Vortex Dynamics in the Turbulent Wake of a Single Step Cylinder

The turbulent wake development of a circular cylinder with a single stepwise discontinuity in diameter was investigated experimentally using flow visualization and two-component Laser Doppler Velocimetry (LDV). A single step cylinder is comprised of two cylinders of different diameters (D and d). Experiments were performed at a Reynolds number (Re_D) of 1050 and a diameter ratio (D/d) of two. A combination of hydrogen bubble and laser induced fluorescence techniques allowed visualization of complex vortex dynamics in the near wake. The results show that turbulent vortex shedding from a single step cylinder occurs in three distinct cells of constant shedding frequency. The differences in frequency and strengths between vortices in the cells lead to complex vortex interactions at the cell boundaries. The results demonstrate that vortex splitting, half-loop vortex connections, and direct cross-boundary vortex connections occur near the cell boundaries. A comparative analysis of flow visualizations and velocity measurements is used to characterize the main vortex cells and the attendant vortex interactions, producing a simplified model of vortex dynamics in the step cylinder wake for Re_D = 1050 and D/d = 2.