On the multiple tones in trailing edge noise of an aerofoil at low-to-moderate Reynolds number flows

In this work, the aeroacoustics of a NACA 0012 airfoil are studied at the Reynolds number ranges from 3.2×10^4 to 3.2×10^5, in which region there are controversies of tonal noise generation, especially for multiple tones. Particle image velocimetry and acoustic measurements are simultaneously performed in an anechoic wind tunnel. A dynamic acoustic feedback loop in different flow regimes is proposed to explain the noise generation mechanism. At the low Reynolds number, the existing tonal noise acts as an external force, trigging the upstream flow structures that will interact with the shedding vortices aft the trailing edge. Consequently, this vortex interaction at the hydrodynamic region can modulate the tonal noise amplitude, resulting in the observed secondary tonal noise. Intermittency of vortex dynamics occurs due to tonal noise amplitudes variation. With the increase of Reynolds number, the boundary-layer transition on the suction side leads to a weak tonal noise. If the Reynolds number is higher, the boundary-layer flow on the suction side is turbulent, leading to the dominance of broadband noise. However, flow instability on the pressure side is triggered by the acoustic waves such that the modulation process takes place again. The quantitative measurements of the boundary layer flow and acoustic signals provide insight into understanding the aeroacoustics of airfoils at low-to-moderate Reynolds number.