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.

Dynamic Characteristics Analysis of Ice-Adhesion Transmission Tower-Line System under Effect of Wind-Induced Ice Shedding

The tower line system will be in an unsafe status due to uniform or uneven fall of ice coating which is attached to the surface of tower and lines.The fall of ice could be caused by wind action or thermal force.In order to study the dynamic characteristics of the self-failure of the transmission line under the action of dynamicwind load,a finite elementmodel of the two-span transmission tower line system was established.The birth and death element methods are used to simulate the icing and shedding of the line.Tensile failure strength is the shedding criterion for ice coating.The fluctuating wind speed time history of the tower line systemis first simulated,and then the fluctuating wind and the average wind are superimposed to generate the instantaneous wind speed and converted into wind load.The dynamic response of the transmission tower line systemunder iced coupling with different wind speeds and different thicknesses of ice coating was studied.This is the first attempt that the coupling dynamic response of the icing shedding and wind load for the transmission tower-line system is discussed in this paper.In addition,the dynamic characteristics of wind are included.In particular,the limiting mechanical conditions are considered.According to the simulation results,it is found:because of the ice shedding,the stress of the conductor changes obviously in the first 20 seconds,and the ground wire changes sharply in the first two seconds;the icing of the conductor(ground)wire is gradually deicing under the action of wind vibration;the displacement of tower top increases with the increase of wind speed and icing thickness.

Numerical Investigation on Vortex-Induced Rotations of A Triangular Cylinder Using An Immersed Boundary Method

A numerical study of vortex-induced rotations(VIRs) of an equivalent triangular cylinder, which is free to rotate in the azimuthal direction in a uniform flow, is presented. Based on an immersed boundary method, the numerical model is established, and is verified through the benchmark problem of flow past a freely rotating rectangular body.The computation is performed for a fixed reduced mass of m~*=2.0 and the structural stiffness and damping ratio are set to zero. The effects of Reynolds number(Re=25-180) on the characteristics of VIR are studied. It is found that the dynamic response of the triangular cylinder exhibits four distinct modes with increasing Re: a rest position,periodic rotational oscillation, random rotation and autorotation. For the rotational oscillation mode, the cylinder undergoes a periodic vibration around an equilibrium position with one side facing the incoming flow. Since the rotation effect, the outset of vortex shedding from cylinder shifts to a much lower Reynolds number. Further increase in Re leads to 2 P and P+S vortex shedding modes besides the typical 2 S pattern. Our simulation results also elucidate that the free rotation significantly changes the drag and lift forces. Inspired by these facts, the effect of free rotation on flow-induced vibration of a triangular cylinder in the in-line and transverse directions is investigated. The results show that when the translational vibration is coupled with rotation, the triangular cylinder presents a galloping response instead of vortex-induced vibration(VIV).

Calculation model and mechanism analysis for rain-wind-induced vibration of stay cable

Rain-wind-induced vibration of cable was studied based on previous research achievements. According to the quasi-steady assumption, the governing equation of vertical motion of the cable was derived and the criterion for unstable motion and occurrence mechanism was studied. A comparison was performed between the oscillation responses of the stay cable obtained from calculated model and previous results. The results indicate that the analysis model can reflect the main characteristics of wind-rain-induced vibrationt of the cable which is amplitude- and velocity-restricted, and it is probably related with the periodic vortex shedding of wake flow. It is essential for the occurrence of rain-wind-induced or wind-induced vibration of cable that the derivative of lift coefficient with respect to transient angle of attack is less than zero. When rain-wind-induced vibration occurs, the aerodynamic force has a dual function for the vibration, and the maximum amplitude of stayed-cable is determined by the relative value of aerodynamic exciting force and aerodynamic damping force.

停机时风力机塔筒与叶片的涡激机理

大型化风力机存在塔筒与叶片的脱落涡相互诱导的现象,极大影响机组的安全运行。为探究两者涡激互扰特性,该文以NREL 5 MW风力机为研究对象,分别选取距离地面65、70、75和80 m高度的塔筒与叶片共存的二维截面,开展6、8、10、12与14 m/s风况下数值模拟,并与单塔筒工况计算结果进行对比分析。研究结果表明,叶片与塔筒的尾涡存在严重相互诱导干涉;与单塔筒绕流工况相比,塔筒特定点压力值增长了1.5倍,升力系数出现多个响应幅值且其最大约为单塔筒的6倍;塔筒与叶片尾涡出现相互合并且脱落频率一致,涡激作用增强;塔筒-叶片工况下的涡脱频率较单塔筒降低,更易与风力机低阶固有频率出现交叉,风力机极可能出现共振。

