两端铰接的细长柔性圆柱体涡激振动响应特性数值研究

目前细长柔性圆柱体涡激振动响应的研究方法主要包括实验方法、计算流体动力学方法以及半经验模型方法.鉴于实验方法研究成本较高、计算流体动力学方法计算时间较长,本文基于尾流振子模型对线性剪切来流下两端铰接的细长柔性圆柱体涡激振动响应特性进行了半经验模型方法研究.先建立了柔性圆柱体结构振子以及尾流振子之间的耦合模型,紧接着基于二阶精度中心差分格式对耦合模型先离散后迭代进行求解.对不同剪切参数下柔性圆柱体涡激振动响应的振动波长、振动频率、振动位移以及响应频率随时间的变化特性等参数进行了分析.分析结果表明:圆柱体的涡激振动响应由驻波和行波混合组成.当无量纲弯曲刚度较小时,在圆柱体两端附近,驻波占主导;而在圆柱体中间段附近,行波占主导.当无量纲弯曲刚度较大时,在圆柱体整个长度区间上均为驻波占主导.随着剪切参数的增大,振动位移以及振动波长均逐渐减小,而振动频率和频率带宽均逐渐增大.     

不同剪切率来流作用下柔性圆柱涡激振动数值模拟

采用浸入边界法对细长柔性圆柱在线性剪切流条件下的涡激振动进行三维数值模拟.细长柔性圆柱振动采用三维索模型模拟,其两端铰接,质量比为6,长细比为50,无量纲顶张力为496.来流为线性剪切流,剪切率从0到0.024变化,最大雷诺数为250.研究发现:剪切流作用下柔性立管横流向振动表现为驻波模式,而顺流向振动表现为行波与驻波混合模式.随着剪切率增大,振动频谱呈现多频响应,振动能量逐渐向低频转移.阻力系数平均值随着展向变化,脉动阻力系数和升力系数的均方根值均表现为"双峰"模式.流固能量传递系数沿立管轴向的分布表明,振动激励区集中于高流速区,而振动阻尼区多位于低流速区.剪切率较小时,圆柱的泻涡为平行交叉模式;剪切率较大时,圆柱的泻涡为倾斜泻涡模式,且由于泻涡频率沿立管轴向变化,尾流发生涡裂现象,形成泻涡频率不同的胞格结构.     

不同剪切率来流作用下柔性圆柱涡激振动数值模拟

采用浸入边界法对细长柔性圆柱在线性剪切流条件下的涡激振动进行三维数值模拟。细长柔性圆柱振动采用三维索模型模拟,其两端铰接,质量比为6,长细比为50,无量纲顶张力为496。来流为线性剪切流,剪切率从0到0.024变化,最大雷诺数为250。研究发现:剪切流作用下柔性立管横流向振动表现为驻波模式,而顺流向振动表现为行波-驻波混合模式。随着剪切率增大,振动频谱呈现多频响应,振动能量逐渐向低频转移。阻力系数平均值随着展向变化,脉动阻力系数和升力系数的均方根值均表现为“双峰”模式。流固能量传递系数沿立管轴向的分布表明,振动激励区集中于高流速区,而振动阻尼区多位于低流速区。剪切率较小时,圆柱的泻涡为平行交叉模式;剪切率较大时,圆柱的泻涡为倾斜泻涡模式,且由于泻涡频率沿立管轴向变化,尾流发生涡裂现象,形成泻涡频率不同的胞格结构。      

涡激响应尾流双振子模型的参数反演方法

尾流双振子模型是研究圆柱结构涡激振动响应的重要模型,模型参数的准确确定对悬浮隧道设计理论具有重要意义.首先通过降阶法将多变量二阶非线性常微分方程组的尾流双振子模型变换为一阶方程组.然后给出一种新型的圆柱结构水槽试验设计方案,其中试验模型的刚度能够较好反映悬浮隧道等实际工程结构的刚度,基于相机动态捕捉和视频识别计算机程序,获取圆柱结构在水平和竖直方向的位移试验数据.基于试验结果和龙格-库塔方法求解一阶方程组,采用BP神经网络智能算法对模型参数进行反演,同时利用遗传算法对神经元的初始权值和阈值进行优化,所得结果平均误差仅为5.50％,优于遗传算法和未优化的BP神经网络模型.结果表明,基于遗传算法优化的BP神经网络智能算法能够精确实现尾流双振子模型的参数确定,为圆柱结构涡激振动响应分析提供理论基础.     

