Control of the Laminar Separation Bubble on a Plunging Airfoil using Plasma Actuators

Abstract

The laminar separation bubble on an X-56A wing section is studied experimentally for static and heaving/plunging conditions at α=12° with Re=200,000 (U_∞=11 m/s), and compared with numerical simulations. Heaving/plunging motion perpendicular to the airfoil chord with k=0.70, and h=0.48% is applied. Bubble shedding dynamics from previous studies dictated these parameters. Active flow control (AFC) in the form of ac-DBD plasma actuation is employed in experiments for both static and plunging wing conditions to influence the laminar separation bubble. Flat-plate PIV data was used to characterize the actuator performance in quiescent conditions. The experimental data is compared to 2D slot blowing/suction in CFD. In both cases, the AFC is applied for 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 estimates of C_μ=0.000580% and C_μ=0.000534%, respectively. AFC in dimensional frequencies of St_c=26,C_μ=0.000368% (800 Hz) and St_c=104,C_μ=0.000662% (3200 Hz) is also applied to characterize its effect. AFC eliminates the LSB and prevents “bursting” which occurs in the unforced oscillating case. AFC at St_c=52 (1600 Hz) is found to be the most effective, as predicted by CFD simulations. The efficacy of the AFC mechanism arises from the excitation of the primary shear layer instability over the bubble. This produces spanwise 2-dimensional coherent structures in experiments (St_c=52) for both static and plunging conditions. CFD simulations suggest that forcing at the primary shear layer instability can delay transition downstream of reattachment. Similar control authority is observed in experiments, but quantitative evidence for transition delay remains elusive.