3种湍流模型模拟水下重力流的比较研究(英文)

运用最新文献记载的3种用于数值模拟水下重力流的湍流模型模拟同一组新的水下重力流的实验。实验数据不仅包括传统的各垂向剖面的速度、浓度,也含有对应剖面的湍流动能。结果表明:在所比较的3湍流模型当中,k-ε模型的模拟性能最好,而MY和k-l模型的模拟均在重力流的上部混合区域呈现过量的扩散。进一步分析表明,MY模型和k-l模型所采用的依据墙面距离定义的宏观长度尺度L尽管可适用于水下重力流底部的壁面边界层,但不适用于重力流上部的自由剪切层,因为重力流上部自由剪切层的湍流长度尺度和依据壁面距离定义的宏观长度尺寸完全不同。     

紊流模型对底坎绕流模拟效果的比较

利用Fluent软件,选取了Spalart-Allmaras模型、标准k-ε模型、RNG k-ε模型和realizable k-ε模型共4种涡黏系数的湍流模型,对底坎绕流问题进行了数值模拟。结果表明:4种模型模拟的底坎附近的流动特征基本一致,但在回流长度、分离主涡的涡心位置及水头损失等参数方面,RNG k-ε模型的模拟效果最好,标准k-ε模型的模拟效果较差,而realizable k-ε模型和Spalart-Allmaras模型的效果居中。     

桩群排列方式对水流结构特性的影响

利用室内水槽试验模拟了桩群不同排列方式工况下水流的运动规律,借助三维超声波多普勒测速仪(ADV)量测了不同断面、不同垂线、不同测点的时均流速,得到了不同工况下的流速分布、紊动强度、雷诺应力,并分析了它们的变化特征。结果表明:桩群使流速重新分布,在桩群区与非桩群区交界处流速明显减小,且桩群布置密度大时,水流绕流流速最大;当桩群排列方式改变时,各断面的三向紊动强度都会发生较大改变,但是对于某一方向上的紊动强度各断面仍有一定的相似性;修桩后雷诺应力分布规律有所不同,水流上侧的雷诺应力较大;桩群排列方式的改变不仅使雷诺应力沿程变化较大,而且改变了雷诺应力时均值的大小及最大值出现的位置。     

NUMERICAL SIMULATION OF PROPAGATION AND BREAKING PROCESSES OF A FOCUSED WAVES GROUP

A two-dimensional numerical wave flume is developed to study the focused waves group propagation and the consequent breaking processes. The numerical model is based on the Reynolds-Averaged Navier-Stokes (RANS) equations, with the standard k-ε turbulence model to simulate the turbulence effects. To track the complicated and broken free-surface, the Volume Of Fluid (VOF) method is employed. The numerical model combines the "Partial Cell Treatment (PCT)" method with the "Locally Relative Stationary (LRS)" concept to treat the moving wave paddle so that various waves can be generated directly in a fixed Cartesian grid system. The theoretical results of the linear and nonlinear waves are used to validate the numerical wave flume firstly, and then a plunging breaking wave created by a focused waves group is simulated. The numerical results are compared to the experimental data and other simulation results, with very good agreements. The turbulence intensity, the flow field and the energy dissipation in the breaking processes are analyzed based on the numerical results. It is shown that the present numerical model is efficient and accurate for studying the waves group generation, the waves packet propagation, and the wave breaking processes.     

TURBULENCE TRANSPORT OF SURFACTANT SOLUTION FLOW DURING DRAG REDUCTION DEGENERATION

Turbulence transport of surfactant solution flow during drag reduction degeneration is investigated experimentally in a two-dimensional channel.Particle Image Velocimetry (P1V) system is used to take t...     

挑流水垫塘坎尾壅水特性研究

挑流水垫塘(消力池)尾坎的壅水特性,对尾坎选型有重要的影响。通过模型试验实测和堰流计算的对比分析,对非淹没和淹没过流状态下尾坎壅水特性进行了研究。研究发现,受水垫塘(消力池)内的水流紊动影响,一般实际工程中尾坎高度可能偏小,因此,选择尾坎高度时需要在堰流计算的基础上适当加高。非淹没过流状态下,当泄洪流量相对较小时,尾坎高度可直接由堰流计算壅水曲线确定;随着泄洪流量的增加,水流紊动对尾坎壅水高度的影响也越来越大,尾坎的实际壅水高度与堰流计算值之间的差值逐渐增大,最高可达16.64%。淹没过流状态下,随泄洪流量的增加,水流紊动对尾坎壅水高度的影响越来越小,尾坎的实际壅水高度与堰流计算值之间的差值也逐渐减小,最小仅为7.43%。     

