Experimental study of flow around polygonal cylinders

Abstract

The wake of polygonal cylinders with side number N=2∼∞N=2∼∞ is systematically studied based on fluid force, hot-wire, particle image velocimetry and flow visualisation measurements. Each cylinder is examined for two orientations, with a flat surface or a corner leading and facing normally to the free stream. The Reynolds number ReRe is 1.0×104∼1.0×1051.0×104∼1.0×105, based on the longitudinally projected cylinder width. The time-averaged drag coefficient CDCD and fluctuating lift coefficient on these cylinders are documented, along with the characteristic properties including the Strouhal number StSt, flow separation point and angle θs????s, wake width and critical Reynolds number RecRec at which the transition from laminar to turbulent flow occurs. It is found that once NN exceeds 12, RecRec depends on the difference between the inner diameter (tangent to the faces) and the outer diameter (connecting corners) of a polygon, the relationship being approximately given by the dependence of RecRec on the height of the roughness elements for a circular cylinder. It is further found that CDCD versus ξ???? or StSt versus ξ???? for all the tested cases collapse onto a single curve, where the angle ξ???? is the corrected θs????s associated with the laterally widest point of the polygon and the separation point. Finally, the empirical correlation between CDCD and StSt is discussed.