近壁面柱体涡激振动的迟滞效应

柱体涡激振动是典型的流固耦合问题,其响应规律大多是在升速流动和远离壁面条件下获得的.而自然环境流动通常不断经历升速和降速过程,近壁面柱体的涡激振动可呈现与远离壁面柱体不同的响应特征.本研究结合大型波流水槽,设计了具有微结构阻尼的柱体涡激振动装置.基于量纲分析,开展系列水槽模型实验,通过同步测量柱体涡激振动位移时程和绕流流场变化,研究了升降流速作用下柱体涡激振动触发和停振的临界速度(即上临界和下临界速度)变化规律,探究了近壁面柱体涡激振动迟滞效应.采用自下向上激光扫射的PIV流场测量系统,对比分析了固定柱体和涡激振动柱体的绕流特征.实验观测表明,近壁面柱体涡激振动触发的临界速度呈现随壁面间距比减小而逐渐减小的变化趋势;但流动降速条件下的涡激振动停振所对应的下临界速度却明显小于升速时的涡激振动触发所对应的上临界速度.采用上临界与下临界约减速度差值可定量表征涡激振动迟滞程度,研究发现该值随着柱体间距比减小呈线性增大趋势.涡激振动迟滞现象通常伴随振幅阶跃,振幅阶跃值则随着间距比减小而非线性减小.     

质量比对D形截面柱体流致振动的影响

利用涡激振动进行海流能收集的VIVACE装置是新能源领域的研究热点.应用FLUENT软件采用k-ωSST湍流模型和Newmark-β法,通过数值模拟探究了四个质量比(2, 5, 7和10)在迎流角90°下D形截面柱体的流致振动响应,系统分析了D形截面柱体在横流向上的振动幅值、频率、平衡位置偏移量、尾涡脱落模式以及能量转化效率.所模拟的雷诺数范围为288～2880,对应的约化速度为2～20.结果表明,质量比对D形截面柱体流致振动的影响明显,质量比会改变D形截面柱体流致振动的响应分支.质量比越大, D形截面柱体进入驰振对应的约化速度越低;质量比增大, D形截面柱体平衡位置偏移量相对减小.随着约化速度的增大, D形截面柱体出现了涡激振动、涡激振动-驰振及完全驰振等响应分支.在所模拟的范围内, D形截面柱体高能量转化效率出现在涡激振动分支,而不是在驰振分支;在质量比为10且约化速度为4.5时,一级能量转化效率达到最大值44%.相关研究可为VIVACE装置的振子选型提供参考.     

串列双方柱体流体动力载荷研究

本文通过流动显示,热线测频和流体动载荷测量在水槽中研究了绕经不同柱间距比S/D(S为双柱间距,D为柱体截面宽)串列双方柱体流动特性。实验雷诺数为Re=6×10~3,柱间距比0.5≤S/D≤10实验测量了涡脱落频率、时间平均阻力、动态阻力和动态升力。通过实验结果综合分析给出临界柱间距范围2.5≤(S/D)_(cr)≤3.0,并将串列双方柱流动随柱间距的变化划分为二种流态区。在临界柱间距,作用于双柱体的流体载荷、涡脱落频率以及流谱都发生跃变。文中分析讨论了两个流态区的特性以及在临界柱间距出现的双稳态特性。     

纵向涡对近壁湍流结构的影响

在封闭水槽中用氢气泡方法观察了纵向涡对近壁湍流结构的影响，涡的下洗侧出现了展向距离较宽、流动较稳定的快斑区，流向速度快；涡的上洗侧出现了含有慢斑的区域，流动结构复杂，流向速度慢，纵向涡使下洗侧速度较快的流体流向壁面，使上洗侧速度较慢的流体远离壁面。     

带有新型涡激振动抑制罩的圆柱体的水动力特性

通过模型实验和数值模拟计算,研究了带有涡激振动抑制罩的圆截面柱体的水动力特性.模型实验主要测试了柱体上附加谐波型和类圆锥型涡激振动抑制罩的单摆结构在不同流速下发生涡激振动的性质;数值模拟则针对谐波型和圆锥型扰动,在雷诺数Re为102到105范围内,研究其水动力参数,如阻力、升力和涡脱落频率等,随扰动波长和波动强度的变化.模型实验结果表明,在直圆柱开始发生共振的流速下,带抑制罩的柱体的振幅显著降低,而在更高流速下则显著增大.数值模拟结果表明,谐波型和圆锥型扰动具有相似的水动力特性;且在不同Re时,阻力、升力和涡脱落频率具有相似的变化规律;随波动强度的增大,阻力一般逐渐增大,升力则在多数情况下先减小而后增大,而涡脱落频率一般逐渐减小.      

