串列双圆柱体绕流特性与互扰效应研究

基于大涡模拟(LES)方法对亚临界雷诺数(Re=3900)下三维串列双圆柱体绕流问题进行了数值计算。首先,通过求解单圆柱算例来验证计算模型及参数的正确性。然后,着重分析了不同间距比对双圆柱体的流体力系数的影响,并阐述了双圆柱体流场特性变化及其互扰效应内在机理。研究表明:雷诺数Re=3900时,串列双圆柱体绕流临界间距比在3.9～4.0之间;随着间距比的增加,双圆柱体临近流场中二次涡团形成的区域与三维涡结构均会发生变化,导致其结构表面所受的流体力系数在时间与空间上变化的规律性逐渐减弱;达到临界间距比时,流体力系数的变化会呈现出较强的规律性。     

基于浸入边界-格子Boltzmann通量求解法的椭圆柱流动特性分析

基于浸入边界-格子Boltzmann通量求解法,开展了雷诺数Re=100不同几何参数下单椭圆柱及串列双椭圆柱绕流流场与受力特性对比研究。结果表明,随长短轴比值的增加,单椭圆柱绕流阻力系数先减小后缓慢上升,最大升力系数则随长短轴比值的增大而减小;尾迹流动状态从周期性脱落涡到稳定对称涡。间距是影响串列圆柱及椭圆柱流场流动状态的主要因素,间距较小时,串列圆柱绕流呈周期性脱落涡状态,而椭圆柱则为稳定流动;随着间距增加,上下游圆柱及椭圆柱尾迹均出现卡门涡街现象,且串列椭圆柱临界间距大于串列圆柱。串列椭圆柱阻力的变化规律与圆柱的基本相同,上游平均阻力大于下游阻力;上游椭圆柱阻力随着间距的变大先减小,下游随间距的变大而增加,当间距达到临界间距时上下游阻力跃升,随后出现小幅度波动再逐渐增加,并趋近于相同长短轴比值下单柱体绕流的阻力。     

基于浸入边界-格子Boltzmann通量求解法的椭圆柱流动特性分析

基于浸入边界-格子Boltzmann通量求解法,开展了雷诺数Re=100不同几何参数下单椭圆柱及串列双椭圆柱绕流流场与受力特性对比研究。结果表明,随长短轴比值的增加,单椭圆柱绕流阻力系数先减小后缓慢上升,最大升力系数则随长短轴比值的增大而减小;尾迹流动状态从周期性脱落涡到稳定对称涡。间距是影响串列圆柱及椭圆柱流场流动状态的主要因素,间距较小时,串列圆柱绕流呈周期性脱落涡状态,而椭圆柱则为稳定流动;随着间距增加,上下游圆柱及椭圆柱尾迹均出现卡门涡街现象,且串列椭圆柱临界间距大于串列圆柱。串列椭圆柱阻力的变化规律与圆柱的基本相同,上游平均阻力大于下游阻力;上游椭圆柱阻力随着间距的变大先减小,下游随间距的变大而增加,当间距达到临界间距时上下游阻力跃升,随后出现小幅度波动再逐渐增加,并趋近于相同长短轴比值下单柱体绕流的阻力。     

三维不等直径串列圆柱绕流的双涡脱落流态频率特征研究

采用大涡模拟方法计算Re=2×10~3三维不等直径串列圆柱(d/D≤1)绕流问题。结果显示,处于双涡脱落流态时,随着串列圆柱间距增加,上游圆柱量纲为一的涡脱频率值St₁总体上升,而下游圆柱量纲为一的涡脱频率值St₂存在先下降后上升的变化规律。在圆柱间距较小的情况下,St₂随着串列圆柱间距的增加而减小,量纲为一的涡脱频率比值、直径比与间距比之间近似满足St₂/St₁∝(L/D)^-1/4(d/D)的幂指数关系;在圆柱间距较大的情况下,圆柱间时均流向速度提高并趋近主流区速度,St₂随间距比增加而上升。在较小直径比串列圆柱情形下,下游圆柱量纲为一的涡脱频率St₂可下降至更低的临界拐点,从而产生“次谐波涡脱锁定”现象。     