剪切流下发生涡激振动的柔性立管阻力特性研究

利用缩尺模型试验的方法研究了线性剪切流下涡激振动发生时柔性立管的阻力特性.文中基于光纤光栅应变传感器测得的模型应变信息,采用梁复杂弯曲理论计算了立管的平均阻力,继而分析了阻力系数沿管长方向和雷诺数的分布特性以及涡激振动对阻力系数的放大效应,并提出了用于估算柔性立管发生涡激振动时阻力系数的经验公式.结果表明:涡激振动对阻力系数有放大效应,使得立管局部阻力系数高达3.2;平均阻力系数在1.0×104到1.2×105的雷诺数区间内的值为1.3~2.0,并随雷诺数的增大而减小.本文提出的经验公式可准确估算高雷诺数下涡激振动发生时柔性立管的阻力系数,此经验公式考虑了流速、涡激振动主导模态以及主导频率对阻力系数的影响.      

砂-膨润土混合屏障材料渗透性影响因素研究

建立了一个新的结构-尾流振子耦合模型. 流场近尾迹动力学特征被模化为非线性阻尼 振子,采用van der Pol方程描述. 以控制体中结构与近尾迹流体间受力互为反作 用关系来实现流固耦合. 采用该模型进行了二维结构涡激振动计算,得到了合理的 振幅随来流流速的变化规律和共振幅值,并正确地预计了共振振幅值$A_{\max}^\ast$ 随着质量阻尼参数$\left( {m^\ast + C_A } \right)\zeta $的变化规律,给出了预测$A_{\max }^\ast $值的拟合公式. 采用该模型计算了三维柔性结构在均匀来流和简谐波形来流作用下的VIV 响应. 结构在均匀来流作用下振动呈现由驻波向行波的变化过程, 并最后稳定为行波振动形态. 在简谐波形来流作用下,结构呈现混合振动形态,幅值随时间呈周期变化.     

圆柱涡激振动的结构-尾流振子耦合模型研究

建立了一个新的结构-尾流振子耦合模型. 流场近尾迹动力学特征被模化为非线性阻尼振子,采用van der Pol方程描述. 以控制体中结构与近尾迹流体间受力互为反作用关系来实现流固耦合. 采用该模型进行了二维结构涡激振动计算,得到了合理的振幅随来流流速的变化规律和共振幅值,并正确地预计了共振振幅值$A_{\max}^\ast$随着质量阻尼参数$\left( {m^\ast + C_A } \right)\zeta$的变化规律,给出了预测$A_{\max }^\ast$值的拟合公式. 采用该模型计算了三维柔性结构在均匀来流和简谐波形来流作用下的VIV响应. 结构在均匀来流作用下振动呈现由驻波向行波的变化过程, 并最后稳定为行波振动形态.在简谐波形来流作用下,结构呈现混合振动形态,幅值随时间呈周期变化.      

