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。     

一端弹性支撑的串列双柱和并列双柱在气流中自由端的振幅响应

本文研究了一端弹性支撑的并列双柱和串列双圆柱在气流中自由端的振幅响应，与单圆柱相比，在小间距时，串列双圆柱中前柱的横向振幅受到较强的激励。而在大间距时，振幅受到抑制，特别当Ｌ／Ｄ＝３．５和４．０时，其振幅响应仅为单柱的１／３左右，而对于后柱，则在大间距时，纵向振幅响应有所增大，而且后柱的振幅响应要比前柱的大得多，而并列双圆柱的自由端振幅基本上受到抑制，在Ｔ／Ｄ＞３．０之后，干扰很快减小到接近单个圆     

Flow-induced vibration of a circular cylinder subjected to wake interference at low Reynolds number

Two- and three-dimensional numerical simulations of the flow around two circular cylinders in tandem arrangements are performed. The upstream cylinder is fixed and the downstream cylinder is free to oscillate in the transverse direction, in response to the fluid loads. The Reynolds number is kept constant at 150 for the two-dimensional simulations and at 300 for the three-dimensional simulations, and the reduced velocity is varied by changing the structural stiffness. The in-line centre-to-centre distance is varied from 1.5 to 8.0 diameters, and the results are compared to that of a single isolated flexible cylinder with the same structural characteristics, m^?=2.0 and ζ=0.007. The calculations show that significant changes occur in the dynamic behaviour of the cylinders, when comparing the flow around the tandem arrangements to that around an isolated cylinder: for the tandem arrangements, the lock-in boundaries are wider, the maximum displacement amplitudes are greater and the amplitudes of vibration for high reduced velocities, outside the lock-in, are very significant. The main responsible for these changes appears to be the oscillatory flow in the gap between the cylinders.     

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.     

大湍流度高雷诺数时并列双圆柱的平均和脉动压力分布

本文通过风洞实验研究了来流湍流度,Iu=10％雷诺数分别为Re=1.95×10~9和Re=6.5×10~5时单个圆柱和不同间距比下并列双圆柱的平均和脉动压力分布。结果表明:在Re=1.95×10~5时单个圆柱的平均压力分布类似于低湍流度高超临界雷诺数时的压力分布;当雷诺数增大至6.5×10~5时,绕圆柱表面流动的分离点前移和背压绝对值提高,总的阻力系数随之增加。并列双圆柱的间距比变化对圆柱表面压力分布影响很大,在极小间距比(N/d=1.05)时,双圆柱间的缝隙流使附近柱面产生高达-5的压力系数峰值(Re=6.5×10~5),同时脉动压力也大为增加;在较小间距比时(1.5     

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。     

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     

同轴双方柱气动力载荷的相互干扰

本文研究了在大气边界层气流中同轴双方柱气动力载荷的相互干扰,讨论了这种干扰与风向角和间距比之间的关系.结果表明处于相邻方柱的干扰下,上游方柱的最大气动力载荷大于单体的载荷;而下游方柱则受到前柱明显的阻挡作用,不仅如此,当风向角为0°,小间距下的后柱,不但没有阻力,还会受到反向的摧力.还发现在串列情况下存在和双圆柱类似的临界间距,阻力和升力在临界间距附近有明显的突变.同时将本实验的结果和一些文献中的结果进行了比较.     

薄矩形柱绕流旋涡脱落的抑制

用窄条形控制件对截面宽度为B、厚度为H的矩形柱体绕流的旋涡脱落进行抑制.实验在风洞中进行, 实验范围为B/H=2.0~5.0,Re=V_∞H/\nu=3.75× 103~1.05×104. 矩形柱的宽边B与来流平行, 窄条与柱体等长, 且两者轴线相互平行放置. 窄条宽度为b/H=0.5, 窄条厚度远小于其宽度; 窄条位置可变, 但窄条表面保持与来流垂直. 尾流脉动速度测量和流动显示结果表明: 当窄条位于一个有效区内时, 矩形柱体两侧的旋涡脱落被抑制; 而当窄条位于一个单侧有效区内时, 矩形体一侧的旋涡脱落被抑制, 在另一侧旋涡脱落却仍存在. 有效区范围从矩形体的上游某点一直延续到矩形体的下游某点. 单侧有效区将整个有效区围在其中. 有效区和单侧有效区范围随着B/H的增大而增大, 但随着Re的增大而减小.      

The near wake structure of a square cylinder

The near wake structure of a square cross section cylinder in flow perpendicular to its length was investigated experimentally over a Reynolds number (based on cylinder width) range of 6700–43,000. The wake structure and the characteristics of the instability wave, scaling on θ at separation, were strongly dependent on the incidence angle () of the freestream velocity. The nondimensional frequency (St_θ) of the instability wave varied within the range predicted for laminar instability frequencies for flat plate wakes, jets and shear layers. For = 22.5°, the freestream velocity was accelerated over the side walls and the deflection of the streamlines (from both sides of the cylinder) towards the center line was higher compared to the streamlines for = 0°. This caused the vortices from both sides of the cylinder to merge by x/d 2, giving the mean velocity distribution typical of a wake profile. For = 0°, the vortices shed from both sides of the cylinder did not merge until x/d 4.5. The separation boundary layer for all cases was either transitional or turbulent, yet the results showed good qualitative, and for some cases even quantitative, agreement with linearized stability results for small amplitude disturbances waves in laminar separation layers.     

