A Conceptual Model of Somali Jet Based on the Biot--Savart Law

We study the relationship between thermal conditions on the earth bottom boundary and the formation of Somali jet based on the Biot-Savart law and the data from National Centres for Environmental Prediction （NCEP）. As the radiation from the Sun gradually moves from the southern meridian, the temperature on the surface of Somali Peninsular and Arabic Peninsular gradually increases. During the same period the surface temperature of the Northern Indian Ocean increases much slower. It is shown that this increase of the temperature difference between the land and sea is inductive to the formation and development of Rayleigh-Benard convection and leads to the increasing relative vorticity strength between positive and negative vertical vortices over the land and sea. According to the Biot-Savart law, increase of vorticity strength will correspondingly induce the horizontal velocity. A pair of positive and negative vorticity fields over the two Peninsulars and the sea surface is effective in forming and maintaining this current. This mechanism is referred to as the ＇Somali suction pump＇. It draws air continually from the Southern hemisphere and releases it at the coastal area of Somali.     

Derivation of the amplitude equation for a hydromagnetic convective problem

The amplitude equation for a convective system under a vertical magnetic field is derived. The coefficients in these equations have been numerically calculated for infinite Prandtl number fluids and for boundary conditions both free and rigid top and bottom. The results confirm that, for realistic parameter values only stationary convection can be present and the pattern is made by convective rolls.     

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.     

Application of the wavelet transform in the laminar turbulent transition for a flow in a mixed convection phenomenon

The aim of this paper is to show the effect of secondary flows caused by natural convection on the laminar-turbulent hydrodynamic transition. It is not a question of measuring a critical threshold value of Reynolds number of transition but only to estimate the degree of turbulence in the transition regime, i.e. weak turbulence in the case of superposition (mixed convection) or not (forced convection) of secondary flows on the forced flow. This is possible thanks to the application of the wavelet transform. The calculation of the H?lder exponent, associated with the maximum value of the singularity spectrum for two configurations, vertical (forced convection) and horizontal (mixed convection) allows the degree of turbulence to be measured in both cases. The variation of the H?lder exponent versus the Reynolds number has enabled it to be shown that the secondary flows stabilise the main flow and stifle the beginnings of the turbulence during the regime of transition to turbulence; these kinds of results have also been shown in literature. Generally, large-sized secondary flows (for example Dean's flows) stabilise the turbulence. Our work confirms this, through an experiment carried out in identical conditions for mixed convection (horizontal flow) and forced convection (vertical flow). Received 30 March 1998 and Received in final form 28 April 1999     

Three-layer magnetoconvection

It is believed that some stars have two or more convection zones in close proximity near to the stellar photosphere. These zones are separated by convectively stable regions that are relatively narrow. Due to the close proximity of these regions it is important to construct mathematical models to understand the transport and mixing of passive and dynamic quantities. One key quantity of interest is a magnetic field, a dynamic vector quantity, that can drastically alter the convectively driven flows, and have an important role in coupling the different layers. In this Letter we present the first investigation into the effect of an imposed magnetic field in such a geometry. We focus our attention on the effect of field strength and show that, while there are some similarities with results for magnetic field evolution in a single layer, new and interesting phenomena are also present in a three layer system.     

Experimental Study on Liquid Free Surface in Buoyant-Thermocapillary Convection

We investigate the surface deformations of buoyant-thermocapillary convection in a rectangular cavity due to gravity and temperature gradient between the two sidewalls. The cavity is 52mm×42 mm in horizontal cross section, the thickness of liquid layer h is changed from 2.5 mm to 6.5 mm. Surface deformations of h = 3.5 mm and 6.0mm are discussed and compared. Temperature difference is increased gradually, and the flow in the liquid layer will change from stable convection to unstable convection. Two kinds of optical diagnostic system with image processor are developed for study of the kinetics of buoyant-thermocapillary convection, they give out the information of liquid free surface. The quantitative results are calculated by Fourier transform and correlation analysis, respectively. With the increasing temperature gradient, surface deformations calculated are more declining. It is interesting phenomenon that the inclining directions of the convections in thin and thick liquid layers are different. For a thin layer, the convection is mainly controlled by thermocapillary effect. However, for a thick layer, the convection is mainly controlled by buoyancy effect. The surface deformation theoretically analysed is consistent with our experimental results. The present experiment proves that surface deformation is related to temperature gradient and thickness of the liquid layer. In other words, surface deformation lies on capillary convection and buoyancy convection.     

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.     

Coupling between aging and convective motion in a colloidal glass of Laponite

We study thermal convection in a colloidal glass of Laponite in formation. Low concentration preparation are submitted to destabilizing vertical temperature gradient, and present a gradual transition from a turbulent convective state to a steady conductive state as their viscosity increases. The time spent under convection is found to depend strongly on sample concentration, decreasing exponentially with mass fraction of colloidal particles. Moreover, at fixed concentration, it also depends slightly on the pattern selected by the Rayleigh Bénard instability: more rolls maintain the convection state longer. This behavior can be interpreted with recent theoretical approaches of soft glassy material rheology.     

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     

水平流作用下行波对流的成长及周期性重复

数值模拟研究了在极其微弱的水平流动作用下，混合流体Rayleigh-Benard对流系统一 维行波斑图的成长及其时空演化问题.揭示了系统行波对流的周期性现象及其对水平流动强 度的依赖性. 关键词： 时空斑图 混合流体对流 水平流     

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.     

