Random vortex induced vibration response of suspended flexible cable to fluctuating wind

A popular dynamical model for the vortex induced vibration(VIV)of a suspended flexible cable consists of two coupled equations.The first equation is a partial differential equation governing the cable vibration.The second equation is a wake oscillator that models the lift coefficient acting on the cable.The incoming wind acting on the cable is usually assumed as the uniform wind with a constant velocity,which makes the VIV model be a deterministic one.In the real world,however,the wind velocity is randomly fluctuant and makes the VIV of a suspended flexible cable be treated as a random vibration.In the present paper,the deterministic VIV model of a suspended flexible cable is modified to a random one by introducing the fluctuating wind.Using the normal mode approach,the random VIV system is transformed into an infinite-dimensional modal vibration system.Depending on whether a modal frequency is close to the aeolian frequency or not,the corresponding modal vibration is characterized as a resonant vibration or a non-resonant vibration.By applying the stochastic averaging method of quasi Hamiltonian systems,the response of modal vibrations in the case of resonance or non-resonance can be analytically predicted.Then,the random VIV response of the whole cable can be approximately calculated by superimposing the response of the most influential modal vibrations.Some numerical simulation results confirm the obtained analytical results.It is found that the intensity of the resonant modal vibration is much higher than that of the non-resonant modal vibration.Thus,the analytical results of the resonant modal vibration can be used as a rough estimation for the whole response of a cable.

Effects of Internal Flow on Vortex-Induced Vibration and Fatigue Life of Submarine Pipelines

With the rapid development of the offshore oil industries, submarine oil / gas pipelines have been widely used. Under the complicated submarine environmental conditions, the dynamic characteristics of pipelines show some new features due to the existence of both internal and external flows. The paper is intended to investigate the vortex-induced vibration of the suspended pipeline span exposed to submarine steady flow. Especially, the effects of the flow inside the pipeline are taken into account. Its influences on the amplitude of pipeline response, and then on the fatigue life, are given in terms of the velocity of the internal flow.

Nonlinear Coupled in-Line and Cross-Flow Vortex-Induced Vibration Analysis of Top Tensioned Riser

The fluctuating furces of the fluid exerted on the top terrsioned riser （＇FIR） in the in-line and cross-flow directions are both modeled by van del Pol wake oscillator model and the nonlinear coupled dynamics of the in-line and cross-flow vortex-induced vibrations （VIV） of the riser are analyzed in time domain in this papar. The numencal shnulation results of the riser＇s in-line and cross-flow displacements and curvatures are compared with experimental measurements and the comparison shows the validity of this method in modeling some main features of the riser＇s VIV. Finally, the effects of the riser＇s top tensions and internal flow velocities on the coupled vibrations of the riser are investigated.

Numerical simulation of a short flexible pipe subject to forced motion and vortex-induced vibration

A series of numerical sinmlations about a small scale （aspect ratio： 63.2） flexible pipe undergoing forced harmonious oscillation and vortex-induced vibration （VIV） have been taken into account. The wake hydrodynamics and pipe deformation were accomplished by ANSYS MFX solution strat- egy designed for fluid-structure interaction （FSI） problem with well-performed LES model. The configuration of structured mesh, multi-domain design, different mesh stiffness admeasured by User Fortran ensured that the numerical task was competent to deal with large deformation related to this case. The introduction of instantaneous amplitude definition and modeless component decom- position method （Chen and Kim, 2008） was helpful to reveal much more information from modal analysis. Most results from numerical simulation are generally consistent with those from model test （Choi and Hong, 2000） via the comparison between them. As supplementary to model test, visualization of the vortex wake was also provided. It has been proved that the forced oscillation doesn＇t only excite a complicated dumbbell-like wake pattern around the outer thimble, but also results in inner flow inside the PVC pipe. The velocity of the inner flow increases with the frequency of forced oscillation.

