The Dynamics of the Re-circulation Bubble behind a Bluff Body in a Flowing Soap Film*

In the present investigation, we study the structure and evolution of the near wake of a circular cylinder experimentally in a flowing soap film at low Reynolds numbers. A high-speed camera is used to obtain instantaneous pictures of the formation and detachment of the vortices and capture the changes occurring within the recirculation bubble. Reynolds number is varied in small steps from Re = 150 to the point of no shedding. Two distinct regimes of the near wake structure are observed, the first one corresponds to the Benard von Karman vortex street in the wake region with a short near wake region comprising of continuous random motion of the fluid particles. Vortices are clearly observed to be peeling off from the shear layers alternately. The other regime is characterized by a significantly large recirculation bubble in the near wake and the wake vortices are seen to originate as a result of deflection of the tip of the recirculation region. The transition between these two regimes occurs for a short range of Reynolds numbers and is observed to display interesting phenomena such as the formation of a vortex chain along the elongated shear layers and upstream movement of the small-scale vortices to form two standing vortices eventually.