Numerical Study on the Suppression of the Oscillating Wake of a Square Cylinder by a Traveling Wave Wall

Traveling waves generated on the side surfaces of a square cylinder are employed to suppress the oscillating wake for improving the flow behavior around a square cylinder; this method is termed the traveling wave wall (TWW) method. This study aimed to evaluate the influence of the key parameters of TWW on the control of aerodynamic forces and the oscillating wake of the flow around a square cylinder. Unsteady numerical analyses at a low Reynolds number (Re) of 100 were performed using a two-dimensional CFD simulation. First, the grid independence and time step independence tests of the simulation were conducted to verify the rationality of the solving parameter settings, and the validation of flow around the fixed square cylinder at Re =100 was carried out. Subsequently, the lift and drag coefficients and the vortex shedding modes under different combinations of three TWW control parameters, including wave velocity, wave amplitude, and wavenumber, were analyzed in detail. The results show that TWW can remarkably reduce the mean value of drag coefficient and the RMS value of the lift coefficient by more than 12% compared to the method involving a standard square cylinder. Two peaks occur in the lift coefficient spectrum, with the low frequency corresponding to the vortex shedding frequency in the wake of the flow around the square cylinder and the high frequency corresponding to the traveling wave frequency. The vorticity contours show that the alternating vortices in the wake of the square cylinder are not completely suppressed under the selected control parameters.     

Numerical simulation of unsteady viscous flow around a circular cylinder

A finite difference solution is obtained for the time-dependent viscous incompressible 2-dimensional flow past a circular cylinder by direct integration of the Navier-Stokes equations expressed in a general curvilinear coordinate system. The solution describes the development of the vortex street developed behind the cylinder. Evolution of flow configuration is studied by means of streamlines, pressure contours, and vorticity contours for different Reynolds numbers. The time-dependent lift and drag coefficients are also obtained.     

Computational study of flow past a cylinder with combined in-line and transverse oscillation

A computational study of the two dimensional flow past an oscillating cylinder is carried out using vorticity and stream function as the dependent variables. With the use of a log-polar coordinate transformation, the nondimensional vorticity transport equations in a non-inertial frame attached to the cylinder are solved using the ADI and SLOR finite difference schemes. The effects of combined in-line and transverse oscillation of the cylinder in the lock-in range of frequency on the time history of the drag and lift are investigated at a Reynolds number of 100. In addition, the influence of position amplitude of the cylinder's transverse oscillation on the lock-in range of frequency, mean drag, amplitude of drag and maximum lift is studied. The time histories of drag and lift forces in the case of combined oscillation are compared with the cases of the cylinder oscillating in the in-line and transverse directions separately. The dominant frequency components in the drag and the lift variations are determined using a Fourier frequency analysis.This research work was financially supported through a University of Windsor Postgraduate Scholarship and grants from the Natural sciences and Engineering Research Council of Canada (Grant Numbers: A-2190 and A-1403).     

直径30cm圆柱的气动力参数和绕流特性研究

为研究光滑圆柱的气动力系数和绕流特性,在均匀流中进行不同风速下的测压风洞试验,试验获得了阻力系数、升力系数、表面风压分布、风压相关性系数、斯托罗哈数等随雷诺数的变化特征,并将试验结果与以往结果进行比较。研究表明:升力系数的脉动值大于阻力系数的脉动值,说明涡脱造成的横风向激励比顺风向紊流激励剧烈;雷诺数位于临界区域时,圆柱表面风压分布呈现出对称-不对称-对称的变化过程,反映了由层流分离转化为湍流分离的全过程;在雷诺数为352000时呈现一侧为层流分离、另一侧为湍流分离的临界流态,风压呈现出左右不对称的单边泡形式;获得层流分离和湍流分离时的表面风压相关性分布特征,层流分离时圆柱同一侧的风压测点均呈较强的正相关,而湍流分离时在分离点前的区域相关性较强,分离点之后的区域相关性较弱;层流分离的升力系数谱有显著的峰值,表明尾流是规则的漩涡脱落,而湍流分离的升力系数谱没有明显峰值,表明尾流是随机的漩涡脱落。     