Large eddy simulation and proper orthogonal decomposition analysis of turbulent flows in a direct injection spark ignition engine: Cyclic variation and effect of valve lift

Large eddy simulation(LES)is used to calculate the in-cylinder turbulent flow field in a direct injection spark ignition(DISI)engine.The computations are carried out for three different maximum valve lifts(MVL)and throughout 100 consecutive engine cycles.The simulated results as well as corresponding particle image velocimetry(PIV)measurement database are analyzed by the proper orthogonal decomposition(POD)method.Through a new developed POD quadruple decomposition the instantaneous in-cylinder flow fields are decomposed into four parts,named mean field,coherent field,transition field and turbulent field,respectively.Then the in-cylinder turbulent flow characteristics and cycle-to-cycle variations(CCV)are studied separately upon the four part flow fields.Results indicate that each part exhibits its specific characteristics and has close connection with others.The mean part contains more than 50%of the total kinetic energy and the energy cascade phenomenon occurs among the four part fields;the coherent field part possesses the highest CCV level which dominates CCV of the bulk flow.In addition,it is observed that a change in MVL affects significantly the in-cylinder flow behavior including CCV,especially for the coherent part.Furthermore,the POD analysis demonstrates that at least 25 sample cycles for the mean velocity and 50 sample cycles for the RMS velocity are necessary for obtaining converged and correct results in CCV.     

Application de l'analyse spectrale autoregressive AU traitement de mesures de turbulence

The Autoregressive (AR) Spectral analysis (also called Maximum Entropy Method) is applied here to velocity turbulence measurements. The validity of such a spectral analysis is tested for classical turbulence data collected in a wind tunnel. The AR estimates are compared with classical Fourier estimates (average of short modified periodograms). Both Levinson's and Burg's algorithms are used for calculating the AR coefficient estimates, and Akaike's “FPE” criterion for choosing the length of the AR filter.Such a method shows much interest in processing some geophysical fluid dynamics phenomena which often are intermittent on a physical point of view. Power spectrum time evolution of some very short length (1 min.) data of oceanic turbulence — they cannot be analysed by Fourier techniques — is a fine example.

Abstract

L'Analyse Spectrale Autorégressive (AR) (dite aussi Méthode de l'Entropie Maximale) est appliquée ici à des mesures de turbulence de vitesse. La validité d'une telle analyse a été testée sur des données classiques de turbulence recueillies en soufflerie. On a comparé les estimations AR avec les estimations classiques (moyenne de courts périodogrammes modifiés). On a utilisé pour l'estimation des coefficients AR les algorithmes de Levinson et de Burg, et pour le choix de l'ordre du filtre AR le critère “FPE” d'Akaike.Ce genre de méthode présente un grand intérêt dans le cas du traitement de certains phénomènes de mécanique des fluides géophysiques qui sont souvent intermittents du point de vue physique. Le calcul de l'évolution spectrale de très courtes (1 mn) données de turbulence océanique en est un bon exemple (ces données ne peuvent être analysées par les techniques de Fourier).     

管壳式换热器数值模拟的流动模型比较

基于计算流体动力学（CFD）技术,分别用LVEL模型、标准k-ε模型对管壳式换热器进行数值模拟,并把内管入口和出口的流动方向速度进行对比,依此判断计算的准确性,来寻找适合于管壳式换热器数值模拟的合理模型.经过计算可以判断：LVEL模型和标准k-ε模型计算出的流场基本一致.LVEL模型已足够准确、耗时也较小,对于管壳式换热器的数值模拟是适用的,是做换热器模拟时一种较好的选择.     

不同出射角度对气膜冷却流场的影响

采用RNGk-ε湍流模型对射流出射角度α为90o、正、负60o和正、负30o的气膜冷却流场进行了数值模拟,结合SIMPLEC算法求解了适体坐标系下的控制方程,近壁区流动采用两层模型的壁面函数法处理,结果给出了射流与主气流速度比R为2和4时的射流轨迹和速度场。结果表明:R和α值是影响气膜冷却流场的重要因素,R值增加,冷气射流对主流场影响区域增加,α值为负时,射流对主流场上游影响区域较大,当α值为30o时,射流影响区域减小;当α为正值时,随α值的减小,射流喷孔下游背风侧的分离现象逐渐消失。