深海采矿系统中悬臂式立管涡激振动分析

不同于传统的海洋立管, 深海采矿系统中的垂直提升管道可以被视为一个底部无约束的柔性悬臂式立管, 工作过程中同样面临涡激振动和柔性变形问题. 本文采用一种无网格离散涡方法和有限元耦合的准三维时域求解数值模型, 系统性地研究了不同流速下悬臂式立管的涡激振动问题. 结果表明: 悬臂式立管的横向振动模态阶数随折合速度增加而增大, 在一定折合速度范围内主导振动模态保持不变; 当主导模态转变时, 对应的横向振幅会发生突降, 但是当新的高阶模态被激发后, 立管振幅随来流速度增加而再次逐渐增大; 在相同的振动模态下, 立管底部位移均方根值随折合速度线性增加, 主导振动频率在模态转变时会出现跳跃现象; 特别地, 本文讨论了三阶主导模态下悬臂式立管的振动响应, 无约束的立管底部呈现出较大的振动能量, 且振幅的驻波特征随折合速度增加而逐渐增强; 本文比较了两端铰支立管与悬臂式立管的涡激振动响应特征, 两者在振幅和主导振动频率两方面均表现出了相同的变化趋势.      

基于非线性吸能机理的涡激振动减振理论与实验研究

流致振动现象广泛存在于机械、航空、土木和石油等重要工程领域, 为防止工程结构因流致振动行为而造成疲劳破坏, 有必要对稳定性、动力学响应及其振动控制做深入研究. 本文提出了一种由弹簧和质量块构成的非线性吸能器(nonlinear targeted energy transfer, NTET), 研究了该非线性吸能器对弹性支承圆柱体涡激振动的被动控制影响机制. 基于能量法推导了圆柱体涡激振动非线性被动控制的耦合动力学方程, 通过设计非线性弹簧?质量块构型的NTET, 进一步开展了涡激振动控制的实验研究, 并与理论预测结果进行了较好的对比, 获得提升涡激振动控制效果的最佳参数值. 研究发现, NTET的质量、弹簧刚度以及弹簧预应力等参数会对涡激振动控制效果产生显著的影响. 本文研究结果表明, 该耦合系统中圆柱体和NTET均表现出周期性的稳态振动响应, NTET质量的改变会显著影响系统的耦合频率. 在无预应力状态下, NTET质量越大、刚度越小时, 有更好的减振效果. 当弹簧预应力逐渐增大时, NTET的非线性刚度逐渐变弱, 会降低涡激振动控制性能. 参数分析表明: 随着涡激振动控制性能的提升, 圆柱体的振幅逐渐较小, NTET的振幅逐渐增大, 能量传递效率逐渐提高. 研究结果可为工程中涡激振动控制策略的高效设计提供有用的理论支撑和实验数据.      

海洋热塑性增强管(RTP)涡激振动数值计算

基于Van der Pol尾流振子模型和多体系统传递矩阵法(transfer matrix method for multibody systems, MSTMM), 建立了可以快速预测海洋热塑性增强管(reinforced thermoplastic pipe, RTP)振动特性和涡激振动响应的动力学模型. 仿真结果与ANSYS软件仿真结果以及文献实验数据对比, 验证了本文模型的准确性. 研究了考虑RTP立管刚性接头, 不同顶张力, 不同来流分布等情况对RTP立管涡激振动响应的影响. 计算结果表明: 流速越大, 立管涡激振动激发出的模态越高; 立管涡激振动主要受低阶模态控制; 立管的刚性接头对立管的湿模态影响较小, 但是对较高阶模态为主所激发出的涡激振动振幅分布影响较大; 剪切流对沿立管轴向的涡激振动振幅分布影响较大, 低流速能量小所引起的涡激振动幅值较小, 但是当剪切流流速达到能激发出较高阶模态时, 相比同等流速的均匀流所引起的涡激振动振幅要大.      

On shear flow single mode lock-in with both cross-flow and in-line lock-in mechanisms

A long flexible cylinder exposed to ocean currents is known to undergo vortex-induced vibration (VIV). In a spatially sheared flow the response of a riser to VIV can vary from single mode lock-in to multimodal. A new experimental facility was designed and built to investigate the above-mentioned areas. The facility consisted of a long flexible cylinder in either a uniform or a simplified vertically sheared flow. The instrumentation consisted of direct local fluid force measurement at two locations on the cylinder as well as accelerometers spaced along the cylinder axis. The simplified shear flow was a 2-slab flow, with each slab having uniform velocity. Test conditions included forcing the cylinder simultaneously at resonance in both regions to investigate modal competition issues and multimodal response patterns. Resonant VIV excitation of two different modes simultaneously, was conducted which revealed single mode lock-in of the higher frequency through an unexpected mechanism. The higher frequency mode's damping region underwent in-line excitation at four times the predicted shedding frequency that provided a power-in effect to support the dominant mode's cross-flow response.     

