Rayleigh-Bénard湍流对流实验研究进展

近十年来,人们的注意力集中到关于Ｒａｙｌｅｉｇｈ－Ｂｅｎａｒｄ硬湍流对流的研究,得到了硬湍流状态下温度场和速度场的主要特征,证实了大惊工充在热对流中起主导作用等,但对大尺度环流与周围流及热郑注的相互作用机理还不很清楚,硬湍流是否为Ｒａｙｌｅｉｇｈ－Ｂｅｎａｒｄ对流的极限状态仍有不同看法,本文将对有关这方面的研究进展作一简要的回顾,并就今后的研究表一些看法。     

剪切湍流大尺度相干结构的模式研究

发展了一种计算剪切湍流大尺度相干结构的新模式．该模式的基础是认为大尺度相干结构为湍流场中流体脉动能量增长最快的那部分，且包含大部分的湍流脉动能量．在此基础上。通过对湍流相干能量方程的推演。建立了描述大尺度相干结构的特征控制方程，并应用Ｃｈｅｂｙｓｈｅｖ多项式方法求得湍流相干能量的最大增长率在波数空间的分布，从而获得对应的大尺度相干结构．应用该模式研究了槽流和一自然对流中的大尺度相干结构，得到的近壁区流动结构与实验现象十分接近．     

ADVANCES AND OUTLOOK IN TURBULENT RAYLEIGH-BENARD CONVECTION

对流现象广泛存在于恒星和行星里.在地球上,对流现象在诸如大气、海洋、地核和地幔等众多动力学系统中起着重要作用.Rayleigh-Bénard（RB）湍流热对流系统是从这些复杂的自然现象中抽象出来的研究对流问题的经典流体力学模型.本文主要从湍流传热、羽流、大尺度流动结构、速度和温度脉动的小尺度统计和非传统RB对流等几个方面着重评述近年来RB对流的若干研究新进展,并对今后的研究做出展望.     

热湍流研究的新十年:从聚焦传统到延伸拓展

热湍流(浮力驱动湍流)作为一种典型的湍流现象,广泛存在于自然界和工程应用中. Rayleigh-Bénard (RB)湍流是从众多自然现象中抽象出来研究热湍流的经典模型, RB湍流的典型特征是系统中存在大尺度环流和羽流等不同尺度的湍流结构,这些结构通过作用于边界层,影响RB湍流的输运效率.因此,明确不同尺度湍流结构的生成、演化和作用机理,对理解RB湍流的输运特性至关重要,也是通过控制湍流结构调控输运效率的科学基础.本文重点从湍流结构的时空演化规律、输运特性、湍流调控和热湍流在其他领域的拓展四个方面评述近十年来RB湍流研究所取得的新进展,并对今后的研究方向做出展望.     

竖直平板间自然对流的湍流频谱特性

根据直接数值模拟的计算结果，对竖直平板间湍流自然对流的脉动动能、速度及温度等物理量的时间序列进行频谱分析．结果表明，流动达到充分发展状态后，小尺度到大尺度的能谱很宽，计算的分辨率足够．从能谱分布可以观察到含能区、惯性子区和耗散区的存在，文中对各区的特性进行分析．由于该流动的强各向异性，惯性子区很窄．并讨论了法向位置对脉动动能的影响以及大尺度结构的特性．     

二维槽道湍流数值模拟与白组织特性

从流体力学基本方程出发,讨论了二维槽道湍流的衰减特性,通过对流场施加合适的体积力,采用拟谱方法对二维槽道强制湍流进行了数值模拟.研究了二维槽道衰减湍流的自组织与逆能量级串特性,再现了二维槽道衰减湍流中湍涡的自组织过程,以及不同波数湍流结构所携能量在自组织过程中的变化,并解释了二维槽道湍流平均速度曲线特征以及海洋环流所特有的自然现象.     

湍流一般机理及其应用

本文综述了湍流机理发展中最重要的一些文章,包括:Brown-Roshko的混合剪切层中大尺度涡的发展;Perry和Chong的湍流边界层中A形涡结构的湍流机理;以及笔者提出的关于3维湍流场中涡结构的涡量脉动对流和扩散并存的一般机理。在一般机理基础上建立了判别流场的准则,列举了在固壁边界附近的猝发现象并展望它的未来应用。     

