利用重正化群方法推导非线性Reynolds应力模型

近年来,Rubinstein和Barton利用Yakhot和Orszag湍流重正化群方法推导非线性Reynolds应力模型,但是他们分析计算过程中存在着一些不自洽的地方.文中利用重正化群方法,对Reynolds应力的数学表达式作二阶微扰展开,从理论上推导得到非线性Reynolds应力模型,其数学形式与从量纲分析和数学物理性质的合理性讨论得到的通用模型形式完全相同,另外从理论上计算了各待定湍流常数.

A Multiscale-Domain Decomposition Method for High Reynolds Number Flows

The high Reynolds number flow contains a wide range of length and time scales, and the flow domain can be divided into several sub-domains with different characteristic scales. In some sub-domains, the viscosity dissipation scale can only be considered in a certain direction; in some sub-domains, the viscosity dissipation scales need to be considered in all directions; in some sub-domains, the viscosity dissipation scales are unnecessary to be considered at all. For laminar boundary layer region, the characteristic length scales in the streamwise and normal directions are L and L Re-1/ 2 , respectively. The characteristic length scale and the velocity scale in the outer region of the boundary layer are L and U, respectively. In the neighborhood region of the separated point, the length scale l<

中等Reynolds数平板绕流的动理论分析

郭永怀先生1953年给出的中等Reynolds数下、不可压缩流体有限长平板绕流的解析解是边界层理论中的经典工作.许多研究者对平板绕流阻力系数的郭水怀公式以及后续工作进行了评估,评估的依据是Janour与Schaaf和Sherman的实验数据.本文的动理论分析和计算表明:Schaaf和Sherman在低亚声速条件下(0.16

Compressible flows at small Reynolds numbers

The problem of the slow viscous flow of a gas past a sphere is considered. The fluid cannot be treated incompressible in the limit when the Reynolds number Re, and the Mach number M, tend to zero in such a way that Re ~ o(M^2 ). In this case, the lowest order approximation to the steady Navier-Stokes equations of motion leads to a paradox discovered by Lagerstrom and Chester. This paradox is resolved within the framework of continuum mechanics using the classical slip condition and an iteration scheme that takes into account certain terms in the full Navier-Stokes equations that drop out in the approximation used by the above authors. It is found however that the drag predicted by the theory does not agree with R. A. Millikan's classic experiments on sphere drag.

The whole question of the applicability of the Navier-Stokes theory when the Knudsen number M/Re is not small is examined. A new slip condition is proposed. The idea that the Navier-Stokes equations coupled with this condition may adequately describe small Reynolds number flows when the Knudsen number is not too large is looked at in some detail. First, a general discussion of asymptotic solutions of the equations for all such flows is given. The theory is then applied to several concrete problems of fluid motion. The deductions from this theory appear to interpret and summarize the results of Millikan over a much wider range of Knudsen numbers (almost up to the free molecular or kinetic limit) than hitherto Believed possible by a purely continuum theory. Further experimental tests are suggested and certain interesting applications to the theory of dilute suspensions in gases are noted. Some of the questions raised in the main body of the work are explored further in the appendices.

Part I: Latent heat of vaporization of n-decane. Part II: Heat and mass transfer from a cylinder placed in a turbulent air stream - Sherwood and Frössling numbers as a function of free stream turbulence level and Reynolds number

Part I

The latent heat of vaporization of n-decane is measured calorimetrically at temperatures between 160° and 340°F. The internal energy change upon vaporization, and the specific volume of the vapor at its dew point are calculated from these data and are included in this work. The measurements are in excellent agreement with available data at 77° and also at 345°F, and are presented in graphical and tabular form.

Part II

Simultaneous material and energy transport from a one-inch adiabatic porous cylinder is studied as a function of free stream Reynolds Number and turbulence level. Experimental data is presented for Reynolds Numbers between 1600 and 15,000 based on the cylinder diameter, and for apparent turbulence levels between 1.3 and 25.0 per cent. n-heptane and n-octane are the evaporating fluids used in this investigation.

Gross Sherwood Numbers are calculated from the data and are in substantial agreement with existing correlations of the results of other workers. The Sherwood Numbers, characterizing mass transfer rates, increase approximately as the 0.55 power of the Reynolds Number. At a free stream Reynolds Number of 3700 the Sherwood Number showed a 40% increase as the apparent turbulence level of the free stream was raised from 1.3 to 25 per cent.

Within the uncertainties involved in the diffusion coefficients used for n-heptane and n-octane, the Sherwood Numbers are comparable for both materials. A dimensionless Frössling Number is computed which characterizes either heat or mass transfer rates for cylinders on a comparable basis. The calculated Frössling Numbers based on mass transfer measurements are in substantial agreement with Frössling Numbers calculated from the data of other workers in heat transfer.

Behavior of spherical particles at low Reynolds numbers in a fluctuating translational flow

The behavior of spheres in non-steady translational flow has been studied experimentally for values of Reynolds number from 0.2 to 3000. The aim of the work was to improve our qualitative understanding of particle transport in turbulent gaseous media, a process of extreme importance in power plants and energy transfer mechanisms.

Particles, subjected to sinusoidal oscillations parallel to the direction of steady translation, were found to have changes in average drag coefficient depending upon their translational Reynolds number, the density ratio, and the dimensionless frequency and amplitude of the oscillations. When the Reynolds number based on sphere diameter was less than 200, the oscillation had negligible effect on the average particle drag.

For Reynolds numbers exceeding 300, the coefficient of the mean drag was increased significantly in a particular frequency range. For example, at a Reynolds number of 3000, a 25 per cent increase in drag coefficient can be produced with an amplitude of oscillation of only 2 per cent of the sphere diameter, providing the frequency is near the frequency at which vortices would be shed in a steady flow at the mean speed. Flow visualization shows that over a wide range of frequencies, the vortex shedding frequency locks in to the oscillation frequency. Maximum effect at the natural frequency and lock-in show that a non-linear interaction between wake vortex shedding and the oscillation is responsible for the increase in drag.

Tipperary, by Frank Delaney: a reading of the rewriting of history of Ireland

O objetivo da presente pesquisa é o de discutir a reescrita da história da Irlanda, mais especificamente aspectos relacionados à construção da identidade nacional e de marcas da tradição, a partir da leitura do romance Tipperary, de Frank Delaney. Publicada em 2007, essa obra aborda de forma singular as querelas sobre identidade nacional, nacionalismo, passado, memória, e seus personagens principais e a trama estão significativamente ligados ao contexto político-social da história da Irlanda. Nessa reconstrução da história, o passado é revisitado através de diferentes pontos de vista. Nossa atenção estará voltada para a seleção de elementos/momentos da história do país que ganham foco na narrativa, e as possíveis repercussões deste processo. Além disso, nos concentraremos na questão das tênues fronteiras entre história e ficção, ou seja, as fronteiras pouco delimitadas entre o discurso histórico e o discurso ficcional. Na escrita da história em Tipperary, Delaney aborda questões relativas a mitos, lendas e tradições como importantes fatores de identidade nacional em uma Irlanda que emerge como uma nação independente. No romance em questão, podemos observar como história e memória se unem na jornada do protagonista, em sua empreitada de narrar a história de sua vida e de seu país