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Covert feathers on bird wings deploy passively on both the upper and lower surfaces and can enhance lift under adverse flight conditions, yet most covert-inspired flap studies have focused on single-side configurations and have not clarified how dual-side flexible flaps interact. In the present study, we perform fluid–structure interaction simulations of a NACA0012 airfoil equipped with flexible flaps on both surfaces at Re = 1000 in the post-stall regime, systematically varying the suction-side and pressure-side attachment locations (xs, xp) and the flap bending rigidity (γ) to examine dual-flap lift enhancement. The optimal dual-flap configuration with (xs, xp, γ) = (0.4, 0.9, and 0.0005) achieves an approximately 73% increase in mean lift relative to the clean airfoil. In this configuration, the suction-side flap at xs = 0.4 not only suppresses upstream propagation of reverse flow induced by the trailing-edge vortex but also works in concert with the pressure-side flap to amplify several additional lift-enhancing effects. With the pressure-side flap placed near the trailing edge at xp = 0.9, the lower-surface positive pressure increases due to enhanced blockage, while the separated wake is reorganized through stronger trailing-edge vortex development and roll-up. These effects intensify the interaction between the leading-edge vortex and trailing-edge vortex and promote secondary-vortex activity, thereby sustaining stronger and longer-lasting suction on the upper surface. Furthermore, flap flexibility provides an additional benefit, with the most flexible case yielding about 6.9% higher lift than the corresponding rigid-flap configuration. Finally, we assess the stability of the resulting unsteady lift response based on wake-mode classification with Lyapunov-exponent analysis.
Kim et al. (Fri,) studied this question.