Impact of Circularity Defect of Helical Fillets HDPE Bridge Stay Covers: Analysis of Bi-Stability at Critical Reynolds Number by Bifurcation Diagrams

Dry galloping of cables is still considered by the scientific community as an important phenomenon lacking investigation on cable stayed bridge. The helical fillet cylinders developed to overcome the Rain-Wind Induced Vibration (RWID) were reported to be affected by the appearance of vibration in dry conditions. This paper focuses on a method developed to process data issued from wind tunnel tests on original High Density Polyethylene (HDPE) cable covers with helical fillets, in a range of Reynolds numbers from the sub-critical regime to the critical regime. Corresponding values range from Re = 9.6x10^4 to Re = 3.6x10^5 for the present work. The experiment consists of investigating the effect of the helical fillets with natural or controlled deformation of his circularity defect. Previous studies ([1], [3] and [4]) reported that dry galloping is caused by the appearance of a negative pressure bubble on one side of a circular cylinder at the critical Reynolds number range, leading to a rapid drop in the drag coefficient and the appearance of a non negligible lift force. Because the flow oscillates between two states, say TrBL0 and TrBL1, giving way to a fluctuating lift force supposed to cause the vibration of cables. However, [1] reports that the TrBL1 regime disappears in the presence of helical fillets. The aim of the current study was to investigate a possible appearance of the TrBL1 regime when the circularity defect threshold exceeds the filet thickness. In this case, the second purpose was to characterize the bi-stability of this phenomenon and provide new tools for wind engineering processing.