流向振荡圆柱绕流中的涡脱落模态分析

本文用有限体积法对平行于均匀来流方向受迫振荡的圆柱绕流问题进行了二维数值模拟。雷诺数选取Re=200、855等几种亚临界雷诺数情况。研究了不同振幅和振动频率下的流场结构、涡脱落模态和一些重要流动参数如升阻力系数、Strouhal数的变化关系。西文计算表明，流向振荡圆柱绕流中涡脱落方式可划分为对称与反对称两种形态，在反对称的涡脱落现象中又观察到了三种基本模态，相互竞争的各种模态间的共同作用对流动特征起决定性作用。同时计算结果还验证了在圆柱绕流实验中观察到的频率锁定现象。     

圆柱桥墩绕流试验研究

一个世纪以来,圆柱绕流一直是众多理论分析、实验研究及数值模拟的对象,但是由于圆柱周围三维水流的复杂性,迄今对该流动现象物理本质的理解仍是不完整的,特别是对于圆柱绕流流态与圆柱桥墩周围冲刷的关系问题,尚未形成系统的理论研究.利用超声波流速仪测量圆柱周围各测点的流速,得出圆柱桥墩绕流纵向、横向和垂向的三维时均流速分布,并对圆柱下游流态结构进行了初步的理论分析,为圆柱绕流流场和桥墩冲刷机理及防护研究提供了参考.     

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。     

方柱绕流CFX三维数值模拟

该文利用计算流体力学软件CFX,对恒定来流条件下黏性不可压缩流体的圆柱和方柱绕流进行了数值模拟。采用有限体积法和SIMPLE计算方法,求解不可压缩Navier-Stokes方程。首先开展雷诺数(Re)等于100和300时的圆柱和方柱绕流问题的二维数值模拟,分析比较了网格大小和时间步长对模拟结果的影响,并比较了不同柱体情况下的流体流动的差异性。进而对Re=100、300时方柱绕流问题进行了三维数值模拟,重点分析了各截面上阻力和升力系数、Strouhal数以及涡量特性与二维结果的差异。发现Re=300时,周向压力、速度场和涡量场与Re=100时相比存在明显的三维特性,并且由于二维流动结构发展成三维流动结构时需要消耗能量,导致三维的阻力升力系数计算结果均小于二维计算结果。研究表明,该文的数值结果与文献结果吻合良好。     

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。     

匀加速流中圆柱起动流动的研究

本文用氢泡流动显示和数值计算计算求解Ｎ－Ｓ方程的方法说细研究了匀加速来流中圆柱起动流动。匀加速起动的圆柱绕流和突然起动的圆柱绕流的流动结构相似，中在低Ｒｅａ数时出现“ａ－现象”；在高Ｒｅａ时出现“β－现象”，但只有一个二次涡，起动涡形成初期，内部基本上是层流；起动涡形成后期，内部流动变为湍流，用多种布置铂丝的方法，测量了流场的速度分布，计算结果和实验结果进行了比较，二者基本一致。     

绕流流场无穷远处流函数边值条件的数值研究

利用流函数－涡量方程求解二维不可压缩低雷诺数绕圆柱流动，在与其它学者计算结果一致的情况下，本文数值研究了无穷远处流函数边值对圆柱定常绕流、圆柱起动流动以及非定常周期性卡门涡街形成的影响。本文计算了各种条件下绕圆柱的阻力系数、流函数涡量分布以及扰动流函数等。计算结果显示，对于定常绕流及圆柱起动流动，几种无穷无流函数边值条件求得的结果基本一致。对于非定常周期性圆柱绕流，不同的无穷远流函数边值条件对计算     

有限元-有限差分法对振荡流加任意方向均匀来流中圆柱绕流的数值模拟分析

本文用有限元—有限差分混合法解不可压缩非定常N—S方程来直接模拟振荡流加任意方向均匀来流流场中圆柱绕流情况。该方法的基本思想为:在同一时间步的不同计算步中,用三阶迎风格式离散动量方程中的对流项,压力修正方程用有限元离散,离散后的方程用直接法解。本文计算了Re=800,K_c=4时,均匀来流与振荡流幅值之比等于0.8,其夹角分别为30°、45°和60°三种绕流情况。首次用该方法系统地计算分析了振荡流中在考虑较多因素时圆柱绕流的涡脱泻现象。重点探讨了有夹角均匀来流情况下振荡流场中圆柱近迹复杂的涡结构及其非定常演化过程。     

近壁水平圆柱绕流漩涡特性流场计算

该文运用计算机数值模拟了均匀流绕固定圆柱的流场.对3种模型进行了分析,分别是:孤立圆柱绕流、串列双圆柱绕流和并列双圆柱绕流.实验中主要模拟了雷诺数为20、40和100时的绕流流动,并得到流场的速度等值线图、速度欠量图和流线图.计算结果发现:当雷诺数为40时,流场有明显变化,有趋势但并没有形成明显的卡门涡街.     