Shear flow past a square cylinder near a wall

A numerical study on the wake behind a square cylinder placed parallel to a wall has been made. The cylinder is considered to be within the boundary layer of the wall, so that the outside flow is taken to be due to uniform shear. Flow has been investigated in the laminar Reynolds number (based on the cylinder height) range. The interaction of wall boundary layer on the vortex shedding at Reynolds number up to 1400.0 has been investigated for cylinder to wall gap height 0.5 and 0.25 times the cylinder height. The gap flow between the cylinder and wall during a period of vortex shedding has been obtained. The governing unsteady Navier–Stokes equations are discretised through the finite volume method on staggered grid system. An algorithm SIMPLE has been used to compute the discretised equations iteratively. Our results show that at the gap height 0.5 times the cylinder height the vortex shedding occurs at a Strouhal number greater than for an isolated cylinder. Vortex shedding suppression occurs and wake becomes steady up to a certain value of Reynolds number at gap height 0.25 time the cylinder height. At higher Reynolds number the formation of a single row of negative vortices behind the cylinder when it is in close proximity to wall is noteworthy. Due to the shear, the drag experienced by the cylinder is found to decrease with the reduction of gap height.     

圆柱绕流的流场特性及涡脱落规律研究

采用粒子图像测速技术对630、800及950三种雷诺数条件下的圆柱绕流场进行了实验,给出了圆柱下游沿流动方向4倍圆柱直径和垂直方向3倍圆柱直径区域内的速度场、涡量场以及涡脱落现象的时空演化规律.结果表明:圆柱尾流区域位于垂直方向约1.5~2.5倍圆柱直径范围内,随着雷诺数增大,这一范围呈现缩小趋势,而主流对涡的拉伸和输运能力有所增强;涡脱落频率随雷诺数增大而增大,小雷诺数时能够较为完整地捕捉到涡生成、脱落、发展和耗散过程,由于PIV采集频率的限制,大雷诺数条件下涡脱落整个过程不易被完整捕捉到.     

串列双方柱干扰效应流动机理研究

为了研究建筑群体周围的流场结构,减小工程设计中由于干扰效应造成的损失,利用粒子图像测速(PIV)结合数值模拟,研究在较大雷诺数及不同间隙工况下,双方柱流场受干扰时的流动特性及流场空间结构。分析升阻力系数、涡脱频率、斯特劳哈尔数等流场特征参数,探究不同间隙对串列双方柱的影响。当Re=3.42×104时,存在临界间隙比G=4使串列双方柱流场结构发生突变,试验观察到流场中出现双稳态现象;当G<4时,下风向方柱平均阻力系数为负值,小于单方柱情况下的阻力,屏蔽效应明显,上风向方柱后方涡脱落被抑制,平均阻力系数出现了明显的降幅,最大降幅约达10%;当G>4时,上下方柱均有涡旋脱落。该结果对于工程应用具有参考意义。     

Effect of the blockage ratio on the flow in a channel with a built-in square cylinder

The structures wakes behind a square cylinder in a laminar channel flow is conducted numerically. The Strouhal number, drag and lift coefficients were studied in a periodic flow for different blockage ratios =1/4, 1/6 and 1/8 at Reynolds number ranging from 62 to 300. The governing equations are solved by using control volume finite element method (CVFEM) adapted to the staggered grid. The SIMPLER algorithm is used for the velocity-pressure coupling. The critical Reynolds numbers corresponding to the onset of vortex shedding and its change from simple periodic to complex periodic motion are established. A discussion about the effect of the blockage ratio on the Strouhal number, the time-averaged drag coefficient and the amplitude of the lift coefficient is also presented.     

海底管道悬跨段流致振动实验研究及涡激力模型修正

对输送液体的模型管道进行了涡激振动试验研究,试验结果表明:当理论涡脱频率与管道的固有频率不一致时,作用在振荡管道上的涡激力并非简谐扰力,而是具有一定带宽的窄带随机扰力。因此,管道的涡激振动响应也是一个随机过程。当理论涡脱频率与管道的固有频率接近时,管道的涡激振动响应逼近简谐振动。试验结果也表明:作用在振荡圆柱体上的涡激力频率不仅是流速和圆柱体直径的函数,也是圆柱体固有频率的函数。     

上游小圆柱对并列双圆柱尾流的影响

采用有限元方法数值模拟了控制小圆柱对并列双圆柱偏流的控制作用.计算区域为60 D×20 D,并列双圆柱距上游10 D,下游50 D,控制小圆柱的大小d/D=0.15和0.25,分别放置于并列双圆柱上游中心线上L/D=2.0,2.5,3.0,3.5的位置处.数值模拟着重研究了流场形态、升阻力系数变化曲线及升力系数频谱图.结果发现,控制小圆柱不能完全抑制并列双圆柱后宽的和窄的尾流,但对并列双圆柱旋涡脱落有一定的削弱作用.     

