Effects of the vibration regime on the spanwise correlation of the aerodynamic forces on a 5:1 rectangular cylinder

It is well known that the spanwise correlation of aerodynamic pressures and forces on vibrating cylinders increases with the amplitude of oscillation. Little knowledge, however, exists on how the nature of the motion, i.e. the degree of freedom in which the body vibrates and the reduced wind velocity, affects such correlation. In this paper, some results are presented of a wind tunnel study aimed at investigating the spanwise correlation of aerodynamic pressure and force fluctuations on a rectangular cylinder with an aspect ratio of 5:1. Measurements were carried out on the stationary cylinder and in four dynamic configurations, each characterised by different values of the heaving and pitching natural frequencies. With varying reduced wind velocity, in each dynamic configuration the three vibration regimes of vortex shedding lock-in, forced motion and flutter were observed in the tests. In the paper only two dynamic configurations are presented, one being that in which the heaving and pitching natural frequencies in still air coincide. This is an interesting case, in which the dynamic response is very much affected by the possibility that the motion has of switching from one degree of freedom to another.     

Simple model of rain-wind-induced vibrations of stayed cables

This paper proposes a single-degree-of-freedom model of rain-wind-induced vibrations in stayed cables. It is assumed that the frequency of the circumferential motion of the upper rivulet is equal to that of cable and the rivulet amplitude is set constant for a given wind speed. The obtained results are verified with the existing experimental data showing that these assumptions capture the qualitative properties of the phenomenon. The explicit, analytical expressions are derived for the aerodynamic damping and exciting force. Finally, a linear SDOF model is derived for simple estimation of the amplitude of cable vibrations induced by wind and rain.     

深圳京基金融中心横风向气动阻尼试验研究

针对439 m的深圳京基金融中心（KFT）工程,采用气动弹性模型技术研究其横风向气动阻尼特性。根据工程地址的100 a重现期风速、缩尺比例以及前期刚体模型同步测压所得到结构风敏感风向（东西向）的广义力功率谱密度特征确定气弹模型试验的风速范围,通过风洞试验获取不同风速下气动弹性模型敏感风向的顶部加速度响应数据,应用随机减量技术计算分析横风向气动阻尼随折算风速的变化规律,试验考虑结构模态阻尼对气动阻尼的影响。结果表明：在折算风速为4~14的范围内,结构气动阻尼值均大于0且随风速的增大而增大,上游地王大厦(DWT)的干扰效应对气动阻尼未产生不利影响,KFT的基本频率和DWT脱落在尾流中的漩涡频率一致的100 a重现期风速时的气动阻尼比为1.25%,表明前期采用刚性模型试验对KFT进行抗风设计时,不考虑气动阻尼影响计算得到的结构风致荷载和风致响应偏于保守和安全。     

The effect of nonharmonic forcing on bluff-body aerodynamics at a low Reynolds number

The aerodynamic response of a circular cylinder to nonharmonic forcing of the inflow velocity is studied by numerically solving the equations of two-dimensional fluid motion on an orthogonal curvilinear mesh. The effect of varying the inflow velocity waveform while maintaining other forcing parameters constant at a Reynolds number of 180 is considered in this study. The forcing frequency is 84% of the natural vortex shedding frequency in the unforced wake while the peak-to-peak amplitude of velocity oscillation is 65% of the reference velocity. Results are reported for the drag and lift coefficients and the flow field in terms of streamline patterns and vorticity distributions. It is shown that the wake is locked-on to the forcing frequency for all cases tested but the aerodynamic response is systematically modified by the imposed changes in the velocity waveform. The magnitude and the phase of the fluctuating drag and lift forces and the mean drag force are affected. These effects are associated with changes in the mechanism of vortex formation and shedding in the wake of the cylinder; it is found that the rolling up of the individual shear layers on both sides can be manipulated to promote shedding of single vortices or vortex pairs.     

Computational and experimental study on flow around a rotationally oscillating circular cylinder in a uniform flow

The effect of rotational oscillation on flow field and fluid forces of a circular cylinder in a uniform flow is studied by three-dimensional computation by large eddy simulation and flow measurement by particle image velocimetry at Reynolds number 2000. The experimental characteristics of the flow indicate the enhancement of vortex shedding at natural frequency, the suppression of vortex shedding at high frequency and the recovery that follows. The above are accurately predicted by numerical simulations except for the slightly smaller estimate of wake suppression effect at high frequency. On the other hand, the fluid force characteristics on the cylinder are studied by numerical simulation, which indicates the drag enhancement and reduction corresponding to the variations of flow field at low and high frequencies, respectively. The drag reduction at high frequency is magnified with an increase in oscillation amplitude, but it can be saturated at higher amplitude. The mechanism of drag reduction is caused by the combined effect of forcing frequency and oscillation amplitude due to the modification of wake flow at high frequency and the delay in flow separation, while the fluctuating lift force slightly increases at a high frequency caused by the small-scale vortex shedding in synchronous with the rotational oscillation.     

