Role of separation bubbles in the presence of airfoil secondary tones for NACA profiles*

We perform numerical simulations to investigate the presence of secondary tones in airfoil trailing-edge noise for various NACA profiles at a moderate Reynolds number. The study focuses on two-dimensional airfoils with symmetric NACA profiles 0006 to 0018, at angles of attack ranging from 1 to 5 degrees. The free-stream Mach number is set as 0.2 and the chord-based Reynolds number is 50,000. Our results indicate that the secondary tones are intrinsically linked to the formation of separation bubbles on either side of the airfoils. The noise spectra become increasingly chaotic as the wake dynamics on both sides strengthen with increasing airfoil thickness and incidence angle. At lower angles of attack, thinner airfoils are dominated by a single tone, while thicker airfoils exhibit broadband spectra due to large separation bubbles near the trailing edge on the pressure side. As the angle of attack increases, secondary tones emerge for thinner airfoils. At higher angles of attack, separation on the pressure side is suppressed for all airfoils, resulting in noise spectra dominated by suction side wake dynamics. This study provides physical insights into the effects of adverse pressure gradients on airfoil tonal noise.