全桥气弹模型颤振导数识别

将节段模型颤振导数识别方法用于全桥气弹模型,对其可行性和便利性进行详细论述;给出气弹模型模态质量的确定方法;采用随机子空间方法和随机搜索方法识别均匀流场和紊流场中苏通大桥气弹模型的18颤振导数,并和特征系统实现算法识别结果进行对比分析。研究结果表明:节段模型颤振导数识别方法是识别气弹模型颤振导数的有效和实用方法;模型模态质量可以方便地根据几何缩尺比由实桥对应值直接推算得到,且比通过模型试验实测精度更高;不同方法识别得到的均匀流场中苏通大桥气弹模型大部分颤振导数基本吻合;相对于紊流场中的气弹模型而言,均匀流场中的节段模型颤振导数识别精度更高。     

Parametric study on flutter derivatives of bridge decks

The method for identification of flutter derivatives of bridge decks developed by the authors is first briefly described in this paper. To investigate the effects of dynamic parameters of a bridge deck model on the flutter derivatives a test of the sectional Jiangyin Bridge deck's models with different dynamic parameters was carried out in a boundary layer wind tunnel using the present identification method. In both smooth and simulated turbulent flow conditions, a plate model and the sectional deck model of the Jiangyin Bridge were tested to further survey the effects of turbulence on flutter derivatives. The identified results tend to indicate that the effects of parameters of the model and turbulence on the flutter derivatives are negligible.     

Identification of aerodynamic parameters for eccentric bridge section model

The equations of motion for bridge deck section model elastically suspended in wind tunnel are formulated about mass center of the system using the Lagrangian approach, accommodating both the elasticity and damping eccentricities in the formulation. The Subsection Extended-Order Iterative Least Square (SEO-ILS) algorithm is developed in the state space for direct identification of system matrices from free vibration data of section model obtained from wind tunnel testing. The flutter derivatives can be extracted straightforwardly from the difference in the system matrices identified at zero wind velocity and at a specific wind velocity, respectively. By making use of complex modal decomposition technique, a procedure is employed to correct the system matrix at zero wind velocity considering both eccentricities. The proposed method is applied to identify the flutter derivatives of a thin plate section model and the section model of a suspension bridge. The results show a favorable agreement between the flutter derivatives of a thin plate obtained with the proposed method and those derived from the analytic formulae. The identified direct flutter derivatives of the suspension bridge section model also are in good agreements with those obtained using Scanlan's method. It is shown that the use of the corrected system matrix at zero wind velocity leads to better accuracy in identifying the flutter derivatives especially at high reduced wind velocity than using the original system matrix, and the eccentricity is found to have more influence on the cross flutter derivatives than on the direct flutter derivatives.     

根据随机交通状况修正横截面尺寸的大跨桥梁的风致性能

现有的交通状况对细长大跨桥梁(SLB)面板主要影响有两类:1)桥梁横截面尺寸发生改变,这可能会反过来改变颤振导数及作用在桥梁上的风致气动弹性荷载;2)作用于桥梁上的附加动力荷载,包括来自于车辆的动力相互作用。与外部动力荷载——车辆相比,通过改变桥梁横截面尺寸来研究其影响是很少见的。在桥面板上分布车辆模型,在风洞实验室模拟随机交通流对按比例制作的桥梁截面模型进行试验。在风洞试验中通过改变桥梁的横截面尺寸来获得不同的颤振导数,目前的研究是从数值上评估其对大跨桥梁的风致性能,如气动弹性性能、风致响应和潜在疲劳累积性能的影响。     

Effect of relative amplitude on bridge deck flutter

Self-excited wind forces on a bridge deck can be non-linear even when the vibration amplitude of the body is small. This phenomenon is evaluated in this paper. Experiments detecting the nonlinearity are performed first, with the concept of “relative amplitude”, i.e. the amplitude of the externally triggered free vibration relative to the envelope of the ambient response of an elastically supported rigid sectional model. Two types of sectional model, a twin-deck bluff model (model A) and a partially streamlined box girder model (model B) are tested with two extreme cases of relative amplitude. Based on the flutter derivatives of model B, a flutter boundary prediction is subsequently carried out on a cable-supported bridge to manifest the changes of critical flutter wind velocity due to different relative amplitudes. The effect of relative amplitude on flutter derivatives and on the flutter boundary reveals, from the structural point of view, a complex relationship between the self-excited forces and the “structural vibration noise” due to turbulence that is inherent in the interaction of the ambient wind with the structure. Although the aeroelastic forces are linear when the body motion due to an external trigger is not affected significantly by this turbulence, they are postulated to be nonlinear when this “vibration noise” cannot be neglected.     

