline convection can explain many of the observations made at the Salton Sea Geothermal System over the years.     

Stability of two-layer systems with respect to convective mixing

The occurrence and development of convection in a two-layer system heated below has been investigated [1–5] under the assumption that the interface of the fluids is horizontal and is not subject to deformations. However, this assumption may not be satisfied if the surface tension on the interface is small and the fluids have either nearly equal densities or the heavier fluid is situated at the top. In the present paper, an attempt is made to study the convection regimes in a two-layer system with deformation of the interface when there is heating from below or above. The simultaneous influence of the convective and Rayleigh-Taylor instability mechanisms is taken into account; the surface tension on the interface is assumed to be infinitesimally small, and thermocapillary effects are ignored. A two-fluid variant of the method of markers and cells [6–9] is used for the numerical solution of the convection equations. A diagram of the regimes is constructed. It is shown that depending on the values of the parameters the system either preserves its two-layer structure, or the development of the conveetive motion leads to the breakup of the interface and complete mixing of the fluids.     

Flow coupling mechanisms in two-layer Rayleigh–Benard convection

 Rayleigh-Benard convection in two-layer systems is characterized by two distinct modes of flow coupling. These are: thermal coupling and mechanical coupling. Intellegible observations of the temperature field for both coupling mechanisms are provided. The flow coupling mechanisms are experimentally characterized as a function of the contrast in the buoyancy driving forces and in the viscosities of the two layers. Aside from the flow coupling between the layers, flow patterns in each layer, and their corresponding spatial transitions are found to be similar to those reported for single layer convection in rectangular boxes. Received: 7 January 1996/Accepted: 4 February 1997     

矩形液池热毛细对流转捩途径研究

主要研究矩形液池热毛细对流的分岔转捩. 通过测量流体内部温度振荡情况, 详细研究了热毛细对流的转捩过程和转捩途径. 实验发现, 矩形液池热毛细对流的转捩过程依次经历了定常、规则振荡、不规则振荡的阶段. 对于不同普朗特数的硅油在不同长高比情况下, 通向混沌的途径不同. 在转捩过程中, 随着温差的增加, 普朗特数在16 (1cSt) 以下和普朗特数为25 (1.5cSt)、长高比为26 的硅油热毛细对流主要以准周期分岔的转捩方式为主;而普朗特数为25 以上的则以倍周期分岔的转捩方式为主;两种分岔有时还会伴随有切分岔形式的出现.实验中还观察到了表面波动和对流涡胞振荡等现象.      

Experimental techniques in natural convection

Experimental techniques in natural convection heat transfer employed in the author's laboratory are introduced. The techniques are mostly related to visualization of flow, temperature field, and heat flux distribution in fluids. Three topics are presented, the first being natural convection in a horizontal rectangular liquid layer driven by surface tension and buoyancy. The patterns of flow were visualized by suspending fine aluminum flakes in the liquid. At the same time, the distribution of the temperature gradient in the liquid was visualized by an optical method making use of the refraction of light. The second topic is the onset of oscillatory convection in the Czochralski growth melt. In this case a forced flow due to rotation of the crystal and the vessel is superimposed on the buoyancy convection, resulting in an oscillatory flow under certain circumstances. The flow pattern and the temperature distribution in the liquid were visualized simultaneously by suspending in the liquid a microencapsulated temperature-sensitive liquid crystal. Periodical oscillation of the flow and the temperature was clearly recognized. The third topic is the rollover of double liquid layers that were stratified stably due to a density difference. A small-scale experiment was carried out to clarify the basic mechanism of rollover. The tracer method was used to visualize boundary layer flow along the vertical side wall and the shadowgraph technique to visualize the density distribution in the liquid layers. The article emphasizes the importance of visual observation in the investigation of natural convection phenomena.     

对流扩散方程的迎风变换及相应有限差分方法

本文提出所谓迎风变换,将对流扩散方程分解为对流迎风函数和扩散方程,并构造相应的有限差分格式。对流迎风函数以简明的指数解析形式反映对流扩散现象的迎风效应,原则上消除了源于不对称对流算子的困难,能够便利对流扩散方程的数值求解。有限差分格式具有二阶精度和无条件稳定性,算例表明其准确性、收敛速度及对边界层效应的适应能力均明显优于中心差分格式和迎风差分格式。