Experimental studies of vortices shed from cylinders with a step-change in diameter

The interactions of vortices shed from two coaxially joined cylinders with a diameter ratio of 0.51 have been studied in a water channel, at Reynolds numbers, based on the large cylinder diameter, between 63 and 1100. Spectral and wavelet analyses of streamwise velocity measurements have identified the formation of three distinct spanwise vortex-shedding cells away from the far ends of the two cylinders, including a low-frequency cell, which appeared in a cyclic manner near the step and behind the large cylinder. Analysis of flow-visualization images has identified two distinct types of vortex connection at cell boundaries: some vortices in a cell of lower frequency connected across the boundary to counterparts in the cell of higher frequency, while others formed a half-loop to connect to a vortex shed from the opposite side of the cylinder in the same cell. Additional cross-boundary vortex connections balanced the total vortex strengths. The interactions of two types of streamwise vortices, identified as edge vortices and junction vortices, with spanwise vortices have also been documented.