Active flow control of the laminar separation bubble on an oscillating airfoil near stall

The laminar separation bubble on an X-56A wing section is controlled in experiments and numerical simulations for static and unsteady plunging conditions. The angle of attack is 12 degrees with Re = 200, 000 (11 m/s). Unsteady plunging motion is applied perpendicular to the airfoil chord with k = π f c/U_∞ = 0.70 and h = a/c = 4.8%. These parameters are selected based on laminar separation bubble shedding dynamics and relevance to the 1/2 scale flight model at the University of Arizona. Active flow control in the form of ac-DBD plasma (experiment) and 2D slot blowing/suction (simulation) is employed for both static and unsteady conditions to influence the laminar separation bubble shedding behavior. For both the experiments and numerical simulations, active flow control is on during 75% of the plunging cycle from 90^◦ < φ < 360^◦ with St_c = 52 (1600 Hz). The simulations used a blowing ratio, B = 5% while the experiments used B ≈ 1%, resulting in C_µ = 0.00058% and C_µ ≈ 0.0007% respectively. The application of active flow control eliminates the laminar separation bubble and prevents “bursting” which occurs in the baseline unsteady case. The active flow control mechanism arises from excitation of the primary shear layer instability over the bubble. This produces 2D spanwise coherent structures in both experiments and simulations for steady and unsteady conditions. Excitation of the primary instability limits the onset of the secondary instability thus delaying laminar to turbulent transition all while maintaining attached flow.