SOME OBSERVATIONS OF TWO INTERFERING VIV CIRCULAR CYLINDERS OF UNEQUAL DIAMETERS IN TANDEM

Analysis of model test results was carried out to investigate the hydrodynamic interaction between a pair of elastically-supported rigid cylinders of dissimilar diameters in a water flume.The two cylinders are placed in tandem with one situated in the wake of the other.The diameter of the upstream cylinder is twice as large as that of the downstream cylinder.The spacing between the two cylinders ranges from 1 to 10 times the larger cylinder diameter.The Reynolds numbers are within the sub-critical range.The cylinders are free to oscillate in both the in-line and the cross-flow directions.The reduced velocity ranges from 1 to 10 and the low damping ratio of the model test set-up at 0.006 gives a combined mass-damping parameter of 0.02.It is found that the lift on and the cross-flow motion of the downstream cylinder have the frequency components derived from the upstream cylinder’s vortex shedding as well as from its own vortex shedding,and the relative importance of the two sources of excitation is influenced by the spacing between the two cylinders.The downstream cylinder’s VIV response appears to be largely dependent upon the actual reduced velocity of the cylinder.     

用SST-DES和SST-URANS方法数值模拟亚临界雷诺数下三维圆柱绕流问题

基于剪切应力运输(shear stress transport,SST)的分离涡模拟(detached-eddy simulation,DES)在近壁面的流动区域采用SST模型,而在其他区域采用亚格子模型求解流场。本研究中分别应用SST-DES和SST-URANS方法对亚临界雷诺数Re=3900下的三维圆柱绕流进行了数值模拟,比较分析了两种数值方法下圆柱表面的时均压力分布、圆柱后方的漩涡脱落特征、圆柱下游流场的时均速度剖面分布等,并将数值模拟结果同前人物理试验比较,结果表明SST-DES方法在亚临界雷诺数下的三维圆柱绕流数值模拟问题上比SST-RANS方法具有更好的效果。     

FLOW PAST TWO ROTATING CIRCULAR CYLINDERS IN A SIDE-BY-SIDE ARRANGEMENT

Measurements were performed using Particle Image Velocimetry(PIV) to analyze the modification of flow by the combined effects of the rotation and the Reynolds number on the flow past two rotating circular cylinders in a side-by-side-arrangement at a range of 425 ≤ Re≤ 1130,0 ≤α≤ 4(α is the rotational speed) at one gap spacing of T / d =1.11(T and d are the distance between the centers of two cylinders and the cylinder diameter,respectively).A new Immersed-Lattice Boltzmann Method(ILBM) scheme was used to st...     

WALL EFFECTS ON FLOWS PAST TWO TANDEM CYLINDERS OF DIFFERENT DIAMETERS

Flows past two tandem cylinders of different diameters placed centrally in a channel with fixed centre-to-centre spacing 6D and diameter ratio are simulated based on the Lattice Boltzmann Method(LBM).In all the simulations,the diameter of the smaller cylinder is chosen as the characteristic length.The Reynolds number based on the average inflow velocity is 20-120 and studies are over the range of blockage ratio 2-8.In both Small-Big Arrangement(SBA)and Big-Small Arrangement(BSA),the effects of the channel width and Reynolds number on the flow structures and force coefficients are studied.Results show that the flows in BSA are more regular than those in SBA for the same flow fields.In BSA with and,the force coefficients all fluctuate with constant amplitudes and a coupled frequency,the coupled frequency becomes small as the blockage ratio decreases and by an exact test we give out the relation of the blockage ratio and Strouhal number.As the blockage ratio decreases to 2,there exist pitchfork bifurcations in both SBA and BSA,and results show that the critical Reynolds numbers of pitchfork bifurcations for SBA and BSA are both between 60 and 80.In SBA with,the flow structure has a static asymmetric mode.It is found that the channel width has also an effect on the critical spacing where the flow changes from single body mode into co-shedding mode.By an accurate survey on flows past two cylinders with equal diameters placed inside a channel with the width,the relation between channel width and the critical spacing is given and results show that the critical spacing increases as the channel width increases.     

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。     

Flow characteristics of the two tandem wavy cylinders and drag reduction phenomenon

This paper presents an extensive numerical study of 3-D laminar flow around two wavy cylinders in the tandem arrangement for spacing ratios (L/Dm ) ranging from 1.5 to 5.5 at a low Reynolds number of 1...     

