Experimental characterization of the noise generated by an airfoil oscillating above stall

The present work investigates the noise emitted by an airfoil oscillating above stall, thanks to synchronized surface pressure and far-field acoustic measurements in an anechoic open-jet wind tunnel. Both NACA0012 and a NACA63₃ 418 airfoils are tested in order to investigate the effects of the airfoil shape on the static and dynamic stall noise. For static configurations, a smoother transition to stall noise is obtained for the cambered NACA63₃ 418 airfoil, with a gradual increase and shift of the noise spectra at low frequencies as the separation point moves closer to the leading edge of the airfoil. Phase-averaged time-frequency analysis of the oscillating airfoil noise reveals that the light-stall noise and deep-stall noise regimes commonly observed for static airfoils also take place during the oscillations. Increasing the frequency of the oscillations leads to an increase of the duration and amplitude of the stall onset broadband noise, and a delay of the dynamic stall noise to greater angles of attack. In the same way, the dynamic stall noise is delayed for the NACA63₃ 418 compared to the NACA0012, causing the stalled phase to be shorter for the NACA63₃ 418. For the NACA0012, the noise does not depend on the angle of attack before the onset on stall. For the NACA63₃ 418, the noise gradually increases in the pre-stall regime and the stall noise is preceded by a high amplitude separation noise.