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.     

低雷诺数下并列双波浪柱尾迹干扰效应数值模拟

该文针对雷诺数Re=100下不同波长参数(/mλD=2、4、6和8)、不同间距比(1.5/3.5m≤L D≤)的并列双波浪柱后尾迹复杂的三维流动结构及干扰效应进行了数值研究。研究表明:三维并列双波浪柱在较小的间距比/1.5mL D=时,波峰对波谷排列的并列波浪柱后尾迹呈现明显偏置双稳流动结构,而波峰对波峰排列工况下基本呈现单一波浪型钝体后尾迹流动结构(尤其是在大波长/mλD=6和8时);当间距比/3mL D≥时,随着波长/mλD的增大,上下波浪柱各自呈现单波浪柱后尾迹流动结构;较大波长的三维波浪形状更好地组织了后尾迹三维流动结构,减少了并列双波浪柱后尾迹的相互干扰,从而减阻减振明显,最高减阻可达14.5%,脉动升力系数上下游波浪柱均趋于0.1。该研究可为近海工程的多立管结构及悬索桥的拉索振动抑制等提供理论支持。     

浅水圆柱绕流流动模式探讨

数值求解了二维浅水方程和RNG k-ε流模型,模拟了不同稳定性系数S和圆柱中心间距G下的单圆柱、两圆柱和三圆柱的尾流流动模式,研究了底部摩擦力对尾流结构的影响。结果表明:随着S增大,尾流在底部摩擦力的作用下趋于稳定,大尺度涡结构逐渐消失。单圆柱尾流区依次出现了涡街（VS）尾流、不稳定漩涡（UB）尾流和稳定漩涡（SB）尾流。多圆柱尾流除了受S数影响外,还与G密切相关。随着G的增大,两圆柱背后依次出现了单钝体绕流、偏转尾流和对称尾流,三圆柱背后则依次出现了单钝体绕流、偏转尾流、对称尾流和非对称尾流。此外,数值结果还表明多圆柱尾流结构不同于单圆柱,多圆柱并排使得尾流更加不稳定。     

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.     

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上在零左右振荡,通过频谱分析发现了它们之间的区别。方柱的尾流存在着强烈的非对称相互干扰,且涡有明显的三维性。     

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...     

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。     

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.     

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...     

双浮体自由垂向振荡响应特性

双圆柱体系统在波能装置和防波构件中得到应用，该系统是一个多自由度受迫阻尼振荡系统，阻力和激励力都来自于波浪力。本文采用线性波理论和特征函数展开皮配法，在平面入射波条件下，求解了双圆柱体垂荡运动辐射速度势和圆柱体固定不动时的绕射速度势，推导了附加质量、阻尼系数、波浪激励力表达式，列出了浮体垂荡运动微分方程。最后讨论了下浮体尺度及位置对系统响应特性的影响。数值计算表明无论下浮体尺度及位置如何变化，它只影响到浮体垂荡响应在低频端的值，下浮体的振幅值总是小于入射波的振幅值，而且它在一个较窄的频率范围内从较大的值很快变化接近于零。下浮体半径与上浮体半径之比的变化不仅影响到两个浮体的响应幅值，而且也影响到曲线的变化趋势；两浮体距离的变化对曲线的变化趋势影响不大，但对曲线的幅值有影响；下浮体厚度的变化只少许改变了响应曲线的幅值。     

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.     

WAKE PATTERNS OF FLOW PAST A PAIR OF CIRCULAR CYLINDERS IN SIDE-BY-SIDE ARRANGEMENTS AT LOW REYNOLDS NUMBERS

A flow past two side-by-side identical circular cylinders was numerically investigated with the unstructured spectral element method. From the computational results at various non-dimensional distances between cylinder centers T/D and the Reynolds number Re, a total of nine kinds of wake patterns were observed: four steady wake patterns, including single bluff-body steady pattern, separated double-body steady pattern and transition steady pattern for sub-critical Reynolds numbers and biased steady pattern for super-critical Reynolds numbers, and five unsteady wake patterns, including single bluff-body periodic pattern, biased quasi-steady pattern, quasi-periodic (flip-flopping) pattern, in-phase-synchronized pattern and anti-phase-synchronized pattern. Time evolution of lift and drag coefficients corresponding to each unsteady wake pattern was given.     

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...     

