半球旋转适应过程的数值试验

为了验证文献[1]和[2]中提出的全球面大气的旋转适应过程理论,本文将先考虑半球问题时的情况,并作了大量数值试验.结果和理论分析完全相符:当半球上只有一个位涡度中心时,流场趋向于带状纬圈环流;若有二个或多个中心存在时,所有位涡度中心都始终存在,流场不趋向于带状纬圈环流.数值试验还给出低指数环流向高指数环流转变时期流场结构的一般特点,并给出旋转适应过程的特征时间依赖于扰动的尺度,一般为四天至半个月左右.     

一类描述大气中重力孤立波的新模型

从两层流体浅水波方程出发,运用尺度分析与扰动方法,建立了一类新的模型(mKdV-BO模型)来描述大气中的重力孤立波。前人建立的KdV模型和BO模型适合描述经向和纬向扰动较弱时重力孤立波的生成和演化,而该模型的非线性更强,适合描述经向、纬向扰动较强时重力孤立波的生成与演化。通过运用试探函数法获得了模型的代数孤波解,并分析了孤立波的生成条件与传播速度。新模型的建立对于进一步解释大气中列队雷雨阵的形成机制,探讨大气中的强对流天气如飑线的形成等具有重要意义。 关键词：重力孤立波;试探函数法;列队雷雨阵     

热源强迫的边界层低涡解及其应用

考虑边界层低涡为受非绝热加热和摩擦强迫并满足热成风平衡的轴对称涡旋系统,采用Boussinesq近似,通过求解柱坐标系中涡旋模式的初值问题,分析了热源强迫对低涡流场结构的影响．结果表明:热源强迫对低涡的流场结构有重要影响,并且这种影响的具体表现形式与加热的径向分布有密切关系．对边界层涡旋解讨论的结果可以解释青藏高原低涡系统的某些重要结构特征．     

Rossby惯性重力孤立波与东、西太平洋副高活动的遥相关

根据季内西太平洋副高活动异常的观测事实，考虑夏季北太平洋地区500hPa层以下的有限纬带区域，应用非线性浅水模式得到了一种孤立波。该波的产生、维持及传播与中、东太平洋副高主体的活动关系密切。 通过对这类孤立波传播特征的讨论和模式大气计算分析，发现它与夏季北太平洋副热带地区存在的位势中心的传播路径和范围比较一致，太平洋中、东部副高主体位置和强度的变化通过这种孤立波的传播可能导致西太平洋副高出现相应的变化。     

有限振幅超长波的发展及对流层大气环流的指数循环

本文由一个描写超长波运动的准地转三层模式,利用多尺度分析法,得到了有限振幅超长波不稳定和西风不稳定的非线性理论.当考虑到超长波扰动和基本西风气流的非线性相互制约后,同时得到了超长波振幅和西风强度作周期性振荡(周期为2—6周),与超长波尺度和初始振幅有关.流场演交表现了类似低指数的阻塞形势和高指数的纬圈环流的转换.超长波发展过程中的热量输送,维持了指数循环的能量转换.因此本文证明了指数循环是大气的超长波和基本纬圈气流相互制约的非线性反馈过程.     

青藏高原东南部低涡的初步模拟实验

青藏高原东南部低涡,是中国西南地区的低压天气系统,它的发展和移动常引起下游地区的大暴雨,对中国东部地区影响较大。本文试用流体转盘模拟实验方法研究了青藏高原东南部的低涡,实验表明,青藏高原地形对西风流场有巨大的阻挡与扰动作用,造成高原附近具有显著特征的槽脊系统,在高原西部上空产生反气旋性流场,在高原的东南部尾流中产生气旋性流场。同时,整个高原作为热源,特别是高原东南部对大气的热力作用,和横断山脉的动力摩擦作用,使得高原东南部最容易产生低压涡旋。而高空急流的位置对低涡的移动和发展关系也很密切,涡旋沿急流方向移动,并从急流基本流场中取得能量,使涡旋维持和发展,此外,对从高原西部向东移的涡旋也进行了模拟试验。     