相同雷诺数下串列双圆柱绕流的仿真分析

为研究均匀水流场中串列排布的柱群之间的干涉影响,本文以三维串列双圆柱为例,通过计算流体力学(CFD)软件FLUENT15.0中双方程k-ε模型,分析模拟了双圆柱所受平均阻力、平均升力、后柱周向压力、斯特劳哈尔数等水动力特性。结果表明:在雷诺数为Re=2×10~4的串列双圆柱绕流中,两圆柱中心间距L与圆柱直径D的比值为L/D=4时,后柱受前柱绕流尾流影响大,明显高于单圆柱绕流的平均阻力系数,后柱的周向压力值也随前柱尾流的摆动呈现显著的不对称性;当L/D=8时,前柱绕流尾流对后柱影响逐渐减弱;当L/D=12时,两圆柱之间的相互干扰几乎可以忽略,可以看作是相互独立的单圆柱绕流。最后,计算的斯特劳哈尔数与单圆柱绕流对应的斯特劳哈尔数相近且仿真数值在计算数值范围之内,验证了整个仿真分析的准确性,也进一步说明了双圆柱绕流的柱群的干涉影响。双圆柱间距越大,前、后柱之间的干涉影响越弱。     

高雷诺数下多柱绕流特性研究

采用改进的延迟分离涡方法数值模拟了高雷诺数下的柱体绕流,包括单圆柱绕流、单方柱绕流、串列双圆柱绕流和串列双方柱绕流,研究了不同雷诺数下圆柱绕流与方柱绕流的水动力特性.计算结果与实验数据及其他文献的数值计算结果吻合良好,研究表明,单方柱绕流在2.0×10~3Re1.0×10~7范围内未出现类似于单圆柱绕流的阻力危机现象,其平均阻力系数C_d、升力系数均方根C'_1及斯特劳哈尔数S t维持在一定范围内波动.串列双圆柱绕流与串列双方柱绕流中,均选取L/D=2.0,2.5,3.0,3.5和4.0这五中间距比进行计算.串列双圆柱绕流中,当Re=2.2×10~4时,在3.0L/D3.5内存在一临界间距比(L_c/D)使得L_c/D前后上下游圆柱的升阻力系数发生跳跃性变化,且当L/DL_c/D时,下游圆柱的阻力系数为负数.而当Re=3.0×10~6时,则不存在临界间距比,且下游圆柱的阻力系数始终为正数.串列双方柱绕流在Re=1.6×10~4和Re=1.0×10~6两种工况下的临界间距比分别处于3.0L/D3.5和3.5L/D4.0区间内,且当L/DL_c/D时,两个雷诺数下的下游方柱阻力系数均为负数.     

并列双圆柱绕流特性和互扰效应数值模拟研究

基于CFD方法,对不同间距比(1.25≤T*≤5)下的二维双圆柱绕流进行了数值模拟研究.初步研究结果表明:当1.25＜T*≤2.2时尾流不能达到稳定,两圆柱交替对下游流场起主导作用;当T*＞2.2时尾流能达到稳定且上下侧圆柱涡的脱落出现同步,即流场趋于稳定的临界值为T ≈2.2.进一步研究发现:当2.2＜T*≤3.3时...     

双圆柱绕流特性的模拟研究

采用格子Boltzmann方法对低雷诺数下气体绕流圆柱的规律进行了研究. 对比计算了双圆柱在不同圆心距、不同Re数、不同来流速度与双圆柱圆心连线角度的情况下,各个圆柱的受力大小和曳力系数. 结果表明,若Re数为20, 改变圆柱间距,圆柱间距在1.2d和1.4d之间时,下游圆柱所受曳力有极小值;双圆柱间距为1.6d时,双圆柱受到总曳力最小;圆柱间距大于2d时,上游颗粒受到的曳力不再受到下游颗粒的影响. 若圆柱间距为1.2d, 改变雷诺数,Re数在30和40之间,下游圆柱所受曳力有极小值. 另外,来流速度角度对圆柱的受力影响很大. 上述规律为低Re数下圆柱绕流的深入研究与应用打下基础.      

