液封Pr和Sc对开口矩形液池热-溶质毛细对流的影响

在基于提拉法制备晶体材料的背景下,采用格子玻尔兹曼方法对具有自由表面的双层矩形液池内热-溶质毛细对流进行数值模拟.通过频谱分析确定双层热-溶质毛细对流的流动类型,在自由表面雷诺数Re2、界面雷诺数Re1、下熔体层普朗特数Pr1和施密特数Sc1分别为600、300、1.0和100的条件下,研究了液封层普朗特数和施密特数对熔体层热-溶质毛细对流的影响.通过双层热-溶质毛细对流和单层系统的对比表明,液封对下层毛细对流可以起到很好的抑制作用.在液封Sc2=100、Pr2=0.4~10.0条件下,毛细对流都是双频准周期型.随着Pr2的增加,第一基频和第二基频都减小,系统稳定性增加;在Pr2=1.0、Sc2=20~100条件下,随着Sc2的增大,系统的振荡频率增大,对流将从周期模型再次转变为准周期型,稳定性降低.     

雷诺数对涡轮叶片换热影响的研究

雷诺数的改变会引起涡轮叶片边界层等流态的改变,进而对叶片表面的对流换热产生强烈影响.该文首先结合实验数据对影响流热耦合计算精度的若干因素的敏感性进行了分析;然后在此基础上,针对MarkⅡ叶型分析了雷诺数对涡轮叶片表面换热的影响.结果表明:流热耦合计算时相关参数的选择对模拟的精度有明显影响,该文针对MarkⅡ叶型得到的一套参数是合理的,相关结果与实验吻合良好;雷诺数的变化使得边界层等流动状态发生改变,从而导致涡轮叶片表面对流换热状态和温度分布产生明显的差异.     

Some predictions of the attached eddy model for a high Reynolds number boundary layer

Many flows of practical interest occur at high Reynolds number, at which the flow in most of the boundary layer is turbulent, showing apparently random fluctuations in velocity across a wide range of scales. The range of scales over which these fluctuations occur increases with the Reynolds number and hence high Reynolds number flows are difficult to compute or predict. In this paper, we discuss the structure of these flows and describe a physical model, based on the attached eddy hypothesis, which makes predictions for the statistical properties of these flows and their variation with Reynolds number. The predictions are shown to compare well with the results from recent experiments in a new purpose-built high Reynolds number facility. The model is also shown to provide a clear physical explanation for the trends in the data. The limits of applicability of the model are also discussed.     

扁平流线型箱梁涡激振动雷诺数效应研究

风洞试验作为研究涡激振动的重要手段,由于其本身条件的限制,可能存在雷诺数效应问题,造成试验值和实际值的偏差.为了研究扁平流线型箱梁涡激振动的雷诺数效应,选取某大跨度斜拉桥的扁平流线型箱梁作为研究对象,利用风洞测振试验,通过调节模型系统自振频率,实现不同雷诺数下的涡激振动,研究扁平流线型箱梁的涡激振动特性及其雷诺数效应;...     

表面损伤的斜拉索平均气动阻力特性的试验研究

斜拉索作为斜拉桥的重要受力构件,其风荷载设计在桥梁抗风设计中具有重要意义。斜拉索在生产、运输和安装过程中有可能受到损伤,针对不同程度表面损伤的斜拉索,利用风洞测力试验,研究了亚临界、临界和超临界雷诺数区域损伤程度对斜拉索气动阻力的影响,并得到了表面损伤斜拉索气动阻力的计算方法。结果表明:在一定的风向范围内,斜拉索表面损伤对其气动阻力影响显著,在亚临界区,表面损伤模型阻力系数大于光滑模型;进入临界区后,表面损伤模型的阻力系数则小于光滑模型;在超临界区,二者又比较接近。随着划痕的加深,临界区起始雷诺数提前,且在临界雷诺数区,随着雷诺数的增大,阻力系数减小。对不同损伤程度斜拉索的阻力系数进行拟合,得出相应的计算公式,以方便斜拉索气动阻力的估算。     

Numerical investigation of the effect of the Prandtl number on the temperature recovery and the Reynolds analogy factors in the boundary layer on a plate

We performed a numerical study of the boundary layer on a wall in a supersonic gas flow using a differential turbulence model. The dependences of the recovery factor and the Reynolds analogy factor on the main flow parameters, such as the Reynolds, Mach, and Prandtl numbers, are obtained for a series of the temperature factor values. The results of calculations are approximated by the dependences of the recovery factor and the Reynolds analogy factor on the Prandtl number.     

