Aerodynamics of bridge cables with concave fillet

This dissertation reports an investigation into the efficiency of two new innovative cable surfaces fitted with concave fillets to prevent wind-induced vibration on cable-stayed bridges. Focus was directed in particular on the phenomenon of rain-wind-induced vibration (RWIV), which is known as the most common and damaging type of vibration. The recent increase in the number of longer and lighter cable-stayed bridges has resulted in an increase in the occurrence of this type of vibration, requiring an active method of damping (e.g. external dampers between bridge girder and cables). Furthermore, due to the subsequent increase of the aerodynamic static loading induced by the wind action on bridge cables due to their increasing number and length, an optimal level of aerodynamic forces must be maintained when introducing passive aerodynamic means of vibration suppression on bridge cables.
A detailed literature review provided understanding and a basic background on bridge cable aerodynamics, wind and cable interaction in terms of instabilities, a categorization of various passive techniques of control, and an identification of key mechanisms for reducing design drag force.
During the research, extensive wind-tunnel experiments were undertaken to examine the aerodynamics of the cable modifications currently used as well as the two innovative cable surfaces fitted with concave fillets. Two currently used systems consisting of helically filleted cables were directly compared with systems using cables with pattern-indented surfaces under the same conditions, and the two cable configurations fitted with helically concave fillets were found to have advantageous properties. Furthermore, a parametric investigation was carried out on the concave fillet shape to evaluate its effect on aerodynamic coefficients, and the structure of the flow’s nearwake and rain-rivulet suppression. The results show a complete suppression of rain rivulets and an early suppression of vortex shedding in the sub-critical regime for both the helically concave (HC) and the helically staggered concave (HSC) filleted surface modifications. In particular, the HSC filleted surface showed a low drag force in the post-critical regime, similar to the pattern-indented surfaces despite a 100% increase in the fillet height compared to a traditional helically filleted surface.
Finally, particle image velocimetry (PIV) tests were carried out in a wind tunnel in a cross-flow set-up on scaled samples of the helically concave and helically staggered concave filleted cable surfaces. Both two-dimensional and stereo PIV measurements were taken to gain a complete quantitative and qualitative overview and understanding of the development of the near-wake structures in both the streamwise and spanwise directions compared to a plain surface cylinder. The HC filleted surface exhibited a more stable near-wake region than the plain surface. This resulted in a weaker interaction and early suppression of vortex shedding, a smooth transition of drag force in the critical regime and a constant zero lift force. In contrast, the HSC filleted surface created high turbulence at separation, weakening a further development of large-scale vortices inside the wake with an increased base pressure behind the cylinder. These results show that the presence of the staggered concave fillets is able to control the flow and prevent the interaction between shear layers creating vortices, which results in a considerable weakening of vortex shedding, and a more stable nearwake turbulent region. This indicates a reduction in the drag force acting on the cylinder, suppression of vortex shedding, and a smooth transition from the subcritical to the critical regime, with a constant zero lift force.