虚拟边界法研究正交双圆柱及串列双圆球绕流

把Goldstein等人提出的虚拟边界法推广到三维情况,研究了 Re=150时不同间距下正交双圆柱绕流,和Re=250时不同间距下串列双 圆球绕流流场. 对于正交双圆柱绕流,当间距比大于3,下游圆柱对上游圆柱尾流的影响只 限定在下游圆柱的尾流所扫过的范围之内;当间距比小于等于3,下游圆柱对上游圆柱尾流 的影响扩大,下游圆柱尾流扫过区上下出现两排三维流向二次涡结构. 对于串列圆球绕流, 研究发现,在小间距比(L/D≈ 1.5)的情况下,由于上下游圆球尾流区的相互抑 制消除了压力不稳定性,整个流场呈现稳 态轴对称特征;间距比为2.0时,周向压力梯度诱发出流体的周向输运,流场呈现稳态非对 称性,但流场中存在特定的对称面;间距比增大到2.5后,绕流场开始周期振荡,原有的对 称面依旧存在;在间距比3.5时下游圆球下表面的涡结构强度有所减弱,导致占优频率发生 交替;间距比增至7.0时,整个流场恢复稳态特征,两圆球尾部同时出现双线涡,这时流场 对称面的位置发生了变动.     

横向振荡圆柱绕流的格子Boltzmann方法模拟

基于格子Boltzmann方法(LBM)对不可压横向振荡圆柱绕流问题进行了数值研究. 与传统的求解宏观的N-S方程的数值方法不同, LBM求解此类问题不需要采用动网格, 而且不需要对网格进行特殊处理, 从而节约了计算成本. 结果显示, 当振荡频率增加到相应的静止圆柱绕流的自然涡脱落频率附近时, 圆柱后最新形成的集中涡距离柱体越来越近, 直到达到一个极限位置. 随后, 集中涡突然转向圆柱体另一侧脱落. 当振荡频率接近于静止圆柱的自然涡脱落频率时, 发生频率同步的现象. 随着振荡频率远离自然涡脱落频率, 同步现象消失. 在几种次谐振荡和超谐振荡下, 尾流区的涡脱落频率仍为相应的静止圆柱绕流的自然涡脱落频率.      

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.      

串列双圆柱流致振动的数值模拟及其耦合机制

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.      

纳米尺度圆柱绕流尾迹区流动形式模拟研究

采用非平衡分子动力学模拟方法,对微尺度低{Re}数下的圆柱绕流问题进行了研究,模拟结果表明:当{Re}<12时,圆柱下游形成对称、无分离的定常流;当{Re}>20时,圆柱下游形成周期性交替出现的对称涡;当12相似文献   

涡激诱导并列双圆柱碰撞数值模拟研究

圆柱类结构物的涡激振动是工程中较为常见的一种现象,如果圆柱结构物之间的距离较小, 就会产生涡激诱导碰撞现象,而涡激碰撞会比涡激振动对结构物疲劳破坏产生更严重的威胁.采用浸入边界法模拟流体中的动边界问题,避免了传统贴体网格方法在求解流体中存在固体间碰撞问题时出现数值求解不稳定问题,采用有限元方法对圆柱的运动和碰撞进行求解,通过数据回归方法建立了流体流动条件下的润滑模型,对不同间隙比下涡激诱导并列双圆柱振动及碰撞过程进行了数值模拟, 数值结果表明,如果两圆柱产生了碰撞将会有连续的碰撞发生, 碰撞时出现了多阶频率,振动主频率要比无碰撞时大, 两圆柱碰撞时的相对速度比自由来流速度小;当两圆柱相互接近时, 随着涡环分离角度的逐渐倾斜, 横向流体力先逐渐减小,当两圆柱间涡环开始相互影响发生挤压时, 横向流体力开始逐渐增大;当两圆柱开始反弹时, 两圆柱间形成了低压区, 改变了横向流体阻力的方向,使两圆柱又产生了接近运动,如此反复从而产生了碰撞后横向流体力和圆柱速度的振荡现象.      