双圆柱在不同排列方式下绕流流态模拟

双圆柱绕流伴随着流动分离、旋涡生成与脱落、旋涡间相互干扰等复杂问题,其流动形态和流动特征受圆柱相对位置影响。使用FLUENT流体软件,选取间距比1.75,2.5和4,在二维层流模型下,模拟了双圆柱串列、30°夹角错置、60°夹角错置和双圆柱并列绕流,分析了双柱绕流流态、旋涡脱落形态、升力、阻力系数随圆柱相对位置改变而变化的规律,并对比已有的试验成果和模拟成果,为桥梁建设和圆柱绕流理论研究提供了基础数据。     

Vanishing of three-dimensionality in the wake behind a rotationally oscillating circular cylinder

The flow behind a three-dimensional rotationally oscillating circular cylinder was studied by a numerical method. The computations were performed at a Reynolds number of 260, which is at a level that the flow wake has developed into a three-dimensional state called Mode-B. The purpose of this paper is to examine the influence of various rotational amplitudes (0.1-0.7) on the wake instability of the flow, while the oscillation frequency is fixed to the value of that measured in the wake of a stationary cylinder. The results show that the rotation with sufficiently high amplitude brings the flow back to its nominal two-dimensional state. Moreover, it is found that the value of the time-averaged drag and the RMS value of the lift are larger than those of a stationary circular cylinder.     

Numerical simulation of turbulent flow around a forced moving circular cylinder on cut cells

Fixed and forced moving circular cylinders in turbulent flows are studied by using the Large Eddy Simulation （LES） and two-equation based Detached Eddy Simulation （DES） turbulence models. The Cartesian cut cell approach is adopted to track the body surface across a stationary background grid covering the whole computational domain. A cell-centered finite volume method of second-order accuracy in both time and space is developed to solve the flow field in fluid cells, which is also modified accordingly in cut cells and merged cells. In order to compare different turbulence models, the current flow past a fixed circular cylinder at a moderate Reynolds number, Re = 3 900, is tested first. The model is also applied to the simulation of a forced oscillating circular cylinder in the turbulent flow, and the influences of different oscillation amplitudes, frequencies and free stream velocities are discussed. The numerical results indicate that the present numerical model based on the Cartesian cut cell approach is capable of solving the turbulent flow around a body undergoing motions, which is a foundation for the possible future study on wake induced oscillation and vortex induced vibration.     

圆柱尾迹流场中横向振荡翼型绕流的数值模拟

该文数值模拟了静止圆柱尾迹中横向振荡翼型绕流的涡结构、力及频谱特性,揭示前方尾迹流场对振荡翼绕流特性的影响，以了解鱼类游动时自主利用前方流场有利干扰的流体力学机理。研究表明圆柱尾迹的影响可导致翼型前缘涡脱落模态发生变化；翼型尾迹的频谱特性更加复杂;在一定的参数下翼型阻力系数可以整体下降。     

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。     

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…     

Numerical simulation of viscous flow past an oscillating square cylinder using a CIP-based model

The flow past an in-line forced oscillating square cylinder at Reynolds number of 200 is studied using an in-house code, named constrained interpolation profile method developed in Zhejiang University(CIP-ZJU). The model is established in the Cartesian coordinate system using the CIP method to discretise the Navier-Stokes equations. The fluid-structure interaction is treated as a multiphase flow of the liquid and solid phases to be solved simultaneously. An immersed boundary method is used to deal with the boundary of the solid body. The CFD model is first applied to the computation of the flow past a fixed square cylinder for its validation. Computations are then performed for the flow past a square cylinder oscillating in the streamwise direction. Considerable attention is paid to the symmetric and anti-symmetric modes of the vortex shedding in the oscillating square cylinder wake. Various oscillation amplitudes and frequencies are simulated and their effects on the vortex shedding modes are analyzed via Lissajous patterns of the unsteady lift coefficient. The relationship among the lift coefficient, the drag coefficient and the lock-on range is also investigated quantitatively.     

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.     

A numerical study on dispersion of particles from the surface of a circular cylinder placed in a gas flow using discrete vortex method

The dispersion of particles emitted from the surface of a circular cylinder placed in a gas flow at the Reynolds number of 200 000 is numerically investigated using the discrete vortex method coupled with a Lagrangian approach for solid particle tracking. The wake vortex patterns, the temporal-spatial distributions and trajectories as well as the dispersion functions for particles with various Stokes numbers（St） ranging from 0.001 to 1.0 are obtained. The numerical results reveal that：（1） Solid particles on the cylinder surface are picked up and then transported away from the cylinder by the wake vortex flow.（2） Solid particles emitted from the cylinder surface always follow the vortices in the cylinder wake, and the response of particles to wake vortices is directly related to their Stokes numbers（particles with St= 0.001, 0.0038, 0.01 can distribute both in the vortex core and around the vortex periphery, whereas those with St= 0.1, 1.0 can not enter the vortex core and congregate mainly around the vortex periphery）.（3） The particles move in rolling state in the wake region, and the dispersion intensity of particles in the lateral direction decreases remarkably as the Stokes number of particles is increased from 0.001 to 1.0.     

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.     

FORCE REDUCTION OF FLOW AROUND A SINUSOIDAL WAVY CYLINDER

A large eddy simulation of cross-flow around a sinusoidal wavy cylinder at Re=3000 was performed and the load cell measurement was introduced for the validation test. The mean flow field and the near wake flow structures were presented and compared with those for a circular cylinder at the same Reynolds number. The mean drag coefficient for the wavy cylinder is smaller than that for a corresponding circular cylinder due to the formation of a longer wake vortex generated by the wavy cylinder. The fluctuating lift coefficient of the wavy cylinder is also greatly reduced. This kind of wavy surface leads to the formation of 3-D free shear layers which are more stable than purely 2-D free shear layers. Such free shear layers only roll up into mature vortices at further downstream position and significantly modify the near wake structures and the pressure distributions around the wavy cylinder. Moreover, the simulations in laminar flow condition were also performed to investigate the effect of Reynolds number on force reduction control.