Vortex formation processes from an oscillating circular cylinder at high Keulegan-Carpenter numbers

Abstract

Development of vortex patterns around a circular cylinder oscillating in quiescent water is investigated using time-resolved particle image velocimetry. Experiments are performed at Keulegan–Carpenter (KC) numbers between 8 and 36 with Reynolds number kept constant at 2400. Similar to previous studies, three modes of vortex patterns are identified and denoted as modes I, II, and III. The development of vortices in each mode at successive phases of cylinder oscillation is studied in details. The classification of modes is based on the development mechanism of shear layers around the cylinder, the number of vortices shed in each half cycle, and the characteristics of the vortex street. Modes I, II, and III are characterized by one, two, and three (or more) vortices generated, respectively, in each half cycle. The appropriate vortex formation length is applied to explain the dependence of number of vortices formed in each cylinder cycle on KC. Vortex shedding in mode I occurs only on one side of the line of cylinder motion. This mode, which occurs at KC between 8 and 16, is observed to have two submodes with different orientations of the vortex street to the line of cylinder motion. Mode II occurs at KC between 16 and 24. The vortex street extends to both sides of the line of cylinder motion and lies at about 45° to it. At KC>24, vortices are shed behind the moving cylinder similar to the case of a towed cylinder. The limited-length vortex street in this mode III pattern lies along the line of cylinder motion. Each vortex pattern is associated with a typical secondary flow stream, which affects distinct evolution stages of vortices around the cylinder and hence the unique vortex pattern. The development of vortices is found to involve complex vortex interaction involving migration, stretching, and splitting.