Dynamics of Laminar Separation Bubble Forced by Rectangular Synthetic Jets

Experiments are conducted to study the response of a laminar separation bubble on an airfoil to rectangular synthetic-jet forcing applied within the fore part of the bubble. The chord Reynolds number is 20,000, and the angle of attack is 8 deg. The flowfields are measured using time-resolved planar and tomographic particle image velocimetry. The three-dimensional data are augmented by a physics-informed neural network to gain insights into the development of coherent structures. Three forcing frequencies, Formula: see text, 0.73 and 1.47, are considered at a constant forcing amplitude. The results show that forcing effectively reduces the bubble at frequencies much lower than the unforced bubble frequency (Formula: see text). This is different from previous optimal control scenarios, where the forcing frequency typically matches the latter. The changes in the bubble topology are associated with the influence of forcing on the shear-layer dynamics. On one hand, the separated shear layer can attach to the surface under the suction effect of synthetic jets, reducing or eliminating the recirculation region. On the other hand, forcing can suppress the formation of hairpin vortices downstream of the orifice and generate high-speed regions near the wall through the interaction between streamwise vortices and hairpin vortices outside the orifice. Linear stability analysis reveals that effective bubble control is achieved through low-frequency forcing due to a significant reduction in the most unstable frequency of the forced flows.