A numerical investigation 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). Its analytical prediction is often based on a flat plate model, the accuracy of which however can be affected by realistic factors such as blade geometry, turbulence characteristic, and the cascade effect. Many studies have been conducted on this topic and most of them were constrained in subsonic cases. With the increase in the Mach number, e.g. a CROR operates at high speed, locally supersonic region and shocks may occur around the airfoil surface, adding new physics processes to the leading edge noise problem. In this work, a local gust ingestion method is developed and implemented to an established computational aeroacoustics (CAA) solver to study the involved mechanisms. It is found sound induced by the gust-airfoil leading edge interaction can be scattered by the shock wave, yielding multiple interference dips at the upstream region. The gust-shock interaction also makes some contribution to noise emission but is relatively weaker than that generated at the leading edge region. In transonic flows, the size and position of the supersonic regions are sensitive to the airfoil angle of attack (AoA), and the sound emission may, therefore, be affected. To this end, simulations using the synthetic turbulence method are conducted to study the broadband noise response with various AoAs. Similar simulations for typical subsonic configurations are also performed for comparison. In this work, the analyses are conducted based on far-field directivities using a new sound extrapolation method with off-body integration surfaces.