Passive airfoil tonal noise reduction by localized flow-induced vibration of an elastic panel

In this paper a novel passive control method is explored for airfoil tonal noise reduction using localized flow-induced vibration of a short elastic panel flush mounted on suction surface of a NACA 0012 airfoil at low Reynolds number. The key idea is to provide local absorption of energy of natural instabilities evolving in the laminar boundary layer by self-sustained flow-induced vibration of a properly designed panel. The panel ultimately acts to weaken the aeroacoustic feedback loop responsible for airfoil tonal noise radiation. Perturbation evolution method with acoustic broadband excitation is utilized to assess the tonal noise reduction potential with various combinations of elastic panel design parameters and optimal resonating and non-resonating elastic panel configurations are determined. Subsequently, direct aeroacoustic simulation of airfoil flow with optimal panel configurations are carried out to uncover the mechanism of tonal noise reduction using localized flow-induced vibration in a quantitative manner. Extensive analysis of numerical results reveals that a resonating panel located just ahead of the sharp growth of boundary layer instability within the airfoil separation bubble provides the strongest reduction of flow instabilities and an overall tonal noise reduction up to 3 dB. Such significant noise reduction is achieved without any sacrifice in the original aerodynamic characteristics of the airfoil. The outcomes of the study evidently suggest that the proposed passive control method with a localized flow-induced panel vibration is effective in suppressing the fundamental mechanism responsible for tonal noise generation of airfoil flow, making it a promising approach for modifying the acoustics of existing aerodynamic or wing profile operating at low Reynolds numbers.