Numerical study of roundness effect on flow around a circular cylinder

Imperfectly round bridge stay cable cross section is speculated to be a key factor for wind-induced large-amplitude cable vibrations observed on site. A delayed detached eddy simulation implemented in Open source Field Operation and Manipulation is used to investigate the flow structure around and in the near-wake of an imperfectly round circular cylinder and the corresponding aerodynamic forces at a Reynolds number of 104 and an attack angle of 0° or 45°. With the increase in roundness imperfection, both monotonic and non-monotonic changes of the mean surface pressure and the wake velocity are found when the cylinder is normal to the flow. At an attack angle of 45° and when the roundness ratio is e/D = 4%, it is found that the geometric imperfection in the cross-sectional shape of the cylinder allows it to the retention of more axial flow in the proximity of the cylinder leeward surface due to a shorter recirculation length. The vortex formed by the intensified axial flow would interact with the conventional von Kármán vortex formation at a frequency a few times lower than that of the latter and lead to intermittently amplified transverse lift. This reveals that imperfect roundness in the cross section of a circular cylinder could be an excitation source of low frequency vibration. Thus, it provides evidence that this kind of geometric imperfection, which commonly exists in real stay cables, could contribute to the mechanisms that trigger large-amplitude or even divergent cable motion, such as dry inclined cable galloping on site.