Frequency-lock reactive control of a separated flow enabled by visual sensors

In this study, a physical-based algorithm is used to control the separated flow downstream of a backward-facing step. It is well known that the spatial extent of the re-circulation bubble can be minimized when acting on the shear layer at the shedding frequency. Using this information in a closed-loop algorithm is not straightforward because of the difficulty of measuring in real time the shedding frequency accurately without disturbing the flow. A novel real-time analysis of 2D velocity fields uses vortex detection to dynamically measure the shedding frequency. Actuation (pulsed jet) is then locked on this frequency. If flow characteristics stray too far from a set point, shedding frequency is updated. The present work demonstrates the efficacy and robustness of this approach in reducing re-circulation, while Reynolds number is randomly varied between 1,400 and 2,400.