双矩形拱肋间的气动干扰效应研究

以某拱桥为例,通过数值模拟研究了串列双矩形拱肋的气动干扰效应,及其对两截面气动力系数的影响。在对计算模型进行验证的基础上,进一步研究了截面宽高比、间距比和来流风攻角对拱肋周围流场的影响,并结合压力云图和湍动能云图解释了气动力系数的变化规律,讨论了不同宽高比截面的漩涡脱落频率与结构自振频率之间的关系,分析了两拱肋升力时程的差异对整体扭矩可能产生的增大效应。结果表明：串列拱肋间的气动干扰效应显著。受上游截面尾流的影响,下游截面的阻力系数明显减小,其值与漩涡的形态、能量大小、移动轨迹等因素密切相关。上、下游截面的升力时程在幅值和相位上存在明显差异,导致拱肋整体的力矩增大,其效应随宽高比或间距比的增大而明显加强,随风攻角的增大而有所降低。漩涡脱落频率随宽高比的增大呈先增大后减小的趋势,而受间距比、风攻角的影响有限。对漩涡脱落频率与宽高比的变化进行多项式拟合,结合结构的模态频率可为拱肋的气动外形设计提供参考。     

Vortex shedding suppression for laminar flow past a square cylinder near a plane wall: a two-dimensional analysis

Summary A numerical study on the uniform shear flow past a long cylinder of square cross-section placed parallel to a plane wall has been made. The cylinder is considered to be within the boundary layer of the wall. The maximum gap between the plane wall to the cylinder is taken to be 0.25 times the cylinder height. We investigated the flow when the regular vortex shedding from the cylinder is suppressed. The governing unsteady Navier-Stokes equations are discretized through the finite volume method on staggered grid system. A pressure correction based iterative algorithm, SIMPLER, has been used to compute the discretised equations iteratively. We found that the critical value of the gap height for which vortex shedding is suppressed depends on the Reynolds number, which is based on the height of the cylinder and the incident stream at the surface of the cylinder. At high Reynolds number (Re ≥ 500) however, a single row of negative vortices occurs for wall to cylinder gap height L ≥ 0.2. The shear layer that emerges from the bottom face of the cylinder reattaches to the cylinder itself at this gap hight.     

串列双圆柱静止绕流的二维数值仿真分析

利用CFX软件建立二维流场模型,采用有限体积法针对串列双圆柱的静止绕流现象进行了数值模拟计算。首先计算了雷诺数Re=200,不同间距时上下游圆柱的斯托罗哈数,并与参考文献的计算结果进行了对比,证明了该文计算的可靠性。然后分析了不同间距时上下游圆柱的升力系数和阻力系数的变化特点,得出了Re=200时双圆柱绕流的临界间距在3.375D~3.5D之间。最后通过对不同间距下流场变化的研究得出:上下游圆柱的间距小于临界间距时,上游圆柱不存在旋涡脱落;超过临界间距时,上游圆柱出现旋涡脱落;下游圆柱始终存在旋涡脱落现象。研究成果能够为计算流体力学和空气动力学技术的发展提供理论基础。     

后置分流平板对并列双圆柱流场特性的影响

数值模拟了Re=200情况下分流板对并列双圆柱流场的影响,并列双圆柱的间距比为1.5 D(D为圆柱的直径),分流板放置在并列双圆柱下游中心线G/D=0.5～1.5的位置;采用有限元法进行数值求解.计算表明,在合适的位置分流板可以抑制并列圆柱后的偏流,减小升阻力.分流板并不能很好地控制旋涡脱落但有一定的削弱作用.     