不等直径串列圆柱绕流大涡模拟

为研究背负式海底管线中增设的小直径附属管线对主管线的水动力影响,将大涡模拟中经典Smagorinsky亚格子模型与特征线算子分裂有限元法结合,并引入出口对流边界条件,完善了基于特征线算子分裂有限元的大涡模拟方法。通过自编程序数值模拟Re=1 000的单圆柱绕流,计算结果与相关文献吻合较好,验证了该算法计算圆柱绕流的有效性,并分析了Re=1 000时不同直径比、间距比情况下的串列双圆柱绕流,根据流场的不同涡脱落形态及两圆柱平均阻力系数、升力系数随直径比、间距比变化的规律得到了不同直径比条件下的临界间距范围。达到临界间距后,流场由单一涡脱落状态转变为双涡旋脱落状态。最后分析了两圆柱平均阻力系数及升力系数在临界间距后急剧增加的原因,为背负式海底管线的布局优化提供了理论依据。     

柔性分离盘抑制圆柱涡激振动的风洞试验

该文利用附属柔性分离盘对圆柱涡激振动(VIV)控制问题进行了风洞试验研究。针对弹性支撑的圆柱及不同长度柔性分离盘的试验模型,在雷诺数7 500-40 000情况下,测定不同约化速度(3.5-18.7)下的振动瞬态位移。结果表明:柔性分离盘长度(L)/圆柱直径(D)比为0.6、0.8和1三种圆柱的横向无因次均方根振幅远小于单圆柱,涡激振动抑制效果很好,均在77.6%以上,其中L=0.8D的圆柱抑制效果最佳,几乎无振动;而L=1.2D和1.5D的圆柱反而出现振动增强,约4.5%和9%。对比单圆柱的共振区,附属柔性分离盘的圆柱共振区在约化速度上更靠后且范围更大。     

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.     

Multi-mode of vortex-induced vibration of a flexible circular cylinder

The vortex-induced vibration of a flexible circular cylinder is investigated at a constant Reynolds number of 1 000.The finite-volume method on moving meshes is applied for the fluid flow, and the Euler-Bernoulli beam theory is used to model the dynamic response of a flexible cylinder.The relationship between the reduced velocity and the amplitude response agrees well with the experimental results.Moreover, five different vibrating modes appear in the simulation.From the comparisons of their vortex structures, the strength of the wake flow is related to the exciting vibrating mode and different vortex patterns arise for different vibrating modes.Only 2P pattern appears in the first vibrating mode while 2S-2P patterns occur in the other vibrating modes if monitoring at different sections along the length of the cylinder.The vibration of the flexible cylinder can also greatly alter the three-dimensionality in the wake, which needs further studies in our future work, especially in the transition region for the Reynolds number from 170 to 300.     

WAKE SHIELDING EFFECTS OF THREE CYLINDERS IN CURRENTS

A large amount of experimental work has been carried out in the past to understand mean and fluctuating forces on a single circular cylinder in cross flows. In comparison substantially less work can be found on the interference between multiple cylinders and the number of publications available becomes drastically smaller as the number of cylinders involved increases. The current experimental work systematically examines the wake shielding effects of three cylinders in a towing tank. Though staggered, the three cylinders are arranged mainly along the flow direction with the downstream cylinders situating in the wakes of the upstream cylinders.The tests are mainly carried out in the sub-critical Reynolds number regime with the upper end close to the critical Reynolds number regime. Significant drag reduction on downstream cylinders is identified.     

Numerical simulation of flow past twin near-wall circular cylinders in tandem arrangement at low Reynolds number

Fluid flow past twin circular cylinders in a tandem arrangement placed near a plane wall was investigated by means of numerical simulations. The two-dimensional Navier-Stokes equations were solved with a three-step finite element method at a relatively low Reynolds number of Re = 200 for various dimensionless ratios of 0.25 ≤ G/D ≤ 2.0 and 1.0 ≤ L/D ≤ 4.0, where D is the cylinder diameter, L is the center-to-center distance between the two cylinders, and G is the gap between the lowest surface of the twin cylinders and the plane wall. The influences of G/D and L/D on the hydrodynamic force coefficients, Strouhal numbers, and vortex shedding modes were examined. Three different vortex shedding modes of the near wake were identified according to the numerical results. It was found that the hydrodynamic force coefficients and vortex shedding modes are quite different with respect to various combinations of G/D and L/D. For very small values of G/D, the vortex shedding is completely suppressed, resulting in the root mean square (RMS) values of drag and lift coefficients of both cylinders and the Strouhal number for the downstream cylinder being almost zero. The mean drag coefficient of the upstream cylinder is larger than that of the downstream cylinder for the same combination of G/D and L/D. It is also observed that change in the vortex shedding modes leads to a significant increase in the RMS values of drag and lift coefficients.