Flow separation from polygonal cylinders in an incident flow

Abstract

In this paper, we carry out large eddy simulation of incident flow around polygonal cylinders of side number N = 5 − 8 at Reynolds number Re = 10 4 . In total, six incidence angles ( α ) are studied on each polygon between the face and the corner orientations, thus covering the entire α spectrum. It is found that the separated shear layers behind the cylinders are highly dynamic, manifesting a flapping motion with frequency matching the Strouhal frequency and strength varying significantly at different incidence angles. The energy of the flapping motion is found to be a significant factor influencing the dynamic flow separation behavior, the distribution of the separation points, and features of the time mean shear layer, such as characteristic length and width. Equations for the separation points are analytically derived and are found to be consistent with available experimental results. The time mean penetration distance of the separated shear layers on the top and bottom of the cylinders is found to be a robust scaling factor for the aerodynamic forces and the near-wake length scales. Based on this, a wake deflection angle is proposed, which is demonstrated to be a universal scaling factor for lift, drag, and Strouhal number, working for all available polygonal and circular cylinder data. Finally, the critical separation angle is empirically derived for the condition at which the Strouhal number is a maximum and drag is minimized.