Evolution of Vortex Shedding Around a Circular Cylinder in a Flowing Soap Film

In this experimental study, we examine, for the first time, the structure and evolution of the near wake of a circular cylinder in a flowing soap film at the onset of vortex shedding. The investigation focusses on the changes occuring within the recirculation bubble and the development of vortex shedding. A notably large recirculation bubble is observed behind the cylinder, marked by the presence of small-scale vortices along its edges, a feature that differs significantly from typical three-dimensional flows. These vortices, driven by the Kelvin-Helmholtz instability, cause a transverse displacement of the recirculation bubble, which in turn leads to the intermittent formation of wake vortices. The instantaneous velocity fields in the wake reveal clear signatures of this intermittent shedding behavior. We further analyze the frequency and wavelength of the small-scale vortex structures on the recirculation bubble and establish a functional dependence on the Reynolds number. This finding appears to be novel and may offer new insights into the mechanisms governing two-dimensional transitions in bluff body wakes.