Prediction Model for Vortex-Induced Vibration of Circular Cylinder with Data of Forced Vibration

A model based on the data from forced vibration experiments is developed for predicting the vortex-induced vibra- tions （VIV） of elastically mounted circular cylinders in flow. The assumptions for free and forced vibration tests are explored briefly. Energy equilibrium is taken into account to set up the relationship between the dynamic response of selfexcited oscillations and the force coemcients from forced vibration experiments. The gap between these two cases is bridged straightforwardly with careful treatment of key parameters. Given reduced mass m^# and material damping ratio of an elastically mounted circular cylinder in flow, the response characteristics such as amplitude, frequency, lock-in range, added mass coefficient, cross-flow fluid force and the corresponding phase angle can be predicted all at once. In- stances with different combination of reduced mass and material damping ratio are compared to investigate their effects on VIV. The hysteresis phenomenon can be interpreted reasonably. The predictions and the results from recent experiments carried out by Wifliamson＇ s group are in rather good agreement.

Experimental Investigation on Vortex-Induced Vibration of Steel Catenary Riser

Steel catenary riser（SCR） is the transmission device between the seabed and the floating production facilities. As developments move into deeper water, the fatigue life of the riser can become critical to the whole production system, especially due to the vortex-induced vibration（VIV）, which is the key factor to operational longevity. As a result, experimental investigation about VIV of the riser was performed in a large plane pool which is 60 m long, 36 m wide and 6.5 m deep. Experiments were developed to study the influence of current speed and seabed on VIV of SCR. The results show that amplitudes of strain and response frequencies increase with the current speed both in cross-flow（CF） and in-line（IL）. When the current speed is high, multi-mode response is observed in the VIV motion. The amplitudes of strain in IL direction are not much smaller than those in CF direction. The seabed has influence on the response frequencies of riser and the positions of damage for riser.

均匀流下水下机械臂涡激振动试验研究

为了揭示均匀流下水下机械臂的涡激振动响应特性,利用运动水下机械臂与静水间的相对运动,搭建了水下机械臂均匀流场涡激振动测试试验台,采集了不同约化速度下水下机械臂不同位置处的振动位移响应.试验结果表明:随着约化速度的增加,振动主频与无量纲振幅均为增长的趋势,横流向无量纲振幅小于0.04D,顺流向出现显著的多频以及更高的幅值,表明水下机械臂在顺流向受到的阻力更加明显.位移标准差随测试高度的增加先增后减,横流向位移标准差空间分布相较于顺流向对称性明显,主导模态控制在1阶.横流向的涡激振动规律性较强,基本符合圆柱绕流时的斯托哈尔规律.顺流向无因次主导频率在0.3~0.8之间,略大于横流向,斯托哈尔数为横流向工况的1~1.7倍,未出现频率锁定现象,但流速及测试高度对水下机械臂的涡激振动存在一定影响.该研究可为构建准确的水动力学模型保证水下机械臂的精准定位与控制提供一定的理论依据.

安装肋条斜拉索的涡激振动和气动力特性研究

以安装肋条斜拉索为研究对象,通过风洞试验系统研究肋条的安装方式(通长安装和间断交错安装)、尺寸和数量对斜拉索涡激振动和气动力的影响,并分析涡激振动的抑制机理.结果表明:和标准斜拉索相比,安装肋条斜拉索能有效抑制涡激振动,且间断交错安装方式的抑振效果优于通长安装方式,肋条数量为8根的控制效果优于6根和12根;增加肋条宽度和厚度都会减小涡激振动振幅;肋条斜拉索的脉动升力系数显著小于标准斜拉索,且功率谱尖峰幅值降低或不存在显著尖峰,表明间断交错肋条可以降低卡门涡脱落强度甚至完全抑制;对于气动力,在试验雷诺数范围内,安装肋条后平均气动力系数无明显雷诺数效应;相比于标准斜拉索,肋条为通长安装时均会增加斜拉索的平均阻力系数,而采用间断交错安装时,不同肋条数量、尺寸和风攻角对斜拉索的增阻减阻情况不同.当采用8根尺寸为20 mm×15mm的肋条间断交错安装时,在所有风攻角下都有良好的抑振效果,最大涡振振幅减小率可达到96.79%,且平均阻力系数均稳定在1.13附近,平均升力系数均稳定在0附近.