涡激诱导并列双圆柱碰撞数值模拟研究

圆柱类结构物的涡激振动是工程中较为常见的一种现象,如果圆柱结构物之间的距离较小,就会产生涡激诱导碰撞现象,而涡激碰撞会比涡激振动对结构物疲劳破坏产生更严重的威胁.采用浸入边界法模拟流体中的动边界问题,避免了传统贴体网格方法在求解流体中存在固体间碰撞问题时出现数值求解不稳定问题,采用有限元方法对圆柱的运动和碰撞进行求解,通过数据回归方法建立了流体流动条件下的润滑模型,对不同间隙比下涡激诱导并列双圆柱振动及碰撞过程进行了数值模拟,数值结果表明,如果两圆柱产生了碰撞将会有连续的碰撞发生,碰撞时出现了多阶频率,振动主频率要比无碰撞时大,两圆柱碰撞时的相对速度比自由来流速度小;当两圆柱相互接近时,随着涡环分离角度的逐渐倾斜,横向流体力先逐渐减小,当两圆柱间涡环开始相互影响发生挤压时,横向流体力开始逐渐增大;当两圆柱开始反弹时,两圆柱间形成了低压区,改变了横向流体阻力的方向,使两圆柱又产生了接近运动,如此反复从而产生了碰撞后横向流体力和圆柱速度的振荡现象.     

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.     

串列布置三圆柱涡激振动频谱特性研究

对串列三圆柱体双自由度涡激振动问题进行了数值计算, 并分析了雷诺数、固有频率比和约化速度对串列三圆柱体结构动力响应及频谱特性的影响. 研究发现: 雷诺数、频率比对上游圆柱的振幅和流体力系数的影响较小. 中游圆柱频率锁定区域随着雷诺数的增大而增大, 其动力响应受上游圆柱尾流的影响较大, 但频率比的影响较小. 同时, 流体力系数在约化速度较小时受雷诺数和频率比的影响较大. 另外, 下游圆柱的振幅和流体力系数受雷诺数及频率比的影响较大. 雷诺数、频率比和约化速度对圆柱流体力系数能量谱密度(PSD)曲线中主峰幅值、频谱成分及波动性的影响较大. 流体力系数PSD曲线波动性的增强, 导致圆柱运动轨迹会从"8"字形转变成不规则形状. 当频率比为2.0时, 上游圆柱尾流出现P$+$S模式, 导致其发生非对称运动, 且升、阻力系数PSD曲线主峰重合. 最后, 激励荷载平均功率值随约化速度的变化趋势与对应的结构动力响应的变化类似. 在同一约化速度区间内, 结构振动响应的强弱与位移的平均功率值成正比. 对不同约化速度区间内的升力系数功率谱密度分析时, 振动频率比($f_{s}/f_{n, y})$对结构振动响应的影响更大.      

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。     

侧柱与串列双柱绕流之间的干扰

本文给出了关于串列双柱与创柱间流动干扰的实验研究结果。当三个圆柱排成等边三角形并靠得很近时，由于三圆柱间强烈的缝隙流动，大大地改变了绕流其中的串列双圆柱的流态。特别，当三圆柱中心距等于二倍圆柱直径时，在串列双柱的前、后柱之间形成了强烈的偏斜的缝隙流，出现了独特的压力分布以及要比单柱高出三倍以上的旋涡脱落频率。     

侧柱对串列双柱脉动压力的干扰

研究了串列双圆柱旁加上一个等直径的圆柱，组成一个等边三角形排列的三圆柱的脉动压力分布．着重研究侧柱对串列双圆柱脉动压力分布的影响．研究的结果表明，绕流等边三角形排列的三圆柱，受影响最严重的是后柱．脉动压力分布出现了严重的不对称，外侧的压力脉动极其强烈，内侧的压力脉动较弱，与时均压力分布，很好的对应关系．另外，侧柱对于串列双圆柱是否达到超过临界间距的绕流流态，有很大的影响     

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.      