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。     

高阻尼比低质量比圆柱涡激振动试验研究

针对圆柱的涡激振动问题,设计开发了高性能循环水槽与超声位移传感器,研究了高阻尼比、低质量比条件下,弹性支撑的刚性圆柱的涡激振动变化规律。结果表明:(1)高阻尼比条件下,振幅主要受独立参数阻尼比、质量比的影响;低质量比条件下,振动频率随流速增大而增大,"锁定区间"较高质量比范围扩大。(2)高阻尼比、低质量比条件下,阻尼的增大会导致振幅减小且"锁定区间"变窄;但振动频率在"锁定区间"内变化趋势一致。(3)高阻尼比、低质量比有利于涡激振动的能量转化,但阻尼不可过大,否则振幅与"锁定区间"均变小,影响电能转化。     

柱体流激振动能量俘获理论与技术研究若干进展

潮流能分布广泛,且储量巨大,具备巨大的规模化开发利用价值.流激振动是一种常见的流固耦合现象,通过柱体流激振动能够在流速较低时实现有效的能量转换,基于柱体流激振动的能量俘获技术在未来具备广阔的工程应用前景.近年来,针对柱体结构流激振动特性和能量俘获性能,出现了大量的实验和数值仿真研究工作.文章全面阐述了多种截面形式的单个柱体、柱群结构流激振动能量俘获理论与技术方面的研究进展:对于单个圆柱流激振动能量俘获,目前已基本揭示了被动湍流控制器参数、系统阻尼、雷诺数和边界条件等因素对能量俘获性能的影响规律,基本完成了理论和技术积累;对于非圆截面柱体流激振动能量俘获,已初步明确特定来流攻角、系统质量比、系统阻尼、系统刚度和雷诺数条件下三角形、四边形、多边形与异形等多种截面形式柱体的流激振动作用机理和能量俘获能力;对于柱群的流激振动能量俘获,各柱体振子之间存在流场干涉,需要合理设计柱体排布形式、柱体间距和系统阻尼等参数,实现流体能量俘获最大化.通过综述国内外流激振动能量俘获理论和技术方面的研究进展,对今后该问题的研究进行了力所能及的展望,期望促进流激振动能量俘获理论的发展和流激振动能量转换装置的工程应...     

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.     

Flow characteristics around a circular cylinder subjected to vortex-induced vibration near a plane boundary

Although vortex-induced vibration (VIV) has been extensively studied, much of existing literature deals with uniform flow in the absence of a boundary. The VIV flow field of a structure close to a boundary generally remains unexplored, but it can have important engineering implications, such as pipeline scour if the boundary is an erodible seabed. In this paper, laboratory experiments are performed to investigate the flow characteristics of an elastically mounted circular cylinder undergoing VIV, and a rigid plane boundary is considered to simplify the problem. The initial gap-to-diameter ratio is fixed at 0.8, and six different reduced velocities are considered. The velocity field is measured using a high resolution particle image velocimetry (PIV) system, which has several advantages over traditional PIV systems, including high sampling rate and the ability to mitigate scatter of laser light near the boundary, allowing accurate measurements at the viscous sublayer. This paper presents the vibration amplitude and oscillation frequency for different V_r; in addition, the mean velocity field, turbulence characteristics, vortex behavior, gap flow velocity, and normal/shear stresses on the boundary were measured/calculated, leading to new insights on the flow field behavior.     

多用途输液立管涡激振动实验研究与疲劳分析

为研究输运不同流体的海洋立管在海流作用下的振动规律,在大型波浪流水槽中进行涡激振动模型实验。实验分别将四种不同质量比的立管模型竖直固定于支架上,立管外部承受不同速度的流体作用,上端施加顶张力。立管模型上均匀布置六个测点,根据每个测点布置的两个应变计,分别测得来流向和横向两个方向振动响应。通过小波变换对实验数据进行去噪处理,利用振型分解法求解立管各点涡激振动位移。考察输运不同流体对立管自振频率以及涡激振动响应的影响,并利用雨流计数法对模型进行疲劳分析。实验结果表明,随质量比增加立管涡激振动频率降低;低质量比的立管更容易产生大位移。     

Three-dimensional numerical simulation of vortex-induced vibration of an elastically mounted rigid circular cylinder in steady current

