Aeroacoustic investigation of an oscillating airfoil in the pre- and post-stall regime

The present study concerns an experimental investigation of the aerodynamic and aeroacoustic characteristics of a sinusoidally oscillating NACA 0012 aerofoil. The experiments were conducted in an aeroacoustic wind tunnel with a uniquely designed Kevlar-walled test section. Prior to experiments, these Kevlar walls were calibrated carefully and shown to provide reliable and accurate aerodynamic and aeroacoustic measurements. Two different regimes of interest, namely the pre- and post-stall angle of attack regimes, have been examined for the lift curve polars, far-field noise spectra and unsteady surface pressure spectra. Interestingly, when the lift polar hysteresis is small at pre-stall angles of attack, the unsteady surface pressure spectra of the oscillating airfoil can be predicted with satisfactory accuracy using a position based weighted averaging approach from its static counterparts. On the other hand, such a method becomes invalid at post-stall angles due to the presence of a significant dynamic stall hysteresis. Instead, an increase in the mean surface pressure and far-field noise spectra are observed at dynamic stall conditions. Furthermore, a short-time Fourier transform analysis reveals that the increase of the surface pressure spectra can be a direct result of the periodic production and convection of dynamic stall vortices.