Dual cylinder inviscid vortex sound of low Mach number

The unsteady motions of an inviscid vortex under the influence of a cylinder pair in the presence of a low Mach number mean flow and the corresponding sound generation are examined in the present study. The two cylinders are in close proximity. A semi-analytical approach using the conformal mapping together with the potential theory is adopted. The results show that the vortex will interact intensively with the cylinders under the right combinations of mean flow direction and initial vortex position. Such interactions result in a high rate of change of vortex propagation velocity, strong fluctuating forces on cylinder and strong sound radiations. However, it is found that much stronger acoustic energy radiation will result when the vortex approaches the cylinder pair from the bottom than from the top, unless the mean flow is nearly perpendicular to the horizontal cylinder pair axis. Stronger sound radiation is also observed for the identical cylinder cases in general, except the flow direction is close to some critical values.     

On characteristics of two-degree-of-freedom vortex induced vibration of two low-mass circular cylinders in proximity at low Reynolds number

Two-degree-of-freedom vortex induced vibrations (VIVs) of two identical spring-supported circular cylinders in proximity with the mass ratio of 2 and zero damping at Re of 100 are numerically studied. Totally 20 arrangements of cylinders are investigated combining four stagger angles and five normalized center-to-center spacings. Results show that the in-line vibration amplitude is comparable to the transverse one for most arrangements and usually accompanies irregular cylinder trajectories. Extremely slender figure-8 cylinder trajectories usually seen in single-cylinder VIVs exist only for the tandem arrangements. Arranging the trailing cylinder to vibrate near the wake boundary of the leading cylinder enhances the possibility of irregular trajectories and impacts of both cylinders. Impact between cylinders must occur in cases with irregular cylinder trajectories; however, irregular cylinder trajectories could be found in impact-free cases. The stagger angle significantly changes the attribute of the transverse vibration frequency, toward either the single-cylinder VIV frequency or natural structure frequency in still fluid. The major transverse vibration frequency and the natural structure frequency in still fluid are decoupled for all the side-by-side arrangements and some far spaced tandem arrangements and highly correlated for non-tandem and non-side-by-side arrangements. The time-averaged impact frequency increases with decreasing normalized center-to-center spacing for most combinations of stagger angle and reduced velocity. Apart from the side-by-side arrangements, high-frequency impacts occur when the trailing cylinder is initially located in or near the wake zone of the leading cylinder. The mechanism of trailing cylinder chopping the gap-flow vortices plays an important role in determining the near-wake vortex structures for all non-side-by-side arrangements.     

Identification of flow regimes around two staggered square cylinders by a numerical study

The flow over two square cylinders in staggered arrangement is simulated numerically at a fixed Reynolds number (\(Re =150\)) for different gap spacing between cylinders from 0.1 to 6 times a cylinder side to understand the flow structures. The non-inclined square cylinders are located on a line with a staggered angle of \(45^{\circ }\) to the oncoming velocity vector. All numerical simulations are carried out with a finite-volume code based on a collocated grid arrangement. The effects of vortex shedding on the various features of the flow field are numerically visualized using different flow contours such as \(\lambda _{2}\) criterion, vorticity, pressure and magnitudes of velocity to distinguish the distinctive flow patterns. By changing the gap spacing between cylinders, five different flow regimes are identified and classified as single body, periodic gap flow, aperiodic, modulated periodic and synchronized vortex shedding regimes. This study revealed that the observed multiple frequencies in global forces of the downstream cylinder in the modulated periodic regime are more properly associated with differences in vortex shedding frequencies of individual cylinders than individual shear layers reported in some previous works; particularly, both shear layers from the downstream cylinder often shed vortices at the same multiple frequencies. The maximum Strouhal number for the upstream cylinder is also identified at \({G}^{*}=1\) for aperiodic flow pattern. Furthermore, for most cases studied, the downstream cylinder experiences larger drag force than the upstream cylinder.     

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.      

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.     

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     

双圆柱斜置排列时下游圆柱压力分布和振荡频率的实验研究

当两个圆柱斜置排列并靠得较近时,绕流下游圆柱的流动明显地不同于单一圆柱.本文研究了下游圆柱压力分布受到的影响,并着重于用FFT来分析圆柱表面某些特征点上的压力频谱,研究频谱与圆柱之间的相对距离和位置的关系.当圆柱间距较远时,压力振荡的频率与单圆柱卡门涡节的频率相接近,随着横向间距的接近,频率逐渐降低,然而当两圆柱十分接近时,外侧压力振荡的频率继续降低,但圆柱内侧压力振荡的频率却突然提高,出现了一个圆柱体两侧作用有不同频率压力的情况.     

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 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.     

Formation of vortex street and vortex pair from a circular cylinder oscillating in water

The flow around a circular cylinder undergoing sinusoidal oscillating movement in still water is investigated by phase-locked PIV measurements. The pattern and development of large-scale vortex structures in the flow are studied from the velocity vectors and vorticity contours obtained at eight successive phases of an oscillating cycle. Experiments are performed at three Keulegan–Carpenter numbers; KC=12, 6.28 and 4.25. Results at KC=12 reveal the mechanism of vortex formation and the development of the shed vortices into a vortex street at a lateral direction to the line of cylinder movement. The role of a biased flow stream and the length of the cylinder stroke in the formation of the vortex street are discussed. At the lower KC numbers, a symmetric pair of vortices is found attached to the leeward face of the cylinder. The vortex pair exhibits an increasing degree of asymmetry when KC increases from 4.25 to 6.28. An explanation in terms of the length of the cylinder strokes and the degree of flow asymmetry is offered for the transition of flow regimes from a vortex pair to a vortex street. The present results are compared with the observations made in previous experimental and numerical studies in the literature.