Flow around stationary and oscillating polygonal cylinders

Abstract

Understanding the intricate flow dynamics around polygonal cylinders is crucial for various engineering applications, including the design of marine cables, offshore platforms and utilising the phenomenon of Flow Induced Vibration (FIV) to harvest energy from wind or ocean currents. In this dissertation, flow around stationary and oscillating polygonal cylinders is studied using numerical and experimental methods. Large eddy simulation (LES) is used to study the effect of the incidence angle on polygons of N=5-8 at a fixed Reynolds number of Re=10,000. In total, six incidence angles are studied on each cylinder ranging from face to corner orientations, thus covering the entire incidence spectrum. It is found that because of the asymmetric nature of polygonal cross sections at most incidence angles, the flow separation characteristics and hence the induced base pressure distribution and the aerodynamic forces exhibit unique and complex dependence on incidence angle and N. Furthermore, it is found that the separated shear layers behind the cylinders are highly dynamic, manifesting a flapping motion with a frequency matching the Strouhal frequency and a strength varying significantly at different incidence angles. Equations for the separation points are analytically derived and found to be consistent with available experimental results. Experimentally, flow induced vibration (FIV) of a polygon of N=5 as well as a circular cylinder is studied in a recirculating wind tunnel. A series of free oscillation experiments are carried out in order to explore galloping behaviour as well as the lock-in region for vortex induced vibration (VIV). It is found that VIV in the case of N=5 is substantially stronger than the circular cylinder in a similar mass ratio. VIV maximum amplitude change non-monotonically with incidence angle, and it is smaller in incidences where galloping is dominant.