带挑臂PK箱梁涡振性能及优化措施机理研究

为研究带挑臂PK箱梁的涡振性能及气动优化措施,通过节段模型风洞试验研究了PK箱梁在0°、±3°风攻角下的涡振性能,并测试了人行道高度、封闭挑臂底部、栏杆透风率改变、不同倾角抑流板等气动措施对PK箱梁气动稳定性的影响,研究表明:主梁在0°、+3°风攻角下出现强烈的竖向涡激共振,并出现多个竖向涡振区间,同时在高风速下主梁出现了明显的扭转涡振;抬高人行道高度能降低各攻角下主梁的竖弯涡振响应,同时高风速下的扭转涡振得到极大程度的改善;通过封闭栏杆来改变栏杆透风率的研究发现,竖向间隔封闭人行道外侧护栏能破坏涡激气动力在展向的相关性,将主梁涡振峰值响应降低至规范限值的55.8%;0.25 m宽抑流板能有效改善主梁气动稳定性,抑流板倾角变化在20°~75°之间时,其均能完全抑制主梁涡振响应;气流在迎风侧人行道护栏处发生分离,在上表面卷起形成规律的大尺度旋涡,从而造成主梁剧烈的涡激振动,抑流板明显破坏了挑臂附近旋涡的形成,无法向下游发展形成规律的大尺度旋涡,从而能有效抑制主梁涡振。

门式钢桥塔涡激振动特性及机理研究

钢桥塔是一种高耸细柔结构,对风荷载十分敏感,易发生涡激振动。为研究某高度为217 m钢桥塔的涡激振动性能,开展了1∶100缩尺比下的自立状态气弹模型风洞试验。试验结果表明,0°~30°风向角下,在低风速区间发生了两塔柱同相涡激振动,在高风速区间发生了反相涡激振动。其中,同相和反相涡激振动的最不利风向角分别为0°和10°,塔柱同相顺风向位移和反相扭转角分别为609.5 mm和4.3°。进一步地,通过计算流体力学(computational fluid dynamics,CFD)方法研究了两类涡激振动的发生机理。数值模拟结果表明,两塔柱附近旋涡交替脱落的频率与桥塔的基阶自振频率接近,由此在塔柱两侧产生的周期性压力差使得桥塔发生同相和反相涡激振动。该研究结果和结论可为同类型钢桥塔的抗风设计提供一定参考。

基于旋涡漂移假设的流线型闭口箱梁竖向涡振机理研究

旋涡脱落和漂移是桥梁涡振发生的关键流场特征,有必要从涡动力学的角度揭示涡振机理。以典型流线型闭口箱梁为例,从气动力做功能量角度,构建简化涡模型,并结合基于风洞试验的断面表面气动力时频特征和基于数值模拟方法的断面周围流场特性,揭示流线型闭口箱梁多阶涡振机理,验证上述模型。研究表明:分离涡斯托罗哈数表征旋涡气动力做功能量特征,其可表达为旋涡漂移速度与来流风速比值的正整数倍,即同一分离点可对应多个涡振锁定区。流线型箱梁断面存在3阶竖向涡振锁定区,其中,第2阶和第3阶涡振锁定区均由来流经过断面前缘附属设施时产生的大尺度旋涡,在分离点至断面尾缘之间发生周期性漂移诱导与维系。在第2和第3阶涡振锁定区内分离涡分别耗费约2个和1个振动周期完成从分离点至截面尾缘的漂移,即二者分别由该分离点诱发的2阶和1阶简化涡模式主导。该研究验证了简化涡方法推演桥梁断面周围旋涡演变特性的合理性。

带肋斜拉索涡激振动和气动力特性的试验研究

为研究肋条安装方式(通长安装与间断交错安装)对斜拉索涡激振动和气动力特性的影响规律,进行了斜拉索节段模型的风洞试验。结果表明:只要安装肋条,就能够在一定程度上抑制斜拉索的涡激振动,在试验参数范围内,斜拉索涡激振动幅值最少可以降低46.16%;当肋条为通长安装时,会明显降低斜拉索涡激振动,但是会造成起振风速的增大或振动区间的延长;当肋条为间断交错安装时,整体抑振效果均好于通长安装情况,风攻角为15°时,肋条斜拉索的抑振效果达到了96.30%,可以用于特定来流风的环境当中;当肋条为间断交错安装时,斜拉索模型的平均阻力系数稳定在1.0~1.3,平均升力系数稳定在-0.07~0.14,具有一定的减阻效果;当风攻角α=15°和30°时,整体脉动气动力系数无明显雷诺数效应,脉动阻力系数稳定在0.025附近,脉动升力系数稳定在0附近,说明安装间断交错肋条后斜拉索具有更好的气动力效应。