On interference between two circular cylinders in staggered arrangement at high subcritical Reynolds numbers

The results of a wind-tunnel investigation on the interference between two identical parallel circular cylinders arranged in staggered configurations, carried out in a uniform smooth flow at high subcritical Reynolds number, are presented. On the basis of the pressure measurements and the flow visualization, three main flow patterns are classified and discussed. Close attention is paid to the switching phenomenon of two different pressure patterns at critical angles. This switching creates discontinuity of large lift forces on cylinders. With measurements of velocity profiles and power spectral analysis in the gap between the two cylinders, the mechanism of the appearance of this extreme aerodynamic force is discussed.     

桥梁箱型吊杆涡振与驰振耦合振动的数值模拟

为研究桥梁箱型吊杆涡振与驰振耦合状态下的风致振动机理,基于某大跨度钢桁架拱桥大长细比箱型吊杆的节段模型风洞试验参数与试验结果,采用计算流体动力学（CFD）软件FLUENT对模型的耦合振动进行了数值模拟,通过FLUENT的后处理功能进一步研究分析了不同振动状态下的气动力和尾流旋涡特征信息。研究结果表明:数值计算得到的风振曲线与试验的实测结果吻合良好,在涡振与驰振的耦合振动过程中,随着振动幅值的不断增大,尾流旋涡脱落由小振幅阶段规则的卡门旋涡形态转化为大振幅阶段的复合模态特征,气动升力也由小振幅阶段的单频特征逐步转化为大振幅阶段的多频特征。     

高墩连续刚构桥线性抖振时程响应分析

采用Miyata T准定常气动力模型,通过双变量泰勒展开,导出12阶单元气动阻尼矩阵和气动刚度矩阵,并在ANSYS中以Matrix27单元输入来考虑桥梁结构风致抖振自激力模型,结合桥址处脉动风场的模拟,对某连续刚构桥最大双悬臂阶段和成桥运营阶段进行线性抖振时程响应分析。最后对脉动风荷载作用下的结构响应值和静阵风荷载作用结果进行了比较,得到脉动增大系数值。     

任意排列两方形断面高层建筑风致干扰机理研究

风致干扰效应是高层建筑群抗风设计中的常见难点问题之一。采用刚性模型测压试验,研究了均匀层流和两种大气边界层风场条件下任意排列两方形断面高层建筑的风致干扰效应,通过平均和脉动基底弯矩系数的干扰因子、风力系数、风压系数分布以及风荷载功率谱的研究,解释了其风致干扰效应的机理。结果表明,任意排列的两方形断面高层建筑风致干扰中,至少存在横风向静力干扰、顺风向静力干扰和横风向动力干扰三个值得注意的干扰区域。 窄道形成的加速效应使受扰结构上形成指向施扰建筑横风向平均吸力和阻塞形成的受扰建筑的横风向平均推力;遮挡效应使得受扰建筑承受指向位于上游的施扰建筑的顺风向风力;漩涡叠加增强位于尾流区受扰建筑上的横风向脉动荷载。不同风场的试验结果表明,提高来流的紊流度有助于减弱上述干扰效应。     

Aerodynamic instability of a bridge deck section model: Linear and nonlinear approach to force modeling

The aerodynamic behavior of a bridge deck section model with a simple single-box shape was characterized in wind tunnel. At large nose-up mean angles of attack, a torsional instability arises, outlining a situation in which nonlinear aeroelastic effects may be critical. Such condition represents an interesting case to develop and validate nonlinear models for the aeroelastic problem. The experimental campaign allowed both to characterize the aerodynamic forces using forced motion tests and to study the aeroelastic behavior of the section model, when excited by actively generated turbulent wind. These aeroelastic tests are used to validate a numerical time-domain model for aerodynamic forces that takes into account the nonlinearities due to the reduced velocity and to the amplitude of the instantaneous angle of incidence. Results are critically analyzed and compared with those obtained with a linear model.     

