Delay of Separation and Transition for a Laminar Airfoil Using Active Flow Control

High-fidelity direct numerical simulations (DNS) and global linear stability analysis (LSA) are employed to investigate active flow control for a wing section with a modified NACA 64₃ - 618 airfoil at a chord Reynolds number of Re = 2 × 10⁵ and a zero-degree angle of attack. Flow control is achieved through two-dimensional harmonic blowing and suction via a narrow spanwise slot. Previous studies have demonstrated the effectiveness and efficiency of active flow control operating at the shedding frequency of the uncontrolled flow for controlling laminar separation at low Reynolds numbers, exploiting the primary shear-layer instability. In this work, it is shown that by forcing the flow with a carefully chosen frequency (guided by global LSA) that differs from the natural shedding frequency of the underlying uncontrolled flow, transition to turbulence downstream of the reattachment location can be delayed, along with a significant reduction in the length of the separation bubble. Furthermore, additional DNS are carried out, where very low-amplitude external random 3-D disturbances (in addition to the 2-D disturbances used for flow control) are introduced into the flow to investigate their possible interactions with the instability waves generated by the applied control forcing, and their ultimate impact on the transition delay and relaminarization.