不同长细比圆柱绕流的大涡模拟

自从人们对层流圆柱绕流现象有了系列研究及清楚的认识后,逐渐把目光投向湍流的圆柱绕流问题,但高雷诺数下的圆柱绕流具有很强的三维特性,因此在数值计算时模型轴向长度的选取是大家关心的问题。该文基于开源软件OpenFOAM运用LES计算了4种不同长细比圆柱的静止圆柱绕流(包括一个二维算例),重点分析和对比了结果中的一些基础参数如St数、平均阻力系数等,与实验结果吻合良好。文中还进一步分析了时均顺流向速度U在流场中沿流向和横向的分布,发现长细比为π的圆柱算例中,计算域的轴向尺度已经能满足圆柱下游靠近圆柱处的流场中的三维结构的发展。     

双圆柱绕流的大涡模拟

基于FLUENT软件平台上的三维大涡模拟方法,对定常来流流场中的单圆柱及不同排列形式的双圆柱进行了水动力计算,并对圆柱周围的瞬时流场、时均流场以及圆柱受力进行了分析.分析结果表明,当Re=6×104时,串列双圆柱中上游圆柱受力明显大于下游圆柱,下游圆柱Gd、Cl的变化频率满足fd=2fl,两圆柱的脱涡频率近似相等,涡街的相位相同;并列双圆柱的Cd基本相等,且变化趋势有一定相似性,脱涡频率均接近单个圆柱,涡街的相位相反;交错45.排列的两个圆柱Cd基本相等,脱涡频率接近,下游圆柱的涡街相位明显滞后于上游圆柱.     

低雷诺数条件下并列三圆柱绕流的尾流模式和水动力系数研究

该文应用浸入边界法对雷诺数Re(28)100和间距比T/D(28)1.1-5.0条件下的等间距并列三圆柱绕流进行了数值模拟。研究发现,尾流模式与无量纲圆柱间距比T/D密切相关,在所研究的间距比范围内共出现六种尾流模式,分别为:单体尾流模式(T/D(28)1.1-1.2)、偏斜尾流模式(T/D(28)1.3)、第一类Flip-Flopping(FF1)尾流模式(T/D(28)1.4-1.5)、第二类Flip-Flopping(FF2)尾流模式(T/D(28)1.7-2.3)、同步尾流模式(T/D(28)2.5-3.2)和调制尾流模式(T/D(28)3.3-5.0)。通过分析近尾流的涡量图、升阻力系数时程曲线和不同圆柱所受升力的相位差,详细研究了各个尾流模式的特点和联系。     

串列布置下倒角柱体绕流数值模拟及水动力特性分析

为研究串列布置情况下,倒角变化对柱体绕流水动力特性的影响,该文采用大涡模拟方法研究了在雷诺数Re=3 900和间距比L/D=3条件下,6种不同倒角(R+=0.0、0.1、0.2、0.3、0.4和0.5)串列布置的柱体三维流场,模拟工作验证有效.分析了柱体后方水动力参数、瞬时流场和时均流场的变化情况,分析结果表明:随着倒角增大,上下游柱体的平均阻力系数Cd-ave变化趋势相反,上游柱体Cd-ave逐渐变小,而下游柱体的Cd-ave逐渐上升且均为负值;随着倒角增大,上下游柱体Sr数值相同并出现先增大后减少的趋势;两柱体之间的回流速度曲线下切深度明显增加,速度最小值出现处从下游柱体两侧向中心不断变化;串列布置下游柱体尾流宽度显著变小,在间距比L/D=3条件下,剪切层的初始分离均发生在上游柱体倒角转折处,并附着于下游柱体前端及两侧.该研究结果可为柱体绕流研究及相关工程应用提供参考.     

高雷诺数下双圆柱绕流的数值模拟

本文使用表面涡法研究高雷诺数下不同排列方式双圆柱绕流的流动状态，计算了双圆柱在并列、串列及级列的情况下的各种流动结构，涡街的变化及作用在圆柱上的受力情况。本文结果清楚地描述了双圆柱绕流复杂的流动状况，计算结果与实验显示的流动状况十分相似，斯特罗哈数和阻力系数与实验结果符合得很好。     

特征线算子分裂有限元的圆柱绕流大涡模拟

为研究圆柱绕流流场特性,将大涡模拟与特征线算子分裂有限元相结合,建立了大涡模拟特征线算子分裂有限元模型,对单圆柱和串列双圆柱绕流问题进行了数值模拟,所得结果与现有研究结果吻合较好。模拟结果表明:对单圆柱绕流,随着雷诺数的增大,圆柱近尾流区上下交替的涡旋逐渐靠近通过圆柱几何中心的水平线,且涡脱落位置逐渐靠近圆柱。对Re=1 000的串列双圆柱绕流,临界间距在圆柱直径的2.25~2.5倍之间;当两圆柱间距小于临界间距时,上游圆柱后方无明显涡旋脱落,间隙处压力较稳定;大于临界间距时,有涡旋脱落,上游圆柱尾流区上下表面交替出现强负压区。     