NUMERICAL SIMULATION OF THREE-DIMENSIONAL VORTEX DYNAMICS IN WAKE OF A CIRCULAR CYLINDER

The studies concerning the wake transition regime of the flow around a circular cylinder have drawn much attention in these years. Many experiments have been conducted for this problem but no accurate three-dimensional numerical simulations have hitherto been made. In this paper, a parallel procedure was developed to solve the three-dimensional Navier-Stokes equations on an SGI Origin3900 machine. Two different parallel strategies on this application were analyzed about their efficiency. It is found that the critical Reynolds number is 195, and the wake flow below this Reynolds number is purely two dimensional one, while the Reynolds number goes beyond this critical point, the wake flow becomes unstable under three-dimensional small disturbances. The transition regime involves two modes of small-scale three-dimensional instability (modes A and B), depending on the regime of Reynolds number ( Re ). It is also found that the two different modes A and B exhibit different physical features of the flow. And many other important questions were addressed in this paper.     

Experimental investigation of flow past a circular cylinder with hydrophobic coating

A series of experiments are performed in a gravitational low-speed water tunnel to investigate the influence of the hydrophobic coating on the flow past a circular cylinder. The mean velocity and turbulence intensity profiles behind the cylinders are measured using the hot-film anemometer while the separation angles are obtained with the flow visualization technology. For the Reynolds number lower than 3 800, the hydrophobic coatings are in the Cassie state, the velocity deficit and the turbulence intensity in the wake of the hydrophobic cylinders are lower than those of the smooth cylinders which implies the drag reduction effect of the hydrophobic coatings. When the Reynolds number becomes higher than 6 600, the hydrophobic coatings turn into the Wenzel state. Through decomposing the velocity data in the turbulent wake into different scales based on the orthogonal wavelet transform, it is found that the total turbulence intensity in the wake of the hydrophobic cylinders becomes almost the same as in the wake of the smooth cylinders while the intensity of the large scales of vortex components in the wake of the hydrophobic cylinders is still lower. Furthermore, the separation angles show the same trend as a function of the Reynolds number but always take smaller values for the hydrophobic cylinders.     

中等Reynolds数下并列方柱统流的数值模拟──MAC－AF1方法及其应用

本文通过采用ＭＡＣ－ＡＦ１方法求解原始变量二维不可压非定常Ｎａｖｉｅｒ－Ｓｔｏｋｅｓ方程，对中等Ｒａｙｎｏｌｄｓ数绕流下的并列方柱绕流进行了数值模拟。通过对Ｒｅ＝３００、５００、１０００、２０００及间隙度为ｓ／ｈ＝２．３时并列方柱的数值计算表明：Ｒｅ数对横向力的变化频率毫无影响，恰好是单方柱绕流横向力变化频率的两倍。阻力的频率与横向力的频率相同，阻力的时间平均值基本上与Ｒｅ数无关，但随着Ｒｅ数的增大，阻力和横向力随时间变化的波动幅度稍微有所增加。本文计算的方柱阻力时间平均值和横向力的频率与实验值符合得很好。     

亚临界雷诺数下波浪型圆柱绕流的数值模拟及减阻研究

采用大涡模拟数值方法对亚临界雷诺数Re=3000下波浪型圆柱的绕流现象进行研究分析。研究结果表明,波浪型圆柱的三维尾迹涡结构能得到很好的控制,它在轴向方向呈现周期性正负涡的交替分布特性。随着幅值的增大,波浪型圆柱表面的自由剪切层得以延展,使得旋涡的脱落发生在波浪型圆柱下游较远处,从而达到减阻的目的。波浪型圆柱与普通直圆柱相比,其幅值对平均阻力系数及脉动升力系数的减少及尾迹控制有着更重要的影响,最大减阻可达16%。     

Large-eddy simulation of the flow past both finite and infinite circular cylinders at Re =3900

The flow past a finite circular cylinder with a height-to-diameter ratio of 1.5 and an infinite circular cylinder of the same diameter at a Reynolds number Re= 3 900 is investigated using the large eddy simulation(LES). The objective of the present study is to explore the differences of the flow mechanisms between the finite and infinite circular cylinders. It is shown that the free end of the finite circular cylinders affects the wake region significantly. The mean drag coefficient and the fluctuating lift coefficient of the finite circular cylinder are smaller than those of the infinite circular cylinder. The three-dimensional separation and the separated shear layer instability of the finite circular cylinder can obviously be observed. The existence of an arch vortex in the average flow downstream of the free end is demonstrated.     