大跨度桥梁箱梁的三分力系数识别研究

该文针对高雷诺数风洞试验研究结果,分别对加拿大国家试验中心NRCC 试验和昂船洲大桥试验建模计算。使用标准k-ε 双方程湍流模型结合边界层网格建模,计算得到的三分力系数与试验数据吻合良好。分析了雷诺数、来流湍流强度、中央开槽宽度对桥梁断面三分力系数的影响。研究结果表明,雷诺数效应和开槽宽度对阻力系数存在影响,但对升力系数与升力矩系数影响很小,来流湍流强度对三分力系数影响均不明显。     

The interaction between inner and outer regions of turbulent wall-bounded flow

The nature of the interaction between the inner and outer regions of turbulent wall-bounded flow is examined. Townsend's theory of inactive motion is shown to be a first-order, linear approximation of the effect of the large eddies at the surface that acts as a quasi-inviscid, low-frequency modulation of the shear-stress-bearing motion. This is shown to be a 'strong' asymptotic condition that directly expresses the decoupling of the inner-scale active motion from the outer-scale inactive motion. It is further shown that such a decoupling of the inner and outer vorticity fields near the wall is inappropriate, even at high Reynolds numbers, and that a 'weak' asymptotic condition is required to represent the increasing effect of outer-scale influences as the Reynolds number increases. High Reynolds number data from a fully developed pipe flow and the atmospheric surface layer are used to show that the large-scale motion penetrates to the wall, the inner-outer interaction is not describable as a linear process and the interaction should more generally be accepted as an intrinsically nonlinear one.     

斜拉桥斜拉索表面损伤状态下风致振动特性研究

斜拉桥斜拉索在生产、运输、安装和运营过程中,可能产生划痕、裂缝等损伤,研究斜拉索在表面损伤状态下的风致振动特性,对于准确评估斜拉索在其服役期内的气动稳定性能具有重要意义。通过对光滑和表面存在凹痕的斜拉索模型进行风洞测振试验,分析了雷诺数、来流风向和凹痕尺寸等参数对斜拉索风致振动特性的影响规律,并对振动状态进行了初步判断。结果表明,表面存在凹痕的斜拉索模型的振动特征随雷诺数的整体变化趋势近似于光滑模型,但存在最不利风向,在此风向下,与光滑模型相比,表面存在凹痕的斜拉索模型的风致振动不仅在较低雷诺数发生,而且振幅较大,发生振动的雷诺数区间也较宽。此外,凹痕尺寸对斜拉索模型在试验雷诺数范围内风致振动特性的影响也非常显著,但并非简单的单调关系,而是与来流风向相关。根据模型风致振动频率与Den Hartog驰振准则,初步判断此风致振动不为涡激振动,而为驰振。     

压电喷墨液滴撞击光滑壁面铺展行为的数值研究

目的 研究典型流体相关无量纲参数对墨滴在光滑承印物表面铺展行为的影响,确定各无量纲参数对铺展直径、铺展因子和稳定铺展时间的影响规律。方法 利用Ansys软件,建立墨滴撞击光滑壁面的数值计算模型,采用VOF模型追踪液滴形状,采用PISO算法计算压力速度耦合。引入韦伯数、雷诺数、奥内佐格数来分析墨滴撞击光滑承印物表面的铺展行为。结果 计算获得不同韦伯数、雷诺数、奥内佐格数下墨滴的最大铺展直径、最终平衡铺展直径、最大铺展因子和最终铺展时间。结论 韦伯数和雷诺数对墨滴最大铺展直径的影响较大,对最终平衡直径的影响较小。韦伯数或雷诺数越小,回缩阶段越短,越快达到平衡。韦伯数、雷诺数与最大铺展因子呈明显正相关。奥内佐格数对墨滴的最大铺展直径、最终平衡直径的影响都较小。奥内佐格数越小,回缩阶段越短,越快达到平衡,奥内佐格数与液滴最大铺展因子呈不明显的正相关性。     