正方形排列四圆柱体绕流特性及互扰效应研究

基于计算流体动力学理论,运用大涡模拟方法对雷诺数Re=3900三维正方形排列四圆柱体结构群的绕流问题进行数值计算,主要分析来流攻角与间距比两个参数对四圆柱体结构群流体参数及流场模态的影响。结果表明:来流攻角与间距比均对四圆柱体结构群绕流特性有较强的影响;来流攻角θ=0°、22.5°、45°下,临界间距比分别为3.5、4.0、3.0;间距比的变化会导致下游圆柱表面压力系数分布发生改变;另一方面,间距比较小时,四圆柱体结构之间的互扰作用均以临近效应为主;随间距比增大,上游圆柱尾流对下游圆柱有显著影响,其互扰作用会转变尾激效应。     

分块法研究圆柱绕流升阻力

使用新的分块耦合方法，分别对单圆柱和串列双圆柱绕流进行了数值模拟．对于单圆柱绕流，低Re下计算所得到的定常涡尺寸与实验非常接近．对于串列双圆柱绕流，研究分析了改变双圆柱中心间距对上下游圆柱的升阻力系数和脉动频率所产生的影响，计算结果与实验非常吻合，为进一步研究涡致振动提供了依据．     

串列双圆柱绕流问题的数值模拟

本文运用有限体积方法,对绕串列放置的双圆柱的二维不可压缩流动进行了数值计算。为研究两圆柱不同间距对圆柱相互作用和尾流特征的影响,选取间距比Ｌ／Ｄ（Ｌ为两圆柱中心间的距离,Ｄ为圆柱直径）在１．５～５．０之间每隔０．５共八个有代表性的间距进行了计算模拟。计算均在Ｒｅ＝２００条件下进行。计算结果表明：对该绕流问题,流动特征在很大程度上取决于间距的大小。且间距存在一临界值,间距比从小于临界值变化到大于临界     

NUMERICAL INVESTIGATION ON FLOW-INDUCED VIBRATION OF TWO CYLINDERS IN TANDEM ARRANGEMENTS AND ITS COUPLING MECHANISMS

对雷诺数Re= 100 条件下串列双圆柱的流致振动进行了数值模拟. 圆柱的质量比m^*均为2.0,间距比L/D 为2.0 5.0. 考虑两种工况:(a) 上游圆柱固定,下游圆柱可沿横流向自由振动;(b) 上、下游圆柱均可沿横流向自由振动. 结果表明:无论上游圆柱静止或者振动,下游圆柱横向振幅明显大于单圆柱的. 工况(b) 的下游圆柱最大振幅要大于工况(a) 的,这是由于两圆柱均振动时,两圆柱之间耦合作用增强,上游圆柱的尾流和下游圆柱的振动之间“相互调节” 作用显著. 对工况(b) 的下游圆柱振动和间隙流之间的作用机制进行了详细的研究,发现当上游圆柱脱落的自由剪切层重新附着于下游圆柱上并且完全从间隙之间通过时,下游圆柱的振幅最大.     

On the origin of wake-induced vibration in two tandem circular cylinders at low Reynolds number