基于非线性涡激力广义模型的涡振幅值换算

节段模型测振风洞试验是研究涡激共振风速锁定区间和振动幅值的主要手段之一,但节段模型涡振振幅并不能由几何缩尺比直接换算至实桥,需要对测试结果进行修正。已有研究表明,涡激力具有强烈的非线性特性,因此基于线性涡激力模型的换算方法不再适用。此外,由于采用的非线性涡激力模型不准确,基于非线性涡激力模型的已有换算方法也不一定适用,并且目前基于非线性涡激力模型的换算方法也没有给出阻尼比的修正方法。为此,文章借鉴量纲分析方法,推导非线性涡激力广义模型,并给出计算涡振振幅的非线性涡激力简化广义模型,然后根据能量相等的原则推导节段模型与实桥的涡振幅值换算公式。研究表明：当节段模型与实桥结构阻尼比一致时,振幅换算关系仅与桥梁阵型函数值有关;当节段模型与实桥结构阻尼比不一致时,振幅换算关系还与涡振生长或衰变过程位移振幅变化率以及涡振稳定状态无量纲幅值有关;不同换算方法的结果对比表明本文换算结果与实测值最为接近。     

Experimental verification on nonlinear dynamic characteristic of a tuned liquid column damper subjected to various excitation amplitudes

In this study, nonlinear dynamic characteristics of a tuned liquid column damper (TLCD) varying with the amplitude of excitation input are investigated through shaking table tests and numerical model of a TLCD. The tuned mass damper (TMD) analogy of a TLCD is used to simplify the formulation, in which it involves equivalent viscous damping of the inherent nonlinear damping term of a TLCD. The equivalent TMD model of a TLCD shows that the dynamic behaviour of a TLCD is affected by the natural frequency, damping ratio and ratio of total liquid mass to the mass in horizontal column of a TLCD. Shaking table test is performed to obtain experimental transfer functions that describe the dynamic behaviour of a TLCD specimen subjected to a harmonic loading with various excitation amplitudes. Transfer functions for various excitation amplitudes are measured from shaking table acceleration to both the liquid displacement within a TLCD container and the control force produced by a TLCD specimen. In addition, the dissipation energy due to the inherent damping of a TLCD is measured from the shaking table test varying with excitation amplitude. The variation of design parameters of a TLCD according to the excitation amplitude is investigated by comparing the transfer functions obtained from the shaking table test to those derived from the TMD analogy of a TLCD. These results showed that both the natural frequency and the mass ratio of a TLCD are independent on the variation of excitation amplitude, while the damping ratio of a TLCD increases with larger excitation amplitude. Copyright © 2010 John Wiley & Sons, Ltd.     

基于完全气弹模型风洞试验输电塔风荷载识别

按照基本缩尺律,设计、制作了输电塔完全气弹模型,并通过大气边界层风洞试验,测试了多种风速、风向条件下输电塔的位移与加速度响应。通过加速度响应功率谱识别出结构的固有频率,并采用Hilbert-Huang变换结合随机减量法识别出包含结构阻尼和气动阻尼的结构总阻尼。利用虚拟激励法建立了由测点位移响应来识别结构顺风向、横风向风荷载的方法。由识别出的风荷载谱曲线,利用非线性最小二乘法拟合得到输电塔顺风向、横风向风荷载经验公式。研究结果表明：风洞试验设计、制作的输电塔气弹模型,测试得到的模型第1阶自振频率与有限元计算结果吻合较好;横风向荷载谱形态与顺风向的相比有较大的区别,其能量分布在一个更宽的频带上,其峰值频率是顺风向的3～4倍。     

An analytical study of the across-wind response of cylinders due to vortex shedding

The across-wind response of tapering cylinder-like structures, such as chimneys, due to periodic shedding of vortices is obtained under lock-in conditions. The method of analysis incorporates aerodynamic effects and describes the lift forces by a linear model monoharmonic in nature. Under lock-in conditions, the critical wind speed and the lift and aerodynamic coefficients are assumed to be uniform throughout the height of the chimney, although its outside diameter varies. The aerodynamic and lift coefficients are derived from a lift oscillator model which adequately describes the experimental results of the vortex-induced oscillation (at lock-in) of an elastically-supported cylinder. With the help of an example problem, the paper shows how the response of the chimney is sensitive to the choice of the critical diameter used for the determination of critical wind velocity and the lift and aerodynamic coefficients. It also emphasizes the importance of the lock-in condition in the second mode in relation to bending stresses.     