用解NS方程和FW-H积分的混合方法计算圆柱绕流噪声

该文所研究的混合方法通过两个步骤模拟流动的声学远场.第一步,在包含所有声源的近场区域内,通过求解NS方程(对于湍流通过大涡模拟)获得非定常流场参数.第二步,通过带可穿透控制面的FW-H积分方法从流体动力近场获得声学远场.FW-H方程的三种积分形式,即三维的Farassat公式、二维的Guo公式和Locard公式被应用,并相互比较.研究了积分控制面位置和展向复制长度对声学模拟结果的影响.把数值模拟的ReD=150时圆柱层流绕流的声学远场和DNS结果进行了比较;把数值模拟的ReD=3900时圆柱湍流绕流的声学远场和实验结果进行了比较.两项比较都能相符合.通过比较FW-H方程三维积分形式和二维积分形式的结果,表明二维积分形式的结果可用作进一步三维精算前的简便估算.     

污染源位置对圆柱形水工建筑下游污染物浓度分布的影响

利用雷诺方程和RNG k-ε紊流模型,对排放源在圆柱形水工建筑上游和下游两种工况下的尾流浓度场进行数值模拟,从污染物时均浓度、浓度脉动强度和浓度半宽度3个方面比较了污染源位置对圆柱形水工建筑下游污染物浓度分布特性的影响。结果表明:随流程的增大,两种工况下污染物时均浓度横向断面分布先后经历双峰结构、过渡阶段和高斯分布3个阶段;排放源在上游时,浓度脉动强度先增大后减小;排放源在下游时,浓度脉动强度一直减小;两种工况的浓度半宽度变化规律一致,当1≤x/D15时浓度半宽度呈非线性增长,当15≤x/D≤30时呈线性增长。     

三方柱绕流的大涡模拟及频谱分析

高雷诺数条件下,无论来流是均匀还是非均匀,多方柱绕流情况下方柱受力及尾流的相互干扰都是相当复杂的.为了研究方柱受力及下游尾流的相互干扰,基于大涡模拟紊流模型对后品字等边布置的三方柱绕流进行了数值模拟,数值模拟结果表明,上游两个方柱的阻力系数要明显小于下游方柱,而上游两方柱升力系数远大于下游方柱,且下游方柱的升力系数基本...     

LARGE-EDDY SIMULATION OF FLOW AROUND CYLINDER ARRAYS AT A SUBCRITICAL REYNOLDS NUMBER

The complex three-dimensional turbulent flows around a cylinder array with four cylinders in an in-line square configuration at a subcritical Reynolds number of 1.5 × 10^4 with the spacing ratio at L/D = 1.5 and 3.5 were investigated using the Large Eddy Simulation （LES）. The full field vorticity and velocity distributions as well as turbulent quantities were calculated in detail and the near wake structures were presented. The results show that the bi-stable flow nature was observed at L/D = 1.5 and distinct vortex shedding of the upstream cylinders occurred at L/D = 3.5 at Re = 1.5 × 10^4. The techniques of Laser Doppler Anemometry （LDA） and Digital Particle Image Velocimetry （DPIV） are also employed to validate the present LES method. The results show that the numerical predictions are in excellent agreement with the experimental measurements. Therefore, the full field instantaneous and mean quantities of the flow field, velocity field and vorticity field can be extracted from the LES results for further study of the complex flow characteristics.     

LARGE-EDDY AND DETACHED-EDDY SIMULATIONS OF THE SEPARATED FLOW AROUND A CIRCULAR CYLINDER

The separated turbulent flow around a circular cylinder is investigated using Large-Eddy Simulation (LES), Detached-Eddy Simulation (DES, or hybrid RANS/LES methods), and Unsteady Reynolds-Averaged Navier-Stokes (URANS). The purpose of this study is to examine some typical simulation approaches for the prediction of complex separated turbulent flow and to clarify the capability of applying these approaches to a typical case of the separated turbulent flow around a circular cylinder. Several turbulence models, i.e. dynamic Sub-grid Scale (SGS) model in LES, the DES-based Spalart-Allmaras (S-A) and k ? ω Shear-Stress-Transport (SST) models in DES, and the S-A and SST models in URANS, are used in the calculations. Some typical results, e.g., the mean pressure and drag coefficients, velocity profiles, Strouhal number, and Reynolds stresses, are obtained and compared with previous computational and experimental data. Based on our extensive calculations, we assess the capability and performance of these simulation approaches coupled with the relevant turbulence models to predict the separated turbulent flow.     