We numerically investigate flow-induced vibrations of circular cylinders arranged in a tandem configuration at low Reynolds number. Results on the coupled force dynamics are presented for an isolated cylinder and a pair of rigid cylinders in a tandem configuration where the downstream cylinder is elastically mounted and free to vibrate transversely. Contrary to turbulent flows at high Reynolds number, low frequency component with respect to shedding frequency is absent in laminar flows. Appearance and disappearance of the vorticity regions due to reverse flow on the aft part of the vibrating cylinder is characterized by a higher harmonic in transverse load, which is nearly three times of the shedding frequency. We next analyze the significance of pressure and viscous forces in the composition of lift and their phase relations with respect to the structural velocity. For both the isolated and tandem vibrating cylinders, the pressure force supplies energy to the moving cylinder, whereas the viscous force dissipates the energy. Close to the excitation frequency ratio of one, the ratio of transverse viscous force to pressure force is found to be maximum. In addition, movement of stagnation point plays a major role on the force dynamics of both configurations. In the case of isolated cylinder, displacement of the stagnation point is nearly in-phase with the velocity. During vortex-body interaction, the phase difference between the transverse pressure force and velocity and the location of stagnation point determines the loads acting on the cylinder. When the transverse pressure force is in-phase with velocity, the stagnation point moves to higher suction region of the cylinder. In the case of the tandem cylinder arrangement, upstream vortex shifts the stagnation point on the downstream cylinder to the low suction region. Thus a larger lift force is observed for the downstream cylinder as compared to the vibrating isolated cylinder. Phase difference between the transverse load and the velocity of the downstream cylinder determines the extent of upstream wake interaction with the downstream cylinder. When the cylinder velocity is in-phase with the transverse pressure load component, interaction of wake vortex with the downstream cylinder is lower compared to other cases considered in this study. We extend our parametric study of tandem cylinders for the longitudinal center-to-center spacing ranging from 4 to 10 diameter.     

Experimental study on dynamic response of smooth and straked risers in tandem arrangement coupling interference effect

An experiment was conducted in a combined wave–current water flume on two tandem risers subjected to uniform flow. The riser model has an effective length of 2.0 m. The aspect ratio is 111.11. The upstream riser is smooth and the downstream riser fitted with three-strand helical strakes with pitch 17.5D and height 0.25D. By varying the external flow velocities and spacing ratios, through comparisons with the dynamic response of isolated smooth and isolated straked riser, the paper observe how interference effect impacts the dynamic characteristics and dynamic response of two risers in tandem arrangement, reveal how the suppression efficiency of the three-strand helical strakes responds to spacing ratio and external flow velocity, and explore the wake excitation effect of inter-riser fluids on the downstream riser and their dynamic feedback to the upstream riser. The results show that the dominant frequency of the upstream smooth riser is sensitivity to the change of the spacing ratio is low, and the displacement response is offset or enhanced in different degree due to the difference of the interference efficiency. The downstream straked riser dominates frequency and displacement higher than the isolated straked riser. The wake vortex of the upstream smooth riser acts on the downstream riser, occupying a dominant position in the vibration of the downstream riser. It degrades the vibration suppression efficiency of the three-strand helical strakes: the suppression efficiency is the highest at spacing ratio of 8, being a merely 70.57%. With the increase of the spacing ratio, the CF displacement of the riser gradually decreases, and the IL displacement gradually increases. The interference efficiency partition and suppression efficiency at different spacing ratios reflects the dynamic feedback of the upstream smooth riser is much smaller than the interference effect of the downstream suppression riser.     

Interference effect of two equal‐sized square cylinders in tandem arrangement: With planar shear flow

Numerical simulations have been performed for flow past two equal‐sized square cylinders in tandem arrangement subjected to incoming planar shear flow. Effect of L/d ratio and the shear parameter has been studied. The range of L/d ratio (ratio of center‐to‐center distance (L) to cylinder width (d)) is varied from 2 to 7 and the non‐dimensional shear parameter (K) is varied from 0.0 to 0.4 in steps of 0.1. For all the cases the Reynolds number (Re) based on centerline velocity and cylinder width is fixed at 100. The results are compared with that of isolated square cylinder with uniform flow. Strouhal number decreases with increasing shear parameter. There are more than one shedding frequency at high shear parameters and L/d ratios. The mean drag coefficient is decreased with shear parameter and lesser than that of the single cylinder. The root mean square (RMS) value of both lift and drag coefficients is higher for the downstream cylinder for all values of shear parameter. With increasing L/d ratio, for both lift and drag, the RMS value increases and then decreases for upstream cylinder, whereas it continuously increases for the downstream cylinder. The stagnation point is moved towards the top leading edge with increasing shear. The critical L/d ratio, which is defined as the distance between two cylinders, beyond which the vortex shedding from the upstream cylinder occurs, decreases with increasing shear parameter. Copyright © 2007 John Wiley & Sons, Ltd.     