尾流干扰下桥梁斜拉索风荷载的吹气控制

为了降低并列双圆柱(桥梁斜拉索的简化模型)在风荷载作用下的剧烈响应,提出了一种能同时抑制并列双圆柱风荷载的主动流动控制方法。采用数值模拟方法研究了并列双圆柱的流动控制效果,结果表明:并列双圆柱间距比为圆柱直径的3倍时风荷载最大,在圆柱尾部施加吹气速度为来流风速的1.5倍时控制效果显著,可以有效降低风荷载并抑制双柱的漩涡脱落。控制机理在于吹气可以有效减小每个圆柱的尾流宽度,抑制两个尾流的相互作用。

基于风振理论的塔式起重机非工作状态风荷载探讨

目前我国起重机械的理论设计中,非工作状态下风荷载采取回避风载共振的方法,从结构件刚性、结构形式等方面采取措施,使起重机的结构自振频率避开漩涡脱落的频率,防止发生共振现象。针对风载较敏感的起重机械,本文通过自制的简单风洞实验,模拟建筑工地施工普通塔式起重机的典型倒塌案例,分析塔式起重机因台风引发的耦合共振现象。就台风危害的特点提出正确对待塔式起重机的结构风工程,为塔式起重机及其附着装置设计和增强抗风防台方案设计等提供参考。

西堠门大桥涡激振动特征分析

准确描述涡激振动的风场特征和动力响应特征是实现桥梁振动控制的前提,基于大跨度悬索桥结构健康监测系统获取的风场和振动加速度数据,对比分析了环境振动和涡激振动的风速、风向、湍流强度、阵风系数、脉动风功率谱密度、振动加速度及其功率谱密度的参数特征。结果表明:加劲梁典型的第6阶竖弯涡激振动中的风速可达7.24 m/s~12.24 m/s,风向基本垂直于桥轴向;三向湍流强度、阵风系数和脉动风功率谱在涡激振动中无明显不同于环境振动的特征。不同于多模态耦合的环境振动,涡激振动呈现单模态振动特征;通过功率谱峰值差异率量化该特征可区分环境振动和涡激振动。

大型风力机塔筒与叶片涡激共振机理研究

大型风力机组停机状态下,塔筒与顺桨叶片尾涡干涉严重,进而产生叶片和塔筒的涡激共振现象,严重影响风力机可靠性。本文以NREL 5 MW风力机为研究对象,采用CFD与有限元方法,分别针对其流场与结构进行数值模拟计算,分析不同来流风速下叶片与塔筒一体化模型的模态频率与振型、尾涡形态与其脱落频率、以及表面压力变化特性。研究结果表明:风力机一体化模型的固有频率基频较小,低阶振型主要为风力机整体的摆振,高阶振型开始出现叶片的挥舞、摆振与扭转;在65~80 m高度区域内、8~12 m/s来流条件下,叶片与塔筒尾涡脱落存在尾涡合并现象,风速越大,合并现象愈明显,尾涡脱落频率也越高;10 m/s与12 m/s速度下,尾涡脱落频率与风力机固有频率较为接近,风力机易出现涡激共振现象。

竖弯涡振作用下桥上列车行车安全舒适性研究

为研究高速铁路桥梁竖弯涡振对桥上列车行车安全舒适性的影响,以某大跨公铁两用斜拉桥和CRH2型动车组为背景,进行风-车-轨-桥耦合系统振动分析。基于ANSYS与SIMPACK联合仿真平台,引入桥梁涡激力数值模型,建立风-车-轨-桥耦合系统振动模型,对比10 m/s平均风速下主梁发生与未发生竖弯涡振时桥梁和列车的动力响应,并分析不同列车速度的影响。结果表明:竖弯涡振会加剧桥梁和列车的竖向响应,而列车的存在会使发生竖弯涡振时的桥梁竖向位移和加速度分别降低31.8%和42.4%,对主梁竖弯涡振具有一定的抑制作用;主梁发生竖弯涡振时列车行车安全性指标峰值和竖向舒适性指标(竖向加速度和竖向Sperling指标)峰值明显大于未发生竖弯涡振时,并均随着车速的增大而增大;当车速超过230 km/h时,列车轮重减载率超过安全限值0.6,当车速超过200 km/h时,桥上列车竖向加速度超过安全限值1.3 m/s^(2)。