Numerical predictions of low Reynolds number flows over two tandem circular cylinders

Flows over two tandem cylinders were analysed using the newly developed collocated unstructured computational fluid dynamics (CUCFD) code, which is capable of handling complex geometries. A Reynolds number of 100, based on cylinder diameter, was used to ensure that the flow remained laminar. The validity of the code was tested through comparisons with benchmark solutions for flow in a lid‐friven cavity and flow around a single cylinder. For the tandem cylinder flow, also mesh convergence was demonstrated, to within a couple of percent for the RMS lift coefficient. The mean and fluctuating lift and drag coefficients were recorded for centre‐to‐centre cylinder spacings between 2 and 10 diameters. A critical cylinder spacing was found between 3.75 and 4 diameters. The fluctuating forces jumped appreciably at the critical spacing. It was found that there exists only one reattachment and one separation point on the downstream cylinder for spacings greater than the critical spacing. The mean and the fluctuating surface pressure distributions were compared as a function of the cylinder spacing. The mean and the fluctuating pressures were significantly different between the upstream and the downstream cylinders. These pressures also differed with the cylinder spacing. Copyright © 2004 John Wiley & Sons, Ltd.     

Vortex Induced Vibrations of a Pair of Cylinders at Reynolds Number 1000

Vortex induced vibrations of two equal-sized cylinders in tandem and staggered arrangement placed in uniform incompressible flow is studied. A stabilized finite element formulation is utilized to solve the governing equations. The Reynolds number for these 2D simulations is 1000. The cylinders are separated by 5.5 times the cylinder diameter in the streamwise direction. For the staggered arrangement, the cross-flow spacing between the two cylinders is 0.7 times the cylinder diameter. In this arrangement, the downstream cylinder lies in the wake of the upstream one and therefore experiences an unsteady inflow. The wake looses its temporal periodicity, beyond a few diameters downstream of the front cylinder. The upstream cylinder responds as an isolated single cylinder while the downstream one undergoes disorganized motion. Soft-lock-in is observed in almost all the cases.     

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     

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.     

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.     

Unsteady incompressible flows past two cylinders in tandem and staggered arrangements

A stabilized finite element formulation is employed to study incompressible flows past a pair of cylinders at Reynolds numbers 100 and 1000 in tandem and staggered arrangements. Computations are carried out for three sets of cylinder arrangements. In the first two cases the cylinders are arranged in tandem and the distance between their centres is 2·5 and 5·5 diameters. The third case involves the two cylinders in staggered arrangement. The distance between their centres along the flow direction is 5·5 diameters, while it is 0·7 diameter in the transverse direction. The results are compared with flows past a single cylinder at corresponding Reynolds numbers and with experimental observations by other researchers. It is observed that the qualitative nature of the flow depends strongly on the arrangement of cylinders and the Reynolds number. In all cases, when the flow becomes unsteady, the downstream cylinder, which lies in the wake of the upstream one, experiences very large unsteady forces that may lead to wake-induced flutter. The Strouhal number, based on the dominant frequency in the time history of the lift coefficient, for both cylinders attains the same value. In some cases, even though the near wake of the two cylinders shows temporal periodicity, the far wake does not. © 1997 John Wiley & Sons, Ltd.     

低雷诺数串列双方柱流致振动质量比效应的数值研究

为澄清串列双方柱流致振动的质量比效应, 采用数值模拟方法, 在雷诺数为150时, 研究了质量比($m^{\ast }=3$, 10, 20)对下游方柱振动响应特性的影响规律, 分析了下游方柱尾流模态的演变过程, 探讨了导致下游方柱振动的流固耦合机制. 结果表明: 质量比对下游方柱的流致振动有重要影响, 低质量比($m^{\ast }=3$)时下游方柱的振动响应更为复杂, 随着折减速度的增大, 下游方柱并未出现传统“锁定”现象(即振动频率比$f_{y}$/$f_{\rm n} \approx1$的锁定), 而发生了“弱锁定”现象(即$f_{y}/f_{\rm n}<1$的锁定); 随着质量比的增加($m^{\ast }=10$和20), “弱锁定”现象消失, 而出现传统“锁定”现象, 且下游方柱横流向最大振幅减小. 质量比对串列双方柱的柱心间距有明显影响, 低质量比($m^{\ast }=3$)时的柱间距在振动锁定区内会急剧减小, 而较高质量比($m^{\ast }=10$和20)下的柱间距则变化不大. 此外, 质量比对串列双方柱的尾流模态和流固耦合机制也有显著影响, 其中低质量比($m^{\ast }=3$)下的情况更为多样.