Impulsive control of low-frequency oscillation of subsonic flow around an airfoil

Recently, active control of separated flow has been studied based on the amplification mechanism of laminar separation bubble (LSB). In aeronautical viewpoint, the time-varying LSB largely affects the aerodynamic performance of flow around an airfoil, especially for high subsonic flow around an airfoil. The purpose of this study is to extract the amplification mechanism in unsteady basic flow field and to construct the flow control strategy.

We extended the nonlinear optimal perturbation method to time-dependent external forces, and we applied the proposed method to flow around NACA0012 airfoil at zero-incident with $M=0.75, Re=20000$. By selecting the initiation time of forcing application, we considered the effect of phase of basic flow field. The extracted optimal forcing showed the high concentration upstream of LSB in each phase. We also confirmed that the repeated application of optimal forcing to an airfoil provides the increase of lift coefficient and suppression of oscillation, which was largely related by the phase of basic flow field. Based on the modal analysis, these changes of aerodynamic performance were obtained by the reattachment of separation of basic flow field. In the presentation, we will discuss the active control of separated flow around an airfoil using the amplification mechanisms of laminar separation bubble in detail.