CFD approach for the induced effects of free wake past rivulets on cables of stayed bridges

Abstract

Large-amplitude oscillations of stayed bridge cables appear under the combined effects of crosswind and rain. "Wave-like" oscillations along the whole cable length are born with large amplitudes even under low-speed wind and only the presence of the rain rivulet motion on the cable cross-section which guarantees an "all-or-none" amplification of the dynamic response of the cable. Even though this peculiar behavior has been studied, in a recent past, through dynamic and structural approaches, the phenomenon is not yet quite well understood, i.e., the quasi-cyclic oscillations seem to be activated to events (rain rivulets motion along the cables) compromising the circumferential geometry shape of the cable cross-section and the frequency of the vortex shedding of the leaving wake past the cable. The aim of the present work is to apply a Computational Fluid Dynamic (CFD) approach having in mind that the unsteady wake interaction with the cable dynamics could be useful to explain and predict how the cable fluctuates in an oscillatory fashion and its time-dependent evolution with the leaving vortex frequency under the combined actions of wind and rain. Due to the available computational resources, the K-epsilon turbulent model simulations of an unsteady and viscous wake flow past the cables is here employed with emphasis on capturing the time-dependent flow structures representing the wave-like nature of the aerodynamic loads under the above mentioned atmospheric conditions in which both the (in-phased) cable and rivulet cross-sections are considered to have the same azimuthal frequency and embedded in a free-stream with constant velocity. In this paper, the rivulet movement on the cable surface has been taken into account while a rigid translatory motion of the cable cross section is disregarded, being the aim of the current computational method to find (and predict) the inception mechanism of the elastic vibrations of the cable. As widely accepted and adopted in the current literature, in this initial computational step, the natural frequency of the cable has been fixed being that the only one a priori hypothesis.