Three-dimensional analysis of a single-step cylinder wake via stereoscopic particle image velocimetry

The present experimental study investigates the turbulent flow field past a single-step cylinder, characterized by a fixed ratio r between the large diameter D and the small diameter d equal to 2, for multiple values of the Reynolds number: R e D = 3000,5000 , and 7000. To reconstruct the highly three-dimensional wake that develops behind this geometry, stereoscopic particle image velocimetry (Stereo-PIV) is applied to acquire measurements in thirty-nine crosswise planes, spanning from 0.75 D to 6 D downstream of the cylinder axis. Time-averaged analysis reveals two backflow regions behind both the larger and the smaller cylinder, while no recirculation is found in the step area. Furthermore, an upwash motion of the wake, followed by a subsequent downwash, is observed developing in the streamwise direction. In addition to Stereo-PIV measurements, simultaneous hot-wire anemometry measurements are carried out to identify the shedding frequencies in the wake of the cylinder and associate each velocity snapshot with a corresponding phase, thus allowing a triple decomposition of the velocity field. Phase-averaged data permits the identification of the large-scale coherent vortical structures. Spanwise vortices from the S-cell and N-cell and their dislocation mechanisms are present for all three Reynolds number cases.