Aerodynamics of bridge hangers in smooth and turbulent flow and implications on aeroelastic stability

The risk of large amplitude vibrations of bridge hangers due to galloping instabilities has posed a challenge to the engineering and research community. Galloping vibrations can lead to serviceability problems and reduce fatigue life. A number of aeroelastic models have been developed to predict the unstable behavior and to design counteracting measures, i.e. shape modifications and increase of structural damping. In particular, the majority of such models is based on the application of the quasi-steady theory to a 2-D model of the cable vibrating in 1 DoF, in-plane, out-of-plane and torsional. Three key issues are relevant to the sectional galloping stability assessment of dry bridge hangers: (i) the complex aerodynamics depending, on the flow conditions (smooth or turbulent), (ii) the deviation of the cable geometry with respect to that of a perfect circular cylinder, and (iii) the choice of a proper stability criterion. In this paper, aerodynamic force coefficients of a real HDPE plain cable cover were measured in the wind tunnel in smooth and turbulent conditions are presented. Cable irregularities (surface roughness, section distortion and axis curvature) are characterized and correlated to the measured aerodynamics. Then, the aerodynamic coefficients are used to investigate aerodynamic stability using different models from the literature. A comparison of the results has highlighted that the use of MDoF models is not justified, as 1-DoF models are sufficient to predict instability; furthermore, it was found that cable irregularities and flow conditions can have a strong influence on the prediction of aerodynamic stability.