Control of vortex breakdown using vortex generators

Wing-tip and leading-edge vortices play an important role in the maneuverability and safety of aircraft. Depending on the application, vortices may need to be consolidated or dispersed. To develop effective means to suppress or enhance vortex breakdown (VB), we study a swirling flow in a sealed cylinder. To understand the mechanism of VB control, such a confined flow is relevant as it is free from ambient disturbances. The basic flow is driven by the rotating bottom disc and is characterized by Reynolds number Rej. A thin vortex core (developing along the axis for large Rej) undergoes VB. The core is viewed as the most sensitive flow region and is forced by a central rotating rod of radius close to that of the core. Flow visualization reveals that even weak forcing by the rod (with Reynolds number Rer ~ Re_d/100) strongly influences the flow. With increasing co-rotational speed of the rod, the size of the initial steady VB ‘bubbles’ decreases, eventually resulting in a conical wake without recirculation. In contrast, with increasing counter-rotation, the number and size of the bubbles increase, and the bubbles become vortex rings, which appear near the top disc and advect toward the bottom disc, undergoing pairing and tearing. We here develop a simple theoretical model to explain the physical mechanism of these flow transformations. The VB control methods in this study can be used in aircraft and vortex devices.