A proportional-integral-differential control of flow over a circular cylinder

In the present study, we apply proportional (P), proportional-integral (PI) and proportional-differential (PD) feedback controls to flow over a circular cylinder at Re=60 and 100 for suppression of vortex shedding in the wake. The transverse velocity at a centreline location in the wake is measured and used for the feedback control. The actuation (blowing/suction) is provided to the flow at the upper and lower slots on the cylinder surface near the separation point based on the P, PI or PD control. The sensing location is varied from 1d to 4d from the centre of the cylinder. Given each sensing location, the optimal proportional gain in the sense of minimizing the sensing velocity fluctuations is obtained for the P control. The addition of I and D controls to the P control certainly increases the control performance and broadens the effective sensing location. The P, PI and PD controls successfully reduce the velocity fluctuations at sensing locations and attenuate vortex shedding in the wake, resulting in reductions in the mean drag and lift fluctuations. Finally, P controls with phase shift are constructed from successful PI controls. These phase-shifted P controls also reduce the strength of vortex shedding, but their results are not as good as those from the corresponding PI controls.     

The initial oscillatory flow past a circular cylinder

This paper presents results obtained from an initial approximation for the flow around a circular cylinder in two-dimensional oscillating flow. The analysis is developed in terms of the scalar vorticity and stream function. An expansion in powers of time from the start of the motion is obtained using an exact analysis which extends the results of boundary-layer theory by taking into account corrections for finite Reynolds number. The time development of the physical properties of the flow are determined both by means of analytical expressions and by an accurate numerical procedure. The surface pressure, drag and surface vorticity are calculated and various estimates of the time of separation and the distance moved in this time are obtained. The phenomenon of steady streaming is not considered in this paper since the time of validity of the expansions is small. The agreement between the analytical and numerical results at small times is excellent.     

Numerical simulation of planar shear flow passing a rotating cylinder at low Reynolds numbers

Two-dimensional shear flow over a rotating circular cylinder is investigated using lattice Boltzmann method. Simulations are performed at a fixed blockage ratio (B = 0.1) while the Reynolds number, nondimensional shear rate (K) and absolute rotational speed range as 80 ≤ Re ≤ 180, 0 ≤ K ≤ 0.2 and −2 ≤ β ≤ 2, respectively. To verify the simulation, the results are compared to previous experimental and numerical data. Quantitative information about the flow variables such as drag and lift coefficients, pressure coefficient and vorticity distributions on the cylinders is highlighted. It is found that, generally, with the increment in |β|, the absolute value of time average lift coefficient increases and time average drag coefficient decreases, and beyond a certain magnitude of β, the vortex shedding vanishes. It is also revealed that the drag coefficient decreases as the Reynolds number increases while the effect of the Reynolds number on lift is almost negligible. At the end, correlations for drag and lift coefficients ([`(C<sub>D</sub> )] ,[`(C_L )]){(\overline {C_D } ,\overline {C_L })} are extracted from the numerical data.     

行波壁抑制圆柱涡激振动的数值模拟研究

采用CFD数值模拟方法完成了圆柱绕流-涡激振动-行波壁流动控制全过程的数值模拟,重点研究行波壁流动控制方法对低雷诺数下两自由度弹性支撑单圆柱涡激振动的抑制作用。详细分析各阶段圆柱横向和流向位移、质心运动轨迹、升力和阻力系数等随频率比的变化。结果表明:行波壁圆柱的波谷处可以产生一系列稳定的随行波壁运动的小尺度旋涡,有效抑制圆柱表面分离涡的产生,达到消除圆柱绕流尾迹和抑制涡激振动的目的;在计算初始和中途启动的行波壁流动控制方法显著抑制了圆柱横向和流向振动、降低了圆柱升力系数脉动值和阻力系数均值,但阻力系数脉动值则明显增大。     

振动圆柱绕流气动力系数的数值模拟研究

给出了在锁定状态下不同振动条件时气动力系数变化的计算结果及其机理分析。首先给出固定圆柱绕流气动力系数的数值模拟结果；其次，在特定的频率比条件下，计算了升力系数均方根值和平均阻力系数随振幅的变化，并从圆柱的底部压力系数和漩涡生成长度两方面对这种变化进行了机理分析。