Control of airfoil flow separation using internal acoustic excitation

Abstract

Internal acoustic excitation has shown considerable potential for suppressing airfoil flow separation. Recent force measurements by Kiley et al. [“Unsteady aerodynamic flow control at low Reynolds numbers via internal acoustic excitation,” Phys. Fluids 36, 094120 (2024)] have demonstrated that acoustic excitation can improve the lift coefficient, yet the associated transient flow evolution and control mechanism remain unclear. In this study, miniature loudspeakers are embedded inside the leading edge of a National Advisory Committee for Aeronautics 0012 airfoil to control large-scale flow separation on the suction side at a Reynolds number of Re=1.5×105 and angle of attack α=20°. Three excitation frequencies, 50, 250, and 500 Hz, are selected for detailed investigation using time-resolved particle image velocimetry. The results show that acoustic excitation significantly suppresses the suction-side separation bubble and promotes a nearly attached mean flow. Reynolds-stress analysis shows that the applied excitations enhance momentum exchange in the separated shear layer. The enhanced momentum-exchange region for the 50 Hz case is located lower than those for the 250 and 500 Hz cases. Transient flow analysis indicates that, without excitation, the leading-edge vortex is relatively weak and dissipate rapidly, merging into the separated shear layer, whereas acoustic excitation strengthens the leading-edge vortex, enhances near-wall momentum exchange, and drives the separation point downstream. The spectral proper orthogonal decomposition (SPOD) for the baseline case reveals no distinct peak, confirming that the natural separated flow lacks a dominant frequency. For the 50 Hz case, SPOD analysis reveals pronounced spectral peaks at the excitation frequency and its harmonics, together with alternating streak-like modal structures downstream of the separation point, indicating that the 50 Hz excitation modifies the dominant coherent structure, thereby suppressing flow separation.