低纬地区非线性孤立波存在性讨论

本文讨论了静力平衡下的绝热自由大气非线性重力惯性波的孤立渡解存在性,设运动在y方向是均匀的,又采用了β平面近似,同时忽略了动量方程中垂直平流项,我们导出了孤立波解的解析解表达式,通过对波速c的讨论,我们指出:在中纬地区,很难形成孤立波;在低纬地区(热带地区),由于科氏力很小,层结稳定度较弱有可能产生孤立波。最后我们讨论了孤立波的波速c与科氏力f,β和稳定参数σ_8之间关系。     

大气、地表水和海洋对地球自转极移季节变化的综合贡献

极移季节变化的地球物理定量激发迄今未得到圆满的解释. 基于大气海洋耦合数值环流模式, 计算了大气和地表水及海洋角动量变化. 与日本气象局(JMA)客观分析资料计算的大气角动量比较, 模式模拟的大气角动量东经90°分量要优于0°分量. 地表水和海洋激发有效地减少了极移与JMA大气角动量之间季节尺度上的差异, 并且结合JMA和美国环境预测中心(NCEP)的大气角动量对极移周年变化的综合激发都优于NCAR-CSM1气候模式的结果.     

涡度方程的时空客观分解与副热带高压的突变与分岔

为了分析研究副热带高压异常活动的动力学机理,基于热力强迫和涡动耗散效应的大气偏微涡度方程,采用Galerkin方法进行方程的时-空变量分离,针对常规方法在空间基函数选择中存在的主观人为性,提出从实际资料场序列中用经验正交函数(EOF)分解与遗传算法结合客观反演空间基函数的思想.选择一组三角函数族作为广义空间基函数,以该基函数与EOF典型场的误差最小二乘和基函数间的完备正交性构造双约束泛函,再引入遗传算法进行空间基函数曲面拟合和系数优化,反演得到客观合理的副热带高压常微动力模型.最后,基于所得非线性动力学模型,对热力强迫作用下的副热带高压的动力学行为和机理进行了分析和讨论,发现太阳辐射加热和纬向海陆热力差异是影响副热带地区位势和流场变化从而导致副高强弱变化和中期进退活动的重要因素,前者以渐变为主;后者则更多表现出突变的特性.通过分析,得到了一些有意义的结果.     

2000年7月磁暴期间120°E附近低纬电离层响应

2000年7月15～16日发生了一次特大地磁暴, 其Dst最小值达到-300 nT. 利用东亚中坜、武汉、Kokubunji和Anyang等电离层台站的数字测高仪数据, 分析了此次磁暴期间120°E附近地磁低纬地区的电离层响应. 总体来看, 2000年7月16日f_oF₂为负相暴, 并出现了G现象; 2000年7月17日起为正相暴特征, 且持续时间很长. 赤道异常2000年7月16日很弱, 但7月17日却非常强, 且持续到子夜后. 低纬电离层出现了夜间增强现象. 7月磁暴低纬电离层另一特征是在武汉和Kokubunji两地区间暴变分量日变化存在一个分界, 分界南北的日变化等明显不同. 分析认为, 7月16日负相暴和G现象很可能由磁暴引起中性成分的改变, 特别是[O₂]/[O]和[N₂]/[O]增加导致. 热层暴时环流及由此引起的中性成分改变可解释7月16日赤道异常很弱. 7月17日的正相暴和非常强的赤道异常, 标示着存在很强的喷泉效应, 但此期间中坜等峰高相对暴前变化不大, 可见非常强的赤道异常不是由暴变东西向电场引起. 持续正相暴和f_oF₂夜间增强表明与暴相关的动力学、电动力学过程在起作用.     

流体动力学的客观性要求

从流体动力学的客观性要求导出了新的流动理论.流动运动的不均匀性产生粘性力,不同观察者的选取会影响这种不均匀分布特征.将粘性力看作一种与观察者的选取无关的客观存在时,粘性力和动量方程在局域旋转变换下的形式不变性要求引入一种新的动力学场——涡旋场,通过构造流体系统的拉朗日密度并利用能量变分方法得到了所有场量的动力学方程.     

