A numerical investigation of the airfoil-gust interaction noise in transonic flows

Leading edge noise is a major source of broadband noise in turbofan engines or on contra-rotating open rotors (CROR). The analytical prediction of the leading edge noise is often based on a at plate model, the accuracy of which can be inuenced by factors such as blade geometry, airfoil angle of attack (AoA), properties of the impinging turbulence and background ow field. Many studies have been conducted on this topic in subsonic cases. However, with an increase in local Mach number, e.g. blades rotating at high speed, locally supersonic regions and shocks may appear around the airfoil that can alter the sound field. More specifically, the sound produced at the airfoil leading edge would be scattered by the shock or refracted by the non-uniform flow, and the interaction between the vortical gusts and the near field flow can generate noise. In this study, numerical experiments are conducted to investigate these effects: (i) pure acoustic dipoles are placed at the leading edge to demonstrate the sound scattering effect by the attached shocks; (ii) a local gust ingestion method is developed to study the sound generation due to the gust distortion in the near field non-uniform ows; (iii) synthetic turbulence is employed to study the broadband noise response. In addition, the effects of the airfoil angle of attack are also studied by conducting both harmonic gust and synthetic turbulence simulations. It is found that the sound directivities are significantly altered for the harmonic gust simulations, especially in the upstream direction. However, the averaged effect can be small for the broadband simulations using the synthetic turbulence. In this study, the far- field directivities are calculated using a new sound extrapolation method, in which off-body integration surfaces are used and the spurious wave contamination issue is avoided.