INVESTIGATION ON THE DEPENDENCE OF BRIDGE DECK FLUTTER DERIVATIVES ON STEADY ANGLE OF ATTACK

Abstract: The prediction of the flutter velocity as well as the response to turbulent wind has become a more and more important issue for flexible long-span bridges. For this reason flutter derivatives have to be measured in the wind tunnel with great accuracy and their dependence on the mean angle of attack, which is sometimes very strong, has to be considered. In addition, if a common free-vibration set-up is adopted, the mean flow incidence may vary with the wind speed, due to the static rotation of the model, thus strongly limiting the generality of the identified aerodynamic coefficients. In this paper a method to determine flutter derivatives at constant angle of attack using Neural Network approximating functions is proposed and an application in the case of a trapezoidal single-box girder deck section model is shown. Moreover, the strange static and dynamic behavior of this bridge cross section for small negative angles of attack has been investigated through the Particle Image Velocimetry technique. It has been found that for these flow incidences the width of the wake is significantly larger than for higher or lower angles of attack, probably due to the fact that separation occurs at the upstream lower corner instead of the downstream one. The physical explanation for this local instability of the flow field has not been found yet.