Aeroelastic effects in a traffic sign panel induced by a passing vehicle

Here, a simple theoretical model of the vehicle induced flow and its effects on traffic sign panels is presented. The model is a continuation of a previous one by Sanz-Andrés and coworkers, now including the flexibility of the panel (and, therefore, the flow effects associated to the motion of the panel). Through the paper an aeroelastic one-degree-of-freedom model is developed and the flow effects are computed from unsteady potential theory. The influence of panel's mechanical properties (mass, damping ratio, and stiffness) in the motion induced forces are numerically analyzed.     

Nonlinear aeroelastic response of slender wings based on Wagner function

This paper presents a method for nonlinear aeroelastic analysis of Human Powered Aircraft (HPA) wings. In this type of aircraft there is a long, highly flexible wing. Wing flexibility, coupled with long wing span can lead to large deflections during normal flight operation; therefore, a wing in vertical and torsional motion using the second-order form of nonlinear general flexible Euler–Bernoulli beam equations is used for structural modeling. Unsteady linear aerodynamic theory based on Wagner function is used for determination of aerodynamic loading on the wing. Combining these two types of formulations yields the nonlinear integro-differentials aeroelastic equations. Using the Galerkin's method and modes summation technique, the governing equations will be solved by introducing an iterative numerical method to predict the aeroelastic response of the problem. The obtained results for a test case are compared with those of linear study which shows good agreement for speeds less than the flutter speed, but the nonlinear model shows limit cycle oscillations for the wing beyond the flutter boundary.     

Application of the k-ω turbulence model for a wind-induced vibration study of 2D bluff bodies

This paper is concerned with the question of an appropriate turbulence model for the computational modelling of bridge deck aero-elasticity. A detailed examination of the suitability of different turbulence models for simulating various aero-elastic phenomena leads to the conclusion that the two equation k-ω RANS turbulence model strikes the right balance between computational efficiency and accuracy in simulating the flow regime. In order to test this hypothesis a rectangular prism with B/D=4 is taken as an example structure and the flutter derivatives are identified from FSI simulations for both low and medium turbulent flow regimes. The simulations are carried out using a block iterative sequential coupling routine that allows for the exploitation of existing fluid and structural solvers. The results show that the k-ω can adequately model the motion induced shear layer dynamics that are necessary for simulating FSI. The results also demonstrate the potential benefits of computational FSI studies in that flutter derivatives (and other aerodynamic coefficients) can readily be obtained without some of the problems encountered in wind tunnel tests.     

Data-driven stochastic subspace identification of flutter derivatives of bridge decks

Most of the previous studies on flutter derivatives have used deterministic system identification techniques, in which the buffeting forces and the associated responses are considered as noises. In this paper, one of the most advanced stochastic system identification, the data-driven stochastic subspace identification technique (SSI-DATA) was proposed to extract the flutter derivatives of bridge decks from the buffeting test results. An advantage of the stochastic method is that it considers the buffeting forces and the responses as inputs rather than as noises. Numerical simulations and wind tunnel tests of a streamlined thin plate model conducted under a smooth flow by the free decay and the buffeting tests were used to validate the applicability of the SSI-DATA method. The results were compared with those from the widely used covariance-driven SSI method. Wind tunnel tests of a two-edge girder blunt type of Industrial-Ring-Road Bridge deck (IRR) were then conducted under both smooth and turbulent flows. The identified flutter derivatives of the thin plate model based on the SSI-DATA technique agree well with those obtained theoretically. The results from the thin plate and the IRR Bridge deck helped validate the reliability and applicability of the SSI-DATA technique to various experimental methods and wind flow conditions. The results for the two-edge girder blunt type section show that applying the SSI-DATA yields better results than those of the SSI-COV. The results also indicate that turbulence tends to delay the onset of flutter compared with the smooth flow case.     