Vortex-induced vibration (VIV) of an elastically mounted rigid circular cylinder in steady current is investigated by solving the three-dimensional Navier–Stokes equations. The cylinder is allowed to vibrate only in the cross-flow direction. The aim of this study is to investigate the variation of the vortex shedding flow in the axial direction of the cylinder and to study the transition of the flow from two-dimensional (2D) to three-dimensional (3D) for VIV of a cylinder. Simulations are carried out for a constant mass ratio of 2, the Reynolds numbers ranging from 150 to 1000 and the reduced velocities ranging from 2 to 12. The three-dimensionality of the flow is found to be the strongest in the upper branch of the VIV response and weakest in the initial branch. The 2S and 2P vortex shedding modes are found to coexist along the cylinder span in the upper branch, leading to strong variations of the lift coefficient in the axial direction of the cylinder. The difference between the flow transition from 2D to 3D in the VIV lock-in regime and that in the wake of a stationary cylinder is identified. The transition mode B found in the wake of a stationary cylinder is also found in the wake of a vibrating cylinder. The critical Reynolds number for flow transition from 2D to 3D of a cylinder undergoing cross-flow VIV at a reduced velocity of 6 is found to be greater than that for a stationary cylinder. For a constant reduced velocity of 6, the wake flow changes from 2D to 3D as the Reynolds number is increased from 250 to 300. Some 2D numerical simulations are performed and it is found that the 2D Navier–Stokes (NS) equations are not able to predict the VIV in the turbulent flow regime, while the 2D Reynolds-averaged Navier–Stokes (RANS) equations improve the results.     

Suppression of vortex-induced vibration of a circular cylinder using suction-based flow control

In the present study, a flow control method is employed to mitigate vortex-induced vibration (VIV) of a circular cylinder by using a suction flow method. The VIV of a circular cylinder was first reproduced in a wind tunnel by using a spring–mass system. The time evolution of the cylinder oscillation and the time histograms of the surface pressures of 119 taps in four sections of the circular cylinder model were measured during the wind tunnel experiments. Four steady suction flow rates were used to investigate the effectiveness of the suction control method to suppress VIV of the circular cylinder. The vibration responses, the mean and fluctuating pressure coefficients, and the resultant aerodynamic force coefficients of the circular cylinder under the suction flow control are analyzed. The measurement results indicate clearly that the steady suction flow control method exhibits excellent control effectiveness and can distinctly suppress the VIV by dramatically reducing the amplitudes of cylinder vibrations, fluctuating pressure coefficients and lift coefficients of the circular cylinder model. By comparing the test cases with different suction flow rates, it is found that there exists an optimal suction flow rate for the maximum VIV control. The cases with higher suction flow rates do not necessarily behave better than those with lower suction flow rates. With the experimental setting used in the present study, the suction flow control method is found to behave better for VIV suppression when the ratio of the suction flow velocity to the oncoming flow velocity is less than one.     

Application of wake oscillators to two-dimensional vortex-induced vibrations of circular cylinders in oscillatory flows

A nonlinear time-domain simulation model for predicting two-dimensional vortex-induced vibration (VIV) of a flexibly mounted circular cylinder in planar and oscillatory flow is presented. This model is based on the utilization of van der Pol wake oscillators, being unconventional since wake oscillators have typically been applied to steady flow VIV predictions. The time-varying relative flow–cylinder velocities and accelerations are accounted for in deriving the coupled hydrodynamic lift, drag and inertia forces leading to the cylinder cross-flow and in-line oscillations. The system fluid–structure interaction equations explicitly contain the time-dependent and hybrid trigonometric terms. Depending on the Keulegan–Carpenter number (KC) incorporating the flow maximum velocity and excitation frequency, the model calibration is performed, entailing a set of empirical coefficients and expressions as a function of KC and mass ratio. Parametric investigations in cases of varying KC, reduced flow velocity, cylinder-to-flow frequency ratio and mass ratio are carried out, capturing some qualitative features of oscillatory flow VIV and exploring the effects of system parameters on response prediction characteristics. The model dependence of hydrodynamic coefficients on the Reynolds number is studied. Discrepancies and limitations versus advantages of the present model with different feasible solution scenarios are illuminated to inform the implementation of wake oscillators as a computationally efficient prediction model for VIV in oscillatory flows.     

正弦振荡来流下柔性立管涡激振动发展过程

在风浪流的作用下,海洋浮式结构物将带动悬链线立管在水中作周期性往复运动,从而在立管运动方向上产生相对振荡来流,这种振荡来流将激励立管悬垂段发生“间歇性” 的涡激振动. 在海洋工程水池中对一个4m 长的立管微段进行模型试验研究,以探索相对振荡来流作用下立管涡激振动产生的机理及其发展的物理过程. 试验通过振荡装置带动模型作正弦运动来模拟不同最大约化速度U_Rmax、不同KC（Keulegan-Carpenternumber）的相对振荡来流,利用光纤应变片测量立管涡激振动响应. 结合模态分析方法处理试验数据得到位移响应时历,继而提出相对振荡来流下柔性立管涡激振动发展的3 个阶段:建立阶段、锁定阶段以及衰减阶段. 并进一步总结了最大约化速度U_Rmax,KC 对涡激振动发展过程的影响规律. 最终获得不同最大约化速度U_Rmax下,涡激振动各发展阶段随KC 所占时间分布比例图.      

