FLUID DYNAMICS AROUND AN INCLINED CYLINDER WITH RUNNING WATER RIVULETS

摘要

This paper presents the experimental study of fluid dynamics around an inclined circular cylinder with and without water running over its surface, covering the water rivulet formation, the fluid forces on the cylinder, the near-wake structure and their interrelationships. Two cylinder inclination angles (α) were investigated, i.e., 80° and 55°, respectively, with respect to incident flow. It has been found that water running over the cylinder surface may behave quite differently, depending on the incident flow velocity (U_∞), which has subsequently a great impact upon 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 U_∞, Category B: the rivulet splits into two, symmetrically arranged about the leading stagnation line, once U_∞ exceeds a α-dependent critical value. The two rivulets may further switch back to one, and vice versa. Category C: two symmetrical straight rivulets occur constantly. Category D: the two rivulets shift towards the flow separation line with increasing U_∞ 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, fluctuating lift rising by a factor of near 5 at α = 80°. Meanwhile, the flow structure exhibits a marked variation, including a declining Strouhal number, reduced vortex formation length, velocity fluctuation and velocity deficit, improved two-dimensionality of the flow, increased coherence between vortex shedding and fluctuating lift, and dipped fluid damping at the vortex shedding frequency. All these observations point to the occurrence of a 'lock-in' phenomenon, i.e. the rivulet oscillation synchronizing with flow separation. 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, fluctuating fluid forces and fluctuating wake velocity. Based on the equilibrium of water rivulet weight, aerodynamic pressure and friction force between fluid and surface, analysis is developed to predict the rivulet position on the cylinder, which agrees well with measurements.