串列双锥柱绕流流动特性的大涡模拟研究

利用大涡模拟研究了雷诺数Re = 3900下串列双锥柱在间距比L/D_m = 2 ~ 10下的升阻力特性及三维流动结构. 研究发现: 上游锥柱在后方形成的两个展向不对称回流区, 使其后方压力分布不对称. 上游锥柱发展的上洗、下洗和侧面剪切层作用在下游锥柱的附着点位置不同是上游和下游锥柱时均阻力系数和脉动升力系数变化的主要原因, 串列双锥柱间流动结构随间距比变化可分为三种状态: 剪切层包裹状态, 过渡状态及尾流撞击状态. 剪切层包裹状态. 上游锥柱的自由端主导来流在下游锥柱迎风面影响范围广, 上游锥柱剪切层完全包裹住下游锥柱, 从而抑制下游锥柱后方回流区形成, 导致下游锥柱时均阻力系数降低; 尾流撞击状态; 上游锥柱尾流得到充分发展, 其回流区大小随间距比增大不再发生变化, 上游锥柱尾流出现周期性脱落, 撞击在下游锥柱表面, 从而使脉动升力系数大幅增加, 最大脉动升力系数较单直圆柱提升约20.7倍; 过渡状态, 此时时均阻力系数和脉动升力系数均会较剪切层包裹状态增加. 该研究可以为风力俘能结构群列阵布局提供理论支持.      

Flow regimes and frequency selection of a cylinder oscillating in an upstream cylinder wake

This paper describes flow around a pair of cylinders in tandem arrangement with a downstream cylinder being fixed or forced to oscillate transversely. A sinusoidal parietal velocity is applied to simulate cylinder oscillation. Time-dependent Navier-Stokes equations are solved using finite element method. It is shown that there exist two distinct flow regimes: ‘vortex suppression regime’ and ‘vortex formation regime’. Averaged vortex lengths between the two cylinders, pressure variations at back and front stagnant points as well as circumferential pressure profiles of the downstream cylinder are found completely different in the two regimes and, thus, can be used to identify the flow regimes. It is shown that frequency selection in the wake of the oscillating cylinder is a result of non-linear interaction among vortex wakes upstream and downstream of the second cylinder and its forced oscillation. Increasing cylinder spacing results in a stronger oscillatory incident flow upstream of the second cylinder and, thus, a smaller synchronization zone.     

低雷诺数下串列布置双圆柱涡激振动特性研究

基于四步半隐式特征线分裂算子有限元方法,对串列布置双圆柱双自由度涡激振动问题进行了数值模拟计算,并分析了间距比、剪切率、频率比以及折减速度4个参数对圆柱结构动力响应的影响.研究发现:不同固有频率比与剪切率对下游圆柱振动幅值影响较大,然而对上游圆柱振动幅值影响较小.上游圆柱在两个自由度方向达到最大值的折减速度不同,然而下...     

Unsteady characteristics of low-Re flow past two tandem cylinders

This study investigated the two-dimensional flow past two tandem circular or square cylinders at Re = 100 and D / d = 4–10, where D is the center-to-center distance and d is the cylinder diameter. Numerical simulation was performed to comparably study the effect of cylinder geometry and spacing on the aerodynamic characteristics, unsteady flow patterns, time-averaged flow characteristics and flow unsteadiness. We also provided the first global linear stability analysis and sensitivity analysis on the physical problem for the potential application of flow control. The objective of this work is to quantitatively identify the effect of the cylinder geometry and spacing on the characteristic quantities. Numerical results reveal that there is wake flow transition for both geometries depending on the spacing. The characteristic quantities, including the time-averaged and fluctuating streamwise velocity and pressure coefficient, are quite similar to that of the single cylinder case for the upstream cylinder, while an entirely different variation pattern is observed for the downstream cylinder. The global linear stability analysis shows that the spatial structure of perturbation is mainly observed in the wake of the downstream cylinder for small spacing, while moves upstream with reduced size and is also observed after the upstream cylinder for large spacing. The sensitivity analysis reflects that the temporal growth rate of perturbation is the most sensitive to the near-wake flow of downstream cylinder for small spacing and upstream cylinder for large spacing.