Eccentric-wing flutter stabilizer: Analysis and wind tunnel tests

IABSE Conference, Guangzhou 2016: Bridges and Structures Sustainability - Seeking Intelligent Solutions, 2016

A device is presented that aims at preventing bridge flutter. It consists of wings positioned along the sides of, and fixed relative to, the bridge deck. Flutter suppression efficiency is high provided the lateral eccentricity of the wings is large. It is a passive aerodynamic device that is more economical than other passive measures or devices. Moreover, it does not contain moving parts. This is an advantage over devices with moving parts that meet resistance due to reliability concerns. Results of a numerical study are presented in which the critical wind speed for flutter onset of a bridge without wings and with wings mounted in various configurations were determined. Preliminary wind tunnel test results are reported and a cost estimate is given.

Periodica Polytechnica Civil Engineering, 2012

The flutter instability analysis of a bridge deck is based on flutter derivatives determined by wind tunnel tests on a section model having two degrees of freedom, heave and pitch (hereafter referred to as the heave-pitch model). The imperfections and the eccentricity that arise during the forced sinusoidal vibration of the section yield erroneous derivatives. This paper studies the relationship between these errors and the imperfections. Rotational excitations around two eccentric axes (hereafter referred to as the pitch-pitch model) of the section model show that the determined derivatives are less error-prone to imperfections. Determining the derivatives, like angular speed flutter derivative A * 2 for the aeroelastic torsion moment, gives a more accurate value, so the flutter instability analysis yields a more accurate estimate of the flutter wind speed. Numerical values are presented for the case of a thin airfoil and a bluff bridge cross section.

2009

The aeroelastic stability of line-like slender structures, e .g. wide-spanned bridges, is often verified by calculating a critical wind speed at which aeroelastic phe nom na like galloping, divergence and flutter occur. Therefore the aeroelastic properties of the des ire bridge deck section are needed and commonly determined in wind tunnel tests. Different methods, free and forced vibration testing, exist. A new experimental rig for forced vibration tests in t hree degrees of freedom, heave, pitch and surge motion, has been constructed at Ruhr-Universität Bochum. With this ne w rig harmonic oscillations in a wind-fixed coordinate system are feasible as well as experiments in a bridge deck-fixed coordinate system under an arbitrary angle of attack. A presentation of the mechanical design and the identification algorithm will be given. Results of wind tunnel experiments, identified flutter derivatives for three degrees of freedom, are presente d for a NACA 0020 airfoil and two bridge ...

2003

A new analysis framework that offers direct and explicit expressions for estimating the bimodal coupled flutter is presented. Its accuracy and effectiveness are demonstrated through a flutter analysis of a cable-stayed bridge. This framework is utilized to emphasize significance of the role played by both structural dynamics and aerodynamics on bridge flutter, which helps in better tailoring of the bridge structural systems and deck sections for superior bridge flutter performance. Based on this framework, guidance on the selection of modes and the role of different aerodynamic force components in multimode coupled flutter are offered. The potential importance of the consideration of inter-modal coupling in predicting bridge flutter dominated by the action of torsional mode is highlighted. Finally, a clear insight to the contribution of drag force to bridge flutter is provided.

Proceedings of the Institution of Civil Engineers - Structures and Buildings, 2012

The first part of this paper is devoted to an approximate approach to flutter, which is attained through simplification of the flutter equations. The critical wind speed and the flutter frequency can be calculated with the proposed formulas by employing only three flutter derivatives instead of the usual eight coefficients. This approach may be seen as an easy engineering tool for a better tailoring of bridge structures at early design stages. In addition, the simplicity of the equations allows better understanding of the mechanism of flutter instability and the role played by structural parameters such as damping. In particular, an explanation is provided for soft- and hard-type flutter. The second part of the paper outlines a model to take into account the uncertainty in the measurement of self-excited forces in a flutter analysis. Ad hoc wind tunnel tests allowed determination of the statistical properties of the measured flutter derivatives. These coefficients are treated as ind...

2002

Flutter prediction methods usually rely on tracking modal damping trends,estimated from flight/experimental data, which are not always accurate indicators of flutter onset. This methods is based on a finite element model of the aircraft and does not directly consider flight/experimental data from the physical model. A new approach to computing flutter instability boundaries based on the structured singular value is presented. This approach is developed that utilizes a theoretical model while directly accounts for the variations using the experimental data. The aeroelastic stability problem is formulated in a fremework suitable for well-developed robust stability theory by parameterizing around velocity and introducing uncertainty operators to account for modeling errors. Experimental data can be used to validate the robust system model and increase accuracy of the flutter margin estimate. Parameterization around velocity allows the generalized equation of motion to be a linear function of wind tunnel flow-speed so that perturbations to this parameter can be entered in the form of linear fractional transformation. The µ-analysis method will treat the perturbation as a system uncertainty. Two uncertainty operators are used to describe the modeling uncertainties in the linear aeroelastic model. The first uncertainty operator is associated with the state matrix of aeroelastic linear model. This uncertainty models variations in both the natural frequency and damping values for each mode. The second uncertainty operator is a multiplicative uncertainty on the force input to the linear model. This uncertainty is used to cover nonlinearities and unmodeled dynamics. The level of both uncertainty is determined from reasoning of the modeling process and analysis on the wind tunnel experiment data. Using this method on an aeroelastic wing section system gives a flutter prediction that is closer to the experimental result, which means it can give a better prediction from safety point of view.