Vortex-induced vibration and mode transition of a curved flexible free-hanging cylinder in exponential shear flows

This paper presents an experimental study of vortex-induced-vibration (VIV) of a curved flexible free-hanging cylinder in exponential shear flows. The emphasis is on previously unexplained phenomena in our early research and in some cases offers insights on the mode transition of nonlinear vibration behavior of long flexible cylinders. The experimental results illustrate that the cylinder undergoes multi-frequency response and the dominant frequency varies spatially. The IL and CF response and transition are out-of-sync. In the second (2nd) mode response, the spanwise response exhibits a mixed pattern with standing wave and traveling wave. The contribution of traveling wave becomes greater as the reduced velocity increases. Only two distinct branches of response, namely the initial and lower branches, are observed in each mode. The lower branches of the first (1st) and 2nd modes present the same normalized frequency. A phase jump around 180°occurs at the transition between initial branches and lower branches, accompanying with a switch between 2S $\leftrightarrow$ 2P or P+S vortex shedding modes.     

Frequency lock-in is caused by coupled-mode flutter

The mechanism underlying the lock-in of frequencies in flow-induced vibrations is analysed using elementary linear dynamics. Considering the case of lock-in in vortex-induced vibrations (VIV), we use a standard wake oscillator model, as in previous studies, but in its simplest form where all nonlinear terms and all dissipative terms are neglected. The stability of the resulting linear system is analysed, and a range of coupled-mode flutter is found. In this range, the frequency of the most unstable mode is found to deviate from the Strouhal law when the frequency of the wake oscillator approaches that of the free cylinder motion. Simultaneously the growth rate resulting from coupled-mode flutter increases, which would lead to higher vibration amplitudes. The extent of the range of lock-in is then compared with experimental data, showing a good agreement. It is therefore stated that the lock-in phenomenon, such as in VIV, is a particular case of linear coupled-mode flutter.     

剪切流场中含内流立管横向涡激振动特性

立管是海洋工程中输送油气或其他矿产资源的必备结构, 外部洋流引起的立管涡激振动影响着立管的疲劳寿命, 危害深海资源开发. 本文基于欧拉?伯努利梁方程, 结合半经验时域水动力模型, 建立剪切流与内流耦合作用下海洋立管涡激振动预报模型, 运用有限元方法和Newmark-β逐步积分法求解方程, 首先将数值模拟结果与实验数据进行对比, 验证模型正确性. 然后, 运用此模型, 对剪切流作用下含内流的顶张立管在不同内流速度和密度下的横向涡激振动响应特性进行研究, 主要分析了立管的横向振动模态、振动频率以及均方根位移等涡激振动参数随内流速度和密度等参数的变化规律. 结果表明, 在剪切流场中, 含内流海洋立管在横向上表现出多模态多频率的涡激振动;立管横向振动的最大均方根位移随内流速度和密度的增大而增大, 特别是当内流速度较大时, 横向最大均方根位移增大明显;立管横向振动的主导频率随内流速度和密度的增大而减小, 并且内流密度的增大同样会引起模态转换和频率转换.      

