Suppression of Low-Reynolds-Number Flow Separation by Controlling the Laminar Separation Bubble with Intermittent Disturbance

The formation of a laminar separation bubble (LSB) at low Reynolds numbers has a significant impact on the aerodynamic performance of an airfoil. An experimental study was conducted to investigate the feasibility of using an intermittent disturbance control method to maintain the stability of this LSB, aiming to suppress the flow separation on a National Advisory Committee for Aeronautics (NACA) 633-421 airfoil model at a low Reynolds ($Re$) number. The dynamic bursting process of the LSB at $\alpha =10°$ and $Re=1.2\times {10}^5$ was reproduced using this flow-control method. During the experiments, dynamic pressure measurements and particle image velocimetry were used to examine the upper-surface pressure distribution and partial flow field of the airfoil model, respectively. It was found that during the bursting process, the LSB could naturally exist for an initial short time period, defined as $T_{\mathrm{maintain}}$, and the laminar separated shear layer kept moving away from the surface, leading to bursting of the LSB as well as a stall. By making use of $T_{\mathrm{maintain}}$, an interval time for the disturbances, $T_{\mathrm{int}}$, was roughly determined and used as the control period for flow disturbances produced from synthetic jet actuators. This intermittent disturbance can interrupt the further development of the natural bursting process of the LSB and trigger a cycle of rebirth of the LSB, forming a stable LSB on the airfoil. A power spectrum of the dynamic pressure measurements at the reattachment point showed that the intermittent frequency dominated the reattachment frequency of the separated shear layer.