不同涡脱落模式下垂直轴风力机叶片的气动响应

为研究尾流中不同涡脱落模式下垂直轴风力机(vertical-axis wind turbine,VAWT)叶片的气动响应,基于攻角变化相似性,进行了叶片正弦俯仰振动的比拟实验。研究发现:在弦长雷诺数O(10^(5))范围内,尾流存在3种涡型结构:前缘离散涡(leading-edge vortex,LEV)、蜿蜒尾流(undulating wake,UW)和反卡门涡街(reverse von Kármán vortex street,Rv KVS);随着叶尖速比λ增大,VAWT叶片缩减频率k增大,攻角幅值α _(m)减小;叶轮叶片几何尺度比R/c较小时,在低λ产生LEV涡型的可能性较小,在高λ产生Rv KVS涡型的可能性较大;R/c较大时,在低λ产生LEV涡型的可能性较大,在高λ产生Rv KVS涡型的可能性较小。LEV涡型导致轻动态失速,造成VAWT叶片发生高频俯仰振动,但对叶轮转矩和VAWT功率影响不大。Rv KVS涡型的出现,伴随叶片升力和转矩幅值增大以及平均推力的产生,会使VAWT叶片扭矩载荷增大,也会使叶轮转矩和输出功率提升。据此提出基于VAWT的新式风墙构型,在继承Nes浮式风墙构型优点的基础上,进一步提升了功率系数和风能输出密度。

尾随涡对风力机叶片气动性能影响机理研究

以PhaseⅥ风力机叶片为研究对象,以r/R=30%、63%和95%处叶素为参考,建立与7、9、15 m/s试验风速下该风力机叶片附着涡环量沿展向分布相同的叶片模型,分析尾随涡对风力机当地翼型气动性能的影响机理。采用带转捩效应的SST k-ω湍流模型,对所建立的叶片模型和二维S809翼型的气动特性进行研究和对比分析。结果表明:旋转叶片尾随涡对分离现象产生抑制作用且随攻角的增大减弱;尾随涡的影响表现出多重效应,除了减小当地翼型的攻角,还降低其吸力面负压系数和压力面正压系数。

风嘴对旋转振荡板旋涡脱落的控制

5∶1矩形柱体被认为是通用桥面几何形状的代表,其简化模型可以用来进行风振控制的研究。风嘴作为一种常用的流动控制装置,能起到减阻和增加矩形板气动稳定性的效果,但控制装置对强迫振荡柱体的控制机理仍缺乏研究。研究者通过对节段模型施加强迫振动,在实验模型前部施加边缘型对称型风嘴,研究控制装置对矩形板尾流旋涡脱落的影响。通过流动显示结果,总结了施加风嘴后的四种旋涡脱落模式。并通过快速傅里叶变化频域分析法,得到实验模型后缘X/D=10处的速度功率谱。施加风嘴控制能增大旋涡脱落频率,并抑制尾流旋涡脱落的能量。而数值模拟得到的不同实验工况下的升力均方根和力矩均方根显著减小,最大降幅分别为52%和23%,表明矩形板气动稳定性的提升和尾流不稳定性的减弱。

C型环对细长圆柱减阻抑振效果的试验研究

长细比较大的圆柱结构易发生涡激振动从而引发结构安全问题,而阻力小的气动抑振措施是解决该问题的理想手段之一。为此,开展了一种C型减阻抑振环风洞试验,研究了不同直径和圆心角的C型环对不同雷诺数下的圆柱气动力的影响,并通过圆柱表面的风压分布揭示了C型环的减阻抑振机理。研究结果表明,通过减弱背风面的负压C型环可以有效减小亚临界雷诺数区圆柱的阻力,而且通过对尾流的干扰可有效抑制旋涡脱落。

考虑风电频率响应的电力系统低频减载控制策略研究

大规模风电机组参与调频会对电力系统的频率稳定产生较大影响。传统低频减载控制策略的研究尚未考虑风电机组频率响应的影响,可能导致低频减载控制策略与传统火电、风电调频资源配置不合理。为此,文章提出一种考虑风电频率响应的电力系统低频减载控制策略。首先,基于风电虚拟惯性和一次调频控制模型,建立考虑风电参与调频的电力系统频率响应简化模型;其次,分析风电参与频率响应的有功响应特性对系统不平衡功率量的影响,进而精细化估算用于指导低频减载的不平衡功率量;最后,利用MATLAB/Simulink仿真平台对所提电力系统低频减载控制策略进行了仿真验证。