壁湍流相干结构和减阻控制机理

剪切湍流中相干结构的发现是上世纪湍流研究的重大进展之一,这些大尺度的相干运动在湍流的动力学过程中起重要作用,也为湍流的控制指出了新的方向.壁湍流高摩擦阻力的产生与近壁区流动结构密切相关,基于近壁区湍流动力学过程的减阻控制方案可以有效降低湍流的摩擦阻力,但是随着雷诺数的升高, 这些控制方案的有效性逐渐降低.近年来研究发现, 在高雷诺数情况下外区存在大尺度的相干运动,这种大尺度运动对近壁区湍流和壁面摩擦阻力的产生有重要影响,为高雷诺数湍流减阻控制策略的设计提出了新的挑战.该文将对壁湍流相干结构的研究历史加以简单的回顾,重点介绍近壁区相干结构及其控制机理、近年来高雷诺数外区大尺度运动的研究进展,在此基础上提出高雷诺数减阻控制研究的关键科学问题.      

二维Rayleigh-Bénard对流问题稳定性的数值追踪

用分解算子法和延续算法对二维Ｒａｙｌｅｉｇｈ－Ｂｅｎａｒｄ对流问题的稳定性进行了数值追踪研究．画出了 Ｐｒ＝ １０时不同 Ｒａ所对应的流线，等涡线和等温线图；并求出了对于不同Ｐｒ数所对应的临界Ｒａ数，其值大约为２７４０，计算结果与物理分析相一致，与三维实验结果比较也合理．     

湍流边界层大尺度相干结构空间相位平均拓扑

湍流边界层大尺度相干结构(large-scale coherent structure,LSCS)是目前湍流边界层研究的热点之一.湍流边界层中存在大尺度相干结构均是通过各种谱的低波数峰值证实的,而对大尺度相干结构各种物理量的空间拓扑形态及其发展演化规律还缺乏研究.应用高时间分辨粒子图像测速多相机阵列系统,测量湍流边界层...     

Numerical analysis of mixed convection in three‐dimensional rectangular channel with flush‐mounted heat sources based on field synergy principle

In this paper the fluid flow and heat transfer characteristics of mixed convection in three‐dimensional rectangular channel with four heat sources are investigated numerically. The SIMPLEC algorithm is applied to deal with the coupling between pressure and velocity, and a new high‐order stability‐guaranteed second‐order difference (SGSD) scheme is adopted to discretize the convection term. The influence of four parameters is studied: Richardson number, heat source distribution, channel height and inclination angle. The numerical results are analysed from the viewpoint of the field synergy principle, which says that the enhanced convective heat transfer is related not only to the velocity field and temperature field, but also to the synergy between them. It is found that the effects of the four parameters on the thermal performance can all be explained with the field synergy principle. To obtain better electronic cooling, the synergy between the velocity and temperature gradient should be increased when other conditions are unchanged. Copyright © 2006 John Wiley & Sons, Ltd.     

Turbulent free convection heat transfer to power-law fluids from arbitrary geometric configurations

The problem of turbulent free convection heat transfer from curved surfaces to non-Newtonian power-law fluids has been investigated using the Nakayama-Koyama solution methodology. The scheme is designed to deal with bodies of arbitrary geometric configurations and hence can be viewed as a generalized version of the Shenoy-Mashelkar approach for turbulent free convection heat transfer from a flat vertical plate to a power-law fluid. The surface wall temperature is allowed to vary in the streamwise direction in an arbitrary fashion, and calculations are carried out for the turbulent free convection about the horizontal circular cylinder and sphere for illustrative purposes. Available theoretical and experimental data have been compared with the predictions of the present analysis and the comparison of results has been found to be reasonably good.     

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.     

Effect of non-uniform temperature gradient and magnetic field on onset of Marangoni convection heated from below by a constant heat flux

This paper investigates the effect of non-uniform temperature gradient and magnetic field on Marangoni convection in a horizontal fluid layer heated from below and cooled from above with a constant heat flux. A linear stability analysis is performed. The influence of various parameters on the convection onset is analyzed. Six non-uniform basic temperature profiles are considered, and some general conclusions about their desta- bilizing effects are presented.     

Convective Flows in a Melt During Crystal Growth under Periodic Temperature Field Conditions

The results of an analysis of thermo-gravitational convection in Czochralski single-crystal growth from a melt under periodic temperature field conditions are presented. The numerical modeling is based on solving the unsteady 3-D Navier-Stokes and heat conduction equations in the Boussinesq approximation. It is shown that using different heating regimes for the crystal medium provides additional opportunities for controlling heat and mass transfer.     

