Numerical and Experimental Studies of Flow over Circular Cylinders Covered with Circumferential Riblets or Grooves

Abstract

In this study, the flow past a circular cylinder partly or fully covered with circumferential riblets or grooves is investigated numerically and experimentally over the Reynolds number (Re D) range of 490 ~ 3900. The purpose is to explore the potential of a passive flow control strategy by placing circumferential riblets or grooves of various arrangements on the cylinder surface. Although many passive techniques have been developed for controlling the flow past circular cylinders, there is a lack of research of flow past cylinders covered with circumferential riblets or grooves. Given the extensive engineering applications of cylindrical structures, it is useful to understand the influence of circumferential riblets or grooves on the flow structures around a circular cylinder and the consequent hydrodynamic behaviour of the cylinder. Further, confined flows over circular cylinders with and without circumferential riblets at a fixed blockage ratio of 2/5 are investigated experimentally and numerically. Existing studies of confined flows (i.e., with a non-zero blockage ratio) exclusively focus on bare cylinders, but cylinders covered with riblets under confinement have not been considered. The outcome of this study may be applied for enhancing mixing and heat transfer. Unveiling the flow process and mechanism underlying the effects of circumferential riblets on the flow structures, wake dynamics and hydrodynamic characteristics of a circular cylinder in both confined and unconfined flows is another highlight of this thesis.