Experimental study of flow around polygonal cylinders

The wake of polygonal cylinders with side number N=2 similar to infinity 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 Re is 1.0 x 10(4) similar to 1.0 x 10(5), based on the longitudinally projected cylinder width. The time-averaged drag coefficient C-D and fluctuating lift coefficient on these cylinders are documented, along with the characteristic properties including the Strouhal number Si. flow separation point and angle theta(s), wake width and critical Reynolds number Re-c at which the transition from laminar to turbulent flow occurs. It is found that once N exceeds 12, Re-c 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 Re-c on the height of the roughness elements for a circular cylinder. It is further found that C-D versus xi or St versus xi for all the tested cases collapse onto a single curve, where the angle is the corrected theta(s) associated with the laterally widest point of the polygon and the separation point. Finally, the empirical correlation between C-D and St is discussed.