EXPERIMENTAL STUDY OF FLOW STRUCTURE BEHIND A CIRCULAR CYLINDER WITH WAVY SURFACE BY PIV

1. INTRODUCTIONThe flow around a s mooth circular cylinderhas been extensively studied by many researchers ,as can be seen fromthe comprehensive reviews ofWilliamson[1]and Rockwell[2]. Wang et al .[3]in-vestigated the 3D flow structure behind a circularcy…     

INTRINSIC FEATURES OF TURBULENT FLOW IN STRONGLY 3-D SKEW BLADE PASSAGE OF A FRANCIS TURBINE

The turbulent flow, with the Reynolds number of 5.9× 105, in the strongly 3-D skew blade passage of a true Francis hydro turbine was simulated by the Large Eddy Simulation (LES) approach to investigate the spatial and temporal distributions of the fully developed turbulence in the passage with strongly 3-D complex geometry. The simulations show that the strong three-dimensionality of the passage has a great amplification effect on the turbulence in the passage, and the distributions of the turbulence are diversely nonuniform, for instance, the rise of turbulent kinetic energy in the lower 1/3 region of the passage is more than 45%, whereas its rise in the upper 1/3 region is less than 1%. With the LES approach, the details of the flow structures at the near-wall surfaces of the blades could be obtained. Several turbulent spots were captured.     

THREE-DIMENSIONAL NUMERICAL SIMULATION OF INTAKE MODEL WITH CROSS FLOW

The hydrodynamics of a pump sump consisting of a main channel,pump sump,and intake pipe is examined using Truchas,a three-dimensional Navier-Stokes solver,with a Large Eddy Simulation(LES) turbulence model.The numerical results of streamwise velocity profiles and flow patterns are discussed and compared with experimental data of Ansar and Nakato.Fairly good agreement is obtained.Furthermore,unlike Ansar et al.'s inviscid solution,the proposed numerical model includes the effect of fluid viscosity and consid...     

DRAG REDUCTION IN A TURBULENT CHANNEL FLOW WITH HYDRO- PHOBIC WALL

This paper investigates a theoretical prediction of friction drag reduction in turbulent channel flow which is achieved by using superhydrophobic surfaces. The effect of the hydrophobic surface is considered to be a slip boundary condition on the wall, and this new boundary condition is added to Large Eddy Simulation (LES) equations. The predicted drag reduction at Reτ=180 is approximately 30%, which concurs with results obtained from Direct Numerical Simulation (DNS). An important implication of the present finding is that the near-wall turbulence structures are modified with streamwise slip velocity. In addition, a noticeable effect on the turbulence structure occurs when the slip length is greater than a certain value.     

CALCULATION OF VISCOUS FLOW AROUND CIRCULAR CYLINDER WITH THREE-DIMENSIONAL NUMERICAL SIMULATION

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NUMERICAL STUDY ON AIR FLOW AROUND AN OPENING WITH LARGE EDDY SIMULATION

Jet characteristics of air supply opening in a ventilating or an air-conditioning system is primarily decided by the folw state in the duct connected to the opening. It is valuable to study the opening jet characteristics and the flow state in a duct. In this study, the Large Eddy Simulation (LES) technique combined with the Tarlor-Galerkin Finite Element Method ( FEM) in Computational Fluid Dynamics (CFD) was applied to the problem. The 3-D flow fields in ducts around air supply opening under typical conditions were investigated by numerical simulation as well as experimental measurements. Numerical results agree well with the available experimental data. It indicates that the LES method is available under the conditions with complicated boundaries and inner accompanied by anisotropic large-scale eddies, and it is credible to predict the jet deflection characteristics around an opening.     

LARGE-EDDY SIMULATION OF TURBULENT FLOW CONSIDERING INFLOW WAKES IN A FRANCIS TURBINE BLADE PASSAGE

Turbulent flow in a 3-D blade passage of a Francis hydro turbine was simulated with the Large Eddy Simulation (LES) to investigate the spatial and temporal distributions of the turbulence when strongly distorted wakes in the inflow sweep over the passage. In a suitable consideration of the energy exchanging mechanism between the large and small scales in the complicated passage with a strong 3-D curvature,one-coefficient dynamic Sub-Grid-Scale (SGS) stress model was used in this article. The simulations show that the strong wakes in the inflow lead to a flow separation at the leading zone of the passage,and to form a primary vortex in the span-wise direction. The primary span-wise vortex evolves and splits into smaller vortex pairs due to the constraint of no-slip wall condition,which triggers losing stability of the flow in the passage. The computed pressures on the pressure and suction sides agree with the measured data for a working test turbine model.     

LARGE EDDY SIMULATION OF PULSATING TURBULENT OPEN CHANNEL FLOW

Pulsating turbulent open channel flow has been investigated by the u.se of I.arge Eddy Simulation (I.ES) technique coupled with dynamic Sub-Grid-Scale (SGS) model for turbulent SGS stress to closure the governing equations.Three-dimensional filtered Navier-Stokes equations are numerically solved by a fractional-- step method. The objective of this study is to deal with the behavior of the pulsating turbulent open channel flow and to examine the reliability of the I.ES approach for predicting the pulsating turbulent flow. In this study, the Reynolds number (Re,) is chosen as 180 based on the friction velocity and the channel depth, The frequency of the driving pressure gradient for the pulsating turbulent flow ranges low, medium and high value, Statistical turbulence quantities as well as the flow structures are analyzed.