Self-noise and directivity of simple airfoils during stall: An experimental comparison

Noise measurements of NACA 0012, NACA 0021 and flat plate airfoils are obtained at a Reynolds number of 96,000, at angles of attack ranging from −30° to 30°. As the airfoils enter a separated flow regime the strength of the dipolar noise due to shed vorticity increases. This noise source grows in strength over a much smaller range of angles as the thickness of the airfoils is increased, causing the sound pressure level of the NACA 0021 airfoil in this frequency range to increase considerably more sharply than the other profiles. In addition the NACA 0021 test model displays noise behaviour that is consistent with an unstable transition between prestall and stalled flow states, compared to transitions involving intermediate flow states observed for the NACA 0012 and flat plate airfoils.