Π型叠合梁斜拉桥涡激振动性能及气动措施研究

型叠合梁为气动钝体结构,容易发生气动失稳。本文以一座Π型截面叠合梁斜拉桥作为工程背景,采用风洞节段模型试验与计算流体动力学(CFD)方法,对主梁的涡激振动性能及相应的气动抑振措施展开了研究。在风洞节段模型试验中获得涡激振动风速区间,讨论了不同气动措施的抑振效果,运用计算流体动力学(CFD)方法对主梁涡振发生机理及气动措施抑振机理进行了初步研究。结果表明:在原始断面下,由于尾迹区旋涡的周期性脱落以及主梁上、下表面旋涡演变的相互作用,导致了涡激振动发生。在采取三种不同抑振措施后,除采用上L型导流板断面在+3°风攻角下发生扭转涡振外,其余优化断面均能使得来流平稳地通过,从而抑制涡振发生。本研究可对Π型叠合梁断面的斜拉桥抗风设计提供一定参考。

基于OrcaFlex的钢悬链立管疲劳损伤预报

涡激振动是钢悬链立管疲劳损伤的重要诱导因素之一,因此涡激振动分析在钢悬链立管设计中占据重要地位。OrcaFlex在海洋工程领域功能强大、应用广泛,能够提供两种涡激振动疲劳损伤预报方法,分别是基于模态叠加法的频域预报方法和基于Iwan-Blevins尾流振子模型的时域预报方法。OrcaFlex的涡激振动时域预报模型只考虑了横流向涡激振动造成的疲劳损伤,基于OrcaFlex的时域预报方法开发了双自由度涡激振动尾流振子模型的接口,考虑了浮式平台运动对钢悬链立管的涡激振动激励作用,实现了钢悬链立管双自由度涡激振动疲劳损伤预报。

大跨度双幅非对称平行主梁涡激振动干扰效应研究

在双幅式桥梁的建设中,公路与铁路同层并排布置的大跨双幅非对称主梁属于较特殊的设计。由于公路桥与铁路桥的动力特性不一致、主梁尾流特性差异较大以及非对称双梁间存在特殊且显著的气动干扰,使得非对称双幅主梁的涡激振动特性更为复杂。为系统探究干扰效应对主梁涡振性能的影响,以一座大跨度公铁两用双幅桥为研究背景,开展了双幅主梁涡激振动风洞试验与流固耦合数值模拟,其中公路梁为Π型叠合梁,铁路梁为流线型箱梁。研究结果表明:公路梁位于迎风侧时出现了较大幅值的竖弯和扭转振动,且风速区间与结构振动频率具有典型涡振“锁定”特征,而位于背风侧时,振动现象基本消失;铁路梁位于迎风侧时未见明显振动现象,而位于背风侧时,试验观察到了大幅振动,且振幅具有迅速上升又迅速下落的特点,无明显“锁定”区间。数值流场表明,公路位于不同位置处(迎、背风侧)时,主梁附近涡结构的尺度与分布均有明显不同。当位于迎风侧时,下桥面腔内大尺度漩涡的存在以及涡结构的周期性变化是位于迎风侧时产生涡振的主要原因;当位于背风侧时,受干扰效应影响,下桥面腔内漩涡尺度减小,升力振荡频率改变,涡振锁定现象消失。铁路附近流场及压力分布表明:当位于迎风侧时,主梁附近流场稳定,自激升力近似为直流力;当位于背风侧时,受公路尾流的干扰,铁路表面存在较大面积的负压区,公路尾流的脉动导致了铁路气动升力的振荡,因而产生了大幅振动。研究了双幅非对称平行主梁间气动干扰效应对涡振性能的影响,揭示了涡振及气动干扰机理,为类似桥梁的工程设计提供了参考依据。