超高宽比矩形烟囱的横风向风振效应探讨

随着社会进步和发展理念的转变,工业建筑的去工业化设计要求越来越多,烟囱作为工业厂房标志性构筑物,常被列为改造对象。为打破圆形烟囱固有的观感,矩形钢筋混凝土烟囱被普遍采用。现在工业烟囱高度一般不低于80m,由于烟囱的功能单一,设计规范对钢筋混凝土烟囱的水平位移限值要求不高(H/100),烟囱的高宽比普遍大于8。横风向风振等效风荷载的规范公式只适用于高宽比4~8的情况,当设计折算风速大于10或高宽比大于8,可能发生不利并且难以准确估算的气动弹性现象,不宜采用附录H2计算方法,建议进行专门的风洞试验研究。但风洞试验需要时间和成本,不是每个项目都具备风洞试验条件,在没有风洞试验数据的情况下如何进行超高宽比横风向风振计算,如何保证烟囱的结构安全,是设计人员面对的现实问题。通过多个设计案例分析,提出解决方案,供同行参考。

考虑UFLS响应的大规模风电并网系统频率特性分析

大规模风电并网系统在受到大功率缺额扰动后,电网会出现频率骤降,而低频减载(Under Frequency Load Shedding,UFLS)控制是防止频率失稳的有效手段。首先,基于含UFLS的改进系统频率响应模型(System Frequency Model,SFR),研究了新型电力系统受到大功率扰动下的频率特征,基于经典频率响应模型建立改进的系统频率响应模型,考虑火电、风电联合调频控制策略,并引入UFLS频率控制;然后,根据所建立的系统频率响应模型对大规模风电并网系统参数进行整定,并建立多资源参与调频下的大规模风电并网系统频率特征表达式;最后,通过Matlab/Simulink仿真平台,验证UFLS控制策略参与调频的可行性及UFLS控制参数对频率调制效果的影响。

VORTEX SHEDDING FREQUENCY DETERMINED BY SURFACE PRESSURE FLUCTUATIONS OF THREE EQUISPACED CYLINDERS

In his paper, the frequency of vortex shedding from three cylinders of the same diameter in equilateral triangular arrangement is determined experimentally by the spectrum analysis of surface pressure fluctuations at a Reynolds number of 3.0×10 4. For α=0°, there exists ‘bistable’ flow at the spacing 2.0. For α=30°, the vortex shedding of the front cylinder is found at small spacing less than 2.0. It is quite different from the case of two cylinders in tandem arrangement only. The side cylinder has great effects on the flow between the other two cylinders in close arrangement. For α=60°, there exists a critical spacing of about 2.5 that the frequency of each cylinders is nearly half of f s below this spacing and equal to f s above it respectively. The interference among three cylinders was negligible in the spacing range above 3.0 at α=0 and 60 deg., and each cylinder behaves as a single cylinder with the same vortex shedding frequency f s. In these small spacing (spacing<3.0), interference occurs seriously and has great effects in a complex fashion on the flow state around three cylinders.

带质量块的微型压电式风能采集器研究

基于风致振动机理的微型风能采集器可以将风能转换为电能,在无线传感等领域具有广阔应用前景。漩涡脱落频率与风速成正比,当漩涡脱落频率与微型风能采集器固有频率接近时,采集器有较高输出功率,因此为了在低风速环境应用风能采集器,需要降低其固有频率。引入质量块可以降低微型压电式风能采集器的固有频率,使其在较低风速下产生较高输出功率。为了预测带质量块的微型悬臂梁压电式风能采集器的固有频率,基于集总参数建模方法得到了固有频率解析表达式。实验结果表明,对于所制作的风能采集器原理样机,质量块的引入使其临界风速由17.2 m/s降低为13.5 m/s。

并列双箱梁桥面风致涡激振动试验研究

基于两座具有并列双箱梁的缆索承重桥梁,做了一系列的弹性悬挂节段模型风洞试验.结果表明,双桥面之间存在不可忽略的气动干扰效应,它对双桥面桥梁的涡激振动会产生不利影响.以平胜桥主梁节段模型为基础的风洞试验研究表明,并列双桥面之间的气动干扰效应随着桥面间距的增加而减弱,从而使主梁的涡激振动特性随两桥面之间的距离而变化,而增加阻尼是抑制双桥面涡激振动的有效手段.