Steady mixed convection stagnation-point flow of upper convected Maxwell fluids with magnetic field

The steady MHD mixed convection flow of a viscoelastic fluid in the vicinity of two-dimensional stagnation point with magnetic field has been investigated under the assumption that the fluid obeys the upper-convected Maxwell (UCM) model. Boundary layer theory is used to simplify the equations of motion, induced magnetic field and energy which results in three coupled non-linear ordinary differential equations which are well-posed. These equations have been solved by using finite difference method. The results indicate the reduction in the surface velocity gradient, surface heat transfer and displacement thickness with the increase in the elasticity number. These trends are opposite to those reported in the literature for a second-grade fluid. The surface velocity gradient and heat transfer are enhanced by the magnetic and buoyancy parameters. The surface heat transfer increases with the Prandtl number, but the surface velocity gradient decreases.     

Entropy analysis in electrical magnetohydrodynamic(MHD) flow of nanofluid with effects of thermal radiation,viscous dissipation,and chemical reaction

The unsteady mixed convection flow of electrical conducting nanofluid and heat transfer due to a permeable linear stretching sheet with the combined effects of an electric field, magnetic field, thermal radiation, viscous dissipation, and chemical reaction have been investigated. A similarity transformation is used to transform the constitutive equations into a system of nonlinear ordinary differential equations.The resultant system of equations is then solved numerically using implicit finite difference method.The velocity, temperature, concentration, entropy generation, and Bejan number are obtained with the dependence of different emerging parameters examined. It is noticed that the velocity is more sensible with high values of electric field and diminished with a magnetic field. The radiative heat transfer and viscous dissipation enhance the heat conduction in the system. Moreover, the impact of mixed convection parameter and Buoyancy ratio parameter on Bejan number profile has reverse effects. A chemical reaction reduced the nanoparticle concentration for higher values.     

Simultaneous heat and mass transfer under unsteady mixed convection along a vertical slender cylinder embedded in a porous medium

Unsteady laminar mixed convection flow (combined free and forced convection flow) along a vertical slender cylinder embedded in a porous medium under the combined buoyancy effect of thermal and species diffusion has been studied. The effect of the permeability of the medium as well as the magnetic field has been included in the analysis. The partial differential equations with three independent variables governing the flow have been solved numerically using a implicit finite difference scheme in combination with the quasilinearization technique. Computations have been carried out for accelerating, decelerating and oscillatory free stream velocity distributions. The effects of the permeability of the medium, buoyancy forces, transverse curvature and magnetic field on skin friction, heat transfer and mass transfer have been studied. It is found that the effect of free stream velocity distribution is more pronounced on the skin friction than on the heat and mass transfer. The permeability and magnetic parameters increase the skin friction, but reduce the heat and mass transfer. The skin friction, heat transfer and mass transfer are enhanced due to the buoyancy forces and curvature parameter. The heat transfer is strongly dependent on the viscous dissipation parameter and the Prandtl number, and the mass transfer on the Schmidt number.     

Magnetic and Gravitational Convection of Air in a Porous Cubic Enclosure with a Coil Inclined Around the Y Axis

The natural convection heat transfer of air in a porous media can be controlled by gradient magnetic field. Thermomagnetic convection of air in a porous cubic enclosure with an electric coil inclined around the $Y$ axis was numerically investigated. The Biot–Savart law was used to calculate the magnetic field. The governing equations in primitive variables were discretized by the finite-volume method and solved by the SIMPLE algorithm. The flow and temperature fields for the air natural convection were presented and the mean Nusselt number on the hot wall was calculated and compared. The results show that both the magnetic force and coil inclination have significant effect on the flow field and heat transfer in a porous cubic enclosure, the natural convection heat transfer of air can be enhanced or controlled by applying gradient magnetic field.     

Natural convection flow under a magnetic field in an inclined square enclosure differentialy heated on adjacent walls

Steady, laminar, natural-convection flow in the presence of a magnetic field in an inclined square enclosure differentially heated along the bottom and left vertical walls while the other walls are kept isothermal was considered. The governing equations were solved numerically for the stream function, vorticity and temperature ratio using the differential quadrature method for various Grashof and Hartmann numbers, inclination angle of the enclosure and direction of the magnetic field. The orientation of the enclosure changes the temperature gradient inside and has a significant effect on the flow pattern. Magnetic field suppresses the convective flow and its direction also influences the flow pattern, causing the appearance of inner loops and multiple eddies. The surface heat flux along the bottom wall is slightly increased by clockwise inclination and reduced by half by the counterclockwise inclination. The surface heat flux along the upper portion of the left side wall is reversed by the rise of warmer fluids due to the convection currents for no inclination and clockwise inclination of the enclosure.