A linear stability approach to vortex-induced vibrations and waves

The motion induced by vortex shedding on slender flexible structures subjected to cross-flow is considered here. This phenomenon of vortex-induced vibration (VIV) is analysed by considering the linear stability of a coupled system that includes the structure dynamics and the wake dynamics. The latter is modelled by a continuum of wake oscillators, distributed along the span of the structure. In the case of uniform flows over a straight tensioned cable, VIV are found to arise as an instability related to the merging of two waves. In the case of a cable of finite length, the selection of modes that experience lock-in with the wake is found using the same stability argument. In non-uniform flows, several unstable wave systems are identified, and competition between them is discussed. Comparison is then made with existing experimental and computational data of VIV of slender structures under uniform and non-uniform flows. Phenomena previously identified in these systems, such as mode switching when the flow velocity is varied, time sharing of the response between two frequencies, or the coexistence of several regions of VIV with different dynamics in the same structure, are discussed with the help of the proposed model.     

DNS-DERIVED FORCE DISTRIBUTION ON FLEXIBLE CYLINDERS SUBJECT TO VORTEX-INDUCED VIBRATION

We use direct numerical simulation (DNS) based on spectral methods to simulate turbulent flow past rigid and flexible cylinders subject to vortex-induced vibrations (VIV). We present comparisons of amplitude, and lift and drag forces, at Reynolds number 1000 for a short and a long cylinder, and we examine differences between a traveling wave response and a standing wave response. The DNS data suggest that the often-used empirical formula proposed by Skop, Griffin & Ramberg in 1977 overpredicts the drag coefficient. We propose an appropriate modification and present preliminary results that indicate that low-dimensional modeling may be an accurate and efficient approach in predicting forces in VIV. Given the lack of any benchmark experiments in VIV currently, the DNS results presented here, both distributions as well as span- and time-averaged quantities, should be helpful to experimentalists and modelers.     

An experimental investigation on vortex induced vibration of a flexible inclined cable under a shear flow

In the present study, an experimental investigation was performed to characterize the vortex induced vibration (VIV) of a flexible cable in an oncoming shear flow. The VIV tests were conducted in a wind tunnel with a flexible cable model. It was found that, under different oncoming velocity profiles, the cable model behaved in single-mode and multi-mode VIVs. The displacement amplitudes of the single mode VIVs were found to be larger than those of multi-mode VIVs, and the cross-flow (CF) response was larger than that of in-line (IL) direction for either the single mode or multi-mode VIVs. For a single mode vibration, the largest CF response occurs in the 1st mode VIV, and the motion trajectory of the 1st mode VIV was found to be an inclined figure of eight shape, while other single mode VIVs behaved in ellipse or straight line trajectories. For multi-mode VIVs, no stable vibration trajectories were found to exist since the vibration frequency bands covered two or more vibration modes. The vortex-shedding frequencies in the wake behind the inclined cable were also characterized in the present study. The shedding frequencies of the wake vortices were found to coincide well with the vibration modes: for a single mode VIV, they were close to the dominant vibration mode; for a multi-mode VIV, they could also cover the appearing vibration modes.     

Numerical study on the suppression of the vortex-induced vibration of an elastically mounted cylinder by a traveling wave wall

In the present paper, the commercial CFD code “Fluent” was employed to perform 2-D simulations of an entire process that included the flow around a fixed circular cylinder, the oscillating cylinder (vortex-induced vibration, VIV) and the oscillating cylinder subjected to shape control by a traveling wave wall (TWW) method. The study mainly focused on using the TWW control method to suppress the VIV of an elastically supported circular cylinder with two degrees of freedom at a low Reynolds number of 200. The cross flow (CF) and the inline flow (IL) displacements, the centroid motion trajectories and the lift and drag forces of the cylinder that changed with the frequency ratios were analyzed in detail. The results indicate that a series of small-scale vortices will be formed in the troughs of the traveling wave located on the rear part of the circular cylinder; these vortices can effectively control the flow separation from the cylinder surface, eliminate the oscillating wake and suppress the VIV of the cylinder. A TWW starting at the initial time or at some time halfway through the time interval can significantly suppress the CF and IL vibrations of the cylinder and can remarkably decrease the fluctuations of the lift coefficients and the average values of the drag coefficients; however, it will simultaneously dramatically increase the fluctuations of the drag coefficients.