The effect of variable viscosity on MHD viscoelastic fluid flow and heat transfer over a stretching sheet

An analysis has been carried out to study the momentum and heat transfer characteristics in an incompressible electrically conducting non-Newtonian boundary layer flow of a viscoelastic fluid over a stretching sheet. The partial differential equations governing the flow and heat transfer characteristics are converted into highly non-linear coupled ordinary differential equations by similarity transformations. The effect of variable fluid viscosity, Magnetic parameter, Prandtl number, variable thermal conductivity, heat source/sink parameter and thermal radiation parameter are analyzed for velocity, temperature fields, and wall temperature gradient. The resultant coupled highly non-linear ordinary differential equations are solved numerically by employing a shooting technique with fourth order Runge–Kutta integration scheme. The fluid viscosity and thermal conductivity, respectively, assumed to vary as an inverse and linear function of temperature. The analysis reveals that the wall temperature profile decreases significantly due to increase in magnetic field parameter. Further, it is noticed that the skin friction of the sheet decreases due to increase in the Magnetic parameter of the flow characteristics.     

Combined effects of non-uniform heat source/sink and thermal radiation on heat transfer over an unsteady stretching permeable surface

The present paper is concerned with the study of flow and heat transfer characteristics in the unsteady laminar boundary layer flow of an incompressible viscous fluid over continuously stretching permeable surface in the presence of a non-uniform heat source/sink and thermal radiation. The unsteadiness in the flow and temperature fields is because of the time-dependent stretching velocity and surface temperature. Similarity transformations are used to convert the governing time-dependent nonlinear boundary layer equations for momentum and thermal energy are reduced to a system of nonlinear ordinary differential equations containing Prandtl number, non-uniform heat source/sink parameter, thermal radiation and unsteadiness parameter with appropriate boundary conditions. These equations are solved numerically by applying shooting method using Runge–Kutta–Fehlberg method. Comparison of numerical results is made with the earlier published results under limiting cases. The effects of the unsteadiness parameter, thermal radiation, suction/injection parameter, non-uniform heat source/sink parameter on flow and heat transfer characteristics as well as on the local Nusselt number are shown graphically.     

A Stream Function Approach to Optical Flow with Applications to Fluid Transport Dynamics

Optical flow is one of the classical problems in computer vision, but it has recently also been adapted to applications from other fields, such as fluid mechanics and dynamical systems. If the goal is to analyze the dynamics of system whose evolution is governed by a flow field that is the gradient of a potential function – which describes many flows in fluid dynamics – it is natural to approach the optical flow problem by reconstructing the potential function, also called the stream function, rather than reconstructing the components of the flow directly. This alternate approach allows one to impose scientific priors, via regularization, directly on the flow itself rather than on its components independently. We demonstrate the stream function formulation of optical flow and its application to reconstructing an oceanic fluid flow driven by satellite measurements. It is also shown how these flow fields can be used to analyze mixing and mass transport in the fluid system being imaged. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermally Enhanced Motions of Variable-Inflow Surface Gravity-Driven Flows

In this paper, we report on theoretical and numerical studies of models for suddenly initiated variable-inflow surface gravity currents having temperature-dependent density functions when these currents are subjected to incoming radiation. This radiation leads to a heat source term that, owing to the spatial and temporal variation in surface layer thickness, is itself a function of space and time. This heat source term, in turn, produces a temperature field in the surface layer having nonzero horizontal spatial gradients. These gradients induce shear in the surface layer so that a depth-independent velocity field can no longer be assumed and the standard shallow-water theory must be extended to describe these flow scenarios. These variable-inflow currents are assumed to enter the flow regime from behind a partially opened lock gate with the lock containing a large volume of fluid whose surface is subjected to a variable pressure. Flow filament theory is used to arrive at expressions for the variable inflow velocity under the assumptions of an inviscid and incompressible fluid moving through a small opening under a lock gate at one end of a large rectangular tank containing the deep slightly more dense ambient fluid. Finding this time-dependent inflow velocity, which will then serve as a boundary condition for the solution of our two-layer system, involves solving a forced Riccati equation with time-dependent forcing arising from the surface pressure applied to the fluid in the lock.
The results presented here are, to the best of our knowledge, the first to involve variable-inflow surface gravity currents with or without thermal enhancement and they relate to a variety of phenomena from leaking shoreline oil containers to spring runoff where the variable inflow must be taken into account to predict correctly the ensuing evolution of the flow.     