An experimental investigation of one- and two-degree of freedom VIV of cylinders

In the paper,an experiment investigation was conducted for one-and two-degree of freedom vortex-induced vibration(VIV) of a horizontally-oriented cylinder with diameter of 11 cm and length of 120 cm.In the experiment,the spring constants in the cross-flow and in-line flow directions were regulated to change the natural vibration frequency of the model system.It was found that,in the one-degree of freedom VIV experiment,a "double peak" phenomenon was observed in its amplitude within the range of the reduced velocities tested,moreover,a "2T" wake appeared in the vicinity of the second peak.In the two-degree of freedom VIV experiment,the trajectory of cylinder exhibited a reverse "C" shape,i.e.,a "new moon" shape.Through analysis of these data,it appears that,besides the non-dimensional in-line and cross-flow natural vibration frequency ratios,the absolute value of the natural vibration frequency of cylinder is also one of the important parameters affecting its VIV behavior.     

倾斜圆柱结构涡激振动研究进展

圆柱结构涡激振动现象在生活中十分常见,如海洋工程中的管道、土木工程中的高耸建筑、桥梁斜拉索,核工程中的热交换器等频繁受到涡激振动影响,诱发结构的疲劳损伤,甚至破坏失效.现阶段,人们对垂直来流作用下圆柱结构涡激振动机理已有较为全面的认识.然而,当圆柱倾斜置于流场中,结构后缘的尾流形态与垂直放置差异显著,结构与流体的耦合作用机理更为复杂.为简化倾斜圆柱涡激振动问题,提出了不相关原则,来流速度被分解为垂直圆柱结构轴向和平行圆柱结构轴向的两个速度分量,仅考虑垂直结构轴向速度分量的影响,忽略平行结构轴向速度分量的影响.近年来,针对倾斜圆柱涡激振动及不相关原则的适用性,出现了大量实验和数值模拟研究成果.为了深化对倾斜圆柱结构涡激振动相关机理的认知,本文全面阐述了倾斜圆柱结构涡激振动响应规律、尾迹流场模式和流体力特性等方面的研究进展,分析了不相关原则的适用范围,探讨了倾斜圆柱结构涡激振动抑制措施,并对今后该领域的研究进行了力所能及的展望.     

Experimental investigation of the flow-induced vibration of a curved cylinder in convex and concave configurations

Experiments have been conducted to investigate the two-degree-of-freedom vortex-induced vibration (VIV) response of a rigid section of a curved circular cylinder with low mass-damping ratio. Two curved configurations, a concave and a convex, were tested regarding the direction of the flow, in addition to a straight cylinder that served as reference. Amplitude and frequency responses are presented versus reduced velocity for a Reynolds number range between 750 and 15 000. Results for the curved cylinders with concave and convex configurations revealed significantly lower vibration amplitudes when compared to the typical VIV response of a straight cylinder. However, the concave cylinder showed relatively higher amplitudes than the convex cylinder which were sustained beyond the typical synchronisation region. We believe this distinct behaviour between the convex and the concave configurations is related to the wake interference taking place in the lower half of the curvature due to perturbations generated in the horizontal section when it is positioned upstream. Particle-image velocimetry (PIV) measurements of the separated flow along the cylinder highlight the effect of curvature on vortex formation and excitation revealing a complex fluid–structure interaction mechanism.     

Numerical simulation of two-degree-of-freedom vortex-induced vibration of a circular cylinder close to a plane boundary

Two-degree-of-freedom vortex-induced vibrations (VIV) of a circular cylinder close to a plane boundary are investigated numerically. The Reynolds-Averaged Navier-Stokes (RANS) equations are solved using the Arbitrary Lagrangian Eulerian (ALE) scheme with a k-ω turbulence model closure. The numerical model is validated against experimental data of VIV of a cylinder in uniform flow and VIV of a cylinder close to a plane boundary at low mass ratios. The numerical results of the vibration mode, vibration amplitude and frequency agree well with the experimental data. VIV of a circular cylinder close to a plane boundary is simulated with a mass ratio of 2.6 and gap ratios of e/D=0.002 and 0.3 (gap ratio is defined as the ratio of gap between the cylinder and the bed (e) to cylinder diameter (D)). Simulations are carried out for reduced velocities ranging from 1 to 15 and Reynolds numbers ranging from 1000 to 15 000. It is found that vortex-induced vibrations occur even if the initial gap ratio is as small as e/D=0.002, although reported research indicated that vortex shedding behind a fixed circular cylinder is suppressed at small gap ratios (e/D<0.3 or 0.2). It was also found that vibration amplitudes are dependant on the bouncing back coefficient when the cylinder hits the plane boundary. Three vortex shedding modes are identified according to the numerical results: (i) single-vortex mode where the vortices are only shed from the top of the cylinder; (ii) vortex-shedding-after-bounce-back mode; (iii) vortex-shedding-before-bounce-back mode. It was found that the vortex shedding mode depends on the reduced velocity.