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

The aerodynamics of circular cylinders featuring geometric imperfections, such as bridge cables, has received much attention in recent years due to the recognition that such imperfections can be the cause of large amplitude vibrations. Bridge cables are usually made of strands or wires protected by an extruded High Density PolyEthylene (HDPE) circular sheath [1]. In the last 20 years, several bridge cable manufacturers have introduced surface modifications on HDPE sheath in order to reduce the drag and to ensure the aerodynamic stability in all climatic conditions. In the case of plain HDPE sheaths, although manufacturers put in place all efforts to obtain smooth, perfectly circular sections, superficial irregularities such as roughness, labeling and ovalling make the aerodynamic behaviour deviate from that of perfect circular cylinder. The imperfections are the result of the manufacturing process, of mechanical damage occurring during transport and installation, as well as of the ageing process due to the exposure to environmental factors. Few experimental works are already available dealing with the effects of imperfections on the aerodynamics of bridge cables. For example, Matteoni and Georgakis and Larose et al. [2,3] confirming previous research showed that the appearance of a negative pressure bubble on one side of a real HDPE tube at the critical Reynolds number range leads to a rapid drop in the drag coefficient and the appearance of a non negligible mean lift force. Moreover, Matteoni and Georgakis, measuring roughness and shape deviation of the wind tunnel model, justified the measured aerodynamic coefficients. Flamand et al. [4], using the Proper Orthogonal Decomposition (POD), measured the spatial and temporal correlation of the pressure pattern along the HDPE tube with surface and section irregularities, to characterise a bi-stabile behaviour occurring at the critical Reynolds number regime; they showed that only three modes are sufficient to faithfully represent the fluctuation of the pressure field around the cable. Matteoni and Georgakis [5] studied the wind induced response of a full scale yawed bridge cable section model, for varying Reynolds numbers and wind angles-of-attack, using passive dynamic wind tunnel tests. They demonstrated that the in-plane aerodynamic damping of a bridge cable section and the overall dynamic response are strongly affected by changes in the angle of attack. This result is in agreement with the prediction of the quasi-steady theory using the result of static tests, although it was not possible to directly compare the regions of instability based on the static and passive dynamic tests. However, although many authors demonstrated the effects of superficial and sectional imperfection on aerodynamics, little attention has been paid to the refined measurement of aerodynamic data of a real cable and to their use in quasi-steady stability criteria. The purpose of the research herewith is to investigate the aerodynamics of a plain bridge hanger in smooth and turbulent flow and to evaluate the prediction of aerodynamic stability using the different models in literature, correlating these results with the measured imperfections of the tested cable.