Turbulent wake of an inclined cylinder with water running

Abstract

This paper presents the results of an experimental study of fluid dynamics around an inclined circular cylinder with and without water running over its surface, covering water rivulet formation, fluid forces on the cylinder, near wake and their interrelationships. The cylinder inclination angle (α) with respect to incident flow was between 55° and 80°. It has been found that water running over the cylinder surface may behave quite differently, depending on the Reynolds number (Re), and subsequently impact greatly upon the fluid dynamics around the cylinder. As such, five flow categories are classified. Category A: one water rivulet was observed, irrespective of α, at the leading stagnation point at a small Re. Category B: the rivulet splits into two, symmetrically arranged about the leading stagnation line, once Re exceeds a critical value that depends on α. The two rivulets may further switch back to one, and vice versa. Category C: two symmetrical straight rivulets occur. Category D: the two rivulets shift towards the flow separation line with increasing Re and oscillate circumferentially. The oscillation reaches significant amplitude when the rivulets occur at about 70° from the leading stagnation point. This increased amplitude is coupled with a rapid climb in the mean and fluctuating drag and lift, by a factor of near 5 for the fluctuating lift at α = 80°. Meanwhile, the flow structure exhibits a marked variation. For example, Strouhal number and vortex formation length decrease, along with an increase in spanwise vorticity concentration, velocity deficit, and coherence between vortex shedding and fluctuating lift. All these observations point to the occurrence of a ‘lock-in’ phenomenon, i.e. the rivulet oscillation synchronizing with flow separation. A rivulet–vortex-induced instability is proposed to be responsible for the well reported rain–wind-induced vibration associated with the stay cables of cable-stayed bridges. Category E: the two rivulets shift further downstream just beyond the separation line; the shear layers behind the rivulets become highly turbulent, resulting in weakened vortex shedding, flow fluctuating velocities and fluctuating fluid forces. Based on the equilibrium of water rivulet weight, aerodynamic pressure and friction force between fluid and surface, an analysis is developed to predict the rivulet position on the cylinder, which agrees well with measurements.