A numerical study of the airfoil leading edge noise in transonic flows

Leading edge noise is a significant broadband noise source in turbofan engines or contra-rotating open rotors (CROR), which has attracted many studies in the subsonic cases. However, with the increase of flow Mach number, more physics could be involved in the problem since there are locally supersonic flows and shocks around the airfoil surface. More specifically, the sound generated at the airfoil leading edge could be scattered by the attached shocks because there are discontinuities, and the sound radiation in the upstream direction can therefore be changed. In addition, sound can be produced when the vortical gusts are distorted by the non-uniform mean flow in the supersonic region. In order have a better understanding of this problem, numerical simulations are conducted using a local gust ingestion method and synthetic turbulence in this work. Additionally, it is known the airfoil angle of attack (AoA) has little influence to the leading edge noise for the subsonic cases. However, in transonic flows the background mean flow pattern is sensitive to the airfoil AoA, and it is desirable to study the effect in the transonic situations. To this end, both single frequency and broadband simulations are conducted in this work and the AoA of a NACA0012 airfoil varies from 0° to 5°. The analyses of the acoustic properties in this work are made based on a new sound extrapolation method that allows us to use the off-body integration surfaces. The method is developed based on filtering out the vortical fluctuations on the data collection surface on account of the divergence and convection properties.