热带低层弱涡旋中扰动的快速发展及其向中心传播的特征

台风发生的必要条件是热带低层具有气旋式扰动, 从卫星云图和诊断分析看,许多低层涡旋中存在分立的云团或中尺度系统. 这些涡旋能否发展成为台风,取决于其中的中尺度波动是否发展集合组成密闭云带. 本文利用柱坐标下的两层动力模式,研究了低层弱涡旋中第二类条件不稳定 机 制驱动下的波动的发展和移动问题. 结果表明:热带弱涡旋中的低层基本流垂直切变 可以很大地加强波动的不稳定性; 波动的相速度和群速度都指向涡旋中心, 波动向中心传播,能量向中心频散. 实例和数值研究也都表明,低层涡旋中的中尺度扰动会迅速发展并且向 中心靠近,促使台风形成.     

New problems of particle transfer in ferrocolloids: Magnetic Soret effect and thermoosmosis

The paper deals with critical reviewing of the experiments on thermodiffusion in ferrocolloids. The observed magnetic Soret effect is much stronger than that predicted theoretically. It is shown that the main reason of that is the influence of the magnetic field on mass diffusion. Besides, some measurements are affected by uncontrolled thermal and solutal magnetic convection. In porous media, when macroscopic convection is suppressed, thermodiffusion is accompanied by thermoosmosis as well as by a microconvective mass transfer induced by particle magnetophoresis on filter grains.     

Studying anomalous scaling and heat transport of turbulent thermal convection using a dynamical model

Turbulent thermal convection is a well-studied problem with various issues of interest. In this paper, we review our work which shows the nature and origin of anomalous scaling and heat transport in the limit of very strong thermal forcing, can be gained by studying a dynamical model, known as shell model, of homogeneous turbulent thermal convection in which buoyancy acts directly at most scales. Specifically, we have obtained two results. The first result is that when buoyancy acts directly at most scales such that the dynamics are governed by a cascade of entropy, the scaling behavior is described by Bolgiano and Obukhov scaling plus corrections that are due to the variations of the local entropy transfer rate. This result indicates the validity of the extension of refined similarity hypothesis to turbulent thermal convection. The second result is that when buoyancy is acting directly at most scales, a damping term acting on the largest scale, which has to be added for the system to achieve stationarity, plays a crucial role in heat transport, and that the heat transport depends on the strength of thermal forcing in the same manner as that predicted for the ultimate state of very strong thermal forcing. With our interpretation of the damping term representing the effect of the boundaries, this result indicates that in the ultimate state of turbulent thermal convection, when buoyancy is acting at most scales, boundaries would play a significant role in heat transport.     

Numerical analysis of the Eckhaus instability in travelling-wave convection in binary mixtures

The Eckhaus stability boundaries of travelling periodic roll patterns arising in binary fluid convection is analysed using high-resolution numerical methods. We present results corresponding to three different values of the separation ratio used in experiments. Our results show that the subcritical branches of travelling waves bifurcating at the onset of convection suffer sideband instabilities that are restabilised further away in the branch. If this restabilisation is produced after the turning point of the travelling-wave branch, these waves do not become stable in a saddle node bifurcation as would have been the case in a smaller domain. In the regions of instability of the uniform travelling waves we expect to find either transitions between states of different wave number or modulated travelling waves arising in these bifurcations.     

Observation of orbital magnetic dipole strength in the actinide nuclei 232Th and 238U

Precise experimental information has been obtained on the distribution of orbital M1 strength using high resolution nuclear resonance fluorescence and inelastic electron scattering. We report on the first observation of those J^π = 1⁺ states in actinide nuclei that are excited by the M1 scissors mode. The M1 strength induced through the convection current part of the M1 operator is closely centered around 2 MeV in both nuclei. The observed spreading of strength is again of the same magnitude that has been observed already in rare-earth nuclei.     

Effects of the network structure and coupling strength on the noise-induced response delay of a neuronal network

The Hodgkin-Huxley (H-H) neuron model driven by stimuli just above threshold shows a noise-induced response delay with respect to time to the first spike for a certain range of noise strengths, an effect called “noise delayed decay” (NDD). We study the response time of a network of coupled H-H neurons, and investigate how the NDD can be affected by the connection topology of the network and the coupling strength. We show that the NDD effect exists for weak and intermediate coupling strengths, whereas it disappears for strong coupling strength regardless of the connection topology. We also show that although the network structure has very little effect on the NDD for a weak coupling strength, the network structure plays a key role for an intermediate coupling strength by decreasing the NDD effect with the increasing number of random shortcuts, and thus provides an additional operating regime, that is absent in the regular network, in which the neurons may also exploit a spike time code.     

MHD mixed convection flow near the stagnation-point on a vertical permeable surface

The steady magnetohydrodynamic (MHD) mixed convection boundary layer flow of a viscous and electrically conducting fluid near the stagnation-point on a vertical permeable surface is investigated in this study. The velocity of the external flow and the temperature of the plate surface are assumed to vary linearly with the distance from the stagnation-point. The governing partial differential equations are first transformed into ordinary differential equations, before being solved numerically by a finite-difference method. The features of the flow and heat transfer characteristics for different values of the governing parameters are analyzed and discussed. Both assisting and opposing flows are considered. It is found that dual solutions exist for both cases, and the range of the mixed convection parameter for which the solution exists increases with suction.     

固体火箭尾舱对流加热机理

对某型固体火箭尾舱热环境进行了研究,发现尾舱热环境存在明显的天地差异,通过数值仿真结果分析了对流热环境产生的机理.在真实飞行环境下,火箭尾舱外部绕流产生的激波和发动机喷流激波之间存在剪切层,剪切层与尾舱连通,在激波与剪切层的相互作用下形成非定常漩涡结构,是尾舱内对流加热的来源.剪切层的强弱与对流热流的大小密切相关,根据对流热流产生机理,对一种减小对流热流的结构形式进行了研究,通过减弱火箭尾部的剪切层,大幅减小了对流热流,固体火箭尾舱热环境得到明显改善.