Measurement of extreme aerodynamic interference forces acting on circular cylinders in turbulent boundary layers

Extreme interference wind loads on circular cylinders are investigated in wind tunnel simulations of atmospheric boundary layers with high turbulence levels. Since for circular cylinders interference effects caused by vortex shedding are pronounced, the present work tries to clarify to which extent turbulence effects are reducing the interference in this case. Aerodynamic forces and moments were measured with a piezoelectric six-component wind tunnel balance. The balance is distinguished by a large dynamic range, low interferences and a high natural frequency. The numerous results include comparisons of mean values, r.m.s. values and extreme values of force coefficients with and without interference with reference to the velocity profiles and the turbulence levels in two different boundary layer simulations. Conclusions are drawn with regard to the application of the results in estimating wind loads on cylindrical structures.     

斜拉桥斜拉索风雨振时索表面水线摆动作用及规律的试验研究

斜拉索表面水线的周向摆动被认为是激发索发生风雨振的重要因素。有水线的索的气动力可以认为是索本身的气动力和水线摆动产生的气动力的组合。在风洞中,通过竖向强制振动试验,测试索的气动力并算得气动导数H1*和H4*。为了模拟水线的摆动,对贴有水线的索施行两端同向强制扭转振动试验,测试振动中的气动力。通过分析水线的摆动对索竖向振动阻尼的贡献,表明水线的摆动既能促使索变得气动不稳定,也可以促使索变得气动稳定,这主要取决于水线摆动与索振动之间的位相差。同时,通过在风洞中模拟人工降雨并对水线进行跟踪观测,发现水线沿索轴向分布及摆动不均一,指出掌握风雨振时水线的各个参数对分析和理解该振动具有重要意义。     

Across-wind loads of typical tall buildings

Previous studies have indicated that the across-wind dynamic responses of super-tall buildings are usually larger than the along-wind ones. With the increase of heights, the across-wind dynamic response of super-tall buildings has been a problem of great concern. In this paper, 15 typical tall building models are tested with high-frequency force balance technique in a wind tunnel to obtain the first-mode generalized across-wind dynamic forces. New formulas for the power spectra of the across-wind dynamic forces, the coefficients of base moment and shear force are then derived. Parametric analyses of the effects of factors on the across-wind loads of the buildings are performed. Besides, a SDOF aeroelastic model of a square tall building with an aspect ratio of 6 is selected from the above buildings and is tested to investigate its across-wind dynamic response and aerodynamic damping characteristics. The power spectrum of the across-wind force of the square building is employed to compute its across-wind dynamic responses with and without considering the effect of the aerodynamic damping. The computed responses are then compared with the corresponding responses from the aeroelastic model test to verify the present formulas of the across-wind loads of buildings.     

Effects of grid curtains on the wind loads of a high‐rise building

The effects of grid curtains on the local and overall wind loads of a high‐rise building are investigated in detail according to a series of wind pressure and wind force tests on rigid model in a wind tunnel. The effects of grid curtains on the mean and fluctuating wind pressures on windward and sideward walls when the wind direction is parallel to the geometrical axes are investigated, along with the effects of the most unfavorable wind pressures for all wind directions. Furthermore, the effects of grid curtains on the mean and fluctuating aerodynamic forces on the entire building are also analyzed for various wind directions, along with the effects of grid curtains on the aerodynamic force spectra when the wind direction is parallel to the geometric axes. The test results indicate that grid curtains affect the mean and fluctuating windward pressure slightly but greatly influence the large sideward negative pressures. Grid curtains increase the mean and fluctuating windward aerodynamic forces and reduce the fluctuating aerodynamic torsions. According to the aerodynamic force spectra, grid curtains can mainly affect the wind forces in the low‐frequency range. Copyright © 2015 John Wiley & Sons, Ltd.     

PT25型高空作业平台工作装置液压缸作用力计算与分析

在高空作业平台工作装置力学分析基础上,运用MATLAB软件对工作装置3个液压缸进行计算,获得其在任意工况下的作用力,并绘制出阻力矩、力臂和作用力曲线,在此基础上对液压缸作用力进行了分析,为工作装置结构设计优化和液压缸选型提供依据。     

Random damping in buildings and its AR model

The full-scale measurements of damping show that damping in buildings exhibits randomness. The randomness of damping is examined in this paper. The factors which govern damping contributions in buildings, at different vibration amplitude levels, are investigated based on the Jeary's damping model [Earthquake Eng. Struct. Dyn. 14 (1996) 733–750]. It is found that, at a high amplitude level, damping in buildings is mainly dominated by random factors. In order to investigate the amplitude-dependent characteristics and randomness of structural damping, a time series analysis method (TSA) is employed to obtain the relationship between damping and vibrating amplitude. The AR (auto-regressive) models of damping in a building have been established and used to predict the damping values at high amplitude level, which are difficult to obtain from field measurements. The predicted data of damping show that damping values, at high amplitude, fluctuate around a plateau value described by Jeary's damping model.