HYDRAULIC CHARACTERIZATIONS OF TORTUOUS FLOW IN PATH DRIP IRRIGATION EMITTER

1. INTRODUCTION The emitter is a device used to dissipate pressure and to discharge water at a constant rate by dissipating the energy when pressured water flows through its narrow and long internal path structure or micro-orifice. It is the main componen…     

Numerical investigation of drag force on flow through circular array of cylinders

A 2-D model for flow through a circular patch with an array of vertical circular cylinders in a channel is established using the Navier-Stokes equations with a hybrid RANS/LES turbulence model–the Scale Adaptive Simulation (SAS) model. The applicability of the model is first validated by test cases where experimental data are available for comparison with the computed results. It is verified that the present model can predict well the average velocity and turbulence structure. The drag force and drag coefficient are then calculated using the present model for a number of cases with different solid volume fractions, cylinder Reynolds numbers and patch diameters. It is shown that the drag coefficient increases with increasing solid volume fraction, but decreases with increasing Reynolds number. However, the drag coefficient is independent of the diameter of circular batch when the solid volume fraction and Reynolds number are kept constant.     

近壁圆柱绕流的壁面积沙现象机理分析

针对近壁圆柱绕流过程中出现的在平板壁面积沙现象用数值模拟方法进行了机理分析。应用标准k-ε湍流模型和流函数-涡量法分析了不同间隙比（圆柱与壁面间的距离对圆柱直径之比）下的尾流特性，得到圆柱表面升阻力和平板壁面压强脉动。结合流动显示的染色粒子图方法，进行了详细的分析并提出近壁圆柱绕流过程中在圆柱下游平板壁面形成沙粒线的可能机理。     

2-D eddy resolving simulations of flow past a circular array of cylindrical plant stems

In the present study, 2-D large eddy simulations(LES) are conducted for flow past a porous circular array with a solid volume fraction(SVF) of 8.8%, 15.4% and 21.5%. Such simulations are relevant to understanding flow in natural streams and channels containing patches of emerged vegetation. In the simulations discussed in the paper, the porous cylinder of diameter D contains a variable number of identical solid circular cylinders(rigid plant stems) of diameter d= 0.048 D. Most of the simulations are conducted at a Reynolds number of 2 100 based on the diameter D and the velocity of the steady uniform incoming flow. Though in all cases wake billows are shed in the regions where the separated shear layers(SSLs) forming on the sides of the porous cylinder interact, the effect of these wake billows on the mean drag is different. While in the high SVF case(21.5%), the total drag force oscillates quasi-regularly in time, similar to the canonical case of a large solid cylinder, in the cases with a lower SVF the shedding of the wake billows takes place sufficiently far from the cylinder such that the unsteady component of the total drag force is negligible. The mean amplitude of the oscillations of the drag force on the individual cylinders is the largest in a streamwise band centered around the center of the porous cylinder, where the wake to wake interactions are the strongest. In all cases the maximum drag force on the individual cylinders is the largest for the cylinders directly exposed to the flow, but this force is always smaller than the one induced on a small isolated cylinder and the average magnitude of the force on the cylinders directly exposed to the flow decreases monotonically with the increase in the SVF. Predictions of the global drag coefficients, Strouhal numbers associated with the wake vortex shedding and individual forces on the cylinders in the array from the present LES are in very good agreement with those of 2-D direct numerical simulations conducted on finer meshes, which suggests LES is a better option to numerically investigate flow in channels containing canopy patches, given that LES is computationally much less expensive than DNS at high Reynolds number. To prove this point, the paper also discusses results of 2-D LES conducted at a much higher Reynolds number, where the near-wake flow is strongly turbulent. For the higher Reynolds number cases, where the influence of the turbulence model is important, the effect of the sub-grid scale model and the predictive capabilities of the unsteady Reynolds averaged Navier-Stokes(RANS) approach to predict flow past porous cylinders are discussed.     