大跨度公铁两用双层钢桁桥涡激振动控制研究

桥梁的涡激振动主要受到主梁断面的气动外形、结构动力特性与来流特性的影响,而大跨桥梁是典型的风致敏感结构,所以在大跨度桥梁的设计中应当引起重视。以某大跨度公铁两用双层板桁组合桁架桥为背景,在XNJD-1回流串联风洞中进行了风洞试验,研究了该主梁的涡激振动现象。通过计算流体动力学(computational fluid dynamics, CFD)数值模拟分析了其主梁断面的涡激振动机理,并且将展向周期摄动法应用于钢桁梁的涡激振动控制,采取了一系列气动控制措施来抑制主梁的涡激振动,其中L型分流板与波浪形风嘴完全抑制了主梁的涡激振动。还通过风洞试验研究了波浪形风嘴的几何参数对抑振效果的影响,试验结果表明,波浪形风嘴的抑振效果对幅值变化比较敏感,增大幅值可以有效抑制主梁的涡激振动。

浅海高压立管涡激振动及疲劳特性分析工艺

随着东海油气资源的不断开发,开发成本较低的自升式平台将得到广泛应用。立管所在海洋环境复杂多变,其结构长度与直径比值较大,在海流作用下会发生涡激振动。采用尾流振子模型对高压立管涡激振动响应进行预报,并基于DNV规范,根据S-N曲线法对结构疲劳进行计算。模型通过有限差分法进行离散求解。研究重点关注顶张力对立管的涡激振动及疲劳特性影响。结果表明,顶张力会影响立管的涡激振动响应,并进一步改变结构的疲劳寿命,影响结构安全性。

深海立管VIV预测模型及影响因素研究

为有效预测多因素作用下深海立管涡激振动(VIV),首先,基于欧拉-伯努利梁弯曲理论和改进范德波尾流振子方程,考虑管内流动流体和立管横向(CF)和顺向(IL)耦合作用,建立海洋立管涡激振动预测模型,用埃尔米特插值函数离散立管微分方程,纽马克-β法在时域内迭代求解立管位移;然后,设计VIV试验,并将试验数据与模型计算结果进行对比验证;最后,研究海流流速、内外流流速和顶张力对深海立管涡激振动特性的影响。结果表明:海流流速越大,立管振动频率越高,振动幅值越大;立管固有频率随内流流速的增大可能降低到锁振频率范围,加速立管的疲劳破坏,顶张力对立管内流影响有明显抑制作用,立管双向耦合作用在涡激振动分析中不容忽视。

可移动自升式井口平台的立管强度和疲劳寿命分析

针对自升式海上平台中两种结构形式的WZ12-8E/W立管,考虑波浪、洋流、土壤特性、附着海生物、温度和腐蚀等综合环境,计算和分析不同设计水位工况下,两种立管在平台移动下的结构强度,并校核是否发生涡激振动,以及预测交变载荷作用下的疲劳寿命。借助ORCAFLEX有限元软件,计算两种立管在多种海况下的动态响应,并把弯矩和张力转化为应力值。利用S-N曲线、雨流计数法和Miner累积损伤准则预测其疲劳寿命,并分别累计0°、45°和90°每个方向下37个工况的疲劳损伤。计算发现两种立管在作业和极端工况下,不会发生涡激振动,强度满足设计要求;但发现疲劳损伤较大且不满足设计寿命要求的结构区域,并提出增加疲劳寿命的改进方法。

浅水隔水导管动态响应及涡激振动疲劳损伤

介绍波浪海流作用下的隔水导管动力学模型,总结极限强度分析、稳定性校核和涡激振动Vortex Induced Vibration(VIV)疲劳损伤分析方法,并就某隔水导管实例分析动态响应和VIV疲劳损伤,得到隔水导管的极限强度结果、稳定性校核结果、VIV疲劳寿命满足钻井作业要求。所提出的隔水导管振动响应、VIV疲劳损伤及寿命分析方法为隔水导管的安全保障提供参考和技术支撑。