Numerical investigation of the aeroacoustic mechanisms of loading noise induced by dynamic stall of rotor airfoils

To investigate the vortex dynamics and loading noise radiation characteristics during dynamic stall of helicopter rotor airfoils, a numerical study is carried out using the unsteady Reynolds-averaged Navier–Stokes method and the Ffowcs Williams-Hawkings equation. First, the aerodynamic and aeroacoustic characteristics of NACA0012 airfoil in dynamic stall were calculated. The numerical method is validated through comparisons with experimental data. Then, the aeroacoustic characteristic of an airfoil in the typical deep stall condition is analyzed, with a detailed discussion on the relationship between flow phenomena and sound pressure (SP). Based on the vortex evolution over airfoil surface, a model for loading noise driven by vortex evolution during dynamic stall is presented. Finally, the impact of reduced frequency (k) on the loading noise characteristics of the airfoil is analyzed, and conclusions are obtained. The dynamic stall loading noise is primarily caused by the shedding and movement of the dynamic stall vortex (DSV) and trailing edge vortex, with DSV shedding inducing rapid and intense SP fluctuations, resulting in high-intensity noise of up to approximately 30 Pa. An increase in k enhances the rate of change in unsteady surface pressure coefficient (Cp), which accelerates pressure variation and leads to a significant rise in overall sound pressure level.