Aeroacoustic Investigation of Thin-Airfoil Stall with a View to Improving the BPM Model

Abstract

This paper presents an in-depth investigation into the near-field pressure and far-field acoustic characteristics of a National Advisory Committee for Aeronautics (NACA) 16-506 airfoil across the prestall, stall, and poststall flow regimes at a Reynolds number of 270,000. The paper specifically focuses on examining the effect of the airfoil’s stalling behavior on its self-noise. The NACA 16-506 airfoil was tested in the aeroacoustic wind tunnel facility at the University of Bristol. Remote sensors were employed to record static pressure and unsteady pressure fluctuation data on the airfoil surface. Additionally, a 78-microphone beamforming array was used to measure the far-field sound, and acoustic spectra subsequently were extracted using a delay-and-sum beamforming technique. The NACA 16-506 airfoil was found to stall by the well-established thin-airfoil mechanism, characterized by the development and growth of a leading-edge separation bubble over a range of angles of attack before bursting and leading to full-chord separation. It was observed that the near-field unsteady pressure field began to change significantly at the onset of the leading-edge separation bubble, whereas the near-field steady pressure field, and consequently the aerodynamic performance, only showed significant changes when the separation bubble burst at a higher angle of attack. It was found that the far-field acoustics changed in concert with the near-field unsteady pressure field, rather than with the aerodynamic performance of the airfoil. In essence, the airfoil self-noise was found to increase in the manner typically associated with stall at an angle of attack significantly earlier than the aerodynamic stall angle. A comparison of the measured acoustic spectra and the trailing-edge noise model of Amiet with two forms of the Brooks, Pope, and Marcolini (BPM) noise model revealed that better noise prediction was achieved when the stall switch within the model was set to the angle of attack corresponding to the onset of the leading-edge separation bubble, rather than to the later aerodynamic stall angle. This simple modification to the BPM model is useful for any airfoil known to stall via a thin-airfoil mechanism.