Aerodynamic instability of circular slender structures due to bistable flow activity

Slender structures with circular cylindrical cross-sections are very common in civil engineering, industrial equipment and bridge engineering. Such long cylinders subjected to wind are columns, piles, long cables, chimney, pylons and others. They usually have smooth surfaces and their shape is supposed to be circular, at first glance. It has been observed that if long cables are subjected to wind load, an aerodynamic phenomenon that generates instability can be experienced. This phenomenon (called dry galloping) has been recently theorized to be linked to an unpredictable jump state occurring once the boundary layers of the circular cylinder changes from laminar to turbulent. With a non-stationary character, the force generated at the critical flow regime is named bi-stable activity. In this work, the birth of bi-stable flow activity around a circular cylinder made of High-Density Polyethylene is studied experimentally. The experiment consists of testing a real cable sheath equipped with a large number of pressure taps in a wind tunnel. The pressure pattern is linearly decomposed and geometric modes governing the bi-stable activity as a function of circularity defect are created. The results show that the energy of the non-stationary load observed at the critical flow regime depends strongly on the circularity defect. Furthermore, the inclination angle of 60° confers a significant increase in the energy of the bi-stable flow mode when an unsteady asymmetric reattachment event occurs. The paper also highlights the variability of the mean drag coefficient as a function of the macroscopic defect.