Effects of Dual-Frequency Forcing on NACA 0025 Airfoil Separation Control

An experimental study was conducted on a NACA 0025 airfoil at [Formula: see text] to investigate the interaction between disparate forcing frequencies on separation control and coherent flow dynamics. Using a dual-row microblower array, forcing frequencies were set to [Formula: see text] and [Formula: see text] at two distinct chordwise locations. While single-row low-frequency forcing promotes lift recovery through the formation of large-scale coherent vortices, it introduces significant periodic unsteadiness. In contrast, dual-frequency forcing reveals a strong coupling mechanism where high-frequency actuation dominates the global response. Proper orthogonal decomposition analyses demonstrate that high-frequency forcing effectively modifies the large-scale structures into smaller, more localized vortices, redistributing fluctuation energy into higher-order modes. Results indicate that the sequence of actuation is critical: upstream low-frequency entrainment combined with downstream high-frequency stabilization yields the most favorable lift-drag tradeoff among the tested cases. This multifrequency strategy effectively preserves lift recovery while mitigating the deleterious unsteadiness associated with large-scale separation.