基于格子玻尔兹曼方法的液滴撞击移动液膜过程模拟

采用可调节表面张力的大密度比格子玻尔兹曼伪势模型模拟了液滴撞击移动液膜的过程,并进一步分析了雷诺数、韦伯数和初始液膜运动速度对液冠演化的影响。结果表明:初始液膜运动速度会增大上游液冠处撞击后液膜运动速度不连续性,促进上游液冠变形,同时减小了下游液冠处撞击后液膜运动速度不连续性,对下游液冠演化具有抑制作用;上游液冠高度随雷诺数增大而增大,但不随韦伯数的变化发生改变,而下游液冠高度则随着雷诺数和韦伯数的增大而减小,但雷诺数和韦伯数均不影响到液冠半径的演化;随着初始液膜运动速度增大,上、下游液冠高度减小,但液冠半径增大。格子玻尔兹曼伪势模型模拟结果与试验结果和理论分析一致,证明格子玻尔兹曼伪势模型可以有效模拟复杂液膜条件下液滴撞击液膜过程。     

电加热圆柱尾迹涡的流动显示

本文在低湍流度风洞及水槽中，就低雷诺数范围内对加势圆柱尾迹涡进行流动显示研究，实验表明在水介质中均匀加热圆柱时可促进其尾涡的脱落、而沿展向局部加热时，其局部涡脱落频率增大，导致原准二维卡门涡街三维化；相反空气中加热圆柱对旋涡脱落有扼制作用，本文首次观察到加热诱发的圆柱尾迹不稳定性的展向波，表现为Ｒｅ＾＊〈Ｒｅ（临界涡脱落雷诺数）时双子涡展向断裂成间距为展向波长λ的涡段。圆柱加热增强了尾迹近区的不稳     

明渠柱体上游马蹄涡的运动学特征研究

马蹄涡是水下柱体结构周围床面冲刷的主要动力。为掌握马蹄涡的运动学特征,引入了基于旋转强度的马蹄涡识别方法,提出了基于椭圆涡拟合的马蹄涡特征提取方法,其中,椭圆涡由Oseen涡和倾斜单向剪切流叠加而成。基于上述方法,通过开展明渠圆柱绕流PIV实验,对柱体上游对称面时均流场中马蹄涡的运动学特征进行了研究。结果表明,在柱体雷诺数大于104的紊流柱体绕流中,马蹄涡上游的流动分离点随柱体雷诺数的增大而缓慢向下游移动,但马蹄涡的位置保持不变,涡中心距柱体中轴约0.67D(D为柱体直径或宽度),距床面约0.06D;随着柱体雷诺数的增大,马蹄涡的半径不变而强度增加,使得马蹄涡作用于床面的剪切应力增大。研究结果建立了客观提取马蹄涡运动学特征的数学方法,得到了流动分离点、马蹄涡位置等运动学特征随水流条件的变化规律,揭示了明渠柱体绕流与其它流动中柱体绕流的差异,初步构建了马蹄涡的动力作用概念模型。     

淹没垂直圆柱周围水流结构及紊流特性试验研究

淹没垂直圆柱虽在各工程领域中均有广泛的实际应用,但还没有关于其绕流场内水流结构及紊流特性的系统研究。通过室内变坡水槽试验对淹没垂直圆柱周围的瞬时流速场进行了精细测量,分析了淹没率对垂直圆柱上下游竖直对称面内三维流速、紊动强度等水力参数垂线分布的影响程度,研究了向下水流、马蹄形旋涡和尾流旋涡等典型水流结构的冲刷力随淹没率的变化规律,揭示了淹没率影响垂直圆柱周围清水局部冲刷的机理。研究结果表明:增大淹没率能够减小垂直圆柱出露于水体中的有效长度,使得垂直圆柱对水体的阻流和扰动作用减弱,也使得垂直圆柱上游迎水面处向下水流、柱周马蹄形旋涡和下游尾流旋涡的冲刷能力被有效削弱,降低了垂直圆柱周围床面泥沙被旋涡体系卷起后顺水流失的概率。     

Numerical simulation of low-Reynolds number flows past two tandem cylinders of different diameters

The flow past two tandem circular cylinders of different diameters was simulated using the finite volume method. The diameter of the downstream main cylinder （D） was kept constant, and the diameter of the upstream control cylinder （d） varied from 0.1D to D. The studied Reynolds numbers based on the diameter of the downstream main cylinder were 100 and 150. The gap between the control cylinder and the main cylinder （G） ranged from 0.1D to 4D. It is concluded that the gap-to-diameter ratio （G/D） and the diameter ratio between the two cylinders （d/D） have important effects on the drag and lift coefficients, pressure distributions around the cylinders, vortex shedding frequencies from the two cylinders, and flow characteristics.