Melting heat and mass transfer in stagnation point micropolar fluid flow of temperature dependent fluid viscosity and thermal conductivity at constant vortex viscosity

Steady mixed convection micropolar fluid flow towards stagnation point formed on horizontal linearly stretchable melting surface is studied. The vortex viscosity of micropolar fluid along a melting surface is proposed as a constant function of temperature while dynamic viscosity and thermal conductivity are temperature dependent due to the influence of internal heat source on the fluid. Similarity transformations were used to convert the governing equation into non-linear ODE and solved numerically. A parametric study is conducted. An analysis of the results obtained shows that the flow-field is influenced appreciably by heat source, melting, velocity ratio, variable viscosity and thermal conductivity.     

Unsteady hydromagnetic Couette flow of dusty fluid with temperature dependent viscosity and thermal conductivity under exponential decaying pressure gradient

In the present paper the unsteady Couette flow and heat transfer of a dusty conducting fluid between two parallel plates with temperature dependent viscosity and thermal conductivity are studied. The fluid is acted upon by an exponential decaying pressure gradient and an external uniform magnetic field is applied. The governing coupled momentum and energy equations are solved numerically using finite differences. The effect of the variable viscosity and thermal conductivity of the fluid and the uniform magnetic field on the velocity and temperature fields for both the fluid and dust particles is discussed.     

Vibration behavior of simply supported inclined single-walled carbon nanotubes conveying viscous fluids flow using nonlocal Rayleigh beam model

A mathematical model is proposed to investigate the dynamic response of an inclined single-walled carbon nanotube (SWCNT) subjected to a viscous fluid flow. The tangential interaction of the inside fluid flow with the equivalent continuum structure (ECS) of the SWCNT is taken into account via a slip boundary condition. The dimensionless equations of motion describing longitudinal and lateral vibrations of the fluid-conveying ECS are obtained in the context of nonlocal elasticity theory of Eringen. The unknown displacement fields are expressed in terms of admissible mode shapes associated with the ECS under simply supported conditions with immovable ends. Using Galerkin method, the discrete form of the equations of motion is derived. The time history plots of the normalized longitudinal and transverse displacements as well as the nonlocal axial force and bending moment of the midspan point of the SWCNT are provided for different levels of the fluid flow speed, small-scale parameter, and inclination angle of the SWCNT. The effects of small-scale parameter, inclination angle, speed and density of the fluid flow on the maximum dynamic amplitude factors of longitudinal and transverse displacements as well as those of nonlocal axial force and bending moment of the SWCNT are then studied in some detail.     

Genesis of a swirling source/sink line into an updraft

Several mathematical models applied to tornadoes consist of exact and axisymmetric solutions of the steady and incompressible Navier—Stokes equations. These models studied by J. Serrin [5] and M.A. Gol'dshtik and V.N. Sthern (1995) describe families of fluid motions vanishing at the ground and are restricted for not developing neither a source nor a sink near the vortex line. Therefore, J. Serrin showed that the flow patterns of the resulting velocity field may have some realistic characteristics to model the mature phase of the lifetime of a tornado, in comparison with atmospheric observations. On another hand, no reason has been explicated to motivate the absence of a source/sink vortex line. We present here a result of local existence and uniqueness of a family of fluid motions, leading to the genesis of such lines inside a non-rotating updraft which do not develops neither a source nor a sink.