高雷诺数下高质量比圆柱涡致振动的数值模拟

圆柱的涡致振动一直是研究者们关注的问题,但是以往的研究大多雷诺数不高,或者质量比较低。该文以高质量比圆柱为研究对象,采用质量-弹簧-阻尼系统,基于SST湍流模型,对结构在高雷诺数下发生涡致振动的过程进行了数值模拟和分析。通过流固耦合数值计算,模拟了圆柱涡致振动的高幅分支试验现象,计算所得的最大振幅比随速度比的变化曲线与试验吻合较好。研究结果验证了流固耦合计算方法的正确性,表明SST模型适合于由强逆压梯度引起的边界层分离流动问题。数值模拟显示在高雷诺数下,高质量比的圆柱涡致振动会出现高幅分支。该文的数值分析方法可以为高雷诺数下结构涡致振动问题的研究提供参考。     

Concerning the existence of wall terms in a three-parameter model of turbulence

We show the existence of a wall term in the equation for «u₁u₂» in a three-parameter model of turbulence by analogy with the known terms in an “E-ε” model. We suggest a convenient system of differential equations for the boundary layer on a flat plate in fluid flow at small Reynolds numberse. We find the limiting calculated value of the turbulent Reynolds number at which wall terms must be taken into consideration.     

A note on the linear instability of the Blasius flow

Summary It is found that in a ribbon-excited Blasius boundary layer, a wave Reynolds number defined here on the basis of phase speed and wave number of the disturbance remains nearly independent of the local mean flow Reynolds number, and so also of the streamwise distance, under the parallel flow approximation. Consequently, a limited similarity feature of the Orr-Sommerfeld equation has been found to exist for the streamfunction in the outer region of the boundary layer.     

Effect of inertia on electrified film flow over a wavy wall

The effect of inertia on the steady flow of a liquid layer down a wavy wall in the presence of an electric field is investigated. Both the liquid film and the region above it are assumed to act as perfect dielectrics. A linearised perturbation analysis is performed for flow down a wall with small-amplitude sinusoidal corrugations, and the free-surface amplitude and phase shift are computed numerically for a broad range of flow conditions. It is shown that the electric field can be used to manipulate the phase shift between the free surface and the wall. In particular, when the Reynolds number lies below a threshold value, an electric field of sufficient strength will bring the free surface precisely into phase with the wall. An electric field can also be used to mitigate the resonance effect identified by previous workers, in which the free surface suffers significant amplification in comparison to the height of the wall corrugations at a particular Reynolds number. Working on the basis of the lubrication approximation, a nonlinear equation for the film thickness is derived featuring a non-local term due to the electric field. Numerical solutions for flow over a wavy wall of finite amplitude reveal that the effect of inertia on the free-surface characteristics depends on the electrical properties of the fluid layer and the strength of the imposed electric field.     

Numerical investigation of supersonic flow past blunt bodies of intricate shape at an angle of attack and slip angle

A supersonic flow of viscous homogeneous gas past blunt bodies of intricate shape at an angle of attack and slip angle is investigated numerically within the model of complete three-dimensional viscous shock layer using the time relaxation method. The main regularities are studied of the general structure of flow and of the distribution of pressure and heat flux along the surface. An analysis is performed of their dependence on the shape of the body, angle of attack and slip angle, Mach and Reynolds numbers, and on other determining parameters of the problem. The accuracy and range of validity of a number of approximate approaches to the solution of the problem are estimated.     

