Experimental study on instability tonal noise reduction using trailing edge gradient porous materials

Instability tonal noise is an important mechanism for airfoil self-noise generation from the trailing edge at low Reynolds number flow under free field conditions. In this paper, an experimental study on the reduction of airfoil instability tonal noise is presented. The experiment in the 0.55 m × 0.4 m aeroacoustic wind tunnel was performed on a set of NACA 0012 airfoils with trailing edge gradient porous materials. Those materials feature a piecewise gradual change in pores per inch (ppi) between one region and another at the trailing edge. The acoustic measurement showed that a noticeable tonal noise reduction (up to 14 dB overall sound pressure level) can be achieved. Moreover, a preliminary parameter study demonstrated that tonal noise reduction enhances when the porous treatments gradually sparsely transiting from aluminum main body to downstream air flow, whereas diminishes with increasing flow speeds.