Study of the impact of angle of attack on tone frequency by thin airfoil at moderate reynolds number

This paper presents an analysis of the effect of the angle of attack on the tonal noise frequency of the airfoil self-noise at moderate Reynolds number and low angle of attack. It is shown that the tone frequency is sensitive to the angle of attack at moderate angle of attack. The current prevailing prediction formula (Paterson formula) follows the conventional Strouhal scaling, and then unable to take the effect of the angle of attack (AoA) into account. Aim to study the effect of the AoA on the tonal noise frequency variation, high accurate numerical simulation is utilized to reproduce the flow past symmetric NACA0012 airfoil with a rounded trailing edge at three low and moderate angles of attack. Then, the near wake flow instability analysis was peroformed and indentified the source location and generation mechanism that the near wake Kelvin-Helmholtz instability is responsible for the tone generation at the location where the wake has a minimum half-width. Finally, a modification that the fixed geometry length scale (the chord of airfoil) in classical Paterson formula is replaced by the flow-related separation length is made, which can account for the effect of the angle of attack on tonal noise frequency. A good agreement can be observed to demonstrate the accuracy of the corrected Paterson formula on the tone frequency prediction by the NACA0012 airfoil simulation results.