Fluid Structure Interaction VII, 2013

In this paper it is shown how to calculate flutter speed on the example of the Great Belt East Bridge in Denmark. Two numerical approaches are shown for prediction of the aeroelastic phenomena on bridges. In the computational fluid dynamics (CFD) simulation turbulence model based on Reynolds Average Navies Stokes (RANS) approach, two-equation shear stress turbulence (SST) models were chosen. Although the SST model needs more computer resources compared to the k-ω and k-ε models, it is still affordable with multi-processing personal computers. In this paper extracted flutter derivatives in the force vibration procedure are shown. Flutter derivatives are later used in the hybrid method of flutter. Final flutter speed was calculated based on flutter derivatives from fluid structure interaction extraction and experimental extraction. Flutter velocity was also determined with a free vibration of deck at the middle of the bridge. The deck section of unit length was clamped into springs and dampers. Flutter speed was reached with time increasing of wind speed until large oscillations occurred. The general procedure of how to formulate the fluid structure interaction and necessary stapes for flutter analysis of the bridge is shown in this paper. Numerically extracted flutter derivatives are compared based on the final flutter speed to experimental measurements of the deck section.

Journal of The Brazilian Society of Mechanical Sciences and Engineering, 2006

A flexible mounting system has been developed for flutter tests with rigid wings in wind tunnel. The two-degree-of-freedom flutter obtained with this experimental system can be described as the combination of structural bending and torsion vibration modes. Active control schemes for flutter suppression, using a trailing edge flap as actuator, can be tested using this experimental setup. Previously to the development of the control scheme, dynamic and aeroelastic characteristics of the system must be investigated. Experimental modal analysis is performed and modes shape and frequencies are determined. Then, wind tunnel tests are performed to characterize the flutter phenomenon, determining critical flutter speed and frequency. Frequency response functions are also obtained for the range of velocities below the critical one showing the evolution of pitch and plunge modes and the coupling tendency with increasing velocity. Pitch and plunge data obtained in the time domain during these tests are used to evaluate the ability of the Extended Eigensystem Realization Algorithm to identify flutter parameter with increasing velocity. The results of the identification process are demonstrated in terms of the evolution of frequency and damping of the modes involved in flutter.

Journal of KONES. Powertrain and Transport, 2013

Aeroelastic phenomena should be considered during the design phase of long span bridges. One of the aeroelastic problems is flutter, the dynamic instability that may cause structural failure at a wind speed called the flutter speed. The prediction of flutter speed of a bridge needs a thorough modelling of bridge stiffness, inertias, and especially its unsteady aerodynamic forces. The potential flow theory is not applicable to calculate unsteady aerodynamics of oscillating bridges due to their non-streamlined complex geometry, and the non-avoidable flow separation. For these reasons, a semi empirical model proposed by Scanlan is used to describe unsteady aerodynamic forces on an oscillating bridge deck.

Journal of Structural Engineering, 2007

Analysis of an aeroelastic bridge system consisting of the fundamental vertical and torsional modes of vibration offers an expeditious assessment of bridge flutter performance. It also produces valuable insight into the multimode coupled bridge response to strong winds. This paper presents closed-form formulations for estimating the modal frequencies, damping ratios, and coupled motions of the bimodal coupled aeroelastic bridge system at varying wind velocities. The derivation of these formulations is based on the assumption of low-level damping of the aeroelastic bridge system. This assumption has also been adopted in current modeling of self-excited forces and the analysis of coupled flutter through complex eigenvalue analysis. This framework leads to a formula for determining the critical flutter velocity of bridges with generic bluff deck sections, which not only provides an analytical basis for Selberg's empirical formula, but also serves as its extension to generic bridges. This formula gives a single parameter or index as a function of flutter derivatives to describe the flutter efficiency of a given bridge section, which facilitates comparison of aerodynamic characteristics of different bridge deck sections. The accuracy of the proposed framework is illustrated through long span bridge examples with a variety of structural and aerodynamic characteristics. Based on the proposed framework, the significance of structural and aerodynamic characteristics on the development of coupled motion and the evolution of modal damping is discussed, which helps to better understand how and where bridges may be tailored for better flutter performance. It is pointed out that coupled bridge flutter is initiated from the modal branch that has a higher modal frequency and is characterized by coupled motion in which torsional motion lags vertical motion. The generation of coupled flutter instability is driven by the negative damping effect caused by the coupled self-excited forces.