Effect of morphed trailing edge on the acoustic and flow characteristics of National Advisory Committee for Aeronautics (NACA) 4418 airfoil

Trailing-edge profile has an important impact on the aeroacoustic behavior of an airfoil, particularly the tonal noise in the low-medium Reynolds number range. Three profiles, NACA 4418 defined by the National Advisory Committee for Aeronautics (NACA) and its morphed variants with the trailing-edge deflections at ±8°, are investigated by wind tunnel experiments to reveal the acoustic and flow characteristics at the chord-based Reynolds number ranging from 1.2×105 to 3.1×105. The consistent observation across all profiles is the dominant feedback loop located on the pressure surface. The aeroacoustic findings show that the morphed trailing edge alters the tonal frequency spacing, mainly due to the change of the vortex convection velocity in the feedback loop. Furthermore, the trailing-edge morphing is observed to modulate the mode switching of the dominant tones. The local flow results reveal that the laminar separation bubble on the pressure side amplifies the flow oscillation. Notably, the profile with the trailing-edge upward deflection, characterized by a laminar separation bubble on the pressure surface, is particularly susceptible to the low-frequency broadband oscillation. The joint analysis of the acoustic and flow fields suggests that the laminar separation bubble heightens the sensitivity of the dominant tone mode switching to the incoming flow velocity.