Forced motion and free motion aeroelastic tests on a new concept dynamometric section model of the Messina suspension bridge

The work presents a new original experimental rig to more deeply investigate the aerodynamic behaviour of long span suspension bridges. Set-up and model are designed to grant an accurate study on main aeroelastic phenomena in bridge engineering. More in details, experimental set-up and a 1:60 scale sectional model of Messina bridge deck are presented. The complete rig—composed by dynamometric model, suspension set, experimental set-up and active turbulence generator—is designed in order to execute both forced and free motion tests, allowing to change the average position in terms of angle of attack and yaw angle and to investigate flutter derivatives, admittance functions and vortex-induced vibrations.     

Multimode flutter of long-span cable-stayed bridge based on 18 experimental aeroelastic derivatives

This paper presents flutter analysis of a super-long span (central span 1020 m) cable-stayed bridge based on experimentally extracted 18 flutter derivatives. In the analysis, the aeroelastic forces to be used in the equation of motion utilize rational function approximation of flutter derivatives. The equation of motion is finally recast into aeroelastically modified modal state space form, which yields an unsymmetric eigenvalue problem. The complex eigenvalues thus obtained are used to identify the critical flutter condition. The numerical analysis is performed on a three-dimensional (3D) finite element model of the bridge. The results show that critical flutter velocity based on theoretical flutter derivatives is exceptionally higher than that based on experimental flutter derivatives.     

A new technique for identification of eighteen flutter derivatives using a three-degree-of-freedom section model

The prediction of flutter instability is of major concern for design of flexible structures. This necessitates the identification of aeroelastic parameters, known as flutter derivatives from wind tunnel experiments. The extraction of flutter derivatives becomes more challenging when the number of degrees of freedom (DOF) increases from two to three. Since the work in the field of identifying all 18 flutter derivatives has been limited, it has motivated the development of a new system identification method (iterative least squares method or ILS method) to efficiently extract the flutter derivatives using a section model suspended by a three-DOF elastic suspension system. The accuracy of a particular flutter derivative was determined by comparing the results obtained from all possible DOF combinations.     

风与张拉薄膜结构的耦合作用

张拉薄膜结构具有自重轻、刚度小的特点 ,因而属于风敏感结构。作用在张拉薄膜结构上的风荷载除与气流本身的特性有关外 ,还与结构在风荷载作用下的位移、速度等有关 ,从而引起附加的气动刚度 (或附加质量 )和气动阻尼。因此 ,在研究张拉薄膜结构的风致动力效应时 ,必须考虑风与结构的耦合作用。为此 ,对张拉薄膜结构的风振研究方法进行了总结 ,主要介绍了考虑风与结构耦合作用的简化气弹力学模型方法 ,并介绍了两个简化气弹性模型试验     

缆索承重桥并列索尾流致气弹失稳研究

为了研究表面粗糙度和雷诺数对并列索尾流致气弹失稳的影响规律,以圆心间距为4D(D为圆柱直径)的双圆柱为研究对象,通过风洞试验,在风向角α=0°～20°、雷诺数Re=18000～168800,研究下游圆柱发生尾流失稳的起振条件、振动幅度及运动轨迹等振动特性,分析增大阻尼比对尾流失稳的减振效果,探讨了圆柱表面粗糙度和雷诺数对尾流失稳的作用效应。研究表明,下游圆柱在不同的风向角及风速条件下会出现尾流驰振和尾流颤振2种气弹失稳形式;增大阻尼比对尾流驰振有明显的减振效果,但对尾流颤振的影响较小。尾流致气弹失稳有明显的雷诺数效应,随着雷诺数的增大,下游圆柱的振动形式会由尾流驰振转变为尾流颤振。增加上游圆柱表面粗糙度对下游圆柱气弹失稳的影响较小;而增大下游圆柱表面粗糙度,则会明显降低下游圆柱出现尾流失稳的可能性,并会使发散性振动转变为“限幅限速”振动。     