Two-dimensional modeling of viscous liquid jet breakup

A two-dimensional computational model has been developed to study the evolution and breakup of a viscous laminar liquid jet, using a boundary-fitted curvilinear coordinate system. A system of elliptic partial differential equations for coordinate transformations has been developed to map the moving boundaries’ physical domain of the jet to a simple rectilinear computational domain. The equations developed for the model comprise the transformed two-dimensional unsteady Navier–Stokes equations for the liquid jet, grid velocity equations, kinematic boundary conditions, and the Geometric Conservation Law. The resulting systems of equations are solved using an implicit finite difference scheme. Effects of inflow oscillation magnitude, wave number, Weber number, and Reynolds number on the breakup process of jets have been studied. The model predicts the instantaneous shape of the jet surface, formation of the main and satellite drops, and the breakup length and time. These results are compared with available experimental data. The comparisons show a good agreement between measured and computed values of drop sizes and breakup lengths for different Reynolds and Weber numbers. However, at a relatively high Reynolds number of 1,254, the model slightly overpredicts the main drop sizes and underpredicts the satellite drop sizes at a wave number of 0.4. At a low Reynolds number of 587, the model overpredicts the main drop sizes at a lower wave number of 0.3. Moreover, the model underpredicts the satellite drop sizes at a lower wave number of about 0.4 and overpredicts the satellite drop sizes at a wave number of 0.8.     

Heat and mass transfer from a laminar humid air stream to a plate at subfreezing temperatureTransfert de chaleur et de masse entre un écoulement laminaire d'air humide et une plaque à une température inférieure au point de congélation

Processes involving the transfer of heat from a humid air stream in laminar flow to a horizontal plate with the simultaneous deposition of frost is of importance in a variety of refrigeration equipment. The accumulating frost layer impedes the heat flow to the cooling surface. The present study is carried out to determine both theoretically and experimentally the factors that influence, frost formation on a cold surface and to correlate the Nusselt and Sherwood numbers with these factors.Experiments were conducted on a 600 mm long, flat horizontal plate, the Reynolds number and the relative humidity of the impinging air stream were varied from 30 000 to 140 000 and from 40% to 60% respectively, while the average surface temperature was near - 18°C. Empirical correlations of the Nusselt number and Sherwood number are presented.     

Relaminarization in supersonic microjets at low Reynolds numbers

Results of measuring the length of the supersonic portion of the air jets that flow out of axisymmetric sonic nozzles 10.4 μm-1 mm in diameter are presented. The measurements are carried out in a range of degree of jet noncalculation of 1–30 and in a wide Reynolds number range, including the laminar and turbulent flow modes. It is shown that the Reynolds number calculated from the nozzle diameter and the outlet parameters of gas is the parameter that governs jet flow. It is found that, for a laminar jet mixing layer, the length of the supersonic portion sharply increases. When the jet mixing layer becomes turbulent, the length of the supersonic portion decreases. The effect of increasing the length of the supersonic portion after its decrease due to the turbulization of flow in a jet and a growth in the Reynolds number is first discovered.     

The recovery factor in a supersonic flow of gas with a low Prandtl number

A three-parameter differential model of turbulence supplemented with a transport equation for turbulent heat flux is used to perform calculations of the boundary layer on a heat-insulated wall in a supersonic gas flow with a value of Prandtl number Pr ≤ 0.7. The dependences on the values of Prandtl, Reynolds, and Mach numbers and of intensity of injection are obtained for the recovery factor.     

Heat transfer and icing of a rough cylinder

Heat transfer from a rough ice surface to air is a fundamental factor determining the icing rate and ice shape on an object under conditions where the impinging water flux exceeds the icing rate. In order to examine the effect of the roughness characteristics on icing, a boundary-layer model of heat transfer from a front half of a rough circular cylinder is proposed. The model is based on the integral equations of the boundary layer, and predicts the local heat transfer coefficient along the cylinder surface and, subsequently, the overall heat transfer rate. Comparison between the results of the model and the available experimental data for three different roughnesses in the cylinder Reynolds number range 5 × 10⁴ < Re < 4 × 10⁶ show that the model simulates the heat transfer more precisely than the previous formulations used in icing models.