Tonal noise generation mechanisms in airfoil transitional flows up to $Re 4\times10^5$*

Large eddy simulations are performed to study tonal noise generation by a NACA0012 airfoil at an angle of attack $\alpha = 3 deg.$ and freestream Mach number of $M_{\infty} = 0.3$. Different Reynolds numbers are analyzed in the range $0.5 \times 10^5 \le Re \le 4 \times 10^5$. The flow patterns responsible for noise generation originate from different laminar separation bubbles, including one observed over the airfoil suction side and another near the trailing edge, on the pressure side. For lower Reynolds numbers, intermittent vortex dynamics on the suction side results in either coherent structures or turbulent packets advected towards the trailing edge. As a consequence, the separation bubbles on the pressure side also become intermittent. Despite the irregular occurrence of laminar-turbulent transition, the noise spectrum depicts a main tone with multiple equidistant secondary tones. Increasing the Reynolds number leads to a permanent turbulent boundary layer on the suction side that reduces the coherence level, causing only small scale turbulent eddies to be observed at the trailing edge. Furthermore, the laminar separation bubble on the suction side almost vanishes while that on the pressure side becomes more pronounced and permanent. As a consequence, the dominant noise generation mechanism becomes the vortex shedding along the wake and the noise spectrum presents a single tonal peak.