A Two‐Step Model Updating Algorithm for Parameter Identification of Linear Elastic Damped Structures

A Frequency Response Functions (FRFs)‐based two‐step algorithm to identify stiffness, mass, and viscous damping matrices is developed in this work. The proposed technique uses the difference between the experimentally recorded FRF and their analytical counterparts by minimizing the resultant error function at selected frequency points. In the first step, only mass and stiffness matrices are updated while keeping the uncalibrated viscous damping matrix constant. In the second step, the damping matrix is updated via changes on the selected unknown modal damping ratios. By using a stacking procedure of the presented error function that combines multiple data sets, adverse effects of noise on the estimated modal damping ratios are decreased by averaging the FRF amplitudes at resonant peaks. The application of this methodology is presented utilizing experimentally obtained data. The presented algorithm can perform an accurate structural identification via model updating, with a viscous damping matrix that captures the variation of the modal damping ratios with natural frequencies as opposed to other conventional proportional damping matrix formulations.     

大缩尺比气弹模型风洞试验紊流积分尺度修正

风洞试验气弹模型缩尺比常选为1/300~1/800以满足紊流积分尺度相似要求,但一些复杂结构气弹模型实际缩尺比为1/40~1/100以减少模型加工难度及提高模型精度。这类大几何缩尺比造成紊流积分尺度的相似性严重偏离,必须对紊流积分尺度不相似时的风洞试验结果偏差进行修正。基于随机振动理论,推导了考虑1阶基本振型的顺风向风振响应及风振系数计算表达式。通过对一格构式输电塔风振响应分析,研究了顺风向紊流积分尺度Lxu对该结构风振响应的影响。研究结果表明：紊流积分尺度对结构抖振响应有显著影响,对峰值响应及风振系数影响也较大。对于该塔1/40大缩尺气弹模型风洞试验,由紊流积分尺度不相似带来的风振系数试验值的最大偏差可达27%,风振系数平均偏差也接近14%,试验结果偏保守。为便于应用,建议了较为通用的、由紊流积分尺度不相似引起的修正系数,这一修正系数随着结构阻尼比、结构频率与风谱卓越频率的比值（频率比）的增加而减小。     

Influence of uncertainty in selected aerodynamic and structural parameters on the buffeting response of long-span bridges

Multimode-analysis methods for the study and derivation of flutter instability and buffeting response are readily available from the literature and have been successfully applied to the assessment of the susceptibility of long-span bridges to wind loading. In both cases flutter critical velocity and buffeting oscillation are usually estimated from deterministic analyses. However, the probabilistic nature of the problem is latent since uncertainties, especially those associated with the definition of wind and aerodynamic characteristics, are intrinsically present. These quantities include, for example, wind-turbulence power spectral density, static coefficients and aerodynamic derivatives, usually derived from either site observations or experimental analysis. Their effects are often neglected or usually addressed through sensitivity analyses only.While in the past uncertainty in flutter estimates has been analyzed by researchers (for example through reliability analysis), little attention has been devoted to buffeting. In this paper the effects associated with the random nature of wind and structural characteristics are analyzed through the derivation of a closed-form solution associated with the single-mode buffeting problem with selected random parameters. A specific example is provided to clarify the role of wind power spectral density, damping and selected aeroelastic derivatives.     

Direct identification of flutter derivatives and aerodynamic admittances of bridge decks

Flutter derivatives and aerodynamic admittances provide basis of predicting the critical wind speed in flutter and buffeting analysis of long-span cable-supported bridges. In this paper, one popular stochastic system identification technique, covariance-driven stochastic subspace identification (SSI in short), is first presented for estimation of the flutter derivatives and aerodynamic admittances of bridge decks from their random responses in turbulent flow. Numerical simulations of an ideal thin plate are adopted to extract these aerodynamic parameters to evaluate the applicability of the present method. Then wind tunnel tests of a streamlined thin plate model and a Π type blunt bridge section model were conducted in turbulent flow and the flutter derivatives and aerodynamic admittances are determined by the SSI technique. The identified aerodynamic parameters are compared with the theoretical ones and the results indicate the applicability of the current method.